Unit IlI—Ignition of Flammable and Combustible Materials and Explosives

EXPLOSIONS

Upper flammable (explosive) limit is defined as the greatest concentration of
gas or vapor that will meet those conditions. Thus, the upper and lower limits
describe the flammable range. The flammable range is sensitive to ambient
temperature and pressure. In particular, increasing the ambient temperature
widens the flammable range by reducing the lower limit and increasing the
upper limit.

When considering the ability of an electrical discharge to ignite flammable
materials, it is useful to consider ignition energies in relation to concentrations
for a gas/air mixture. Figure III-4 shows a typical curve for a given substance
relating ignition energies over the range of upper and lower explosive limits.
Table II-2 lists minimum spark ignition energies for various fuels.

An explosion is generally defined as a sudden and violent release of large
amounts of gas. This definition includes rupture of high pressure vessels, such as
a boiler, and chemical reactions. Here we are concemed with chemical reactions,
which can be either a deflagration or a detonation.

Figure D-4. Typical curve relating ignition energy
to concentration for a gas/air mixture,

Table III-2. Minimum Spark Ignition Energies
for Various Fuels*
Ignition energy
A Fuel Minimum Ignition Energy
(104 Joules)
Acetylene 02
Hydrogen 0.2
| j Ethylene 0.96
Methanol 2.15
| | Furan 2.25
| I Propylene 2.82
| | Ethane 2.85
| | Propane 3.05
MLE.-- —.— — Methane 47
| 100 Isobutane 5.2
= Ta ! Cyclohexene 5.25
% of Benzene 55
Flammable gas Isopropyl alcohol 65
Vinyl acetate 7.0
Acetone 1L5

Adapted from Risinger, 1985.

where MIE is the minimum ignition
energy and LEL is lower explosive
limit and UEL is the upper explosive

limit.

IIl-7


Ni!

ALE

REQUEST FOR ASSISTANCE IN

JULY 1989

Preventing Electrocutions

of Workers Using Portable
Metal Ladders Near
Overhead Power Lines

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service

Centers for Disease Control

National Institute for Occupational Safety and Health



Unit 1I—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Contact with
conductors when the
insulation has failed

Mechanical

High-voltage sources are not restricted to overhead lines. Although safety
codes require high-voltage equipment to be enclosed, injuries and fatalities
continue to occur. Failure to secure the enclosures against unauthorized entry
has led to injuries involving people from the curious to the vandal. At a mini-
mum, such enclosures should always be designed with a locking system that
can only be opened by trained and authorized individuals. Other methods, such
as a system that will automatically cut power when an enclosure is opened,
will be discussed later in this module. We will also discuss necessary safety
precautions for authorized and skilled workers when bare energized conduc-
tors are present (Hammer, 1989).

Electrical consumer products and appliances in the home are also potential
sources of shock if protective covers are damaged or removed. Typically, some
conductors inside electrical equipment are not insulated because unskilled
human contact is not expected. When unskilled or unsuspecting people dis-
mantle or break through protective enclosures, there is a potential for shock.

In many cases a conductor is covered with an insulating material that keeps a
current from passing to another conductor, including the human body. If, how-
ever, the insulation is defective by design, deterioration, or damage, there is a
high potential for equipment failure or electric shock. Deterioration or damage
can be caused by a number of factors and in a number of ways. We will pre-
sent an overview on the breakdown of insulating materials here, but be aware
that there is a wealth of information and theory on the subject, which you
should consult as needed.

There are four basic ways, listed below and summarized in Figure II-2, that
insulation may fail. As you read this section, understand that these categories
are not mutually exclusive—that these factors often combine to cause an insu-
lator to fail.

1. Mechanical stress, as with friction, can cause tearing and crushing.

2. Excessive electrical stress can occur when equipment experiences volt-
ages significantly beyond the levels it was designed for.

3. Temperature, either excessively high or low or temperature cycling, can
cause differential expansion or contraction of a material.

4, Chemical and physical reaction, such as oxidation, can contaminate or
leach important ingredients (Cooper, 1986).

Concerning mechanical failures, the best defense is through design. For exam-
ple, design the physical product or device so that insulated conductors are not
subject to mechanical loading. If that is not possible, be sure that the insulation
used can withstand all foreseeable mechanical loading. An electro-mechanical
industrial robot, for instance, may have insulated wires passing through multi-
ple articulations and being subjected to continual bending, friction, and possi-
ble pinch points. As a designer, you would want to minimize as much bending
as possible; minimize the friction either by guiding the wire so that it won’t
rub against (or be pinched between) other parts or by covering such areas with
low friction materials. As always, proper inspection and maintenance proce-
dures must be recommended by the designer and be followed by the user to
ensure that, in this case, the insulation is in good condition.

IL-5


Page 3 - Request for Assistance in Preventing Electrocutions Due to
Damaged Receptacles and Connectors

welder into a broken "female" connector of an extension cord. As in
the previous case, the victim inserted the ground prong of the
welder cord 90 degrees clockwise away from the appropriate ground
terminal of the extension cord, and the metal casing of the welder
connector became energized. It appeared that the connector on the
extension cord had been damaged by everyday use or abuse (being
thrown down on and dragged across concrete floors, being run over by
industrial equipment, etc.).

REGULATORY STATUS

Although, in these investigations the receptacles and connectors in
these investigations were listed by a nationally recognized testing
laboratory, the damaged state of the receptacles negated their
conformance ‘to these listings*, to the manufacturers’ specifications,
and to the safety features inherent in their design. NIOSH strongly
urges periodic inspection and maintenance of electrical systems to
assure compliance with applicable sections of the National Electric
Code, OSHA standards, and other listing requirements. Electrical
components should be used only in accordance with the manufacturers'
specifications, and should be tested and approved by a nationally
recognized laboratory (such as Underwriters Laboratory, Factory Mutual,
etc.).

CONCLUSION

The investigations by NIOSH indicate that damaged receptacles may
physically permit improper electrical connections to be made, negating
the intended safeguards designed into them. Furthermore, workers may
not recognize a hazard of electrocution associated with the use of worn
or damaged receptacles and connectors. Electrical hazards of this sort
are of particular concern because of the large number of users of
electrical equipment in all kinds of workplaces. Investigations of such
incidents suggest failures in the areas of PROPER UTILIZATION OF
ELECTRICAL COMPONENTS, HAZARD RECOGNITION, and PERIODIC INSPECTION
AND MAINTENANCE OF ELECTRICAL SYSTEMS. These basic safety activities
are potentially lifesaving in preventing such incidents.

Caution should be used around ALL electrical circuits and equipment.
The potential for electric shock should never be underestimated.
Employers and other groups should regularly emphasize the safe use of
electricity in the workplace. Continuous efforts must be made to
prevent electrical injuries and deaths due to damaged receptacles and
connectors.

*A listing means that the piece of equipment has met the safety criteria
established by the testing laboratory.




Page 3 - Request for Assistance in Preventing Electrocutions from Contact
Between Cranes and Power Lines

Additionally, 1926.550(a)(15) requires that: a person be designated to observe
the clearance of the crane when it is difficult for the crane operator to use
direct observation; cage-type boom guards, insulating lines, or proximity
warning devices may be used, but their use does not eliminate the need to
adhere to the other parts of the regulation; any overhead wire is to be
considered energized until the owner of the line or the electric utility
indicates that it is not energized and that it has been visibly grounded;
transmitter towers should also be de-energized or tests shall be conducted to
determine if an electrical charge has been induced on the crane. Induced
charges shall be dissipated by providing an electrical ground directly to the
upper rotating structure supporting the boom; ground jumper cables shall be
attached to materials when an electrical charge is induced; crews shall be
provided with nonconductive poles to attach the ground cable to the load;
combustible and flammable materials shall be removed from the immediate area
prior to operations.

The Construction Safety Association of Ontario, Canada (CSA-Ontario),
recommends safe work practices [4] beyond those addressed in the OSHA standard
including the use of nonconductive taglines to guide loads and the use of
insulating personal protective equipment by exposed workers.

APPLICATION OF EXISTING STANDARDS AND RECOMMENDED WORK PRACTICES

Table 1 presents an analysis for each of the five cases described in this
alert regarding compliance with the OSHA standard or CSA-Ontario recommended
work practices. In two of the cases, neither the OSHA standard nor the
CSA-Ontario recommended work practices were being followed. In the remaining
three cases, only one of these safe work practices (avoiding the storage of
materials directly under power lines) was being followed. In each of these
five cases, there was demonstrable lack of compliance with the OSHA standard.

CONCLUSION

The principal objective of the investigations undertaken by NIOSH as part of
its Fatal Accident Circumstances and Epidemiology (FACE) Project is to
determine what factors enabled the fatality to occur. The goal is to learn
how such fatalities can be prevented. In this context, whether or not an
operation was "in compliance" with existing standards is but one of many
variables which may or may not have contributed to the fatality. However, in
the course of the investigations reported here, it became obvious that full
compliance with relevant OSHA standards and full use of the CSA-Ontario work
practices would have prevented each fatality.

As an obvious first step in preventing such fatalities in the future, we
conclude that all such operations should be done only in compliance with
existing OSHA standards.


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Pressurization

Arcs and sparks

Seals are », contained within an

placed in i approved housing.

threaded 73

conduit, Seals limit the
explosion to an
area close to the

source.
he

Specially designed
equipment can be
located in hazardous areas.

The equipment is designed so that
only cool gases are allowed to vent
to the surrounding hazardous area.

Figure III-6. When sparks or arcs ignite flammable gases,
vapors, or liquids, explosion-proof equipment con-
tains the explosion and vents only cool gases into
the surrounding hazardous atmosphere (adapted
from U.S. Department of Labor, 1981).

When explosion-proof equipment generates sufficient heat to increase the
potential of ignition, that equipment must be cooled so that the casing does not
reach temperatures that may contribute to an external atmosphere explosion.

If appropriate explosion-proof equipment is not available or feasible, pressur-
ization systems may be possible alternatives for electrical equipment located
in hazardous areas. Explosion-proof equipment is generally most appropriate
and economical for individual units. Pressurization may be suitable when a
large unit or area or a number of units are to be contained in one enclosure.
Pressurization may also be advisable or necessary for some types of equipment
operating in high altitude. Because the lower pressure reduces the dielectric
effects of air, electrical discharges may occur at lower operating energies than
those at sea level.

Pressurization is accomplished by providing a constant flow of air or inert gas
to the inside of the enclosure so that the internal pressure is higher than that of
the surrounding atmosphere. The increased, positive pressure prevents the
flammable atmosphere from entering the enclosure where it might be ignited
by the electrical equipment. The constant flow also clears the enclosure of any
contaminants that may enter. Using an inert gas has an added advantage in that
it contains no oxidizing agents, a necessary element for fire or explosion.
Because a failure could be very hazardous, the enclosures are designed with a
pressure-sensing switch that prevents activation or halts operations if the pres-
surization fails.

IIl-16


Page 6 - Request for Assistance in Preventing Electrocutions Due to
Damaged Receptacles and Connectors

REFERENCES

1. NIOSH Alert: Request for Assistance in Preventing Electrocution

from Contact Between Cranes and Power Lines. DHHS (NIOSH)
Publication No. 85-111, National Institute for Occupational Safety

and Health, 6 pages (1985).

2. NIOSH Alert: Request for Assistance in Preventing Grain

Electrocutions. DHHS (NIOSH) Publication No. 86-119, National

Institute. for Occupational Safety and Health, 4 pages (1986).

# U.S. GOVERNMENT PRINTING OFFICE: 1993: 75-193/90001


Unit I1—Ignition of Flammable and Combustible Materials and Explosives

Experiments have shown that oils generally need a resistivity greater than
10 Q-centimeters to generate appreciable static charge accumulations and
that charge generation increases with the velocity of flow. Although heavy oils
are good generators of static charge, alone they are generally not a hazard
because of their low volatility. They have contributed to many fires and explo-
sions, however, when they are loaded into a vessel that previously contained a
lighter fuel such as gasoline. If residual vapors from the previous fuel reach a
flammable mixture, a static discharge from the loading process will result in
explosion or fire. Protection against this hazard usually involves making the
vessel atmosphere inert or purging the atmosphere during the loading process
(Howard, 1985).

Several general approaches to controlling static hazards during loading and
unloading tasks are worth mentioning here, but you should, of course, consult
specific texts, standards, and experts should your responsibilities ever include
such activities.

As always, controlling the potential for a flammable atmosphere is the first
step; however, the nature of hydrocarbon products makes this difficult in some
cases (inerting the atmosphere or purging are possible methods). Controlling
the generation, accumulation, and discharge of static electricity is done in sev-
eral ways.

One important control of static charge generation in all applications is mini-
mizing or eliminating water from settling through an oil. Keeping water from a
tank and keeping any water that does enter from being agitated are necessary
procedures. Limiting the velocity of flow in pipes, reducing splashing, and
minimizing contaminants are all common ways to reduce generation of static
charges.

Bonding and grounding can control accumulation of charges. Pipes carrying
the liquids should be of conducting materials, and all flanges and connections
should be bonded if not of conducting material. For example, loading fuel into
a tank truck would involve making an effective conducting bond between the
loading hose and the tank and ground. This does not, however, eliminate
charge accumulations on the surface of the fluid in the tank nor is there an
effective method to do this. That is, although the potential difference between
the hose and ground are minimized, a potential difference between the fluid
surface and the inner walls of the tank is still present, and sparking over the oil
surface can occur. Sometimes the only way to control the sparking-over hazard
is to minimize charge generation; this, however, depends on the type of tank
and process. With stationary tanks, a floating roof can be used to eliminate
sparking over the fluid surface. The potential for static discharge can be
reduced by minimizing protruding surfaces inside the tank (see Figure III-10).

Til-27


Unit I—An Introduction to Safety and Some Basic Electrical Engineering Concepts

Impedance

Voltage

Direct current (dc)

Alternating current (ac)

Capacitance

As an alternating current passes through a circuit under an applied voltage, it
is impeded. This impedance may be due to resistance, inductance, capacitance,
or a combination thereof and is measured in Ohms (Q).

If electrons are to move in some concerted manner through a conductor (i.e.,
as an electric current), there must be some potential difference applied to the
material. For example, the ends of a wire must have a charge difference
(potential difference), so that the electrons will flow to equalize it. Such a
potential difference is referred to as voltage. In other words, if we want to
move charge through a wire, we must apply a force (electromotive force
[EMF)) that causes a potential difference, or voltage. Without a voltage, there
will be no current.

A direct current, or de, is one in which the direction of flow does not change
and may remain at a constant level over time, as shown in Figure 1.2 (a).

An alternating current, or ac, is one that changes with time, as shown in Figure
1.2 (b). In the United States, household current is a sine function with a fre-
quency of 60 cycles/second (60 hz). That is, the current alternates back and
forth 60 times each second. As we will see later, the frequency of an alternat-
ing current has significant implications for human safety. Depending on the
application, alternating currents may have many different waveforms other
than a sine wave.

(a) (b)

Figure 1.2. (a) Direct current is constant over time; (b)
alternating current fluctuates over time.

A capacitor is a device consisting of two conducting materials separated by
a dielectric medium. A dielectric is a nonconducting material, such as air.
Because of this arrangement, the flow of current in a capacitive element is
nonlinear, taking the form
. dy
i=C =
dt

where i is the current (measured in amps), C is the capacitance (measured in
farads) and dv/dt is the rate of change of voltage (measured in volts/second).
The higher the frequency of the voltage, the lower the resistance to current
flow. For a steady state direct current (dv/dt) = 0), then, a capacitor would act
as an opening in the circuit and no current would flow through the element.

19


Suggestions, requests for additional informa-
tion on safe work practices, or questions re-
lated to this announcement should be directed
to Dr. Thomas R. Bender, Director, Division of
Safety Research, 944 Chestnut Ridge Road,
Morgantown, West Virginia 26505-2888;
telephone (304) 291-4595.

We greatly appreciate your assistance.

. Donadd Millar, M.D., D.T.P.H. (Lofnd.)

Assistant Surgeon General

Director, National Institute for
Occupational Safety and Health

Centers for Disease Control

REFERENCES

CFR [1988]. Code of federal regulations.
Washington, DC: U.S. Government Printing Of-
fice, Office of the Federal Register.

NIOSH [1984]. Request for assistance in
preventing electrocutions of workers in fast food
restaurants. Cincinnati, OH: U.S. Department of
Health and Human Services, Public Health Ser-
vice, Centers for Disease Control, National In-
stitute for Occupational Safety and Health,
DHHS (NIOSH) Publication No. 85-104.

NIOSH [1985a]. Fatal accident circumstances
and epidemiology (FACE): Billboard worker dies
when metal ladder contacts 7,200 volt power line
in Kentucky. Morgantown, WV: U.S. Depart-
ment of Health and Human Services, Public
Health Service, Centers for Disease Control,
National Institute for Occupational Safety and
Health, Division of Safety Research, FACE-85-
21-ll.

NIOSH [1985b]. Request for assistance in
preventing electrocutions from contact between
cranes and power lines. Cincinnati, OH: U.S.
Department of Health and Human Services,
Public Health Service, Centers for Disease Con-
trol, National Institute for Occupational Safety
and Health, DHHS (NIOSH) Publication No. 85-
111.

NIOSH [1986a]. Request for assistance in
preventing electrocutions due to damaged
receptacles and connectors. Cincinnati, OH:
U.S. Department of Health and Human Ser-
vices, Public Health Service, Centers for Dis-
ease Control, National Institute for Occupational
Safety and Health, DHHS (NIOSH) Publication
No. 87-100.

NIOSH [1986b]. Request for assistance in
preventing fatalities of workers who contact
electrical energy. Cincinnati, OH: U.S. Depart-
ment of Health and Human Services, Public
Health Service, Centers for Disease Control,
National institute for Occupational Safety and
Health, DHHS (NIOSH) Publication No. 87-103.

NIOSH [1986c]. Request for assistance in
preventing grain auger electrocutions. Cincin-
nati, OH: U.S. Department of Health and Human
Services, Pubic Health Service, Centers for Dis-
ease Control, National Institute for Occupational
Safety and Health, DHHS (NIOSH) Publication
No. 86-119.

NIOSH [1987a]. Fatal accident circumstances
and epidemiology (FACE): Construction worker
electrocuted in North Carolina. Morgantown,
WV: U.S. Department of Health and Human
Services, Public Health Service, Centers for Dis-
ease Control, National institute for Occupational
Safety and Health, Division of Safety Research,
FACE-88-5-II.

NIOSH [1987b]. Fatal accident circumstances
and epidemiology (FACE): Painter electrocuted
in North Carolina. Morgantown, WV: U.S.
Department of Health and Human Services,
Public Health Service, Centers for Disease Con-
trol, National Institute for Occupational Safety

Metal Ladders


Page 2 - Request for Assistance in Preventing Grain Auger Electrocutions

BACKGROUND

The grain auger is a portable piece of farm equipment, 50 to 60 feet
long, and weighing several hundred pounds. It is used to move grain
from one location to another (e.g., unloading grain from a truck or
trailer and loading it into a dryer or storage bin). It is moved to a
desired location on inflatable-type car tires and then raised into
position by means of a hand crank attached to a steel pully system;
the discharge end is elevated to the top of a dryer or bin, and the
opposite end is flowered in order to pick up the grain to be moved.
The auger is usually powered by connecting a universal joint to the
power takeoff on a tractor or other piece of farm equipment. After
transferring the grain, the auger should be lowered to a horizontal
position for safe transportation to another location. However, the
auger is not always lowered before being moved, and this unsafe
practice could pose a life threatening hazard if the auger comes into
contact with overhead electrical lines or if it were to tip over
during transport.

CASE REPORTS OF TWO FATAL INCIDENTS

These case reports resulted from NIOSH investigations of the
circumstances that led to the five fatalities described below. The
investigations were conducted as part of the NIOSH Fatal Accident
Circumstances and Epidemiology Program.

Case #1 - TWO ELECTROCUTED, THREE INJURED

During mid-morning on October 14, 1985, five farm workers
were in the process of moving a portable grain auger. To
move the auger from a 30-foot tall grain drying bin to
another location, it was raised to approximately 35 feet (an
angle of about 45 degrees) so that the top could clear the
bin. The workers then pulled the grain auger machine back
approximately 15 feet from the grain bin, rotated the rear of
the auger 90 degrees, and began pushing the auger to the new
location. As the workers pushed the auger forward,
approximately 90 feet, it contacted an electrical fine which
was about 25 feet above the ground. Two of the workmen were
electrocuted and three others were injured.

Case #2 - THREE ELECTROCUTED

During the early morning of October 18, 1985, two farm
workers and the farm owner were moving a portable grain auger
from a grain bin, approximately 30 feet high, to another
location. The auger was first raised to 35 feet to clear the
top of the grain bin and then pulled back approximately 15
feet. The workers swiveled the auger 90 degrees to allow a


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Table IlI-4. Examples of Static Charge Generation
Between Flowing Materials*

Media Flow

Liquid/solid Flow Liquid flowing through a pipe
Liquid passing through a filter
Splash filling a tank

Liquid/liquid Drops of one liquid settling
through another
Stirring immiscible liquids

Gas/solid Filling a container with powder
Dust filter

Discharge of carbon dioxide (or
any other nonconducting gas)

Gas/liquid Liquid spray (even water)
Discharge of wet steam

Solid/solid Reeling or unreeling of plastic
or paper sheets

Conveyor belt passing over a
pulley/roller

Movement of the human body

Adapted from Hirst, 1989.

A low conducting liquid flowing through a pipe can generate a continuous
leakage current (resulting from continuous static charge accumulation) of
between 0.001 and 1 microampere. Powder being discharged from a grinding
mill can produce currents of 0.1 microampere. Such low currents are generally
not a problem. If static charge cannot leak away, however, and it thus accumu-
lates a high voltage, a significant hazard exists (Hirst, 1989).

Once a static charge has accumulated on a body, it will either discharge as a
capacitor would or slowly leak away. Relaxation time is a term used to
describe the time, or time constant, it takes to leak away 63 percent of the orig-
inal charge. This time varies from material to material depending on conduc-
tivity and with environmental conditions such as humidity.

For instance, if a container that is insulated from ground (i.e., no path to
ground to leak the charge) is being filled with a liquid and gaining a charge at
1.0 kilovolt per second, 10 seconds of filling would result in a static charge of
10 kilovolts. The charge may slowly dissipate by leaking to ground, or it may
discharge as a spark to ground (directly or via a person or other object). The
high voltage makes it possible that a subsequent spark would be of sufficient
length and energy to ignite a variety of flammable atmospheres. A person can
develop a potential difference as high as 10 kilovolts between his body and
ground simply by briskly scuffing one’s shoes across a wool or nylon carpet.
This is enough voltage to produce an arc in a 1/8-inch gap through air
(Hammer, 1989).

TH-22


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

EFFECTS OF
SHOCK ON THE
HUMAN BODY

If you are outside during an electrical storm, you can do several things to pro-
tect yourself from injury. If an all-metal shelter or a structure that has lightning
protection is available, seek shelter within the structure. Once inside, however,
avoid contact with metal surfaces (e.g., any part of a metal structure) or elec-
trical equipment that may become energized in the event of a lightning strike.
The interior of the steel body of an automobile is considered an excellent shel-
ter during an electrical storm, even if a high voltage line is touching it (the
rubber tires insulate the vehicle from ground). If you attempt to leave the car
while the line is energized, however, you will provide the path to ground and
will be severely, probably fatally, shocked.

In an open area or in a boat on a body of water, away from high points, it is
advisable to lie down so that you are not the high point acting as the least
resistance path. On land ravines or depressions are the best place to lie down.
A common error people make is to seek shelter from storms under large trees.
The only tree in an area, or the highest tree, should be avoided because a light-
ning strike in the vicinity is most likely to hit that tree. In a wooded area, seek
shelter near the smallest trees, well away from the tallest tree in the area.
Always avoid contact with metal structures that may act as a path to ground
for lightning, as you too will become a path to ground in the event of a strike.

Static electricity and lightning hazards deserve much more attention than we
have afforded here. In Unit III we will investigate how static charges are gen-
erated, accumulated, and discharged. With that information, we will discuss
methods to reduce and eliminate the hazards associated with lightning and sta-
tic electricity.

The effect of electric shock on a human depends on characteristics of the cur-
rent, the time the body is exposed to the current, and the path the current takes
through the body. The magnitude, duration, frequency, and wave form of the
current all affect the severity of the shock. In addition, the area of the body
that receives the current, where it exits, the path it takes through the body, the
health or physiological state of the body, and the resistance of the body tissues
in the current pathway will affect the outcome. Depending on these factors,
electric shock can have two damaging effects on a living body: tissue can be
burned or normal physiological functions can be inhibited.

Whenever an electric current meets resistance, a.certain amount of energy is
released in the form of heat. The amount of heat, and thus the severity of the
electrothermal burn of human tissue, depends on the magnitude, the voltage,
and the duration of the current (Cabanes, 1985). A shock victim may receive
superficial burns at the points of entry and exit of current and internal burns
wherever the current passed within the body.

Ti-12


Unit V—Systems Safety Analysis ~

Table V-1. Preliminary Hazard Analysis of Flight of Daedalus and Icarus*

PRELIMINARY HAZARD ANALYSIS.

IDENTIFICATION Mark I Flight System SUBSYSTEM Wings DESIGNER Daedalus
Probability of Corrective or
Hazard Cause Effect Accident Due to Hazard Preventive Measures
‘Thermal radiation from sun. Flying too high in presence Heat may melt beeswax Reasonably probable. Make flight at night or at
of strong solar radiation, _ holding feathers together. time of day when sun is not
Separation and loss of very high and hot.
feathers will cause loss of oo
aerodynamic lift. Aeronaut Provide waming against
may then plunge to his flying too high and too
death in the sea. close to sun.
Maintain close supervision

Moisture. Flying too close to water

surface or from rain.

Inflight encounter.

a. Collision with bird.

b. Attack by vicious bird.

Hit by lignting bolt. Bolt thrown by Zeus

angered by hubris displayed

by aeronaut who can fly.

Feathers may absorb mois- Reasonably probable.
ture, causing them to
increase in weight and to
flag. Limited propulsive
power may not be adequate
to compensate for increased
weight so that the aeronaut
will gradually sink into the
sea. Result: loss of function
and flight system, Possible
drowning of aeronaut if sur-
vival gear is not provided.

Injury to aeronaut. Remote probability.

Injury to acronaut.

Remote probability.

Death of aeronaut.

Happens occasionally.

over aeronauts. Use buddy
system.

Provide leash of flax
between the two acronauts
to prevent a young, impetu-
‘ous one from flying too
high,

Restrict area of aerodynamic
surface to prevent flying too
high.

Caution aeronaut to fly
through middle air where
sun will keep wings dry or
where accumulation rate of
moisture is acceptable for
time of mission.

a. Select flight time when
bird activity is low. Give
birds right-of-way.

b. Avoid areas inhabited by
vicious birds. Camry
‘weapon for defense.

‘Acronaut should not show
excessive pride in being
able to perform godlike
activity (keep a low profile).

Adapted, with permission from Hammer, W., p. 553.

V-3


Unit [l—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Isolation

Marking

Warning devices

Grounding

Enclosures or barriers should be used to isolate unauthorized or untrained per-
sonnel from shock hazards, especially from high-voltage systems. The isolat-
ing barriers must be so designed and located to eliminate contact with ener-
gized equipment and must be available only to trained and authorized people.

Warnings posted at points of access to hazardous electrical systems should be
marked with appropriate hazard information. As a designer, you should design
to eliminate hazards. Some equipment is, however, inherently hazardous, espe-
cially if other hazard control methods are bypassed, as often happens during
maintenance procedures. Warning information, both written and color coded,
must be located so that personnel will be informed at critical points during
operation, installation, or maintenance procedures.

Warning devices, such as a sound or light, may be connected to:an electrical
system to indicate when it is energized. These are especially important in com-
plicated, multi-function systems for which an operator must monitor a number
of system functions. The warning device must indicate when the particular
system is actually energized. An analysis of the Three Mile Island nuclear
power plant incident showed that a warning device (a light on the operators
panel) indicated that a critical step in the shutdown procedure had been acti-
vated. In fact, although the system (a pump) had failed to activate, the warning
device indicated it had. The warning light was wired to the switch on the oper-
ators panel, not to the actual system. Therefore, the operators assumed the sys-
tem actually energized and performed its intended function when it didn’t. The
result was a release of nuclear material. A warning device must monitor the
actual system, not the switch intended to activate the system.

Warning devices can also be used to warn operators in the event of system fail-
ure or malfunction.

The earth can be looked at as an infinite store for electrons. Given a conduc-
tive path to earth, or “ground,” a charge difference will either draw electrons
from or return them to the soil (Hammer, 1989). Realistically, the idea that the
earth is an infinite electrical sump is not true. All current must flow in loops;
any current that enters the earth must also leave. In fact, this is often the cur-
rent path in electrocution or shock incidents; the person becomes part of the
circuit—a conducting path to ground. Note from Figure II-6 that the neutral
wire in a circuit is also connected to ground, completing the loop.

I-19


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS.

Class HI locations are those that are hazardous because of the presence of easi-
ly ignitable fibers or flyings; such fibers or flyings are not, however, likely to
be suspended in the air in quantities sufficient to produce ignitable mixtures. wi

Division I conditions are those where an ignitable atmosphere is expected to
prevail at any time during the course of normal operations. This represents the
worst case.

Division 2 conditions are those where no ignitable atmosphere exists under
normal operating conditions, but where abnormal circumstances such as
equipment malfunction or operator error might create a hazardous environ-
ment.

To determine whether a location is considered hazardous, ask yourself the fol-
lowing questions. A “yes” answer to.any question within a class indicates that
the location is considered hazardous. (These questions and their assignments
are adapted from NSC’s “Accident Prevention Manual for Industrial
Operations, Administration, and Programs,” 1988).

Class I:
—Are flammable liquids, vapors, or gases likely to be present?
—Are liquids having flash points at or above 100 degrees Fahrenheit likely
to be handled, processed, or stored at temperatures above their flash
points?

Class IE:
—Are combustible dusts likely to be present? ww
—Are combustible dusts likely to ignite as a result of storage, handling, or
other causes?

Class HI:
—Are easily ignitable fibers or flyings present but not likely to be in sus-
pension in the air in sufficient quantities to produce an ignitable mixture
in the atmosphere?

Each of the classes above have been further subdivided into two divisions
(Division 1 and Division 2) depending on the likelihood that an ignitable
atmosphere might be present. To determine those conditions that should be
assigned to Division 1, ask yourself the following questions. Again, a “yes”
answer indicates that an ignitable atmosphere might be present .

Class I, Division 1:

—Is a flammable mixture likely to be present under normal operating con-
ditions?

—Is a flammable mixture likely to be present frequently because of repair,
maintenance, or leaks?

—Would a failure of process, storage, handling, or other equipment be like-
ly to cause an electrical failure coinciding with the release of flammable
gas or liquid?

—lIs the flammable liquid, vapor, or gas piping system in an inadequately
ventilated location, and does the piping system contain valves, meters, or wi
screwed or flanged fittings that are likely to leak?

TH-12


Page 2 - Request for Assistance in Preventing Electrocutions Due to
Damaged Receptacles and Connectors

BACKGROUND

Occupational electrocutions continue to be a serious problem throughout
the United States. Data obtained from the Bureau of Labor Statistics’
Annual Survey indicate that approximately 10% of all occupational
fatalities are due to electrocutions. Those data, as well as other
information collected by the National Institute for Occupational Safety
and Health (NIOSH), demonstrate that fatalities due to electrocutions
occur in a variety of ways. For example, previous NIOSH Alerts have
described cases in which workers have been electrocuted as a result of
contacting improperly grounded equipment, or when cranes or grain augers
have contacted overhead power lines [1,2]. This Alert presents
information on two fatal electrocutions that occurred as a result of
using damaged receptacles and connectors.

Two investigations by NIOSH found evidence to suggest that the victims
were unaware of hazards associated with the use of damaged connectors.
In both cases, it was assumed that because a connector fit into a
receptacle, the connection was proper and no hazard existed. The
prevalence of this particular hazard is not clear. However, the cases
described below point out the insidious nature of this hazard. The
presence of receptacles and connectors in all workplaces, and the
repetitive nature of their use (which in certain workplaces increases
the possibility of damage) suggests that the potential hazard is
widespread. These investigations also demonstrate that careful routine
inspection and aggressive maintenance might well prevent such
fatalities.

CASE REPORTS OF FATAL INCIDENTS

Case #1 - (ONE FATALITY)

On July 23, 1985, a 24-year-old employee of a textile mill was
electrocuted when he touched a loom frame while performing his
routine duties at the loom. The loom had become energized when an
electrical, three-prong connector from a thread feeder machine was
inserted into a damaged receptacle mounted on the loom. The damage
to the receptacle permitted the ground prong of the plug to be
improperly inserted into one of the phase terminals (90 degrees
clockwise away from the appropriate ground terminal). . This resulted
in energizing the ground prong and the frame of the loom. When the
worker touched the energized loom, he was electrocuted. It
appeared, upon subsequent inspection, that the receptacle had been
damaged because of a lack of adequate strain relief for the
electrical cord from the thread feeder.

Case #2 - (ONE FATALITY)

On July 29, 1985, a 29-year-old welder was electrocuted when he
inserted the "male" end of an electrical plug on a portable arc

©


Unit I—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Burns

NIOSH ALERT: Request for Assistance in Preventing Fatalities of
Workers Who Contact Electrical Energy

Recent incidents...have shown that electrocution victims can be revived if
immediate cardiopulmonary resuscitation (CPR) or defibrillation is pro-
vided. While immediate defibrillation would be ideal, CPR given within
approximately 8 minutes can be lifesaving.

*This is.an excerpt; see reference materials at the end of this book for a com-
plete copy of this Alert.

Burns, the most frequent injury associated with electric current, take sev-
eral forms. Some are caused by the intense radiation of heat from an
electric arc—arc burns. Others result from the flow of current through
tissue—electrothermal burns. Still others, particularly in industrial set-
tings, result from electrical malfunctions that cause hot oils or other liq-
uids or heated metals to splash. More than one of these burn types may
occur together. For example, current can flow to a body by an electric arc
and cause severe skin burns. The burned skin will have a lower resistance
than healthy skin, allow more current to pass directly to the internal tis-
sues, and result in electrothermal burns. The severity of electrothermal
burns is governed by Joule’s law which states that the amount of energy
converted to heat (W) in a time interval (t) at a constant current (I) is
proportional to the square of the current.

W =RPt joules = VIt (watt-seconds)

Thus, the energy released as heat depends on voltage and current magnitudes
and the total time they are applied. Note the importance of voltage in the equa-
tion; it is, in fact, high-voltage contacts that result in the most severe burns
(Cabanes, 1985).

In review, the intensity of an electrical shock depends on the voltage, current
magnitude, current path, current frequency, and time the current flows through
a person. Many factors affect the severity and type of injury associated with a
shock. For example, any current path that includes the heart (e.g., a current
running from hand to hand) is more likely to result in heart fibrillation than
one that doesn’t.

The amount of current flow depends on the applied voltage and the resistance.
For the human body, most of the resistance is provided by the skin. As Table
JJ-2 shows, when the skin is wet there is a significant decrease in resistance.
Once the resistance of the skin is overcome, the internal tissues offer very little
resistance to the current (Daziel, 1972). The values contained in the table are
best estimates, as individual differences and the physiological state of a person
will affect actual resistance values.

I-15


Unit [1—Ignition of Flammable and Combustible Materials and Explosives

intrinsically safe
designs

Encapsulation
and embedment

Hermetic sealing

Liquid filling

Explosion-proof
equipment

An intrinsically safe design (sometimes called an inherently safe design) is one
that is not capable of igniting because its operating energies are less than the
energy required to ignite the known flammable materials or mixtures in the
area. An inherently safe design could be one that employs miniaturized equip-
ment utilizing components that operate with far less current and voltage than
those that would present an ignition hazard for flammable mixtures.

To say a design is “intrinsically (inherently) safe,” however, may lead to a
false sense of security. No design can be totally fail proof. Certainly, we can
minimize the likelihood of failure to an almost negligible level but never to
zero. Many of the failures that have led to ignitions in the past were never
imagined before they happened.

Embedment and encapsulation are methods designed to separate the potential
electrical ignition sources from a flammable environment. The materials used
are nonflammable, sufficiently thick to prevent sparks from reaching flamma-
ble mixtures, and thermally nonconductive so the outer surfaces are kept well
below ignition temperatures for the known flammable environment.

Similar to-encapsulation, hermetic sealing was originally developed to protect
electrical equipment from moisture and environmental contaminants. As the
name implies, hermetic sealing completely separates the components by con-
taining them within a tightly sealed enclosure. The container can be filled with
liquid or gas and is designed to dissipate heat at levels safely below ignition
temperatures for flammable mixtures.

Liquid-filled containers surrounding electrical equipment, such as transform-
ers and circuit breakers, have been used for some time as a way to cool and
protect components. Liquids can be chosen that can also eliminate sparking
and arcing that otherwise could ignite a flammable or explosive atmosphere.
The enclosure and liquid filling separate the flammable atmosphere from the
potential electrical ignition source. A disadvantage with liquid filling is that
some of the most effective liquids are also very hazardous to humans (e.g.,
PCB’s). This has presented problems for maintenance people who may be
unaware of the hazards and for people and fire fighters exposed to these chem-
icals during fires or when such equipment explodes (for instance, a trans-
former overloaded by a power surge from a lightning strike may explode).

The idea behind explosion-proof equipment is to enclose electrical compo-
nents in a way that allows access of flammable atmospheres and yet contains
any resultant explosion without igniting the surrounding atmosphere. Design
regulations are largely concerned with the strength of the enclosure; details of
the joints between removeable covers and the main housing; and the design of
any other necessary apparatuses in the housing, such as shaft exits (Bass,
1984). Because of the mechanisms of explosions and flame-propagation,
explosion-proof enclosures are only appropriate for small components, such as
switches, For larger enclosures, a small explosion-proof apparatus prevents
external gases or vapors from igniting by cooling an escaping gas flow. This is
done through conduction of heat by the gas flow; either the enclosure material
conducts the heat or the escaping gas expands and mixes with the external
atmosphere. Figure III-6 illustrates the idea behind explosion-proof equip-
ment.

1-15


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Job Safety
Analysis (JSA)

Failure Mode and
Effect Analysis
(FMEA)

Job Safety Analysis is a written procedure designed to review job methods,
uncover hazards, and recommend safe job procedures. The following four
basic steps are included in making a JSA:

1. Select the job, usually basing selection on potential hazards or high inci-
dence rates.

2. Break the job down into a sequence of steps. Job steps are recorded in
their normal order of occurrence. Steps are described in terms of what is
done (“lift,” “attach,” “remove”) not how it is done.

3. Identify the potential hazards. To determine what events can happen one
should: observe the job, discuss the job with the operator, and check acci-
dent records.

4. Recommend safe job procedures to avoid the potential incident or injury.

A basic JSA form should include the job steps, the hazards associated with
these steps, and recommended safe procedures. The form may be altered if
needed to meet specific organizational needs by including such information as
the name of the person performing the analysis, name of the operator and
supervisor, or the name of reviewers or approvers of the analysis.

Failure Mode and Effect Analysis is both a system safety and reliability analy-
sis used to identify critical failure modes that seriously affect the safe and suc-
cessful life of the system or failure modes that could prevent a system from
accomplishing its intended mission. This technique permits system change to
reduce the severity of failure effects. FMEA is organized around the basic ques-
tion “What if?” The areas covered and the questions asked move logically
from cause to effect.

1. Component—What individual components make up the system?
2. Failure Mode—What could go wrong with each component in the system?
3. System Causes—What would cause component failure or malfunction?

4. System Effects—What would be the effect of such a failure on the system
and how would this failure affect other components in the system?

5. Severity Index—-Consequences are often placed into one of four reliability
categories:

1—Catastrophic—may cause multiple injuries, fatalities, or loss of a
facility.

2—Critical—may cause severe injury, severe occupational illness, or
major property damage.

3—Marginal—may cause minor injury or minor occupational illness
resulting in lost workday(s) or minor property damage.

4—Negligible—probably would not affect the safety or health of per-
sonnel but is still in violation of a safety or health standard.

v-4


electrical conductors." Other pertinent regula-
tions require that "portable ladders in use shall
be tied, blocked, or otherwise secured to
prevent their being displaced" [29 CFR
1926.450(a)(10)]. Additional OSHA regula-
tions require employers to instruct each
worker to recognize and avoid unsafe condi-
tions [29 CFR 1926.21(b)(2)], and to provide
prompt medical attention in case of serious
injury [29 CFR 1926.50].

CASE REPORTS

As part of the Fatal Accident Circumstances
and Epidemiology (FACE) Program, NIOSH
investigated five incidents (resulting in six
electrocutions) that occurred between 1985
and 1987 and that involved contact between
portable aluminum ladders and overhead
power lines.

Case No. 1 - One Fatality

On May 4, 1985, a 28-year-old male worker
removed the bottom of a poster on a 12-by-
24-foot (-ft) billboard that was scheduled for
reposting. He then removed a 24-ft aluminum
hook ladder from the service truck. While the
worker was positioning the ladder to reach the
top section of the billboard, the ladder con-
tacted a 7,200-volt (-V) overhead power line
that was located 8 ft from the top of the
billboard, and he was electrocuted [NIOSH
1985a].

Case No. 2 - One Fatality

On July 21, 1986, a 27-year-old male painter
was standing on a fully extended 24-ft
aluminum ladder while painting a rain gutter
on an apartment building. After painting a
section of the gutter, the worker descended
the ladder to move it to a new location. As he
was repositioning the ladder, it contacted a
7,200-V overhead power line that was located
8 ft from the gutter, and he was electrocuted
[NIOSH 1987d].

Case No. 3 - Two Fatalities

On November 17, 1986, two male painters (20
and 21 years old) were using a 36-ft aluminum
extension ladder to paint a 20-ft-high metal

light pole. One worker was standing on the
ladder painting, and his coworker was on the
ground holding the ladder. The ladder slipped
away from the pole and contacted a 12,460-V
overhead power line that was located within 2
ftof the pole. Both painters were electrocuted
[NIOSH 1987c].

Case No. 4 - One Fatality

On September 1, 1987, a 28-year-old male
painter and a coworker were using an
aluminum extension ladder while cleaning the
outside brick wall of a three-story convales-
cent home before painting. After cleaning one
section, the workers moved the ladder to
another location. The painter held the base of
the ladder as the coworker simultaneously
climbed and raised the extension of the 40-ft
ladder. When the ladder was extended to
approximately 34 ft, it tipped backward, con-
tacting a 7,200-V overhead power line that
was located 15 ft from the structure. The
coworker on the ladder received an electrical
shock and fell to the ground. The painter hold-
ing the ladder provided a path to the ground
for the electrical current and was electrocuted
[NIOSH 1987b].

Case No. 5 - One Fatality

On September 24, 1987, an 18-year-old male
construction worker and two coworkers were
looking for an area on an office building roof
to store shingles. The 18-year-old and a
coworker were holding a fully extended, 32-ft
aluminum ladder as the other coworker de-
scended it. The ladder tipped backward, con-
tacting a 7,200-V overhead power line that
was located 6 ft from the building, electrocut-
ing the 18-year-old worker holding the ladder,
and ang the other two coworkers [NIOSH
a].

APPLICATION OF EXISTING
REGULATIONS

Data demonstrated that employers and
workers in all five fatal incidents violated the
following applicable* OSHA regulations:

“Regulation No. 3 did not apply to Case No. 1.

Metal Ladders


Unit Il—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Bonding

Ground-fault
circuit interrupter
(GFCI)

If a receptacle is correctly wired, V and V3 (V2 = 0) will register the same
voltage. If the hot and neutral are reversed (reversed polarity), V2 and V3
(V; =0) will register the same voltage. If the hot is open, all votages will be
zero. If the ground is open, only V3 will register. If the neutral is open, only
V, will register.

If the ground and neutral are reversed, only the method pictured in figure 2.11
(b) will accurately reveal the error. This test is performed by applying a load to
anearby receptacle on the same circuit and measuring the voltages shown with
an oscilloscope. When properly wired, current will flow only through the hot
and neutral wires. The U-ground line will only carry a small leakage current. If
the ground and neutral were reversed, the ground would carry a larger current
than the neutral (i.e., V4 > V2). Commercial devices designed to test receptacle
wiring are available.

NIOSH ALERT: Request for Assistance in Preventing Electrocutions
Due to Damaged Receptacles and Connectors

... Results of ... investigations indicate that periodic inspection, recognition

of hazards, and proper use of receptacles and connectors, and prompt
repair of damaged connectors and receptacles,.could prevent [electrocu-
tions]...

*This is an excerpt; see reference materials at the end of this book for a
complete copy of this Alert.

Bonding is a form of grounding in which all major parts of a system or pieces
of equipment are linked with an effective conductor to provide a continuous
path to ground. A bond is a mechanical connection between physically sepa-
rated parts that provides a path of low resistance for current flow between the
parts and to the ground. Bonding is also an important concept for flammable
liquid transfer systems and procedures, which will be covered in depth later in
this text. Briefly, fluid flow can create opposing electrostatic charge buildup
between containers when transferring liquids. Without bonding, which pro-
vides a conductive path between containers that will neutralize the opposing
charge build-up, the charge difference may result in a spark between contain-
ers. This spark may have sufficient energy to ignite the flammable liquid or
vapors.

A GFCI monitors the current flow in both the hot wire and the grounded neu-
tral wire. For example, a current flows into a toaster through the hot wire and
returns via the grounded neutral wire. Even though energy is lost in the form
of heat, the current leaving the toaster will be equal to that entering.

TI-25


Unit IV

SAFETY STANDARDS, ORGANIZATIONS PROMULGATING
STANDARDS, AND PROFESSIONAL LIABILITY

PURPOSE:

OBJECTIVE:

SPECIAL TERMS:

To introduce the student to pertinent electrical safety standards, the organizations
which produce them, and professional liability for not applying the standards in
one's work practices.

To acquaint the student with:

. The different types of safety standards

.. The organizations which develop electrical safety standards

. The concept of professional liability for not following these standards

. Standard

. Code

. Recommended practice
. Voluntary standard

. Mandatory standard

. Consensus standard

. Horizontal standard

. Vertical standard

. Specification standard
. Performance standard
. Liability

. Negligence

. Strict liability

. Warranty



AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

INTRODUCTION

A few definitions

Hazard

Danger

Damage

Risk

The struggle to provide an acceptable degree of safety in the modern world
depends on the give-and-take relationship between the cost of accident pre-
vention and the moral regard for human life and well-being. Conceptualizing
safety with such a simple statement, however, disguises the complexity of the
issue. What is the "modern world”? What is the "cost of accident prevention”?
What does "moral regard for human life and well-being" entail? What exactly
is "safety"? What is an "acceptable degree" of safety? First, we'll try to answer
these questions, and then we'll see how this all applies to engineers.

“Modern world,” as used here, applies to the time period following the
Industrial Revolution. During the last half of the nineteenth century, the
Industrial Revolution made a profound impact on the United States. American
production methods changed from craft shops to mechanized factories, first in
New England in the textile industry and soon throughout the entire country.
The National Safety Council (1988) notes that these changes in production
methods can be summarized as:

¢ the substitution of steam and other inanimate power sources [e.g.,
electricity] for animal sources;

* the substitution of machine power for human power and skills;

* the development of new processes for making iron, steel, industrial
chemicals, and other materials; and

the reorganization of work from craft shops to large factories or mills
with an efficient division of labor.

These production methods greatly expanded the quantity and types of products
available to the average American. They also expanded the magnitude and
types of hazards present in our everyday activities. The discovery of electricity
and its subsequent use in industry and in the home was an integral part of this
period and presented a variety of new and unique hazards to the public.

Since we have used the term "hazard" several times so far, this may be a good
time to define some of the terms we'll be using throughout this book
(Bloswick, 1992).

A hazard is a condition that has the potential to cause injury, damage to equip-
ment or facilities, loss of material or property, or a decrease in the capability to
perform a prescribed function.

The danger inherent in a situation depends on the relative exposure to a haz-
ard. For example, a high-voltage transformer is a significant hazard but may
present little danger if locked in an underground vault.

Damage is the severity of injury or magnitude of loss that results from an
uncontrolled hazard. A worker near a 220 volt line is exposed to the same haz-
ard (potential for electric shock) and is in the same danger (exposure to elec-
tric energy) as a worker near a high voltage (1000 volts and greater) line. The
possibility of damage (severity of injury in this case), however, is much
greater in the latter case.

Risk is a function of the probability of loss (danger) and the magnitude of
potential loss (damage). It can be thought of as (probability of loss) x (magni-
tude of potential loss).

1-2


CONTENTS

Page
UNIT I—AN INTRODUCTION TO SAFETY AND SOME
BASIC ELECTRICAL ENGINEERING CONCEPTS
Purpose; Objective; Special Terms ....sessoessssosssssssssssssessesnscsseesseeseecneenncensenseensannecnnenseensessneeae T-1
INTRODUCTION. 1-2
A few definitions; Hazard; Danger; Damage: Risk. 1-2
Accident; Safety; Economics and safety...... 1-3
More than $; Moral obligations; Perception of risk; Exposure to risk; Voluntary . 1-4
Risk on the job; Assessment of risk; Skewed perceptions L5
Drawing the line; Dispelling a myth........... 16
HAZARD CONTROL AS A DESIGN CRITERIA . 1-7
Design for people...... 1-7
REVIEW OF THE FUNDAMENTALS OF ELECTRICITY AND CIRCUITS 1-7
Electric current; Conductors; Insulators; Resistance .. 1-8
Impedance; Voltage; Direct current (de); Alternating cu current (ac); Capacitance. 1-9
Review of basic circuit analysis..... 1-10
REFERENCES AND SUGGESTED READINGS .......ssesesseessssseesesstenneesenn 1-12
SAMPLE QUIZ QUESTIONS ..ssccssssssssssessssssccssusssssesssnnnseceesnsseeeceuunsntesennneesees 1-13
UNIT I—ELECTRIC SHOCK HAZARDS AND CONTROLS,
AND INADVERTENT ACTIVATION OF EQUIPMENT

Purpose; Objective; Special Terms ....csssssssssssssseseseesssssssessssenmeseeeececcnnntiesssennnesssssssnnenensssess Ue]
AN EXAMPLE OF ELECTRIC SHOCK.......-sssssssssccsssessssseessssntesssssneessssseesssseeserssstesssnneeccssenseessses U2
WHAT IS ELECTRIC SHOCK IL-3
Contact with a normally bare energized conductot T-3
Contact with conductors when the insulation has failed... TI-5
Mechanical ............++ IL-5
Electrical stress effects; Temperature effects Il-6
Chemical and physical reaction: Il-7
Moisture .... 11-8
Equipment failure that results in an open or short circuit IL-8
Short circuit...... IL-8

Static electricity and lightnin, Tl-11
EFFECTS OF SHOCK ON THE HUMAN BODY... I-12
Generalized effect of 60 Hz alternating currents; 11 mA . I-13
I-14

I-15

I-17



Page 4 ~ Request for Assistance in Preventing Electrocutions Due to
Damaged Receptacles and Connectors

RECOMMENDATIONS
NIOSH makes the following recommendations in these areas:
1. PROPER UTILIZATION OF ELECTRICAL SYSTEMS

All receptacles and connectors should be used only in accordance
with the manufacturers' specifications, and the specific listing
for the item as set forth by nationally recognized testing
laboratories. Users should be advised of the importance of
using receptacles and connectors only for applications for which
they have been designed. When a component is selected for use,
it should be evaluated to determine if it can tolerate the
environment to which it will be exposed. Physical abuse and
stress on these components should be minimized by the selection
of a safe location and by the use of stress/strain relief
devices.

2. AWARENESS AND RECOGNITION OF HAZARDS

Policies that address the proper use of receptacles and
connectors should be developed and implemented by qualified
safety personnel. Safety training should emphasize awareness
and recognition of electrical hazards associated with
receptacles and connectors (i.e., broken receptacles and
connectors, improper electrical connections, damaged cords, the
importance of grounding, etc.). Immediate corrective action
should be taken when damaged components or safety hazards are
encountered. When safety policies and procedures are developed,
they should be enforced.

3. PERIODIC INSPECTION AND MAINTENANCE OF ELECTRICAL SYSTEMS

Periodic inspections should be conducted for all electrical
system equipment and components in order to identify all
electrical hazards present. Records should be kept of any
electrical hazards identified, and appropriate corrective action
should be taken immediately. These periodic inspections should
be supplemented with daily inspections by the personne! using
this equipment.

We urge safety and trade associations interested in job site safety to
bring these recommendations to the attention of employers.

Requests for additional information, and comments or questions
concerning this announcement, should be directed to Mr. John Moran,

©


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

If short occurs here motor will

Live or Hot ‘
conductor continue to v

Ground 7 t

conductor .
Equipment
grounding
conductor

@)

Live or Hot
conductor

Source 1

Ground 7 *

conductor

Equipment
grounding
conductor

()

Live or Hot
conductor

Source 4

conductor + *
Perley Live drill housing will
conductor cause a potentially fatal

electric shock to anyone
who attempts to handle
drill while in contact with
a grounded surface

()

Figure II-9. Examples of incorrect wiring of receptacles (a) hot and neutral
reversed; (b) ground and neutral reversed; (c) hot and ground
reversed, (adapted from U.S. Department of Labor, 1981).

Ti-22


Unit HI—Ignition of Flammable and Combustible Materials and Explosives

tors, however, the charge difference will remain as the surfaces are separated.
Since the potential to develop a static charge is inversely related to the conduc-
tivity of the materials, the presence of water or humidity (high conductivity)
will reduce the potential for static charge accumulation. Depending on the
materials involved and environmental conditions, voltages between 1 volt and
1 kilovolt can result from simple separation (Hammer, 1989; Hirst, 1989).

Bringing a charged body near a second with no initial charge can induce a
charge on the second. If the materials are conductors, the charge will move to
the outside of the bodies. If the body with an induced charge (with an unlike
charge equal and opposite to the charged body) is momentarily grounded, the
like charge disappears but the unlike (equal and opposite) remains (Figure II-7).
Separating the bodies then leaves the equal and opposite induced charge on the
second body, until either a static discharge or grounding releases the potential
difference between the body and ground (Hirst, 1989; Wright, 1985).

3. Grounded 4, Remove ball

Figure III-7. Static charged by induction. The
shaded areas represent insulation
(adapted from Wright, 1985).

This type of static charge buildup is probably not surprising to you. What does
come as a surprise to many, however, is that one material flowing through or
over another can develop high static charges. Flow constitutes a continuous
separation condition and can occur between a liquid and solid, two liquids, a
gas and a solid, a gas and a liquid, or two solids. Flow over protrusions or
rough surfaces increases the generation of charge. Table III-4 contains some
examples for each of these situations. Figure III-8 illustrates a static charge
accumulation caused by a fluid flow through a pipe and through a filter (Hirst,
1989).

Til-21


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Lightning

To summarize, static electricity is a significant hazard in any industry or task
that is likely to have flammable fuel/air mixtures. Any time there is movement
between materials (solids, liquids, gases, or dusts), static charges will be gen-
erated. Static charges can be transferred to other materials by sparking or cur-
rent during contact, and a charge can be induced on nearby materials from
another charged material. Accumulation of static. charge depends on the con-
ductivity of the materials involved: conductors will leak away charges if they
have a path to ground or have enough time to safely relax a charge through
poor conducting materials. Poor conductors will readily accumulate and hold
static charges. Humidity will reduce the accumulation of charge.

Controlling the potential for flammable atmospheres, charge generation, and
accumulation are the most effective methods to minimize static discharge as
an ignition source.

Lightning is a massive discharge of static charge involving very high poten-
tials and currents (Hammer, 1989). In electrical storms, clouds gradually accu-
mulate static charges until the voltage is high enough to cause a discharge
through a lightning stroke. As a cloud accumulates charge, an equal and oppo-
site charge is induced on the ground (or structures) beneath it, as illustrated in
figure III-11. Because of this equal and opposite charge, lightning will some-
times travel upwards rather than downwards. The induced charge on structures
may also, under certain atmospheric conditions, result in corona discharge (a
glow, sometimes accompanied by a hissing sound, “St. Elmo’s Fire”) capable
of igniting flammable atmospheres.

or

Figure TII-11. When a cloud becomes charged due to
atmospheric flow, an equal and opposite
charge develops on the ground. The charge
on the ground may accumulate the most on
projections (such as the house pictured).

TII-30


Sofety/Health Awareness
te

Preventive Engineering

Public Health Service
Centers for Disease Control and Prevention
COMERS FOR DatAse conTaD

a U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
:
= National Institute for Occupationcl Safety and Health fayt


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

AND condition: All events leading into it (A and B)
must occur before the event at the top (C) can
occur. This condition is generally expressed as

AB, A‘B or (A)(B) = C.

Cc
OO
Cc
OR condition: Any event leading into it (A or B)
will cause the event at the top (C) to occur.
This condition is generally expressed as
A B A+B=C.
OC)

A basic event: one that starts a sequence of causal
events.

A top event or contributory event.

Figure V-1. Fault Tree Symbols.

Prob (output) = (Pr Input 1) + ( Pr Input 2) + (Pr Input 3)

— [(Pr Input 1) x (Pr Input 2) + (Pr Input 2) x (Pr Input 3)

+ (Pr Input 1) x (Pr Input 3) + (Pr Input 1) x (Pr Input 2) x (Pr Input 3)]
Where the probabilities of the input events are small (less than 0.1, for exam-
ple), the probability of the output event for an OR gate can be estimated by the
sum of the probabilities of the input events or:

Prob (output) = (Pr Input 1) + (Pr Input 2) + (Pr Input 3)

Assume for example, in Figure V-2, that the following probabilities exist:

Probability of power supply failing = .0010
Probability of switch open = .0030
Probability of blown fuse = .0020
Probability of light 1 out = .0300
Probability of light 2 out = .0400

V-8


NIOSH

ALERT

ALERT

ALERT

ALERT

ALERT
ALERT

p Request for Assistance in

Preventing Grain Auger
Electrocutions

July 1986

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service

Centers for Disease Control

National Institute for Occupational Safety and Health


ACKNOWLEDGEMENTS

We wish to thank Professor Dale I. Rummer, University of Kansas, for his review of this document. We
thank the following National Institute for Occupational Safety and Health (NIOSH) personnel for their
review of this document: Heinz Ahlers; Donald F. Knowles, Jr; Ronald J. Kovein; Louis D. Smith; and
James B. Walters. The report was edited by Marion G. Curry. Layout and typesetting was provided by
Pauline J. Elliott,

This report was prepared in support of NIOSH Project SHAPE (Safety and Health Awareness
for Preventive Engineering). Project SHAPE is designed to enhance the education of engineers
in the safety and health field.

DISCLAIMER

The opinions, findings, and conclusions expressed are not necessarily those of the National Institute for
Occupational Safety and Health, nor does mention of company names or products constitute endorsement
by the National Institute for Occupational Safety and Health.

NIOSH Project Officer
Stephen D. Hudock, CSP

NIOSH Order No. 89-39341


Unit V—Systems Safety Analysis

y Light 1
Fuse _
Switch
el Power Light 2
Source

Room
dark
Power A Both lights
off burned out
i 1 i 1
Power supply Fuse Light 1 Light 2
failed blown bumed out burmed out
Switch
opened

Figure V-2. Fault Tree Analysis of light bulbs
(adapted from EG&G).

Since the power will be off if the the power supply fails, or, the switch is open,
or the fuse is blown, the probability of the power being off is the sum of the
probabilities of the power supply failing, the switch being open, and the fuse
being blown or (.0010) + (.0030) + (.0020) = .0060. Both lights will be burned
out if light 1 is out and light 2 is out. The probability of both lights being out is
(.0300) x (.0400) or .0012. The room will be dark if the power is off or both
lights are burned out which is (.0060) + (.0012) or .0072.

The concepts of “cut sets” and “path sets” are useful in the analysis of fault
trees. A cut set is any group of contributing elements that, if all occur, will
cause the end event to occur. A path set is any group of contributing elements
that, if none occur, will prevent the occurrence of the end event.
For the example in Figure V-2, the end event will occur if:

— the power supply fails,

— the switch is open,

— the fuse is blown, or

— light 1 and light 2 are burned out.

Each of the four sets above represents a cut set since, if all of the events in any
of the sets occur, the end event (room dark) will occur.

v-9


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

When a short sends current to an unintended conductor, such as the casing of
an electric hand tool, the path to ground may be through a person, as in the
case study at the beginning of this section (fortunately, newer power tools are
being designed to minimize this type of failure and hazard).

When a short occurs between conducting elements within a circuit, some com-
ponents may be loaded beyond their design capacity. This type of short is not
necessarily an immediate shock hazard, but the overloading condition may
lead to component failure that results in a shock hazard. An overloaded circuit
is also a fire/explosion hazard; this will be covered in Unit III.

An open circuit will occur when a conductor in a circuit is severed. If the sev-
ered wire is in parallel with other conductor(s) in the circuit, it will increase
the current in the parallel path(s) and possibly create an overload condition
(see Figure II-4 (a)). The potential for shock is very high if a person comes
into contact with the free end of a severed, energized conductor. The downed
power line (Figure II-4 (b)) illustrates how a person will be severely shocked
should he/she contact the energized wire and become a path to ground.

- —-—-— __ Intended current path
wees Alternate current path

(a)

Figure Il-4. (a) An unexpected open circuit may overload parallel
components.

TI-10


Unit IV—Safety Standards, Organizations Promulgating Standards, and Professional Liability

Voluntary versus
mandatory

Consensus

Horizontal versus
vertical

Except where otherwise indicated, we will use “standard” as a generic descrip-
tor for standards, codes, criteria, recommended practice, or any other safety-
related document.

Standards may be applied voluntarily, or they may be mandated by some fed-
eral, state, or local regulatory agency. Voluntary standards may be those devel-
oped and followed by a single company (i.e., “in-house”), or they may be con-
sensus standards decided on by a group of representatives for a given industry.
For example, the National Electric Code (NEC) is a consensus standard devel-
oped and agreed upon by interested member organizations and citizens. The
NEC is not mandatory; it is not a law. It is, however, often adapted and man-
dated by regulatory agencies.

Consensus implies more than simple majority approval, but it dees not neces-
sarily mean unanimity. Unanimous agreement usually cannot be achieved
without compromising the effectiveness of the standard (Bass, 1986). Some
contend that the effectiveness of consensus standards is still compromised by
attempting to reach near unanimous agreement on language. They contend fur-
ther, that members of the sponsoring organizations are not bound to adhere to
the standards and, therefore, might not do so. They believe that, at a minimum,
government’s role should be to ensure that all segments of an industry adhere
to a given standard.

Mandatory standards, codes, practices, etc., are those that have passed into
law, such as the laws under the jurisdiction of the Occupational Safety and
Health Administration (OSHA). OSHA standards are mandatory; they are law.
The OSHA standards are, for the most part, standards that were once voluntary
consensus standards that have since been adopted and enforced under the
OSHAct. We will discuss the OSHA in detail later in this unit.

Government involvement in safety standard activities is not without critics.
Among other arguments, the critics believe that by mandating (by law, with
penalties for noncompliance) standards, which most agree are minimum safety
requirements, regulatory agencies actually stifle progress toward more effec-
tive safety policies.

Standards can be classified as horizontal or vertical.

A horizontal standard is one that is applicable across all industries—one that
is generalized by a particular hazard type, regardless of the industry.

A vertical standard is one that applies to one type of industry only.

Because of the huge variety of industries under the jurisdiction of OSHA, most
of their regulations are horizontal, although vertical standards are included
where appropriate. For instance, OSHA standard 1910.106 is basically a hori-
zontal standard pertaining to flammable and combustible liquids. Section
1910.106(i), however, within the same standard, contains a vertical portion
entitled “Refineries, chemical plants, and distilleries.”


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Again, certain industries and situations present greater hazards than others. A
storage tank holding flammable liquids presents one type of situation that has
experienced many lightning-related fires in the past. Investigate each specific
situation, and when necessary, implement minimum protective schemes. These
frequently involve effectively bonding and grounding all conducting parts in
and around a tank.

Specific precautions beyond the general scope of this material must be taken
to protect high-voltage power lines and transformers from the hazards of light-
ning. Power lines are a relatively common target for lightning, and the associ-
ated power surges can cause severe damage to equipment (transformers have
been known to explode under such conditions). An elaborate combination of
grounding strategies and surge protection is required to reduce these hazards.

REFERENCES AND SUGGESTED READINGS

. Bartknecht, W., “Dust Explosions,” (ISBN 0-387-50-100-2) Springer-Verlag, New York,
NY, 1989.

2. Bass, H.G., “Intrinsic Safety, Instrumentation for Flammable Atmospheres,” Quartermaine

House Ltd, Middlesex, U.K., 1984.

Cooper, F., “Electrial Safety Engineering,” Second Edition, Butterworths, London,

England, 1986.

4. Earley, M.W., Murray, R.H., Caloggero, J.M., “The National Electrical Code Handbook,”
Fifth Edition, National Fire Protection Association, Quincy, MA, 1990.

. Field, P., Edited by Allen, T. and Williams, J.C., “Handbook of Powder Technology, Dust
Explosions,” Vol. 4, Elsevier Science Publishers BV, Amsterdam, The Netherlands, First
Edition, 1982, Reprinted with Amendments, 1989.

6. Hammer, W., “Occupational Safety Management and Engineering,” Fourth Edition,

Prentice-Hall, Englewood Cliffs, NJ, 1989.

. Hirst, R., “Underdown’s Practical Fire Precautions,” Third Edition, Gower Technical,

Hants, England, 1989.

Howard, J.C. Edited by Vervalin, C.H., “The Hazards of Static Electricity,” in “Fire
Protection Manual for Hydrocarbon Plants,” Vol 1, 3rd Edition, Gulf Publishing Company,
Houston, TX, 1985.

National Electric Safety Code Committee, ANSI C2, “National Electrical Safety Code
Interpretations, 1981-1984 Inclusive,” Institute of Electrical and Electronics Engineers,
Inc., New York, NY, 1984.

National Safety Council, “Accident Prevention Manual for Industrial Operations,
Administration and Programs,” Ninth Edition, R.R. Donnally and Sons, NSC, Chicago, IL,
1988.

11. Ridley, J., “Safety at Work,” Second Edition, Butterworths, Boston, MA, 1986.

12. Risinger, J.L., Edited by Vervalin, C.H., “Control Floating Roof Tank Fires,” in “Fire
Protection Manual for Hydrocarbon Plants,” Vol. 1, 3rd Edition, Gulf Publishing Company,
Houston, TX, 1985.

Risinger, J.L., Vervalin, C.H., “Sources of Ignition,” in “Fire Protection Manual for

Hydrocarbon Plants,” Vol. 1, 3rd Edition, Gulf Publishing Company, Houston, TX, 1985.

14. U.S. Department of Labor, “An Illustrated Guide to Electrical Safety," Occupational Safety
and Health Administration, Washington, DC, 1981

15. Wright, L., Ginsburg, I., Edited by Vervalin, C.H., “What Experimentation Shows about

Static Electricity,” in “Fire Protection Manual for Hydrocarbon Plants,” Vol. 1, 3rd Edition,

Gulf Publishing Company, Houston, TX, 1985.

16. Wright, R.V., Edited by Vervalin, C.H., “Fundamentals of Static Blectricity,” in “Fire
Protection Manual for Hydrocarbon Plants,” Vol. 1, 3rd Edition, Gulf Publishing Company,
Houston, Texas, 1985.

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TII-32


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Drawing the line

Dispelling a myth

flying than driving. Yet, the magnitude of an air disaster skews people’s per-
ception of the risks, leaving many overly fearful of flying. (Relating to the
above discussion, people may also be affected by the lack of control they feel
when flying, since they are mere passengers at the mercy of the pilot(s) and
technology.) :

Perception of potential risk may also be skewed if we or people we identify
with or have relationships with are the potential victims. Again, imagine our-
selves or a loved one as the victim of a plane crash shown on TV, or imagine a
child trapped in a well as the Nation follows the rescue attempts for several
days. Such things can affect us more acutely than does the continuous, but
anonymous, death and injury occurring on the roadways (until it happens to
someone we know, that is).

An individual's perception of risk and whether or not that risk is acceptable is
a complicated function of many external and internal factors. Arriving at
absolute safety, then, or providing a measure of safety that satisfies all individ-
uals and organizations under all conditions, is neither attainable nor affordable.
As engineers, however, we must do all we can to provide safe products and
environments.

Where and how do we draw the line on what is safe and what is acceptable?
Because of the numerous concerns throughout the design, manufacturing, dis-
tribution, sales, use, maintenance, and disposal of a product, there is no defini-
tive answer to this question. The following case example illustrates the diffi-
culties involved.

When the Ford Pinto was in the development stage, crash tests revealed that
the windshield could not withstand a prescribed test crash without breaking;
this violated a standard. To solve this problem, the drive train was moved
back, changing the way energy was absorbed during impact. The new location,
however, was close enough to the gas tank that during certain rear-end colli-
sions the drive train would collapse the fuel tank and cause an explosion
(Martin and Schinzinger, 1989). At least one engineer was concerned by the
potential safety consequences, but he was ignored by corporate officials. The
Pinto was produced and sold with this known defect, probably because of the
up-front costs of redesigning. At the time, Ford was in a serious competition
with foreign car makers to put a fuel- efficient model on the market at a com-
petitive price. Once on the market, Pintos did explode during rear-end colli-
sions; deaths and injuries and huge civil liability lawsuits resulted. It is debat-
able whether the final economic costs to Ford, in lawsuits, insurance, and dam-
aged reputation, outweighed changing the defective design in the first place.

A myth that engineers can no longer claim is that operator error and negli-
gence is the principal cause of most adverse events. An engineer is a profes-
sional problem solver. When these events repeatedly occur during the use of a
particular product, a problem undeniably exists. Just as a part that fails due to
fatigue before a product's useful life is complete, this event is an abnormal and
undesirable incident—a failure—and must be treated the same. We cannot
blame the user every time someone is electrocuted. "He knew it was a high
voltage line—it’s his own fault for touching it." Obviously, if "he knew...," he
wouldn't have touched it, or he touched it inadvertently.

1-6


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

INTRODUCTION Unexpected behavior or operating characteristics physically inherent to some
electrical equipment may release energy at levels sufficient to ignite flamma-
ble materials or fuel-air mixtures. Indeed, 22 percent of 25,000 fires studied by
Factory Mutual Engineering Corporation between 1968 and 1977 were caused
by electrical ignition sources—the leading cause of fire in industry (National
Safety Council, 1988). The Fire Protection Association (FPA), the British
equivalent to the National Fire Protection Association (NFPA) in the United
States, also ranks “electrical installations and apparatus” as the most common
cause of “major” fires.

The occurrence of residential and commercial fires may seem rather low, but
the loss of life and property when it does happen is quite disturbing. All rea-
sonably practicable steps to eliminate the potential for a fire or explosion must
be taken, or the owner, insurance carrier, and regulatory agencies must agree
to reduce the hazard to some minimally acceptable probability.

In this course unit, we will begin by exploring the mechanics of fire and explo-
sions. A complete understanding of this science, which is well beyond the
scope of this course, draws heavily on the field of chemistry. What we present
here, however, is enough to familiarize you with some of the terms and con-
cepts you will be exposed to in the future. Of the greatest importance is the
potential contribution of electrical equipment as an ignition source and the
methods you can use to control the hazards of fire and explosion. When you are
faced with design or operations, however, consultation with experts in fire pre-
vention and control helps minimize the potential for fire and explosion hazards.

MECHANICS Fire, or the process of combustion, is a complex mix of heat, fuel, oxygen, and

OF FIRE chemical chain reactions. The “fire triangle” (Figure III-1 (a)) is a convenient
way to remember the basic elements (Ridley, 1986). The “fire pyramid”
(Figure III-1 (b)), symbolizes the entire process of combustion by including
“chain reactions” (National Safety Council, 1988).

Fuel Oxygen

Heat
The Fire Triangle

Chain
Reactions

The Fire Pyramid

Figure DI-1. (a) The fire triangle; (b) the
fire pyramid.

Til-2



AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

As engineers, we create things that have profound effects on the lives of individ-
uals in society. Through our studies and our work, we become very familiar with
new and complicated technologies. Technical concepts become intuitive to us:
“.. it’s obvious ... everybody knows that.” We tend to forget the years spent
developing our expertise—years developing basic knowledge of the physical
laws of our world. Can we assume, or even expect, the general public possesses
the technical skills and knowledge we take for granted? Of course not.

Certainly some people understand, for instance, that submerging a hair dryer
in water is hazardous. More important, however, is that many others do not
realize that mixing electric appliances and water creates a very dangerous situ-
ation. A child has no concept of electricity. An adult may have an understand-
ing of electricity, but not understand that it can dangerously energize the bath-
tub water. Furthermore, even if the hazard is understood, that knowledge
won’t help when the hair dryer accidentally falls into the bath water. We know
that many people are unaware of some electrical hazards.

We know that’ people—even skilled people—make mistakes and that adverse
events happen. If we don't make every effort to provide safe products, systems,
components, homes, and workplaces, we are ignoring our ethical obligations.

This, of course, over simplifies ethical and moral dilemmas that face an engi-
neer in the normal conduct of a career. Often it is not one engineer that makes
risk-benefit decisions that affect the safety of the design or operation of prod-
ucts and systems. For highly complex systems, such as a nuclear reactor, the
effort and compromises of a wide variety of interests and expertise decide the
final course of action in design and operations. As an example, study of the
Three Mile Island nuclear power plant incident highlights the difficulty in sat-
isfying the interests of all parties involved in a project, and the incident exem-
plifies the hazardous outcome when those involved have not held paramount
the safety, health, and welfare of the public in the performance of their profes-
sional duties.

You are encouraged to do additional readings regarding your responsibilities
and your rights as an engineer.

When accidental injury or death does occur, who was at fault? If an injury
occurs in the workplace, workers’ compensation laws generally provide limit-
ed compensation to an injured employee, and the employer is exempt from
civil lawsuit. Designers, manufacturers, and distributors, however, are subject
to civil lawsuits whether an injury or death was work related or not. In some
instances, individual engineers can be held civilly or even criminally liable.

The basis on which parties named in a products liability suit are judged is
called the standard of responsibility. The legal standards used to test liability
are the theories of negligence, strict liability, warranty, and misrepresentation.

Negligence focuses on conduct and asks whether the manufacturer acted rea-

sonably. Under this theory, the standard of responsibility is the duty to exercise
due care in providing a reasonably safe product.

IV-10


Unit 111—Ignition of Flammable and Combustible Materials and Explosives

element

Figure III-8. (a) Equal and opposite charge sepa-
ration between a low conduction
fluid and a pipe without flow;
(b) when fluid flows in the pipe, the
charges begin to accumulate and
move with the fluid (“streaming cur-
rent”); (c) charge separation in a fil-
ter (adapted from Hammer, 1989).

The ability to store electric charge is known as capacitance. When a static
charge discharges as a capacitor, static electricity is a hazard. If a charge leaks
away over time, the energy released is not enough to constitute a fire or explo-
sion hazard. A capacitive discharge, however, may release sufficient energy to
ignite a variety of flammable mixtures. The energy in a capacitor, and thus a
static charge insulated from ground or other conductor, is

_ cv?
a)

where E is energy in joules, C is capacitance in farads, and V is voltage
(Howard, 1985).

When this energy is released, there is, indeed, a concern wherever flammable
environments could be present, as the following case histories illustrate.

A fatality occured in a hospital operating room when a static discharge ignited
ether vapor, which was being used as an anaesthetic.

IH-23


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Table I-2
Rough Estimates of Human Resistance to Electrical Current

Body Area Estimated Resistance (Ohms)
Dry skin 100,000 to 600,000
Wet skin 1,000
Internal body (hand to foot) 400 to 600
Internal body (ear to ear) (about) 100

Adapted from Firenze and Walters, 1981.
Let’s return to the case study at the beginning of this section and analyze the
man’s electrical shock with the information we now have. Assume a 110-volt
source (i.e., 110-volt potential between each hand and ground—trefer to Figure
IE-5), and calculate the amount of current that flows through his body when the
skin is wet and when dry, using the values shown.
V = 110 = (Ry + Ry) * iy + Rg * is = (Ry + Ry) * ip + Rg * iy
i =branch current
ip t+in=i5
110 = (Ry + Ro) * iy + Rg * (i, + ip) = (Ri + Ro) Fi + Rg *2* iy

_ 110
~ (Ri +R) + (2* Rs)

i=in
So, the current in each arm is
iy (dry skin) = ig (dry skin) = 0.000577 = 0.577 mAmps
i; (wet skin) = ip (wet skin) = 0.034375 = 34.375 mAmps
The total current that passed through his body is

iz (dry skin) = 1.154 mAmps

iz (wet skin) = 68.75 mAmps

I-16


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

CONCLUSIONS This has been a brief review of electrical shock hazards and of some of the
REGARDING controls available to reduce or eliminate the hazards. When you begin design-
SHOCKS ing, maintaining, or operating electrical equipment or systems, or when you

are responsible for others performing such duties in your career, you must
study, understand, and implement these controls to eliminate shock hazards
whenever possible. When it is impossible to eliminate hazards by design,
every attempt must be made to reduce the hazard to an acceptably safe level.
Later in this text, we will introduce standards and regulations that address
these concepts and will assist you in making your designs and procedures safe.

INADVERTENT Inadvertent activation of electrically powered equipment frequently causes
ACTIVATION OF death, injury, and equipment damage. Examples where a worker was inside a
EQUIPMENT large piece of industrial equipment performing maintenance or cleaning proce-

dures when the machine was activated are not uncommon, and the outcome is
often fatal. In other cases, workers may be struck and injured by moving parts
when equipment powers-up without warning or operates differently than
expected. Severed fingers, hands, and limbs are not uncommon in these situa-
tions. A combination of safety designs and operating procedures are often used
to minimize this hazard.

Case study Before a worker inspected and maintained a large industrial fan, he turned off
all the power at the main control panel. Normally an operator controls and
monitors the equipment from this control panel. The worker assumed the oper-
ator had gone to lunch and decided he could complete his work before the
operator returned. He entered the fan housing to inspect the blades, which
measure some 20 feet in length. Meanwhile, the machine operator returned to
his station to find the fan not running. He immediately switched all the power
on to test the fan. The maintenance worker was killed within seconds, unbe-
knownst to the operator.

Lockout/tagout This event could have been avoided by using a number of safety designs and

procedures procedures, including locking and tagging out the control panel.
Lockout/tagout procedures would have been the responsibility of the mainte-
nance worker. Had they been a part of the company procedures, he would have
physically locked the power controls in the off position and placed tags
describing the work being done, the person’s name, and the department
involved. He would then keep the only key to that lock with him (typically, the
supervisor keeps a master key to the lock in his office). These procedures
should be normal in every industry. All personnel should be trained to follow
and understand their use. When they are followed and assuming no other sys-
tem faults, the risk of injury due to unexpected activation is significantly
reduced.

No matter what method is used to lock electrical switches off, the procedures
must be constantly supervised and employees thoroughly trained and required to
follow them. Here is one minimally acceptable lockout program for maintenance
procedures (a more detailed plan can be found in the reference materials):

1. Alert the operator that maintenance is to be performed.

2. Before beginning work, make sure that no part of the system or machine
can be activated or set in motion without permission from the mainte-
nance worker.

II-28


REQUEST FOR ASSISTANCE IN PREVENTING

ELECTROCUTIONS FROM CONTACT BETWEEN CRANES AND. POWER LINES

BACKGROUND

Contact between cranes and overhead power lines is a major cause of fatal
occupational injuries in the United States. Based upon an analysis by the
National Institute for Occupational Safety and Health (NIOSH) of the data from
the Supplementary Data System [1] of the Bureau of Labor Statistics, there
were approximately 2,300 lost workday occupational injuries in the United
States in 1981 which resulted from contact with electrical current by crane
booms, cables, or loads. These 2,300 injuries were extremely severe,
resulting in 115 fatalities and 200 permanent total disabilities. Comparable
statistics obtained in studies conducted by the National Safety Council from
1964 to 1976 produced an estimated annual average of 150 fatalities resulting
from such incidents [2]. NIOSH believes that this type of event is the most
common cause of fatalities associated with mobile crane operations [3] and is
responsible for approximately 1.5% of all fatal work-related injuries each
year.

CASE REPORTS
As part of the Fatal Accident Circumstances and Epidemiology (FACE) Project

conducted by NIOSH, six fatal injuries involving crane-related electrocutions
were investigated. The synopses of these cases are as follows:

Case #

A 28-year-old construction worker was holding on to a steel
ladder being moved by a telescoping boom crane. As the
crane's boom was swung in the direction of 7,200 volt power
lines, the cable contacted the closest of the lines and the
worker was electrocuted.

Case #2:

A co-owner of a steel erection company and three workers
were using a telescoping boom crane to move a section of a
steel framing member at the construction site of a
commercial storage shed. As the section was moved, it came
into contact with a 23,000 volt overhead power line. Two
of the three workers who were in direct contact with the
load were electrocuted while the third received serious
electrical burns.

July 1985


Unit Ill

IGNITION OF FLAMMABLE AND COMBUSTIBLE
MATERIALS AND EXPLOSIVES

PURPOSE:

OBJECTIVE:

SPECIAL TERMS:

Fd

ee

To introduce students to the consequences of electricity acting as an ignition
source.

To acquaint the student with:

1.
2.

The basic characteristics of fires, combustions, and explosions
Electricity as an ignition source

3. Methods to control ignition hazards

4.
5.

WCONANAWNE

Consequences of electrical overheating
Chararcteristics of static electricity

- Combustible
. Flammable

Deflagration

. Detonation

Electrical overloading
Spark
Are

. Corona

. Intrinsically safe
. Encapsulation

. Embedment

. Hermetic sealing
. Explosion-proof
. Pressurization

. Fuses

. Circuit breakers

. Static electricity

T-1



DISCLAIMER

Mention of the name of any company or product
does not constitute endorsement by the National
Institute for Occupational Safety and Health

DHHS (NIOSH) Publication No. 87-103

Copies of this and other NIOSH documents are available from

Publications Dissemination, DSDTT
National Institute for Occupational Safety and Health
4676 Columbia Parkway
Cincinnati, Ohio 45226
(513) 841-4287


Unit I]—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Interlock

3. Place a personal padlock and tag on the master control switch, lever,
valve, or local disconnect. This should be done even if another lock is
already in place (for large operations, others may also be performing
maintenance in other areas). A worker can only be protected by using his
own padlock.

4. When work is complete, remove the lock and tag. Only the worker per-
forming the work should remove the lock. Before the machine is activated,
make sure no other person is in danger before activating the equipment.

5. Provide each maintenance worker with his/her own locks with unique
keys. Only the worker and the supervisor should have copies of the keys.
Lost keys should be reported immediately and new locks issued.

Unfortunately, sometimes a worker will forget to follow procedures or take a
short-cut. Therefore, additional safety measures should be designed into the
system to further reduce the risk of injury. In the case study, an interlock
would have been an additional safety measure that would have eliminated the
electrical contact. An interlock is designed to disconnect all power to a system
when a protective enclosure has been opened.

In other cases of inadvertant activation, a person may bump a power switch on
a piece of equipment and unexpectedly activate the power, or release mechani-
cal energy, or both. Locating the controls at a distance from moving or ener-
gized parts will preclude injury should they be accidently activated. Other
methods include the use of multiple controls and actions to activate a machine.
For example, power presses, such as those that stamp metal parts using huge
descending weights require three separate actions to activate the press. Before
each cycle, a supervisor must use a key to set the machine for operation. The
operator then must activate a preset button before being able to activate the
punch with dual hand buttons located a safe distance from the machine. The
dual hand buttons are designed to keep both hands occupied during the
machine cycle and thus keep them clear of the danger area. All buttons are
guarded to minimize the possibility of accidental depression. In some cases,
the machines are equipped with powerful brakes that will stop the punch from
descending if the operator removes a hand or hands from the buttons.
Although this system sounds fool proof, adverse events still occur because of
design, manufacturing, or installation errors or because operators devise ways
to bypass the safety features.

The punch press example illustrates the difficulty engineers have in trying to
design a safe product. Not only will people using our designs make mistakes
(we all make mistakes; we all know mistakes happen), but they may intention-
ally bypass safety designs to make their task easier or to reduce the time nec-
essary to.complete it. A design that is both safe and efficient is an effective
design.

T-29


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Control of static
hazards in
hydrocarbon
industries

Choosing materials that are good conductors or coating nonconducting bodies
with conducting materials will minimize potential static charge accumulations.
Since the presence of water (a good conductor) reduces static accumulations,
humidifying the atmosphere might also be a reasonable and effective method
to reduce charge accumulation.

Grounding and bonding are also effective ways to eliminate potential differ-
ences (voltage) between two conducting bodies or a body and a ground. Recall
that it is the voltage and capacitance that determine the energy released in a
discharge of static charge. By bonding two conducting bodies together and
grounding the system, the voltage between neighboring bodies can be nearly
eliminated. Bonding decreases the potential difference between conducting
bodies, and grounding the system decreases the potential between the conduct-
ing bodies and the ground.

For some applications, electrostatic neutralizers may be appropriate. There are
three major types of charge neutralizers: radioactive, high-voltage, and, induc-
tion. The radioactive source emits alpha particles with two positive unit
charges that act to neutralize the net negative charge on a material. Because
radioactivity may be harmful to human tissue, this type of neutralizer should
be used only when humans will be protected from harm (Hammer, 1989).

High-voltage neutralizers produce a high potential in the air near the surface of
the material of concern. This type of device ionizes the air by producing both
positively and negatively charged particles that act to neutralize any charges
that may develop on the nearby material. These high-voltage devices may,
however, create their own fire hazard where flammable atmospheres are a con-
cern and may present a shock hazard to personnel as well (Hammer, 1989).

Induction neutralizers produce opposite polarity to that causing static charge
generation and accumulation and, thus, neutralize any potential difference
(Hammer, 1989).

Some industries are particularly prone to hazards related to electrostatic dis-
charge and have developed specific safeguards and procedures over the years.
Hydrocarbon industries and industries handling or producing dusts and pow-
ders are worth special mention, and the general concepts should be extended to
other industries as well.

Because of the flammable characteristics of many hydrocarbon products (e.g.,
oils, gasoline, etc.), it is not surprising that this industry has experienced many
ignitions blamed on static electricity. Liquid transfer often presents the greatest
hazard. Fluid movement may cause flammable vapors, and the flow through
pipes, filters, spraying the product through air, and impurities present in the
product can all generate static charges. In particular, the following operations
have been identified as static charge hazards when handling dry refined oils
such as gasoline, kerosene, and jet fuels:

— flow through a pipe,

— agitation by propeller mixing or a centrifugal pump,
— overhead splash filling of tanks,

— settling of water or other contaminants through an oil,
— filtering, and

— rapid evaporation.

TII-26


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

HAMMURABI’S
CODE
{approximately
1750 B.C.)

THE ROLE OF
SAFETY
STANDARDS

A few definitions

“If a builder constructs a house for a man and does not make it firm and
the house collapses and causes the death of its owner, the builder shall be
put to death.”

Hammurabi’s Code contains what may be the first written statements regard-
ing responsibilities of a party in accident prevention. Since 1750 B.C., the
penalties have softened, but the volume of standards, codes, and other written
safety documents has proliferated in step with technological advances over the
3700 years (Hammer, 1989).

Debate over the language contained in standards, and whether the standards
should be enforced by a government or be voluntary, probably began when
Hammurabi’s law was pressed in cuneiform wedges and baked; certainly the
debate continues today. There is no arguing, however, the valuable role of
standards in making products, homes, and workplaces safer than they would
without such guidance.

Safety, in part, depends on the development and use of safety documents sum-
marizing previous knowledge designed to protect individuals in a society. As
engineers, we rarely design every aspect of a product or process from scratch.
By using standards, we are free to spend more time and energy on the new and
unique aspects of a design.

Ideally, the standards we refer to and depend on would incorporate all the
lessons learned by those that passed before us, including the successes and the
failures. Realistically, though, safety standards generally provide only mini-
mum guidelines. The safety of our designs should never be limited by merely
satisfying some minimally acceptable requirements of applicable standards.
Wherever possible, we must exceed those safety requirements. It is our moral
obligation (and often legal obligation, as well) to provide products and sys-
tems that can be used safely by the general public (Bass, 1986).

In this course unit, we will discuss the nature of standards and some of the
standard-producing organizations you should be familiar with. We will con-
clude with some thoughts on what can happen if we are derelict in exercising
our professional responsibilities as engineers...

A standard is a set of rules, requirements, or principles.

A code is a collection of laws, standards, or criteria relating to a particular subject.

A recommended practice is a series of recommended methods, rules, or
designs.

A test method defines such things as specimen selection, testing procedures,
sampling, and analysis methods to determine the properties, composition, or
performance of materials or products.

A definition explains standardized meaning of terms as applied to materials,
products, systems, and methods.

Iv-2


Unit II—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

In the event of certain fault conditions, reversed polarity may result in internal
circuits in electrical equipment being energized even if the switch is “off.”
Reversing the ground and neutral wires is not a hazard in itself. If such a rever-
sal was in series with another receptacle that had reversed polarity, a hazard
would exist because the neutral wire would then become hot. Reversing the hot
and ground wire is the most hazardous condition shown: in this case the drill
casing may become energized if a fault occurs and may cause a shock to any-
one that touches it. The drill motor will not run in this situation. Figure IJ-10
shows a correctly wired receptacle.

a ‘*— Brassed-colored
White wire terminals
Nickel or light
colored terminals

Green or bare-v
grounding

conductor a

Green
hexagonal
head terminal
screw

[ L Hot (black) Load
—_ : LA

iy 1U
Neutral (white)

Grounded (green or bare)

®)

Figure II-10. (a) Duplex receptacle correctly wired to designated terminals
(adapted from U.S. Department of Labor, 1981); (b) schematic
of the entire, correctly wired, three-wire receptacle and circuit.

I-23



and Health, Division of Safety Research, FACE-
88-4-I1.

NIOSH [1987c]. Fatal accident circumstances
and epidemiology (FACE): Two painters
electrocuted in Ohio. Morgantown, WV: U.S.
Department of Health and Human Services,
Public Health Service, Centers for Disease Con-
trol, National institute for Occupational Safety
and Health, Division of Safety Research, FACE-
87-28-I1.

NIOSH [1987d]. Fatal accident circumstances
and epidemiology (FACE): 27-year-old
painter electrocuted in Georgia. Morgantown,
WV: U.S. Department of Health and Human
Services, Public Health Service, Centers for
Disease Control, National Institute for Oc-
cupational Safety and Health, Division of
Safety Research, FACE-87-32-II.

NIOSH [1987e]. Request for assistance in
preventing electrocutions by undetected feed-

ack electrical energy resent in power lines.
Cincinnati, OH: U.S. Department of Health
and Human Services, Public Health Service,
Centers for Disease Control, National Institute
for Occupational Safety and Health, DHHS
(NIOSH) Publication No. 88-104.

NIOSH [1988]. National traumatic occupation-
al fatalities (NTOF) data base. Morgantown,
WV: U.S. Department of Health and Human
Services, Public Health Service, Centers for
Disease Control, National Institute for Oc-
cupational Safety and Health, Division of
Safety Research. Unpublished data base.

Metal Ladders




Page 6 - Request for Assistance in Preventing Electrocutions from Contact
Between Cranes and Power Lines

REFERENCES

1. Bureau of Labor Statistics: Supplementary Data System Microdisk Files
User's Guide, 1976-1977, No. PB288258. Springfield, Virginia: National

Technical Information Service, 1978.

2. National Safety Council utility study, 1964-1968. Accident Facts.

Chicago, Illinois: National Safety Council, 1969-1976.

3. Coleman PJ, Gottlieb MS, Kaplan MC, Knutson SJ, McPeek JS. A human factor
analysis of material handling equipment. Madison, Wisconsin: State of
Wisconsin, Department of Industry, Labor and Human Relations,

January 19783;132-4.

4, Crane handbook. Toronto, Ontario, Canada: Construction Safety Association

of Ontario, October 19753;133-50.


1. Portable metal or conductive ladders
shall not be used for electrical work or
where they may contact electrical con-
ductors [29 CFR 1926.450(a)(11) or
1926.951(c)(1)].

2. Employers shall instruct each employee
in the recognition and avoidance of un-
safe conditions and the regulations ap-
plicable to his work environment to
control or eliminate any hazards or other
exposure to illness or injury [29 CFR
1926.21 (b)(2)].

3. Portable ladders in use shall be tied,
blocked, or otherwise secured to prevent
their being displaced [29 CFR
1926.450(a)(10)].

Compliance with these regulations might have
prevented ail five deaths.

CONCLUSIONS

The principal objectives of the NIOSH FACE

Program are to identify potential risk factors

that may contribute to traumatic worker
deaths and to recommend measures that

might prevent similar fatalities. Whether or not
awork operation complies with existing OSHA
regulations is only one variable that may con-

tribute to a fatality. However, in the investiga-
tions reported here, full compliance with

relevant OSHA regulations would probably
have prevented these deaths. The lack of
compliance with existing regulations in the five
incidents described suggests that many
employers and workers may be (1) mons

unaware of these OSHA regulations, (2
misinterpreting the requirements of the

regulations, or (3) failing to inform their

workers about the dangers of using metal

ladders around overhead power lines.

RECOMMENDATIONS

The following recommendations will help

prevent deaths and injuries resulting from
contact between metal ladders and overhead
power lines:

Metal Ladders.

« NIOSH recommends that employers and
workers comply with the OSHA regulation
prohibiting the use of portable metal or
conductive ladders for electrical work or in
locations where they may contact electrical
conductors. Nonconductive ladders such
as those made of wood or fiber glass should
be used instead.

¢ Employers should fully inform workers
about the hazards of using portable metal
(including aluminum) ladders near ener-
gized power lines.

¢ If portable metal ladders are used in the
vicinity of energized power lines, NIOSH
urges that all employers and workers strict-
ly adhere to the OSHA safety regulations
[29 CFR 1926.450 and 1926.951(c)(1)] for
providing proper balancing and securing of
ladders, and for maintaining safe working
distances to avoid contact with electrical
conductors.

* To assure proper protection for anyone
working near electrical power lines, ar-
rangements should be made with the
power company to de-energize the lines or
to cover the lines with insulating line hoses
or blankets.

¢ Employers. should provide workers with
training in emergency medical procedures
such as cardiopulmonary resuscitation.
Fatalities may be prevented by prompt
emergency medical care.

NIOSH also urges safety and trade associa-
tions, electrical utility companies, product
manufacturers, and OSHA State consultative
services to bring these recommendations to
the attention of employers and workers using
portable metal ladders. Further information on
electrical energy hazards can be found in six
previously published NIOSH Alerts [NIOSH
1987e, NIOSH 1986a, NIOSH 1986b,
NIOSH 1986c, NIOSH 1985b, NIOSH 1984].


Page 5 - Request for Assistance in Preventing Electrocutions Due to
Damaged Receptacles and Connectors

Director, Division of Safety Research, National Institute for
Occupational Safety and Health, 944 Chestnut Ridge Road, Morgantown,
West Virginia 26505, Telephone (304) 291-4595.

We greatly appreciate your assistance.
f

\ (~
MW
1 USDonald Millar, M.D., D.T.P.H. (Lond. )
Assistant Surgeon General
Director, National Institute for

Occupational Safety and Health
Centers for Disease Control



AN INTRODUCTION TO ELECTRICAL SAFETY
FOR ENGINEERS

Authors
Donald S. Bloswick, P.E., Ph.D.
Assistant Professor, Department of Mechanical Engineering
and
Peter M. Budnick, M.S.E.
Graduate Research Assistant

The University of Utah
Salt Lake City, Utah

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Centers for Disease Control and Prevention
National Institute for Occupational Safety and Health
Division of Training and Manpower Development
Cincinnati, Ohio
June 1993


NIOSH

ALERT

ALERT

ALERT

ALERT

ALERT
ALERT

Request for Assistance in

Preventing Electrocutions Due to
Damaged Receptacles and Connectors

October 1986

U.S. DEPARTMENT OF. HEALTH AND HUMAN SERVICES
Public Health Service

Centers for Disease Control

National Institute for Occupational Safety and Health


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Systems Hazards
Analysis (SHA)

For example, if the commutator were to fault to the case of the drill (see Unit
II), the potential injury severity is very high (death due to shock). The proba-
bility is also relatively high. This leads to a high criticality index. In this case,
the hazard can be very significantly reduced by grounding the case of the drill
or using double insulation. (Both of these measures were discussed in the
module dealing with shock.)

Systems Hazards Analysis includes the human component (a strength of Job
Safety Analysis) and the hardware component (a strength of Failure Mode and
Effects Analysis).

SHA concentrates on the worker/machine interface. What process is being per-
formed on what equipment? What major operations are required to complete
the process? What tasks or activities are required to complete an operation?
The thesis of SHA is that failures (undesired events) may be eliminated by
systematically tracking through the system, looking for hazards that may result
in a failure situation.

In the language of SHA, the terms process, operation, and task have specific
meanings. Process means the combination of operations and tasks that unite
physical effort and physical and human resources to accomplish a specific pur-
pose. An operation is a major step in the overall process (e.g., drilling and
countersinking stock on a drill press). A task is a particular action required to
complete the operation (e.g., placing a cutting tool in a holder before sharpen-
ing the tool on the grinder).

Once the process to be analyzed has been identified, it is subdivided into its
operations and tasks. To do this, the analyst must be familiar with the tasks
involved in the operation and the interactions between and within the system
being analyzed and associated systems and subsystems. Often a flow diagram
is constructed to record what is taking place throughout the flow of operations
and tasks that fulfill process demands. This enables the analyst to see the perti-
nent subsystems, methods, transfer operations, inspection techniques, and
man/machine operations.

One type of hazard analysis process (adapted from Firenze, 1991) includes
identifying and recording information relating to:
1. process (turning between centers on a machine lathe),

2. major unit operations required to complete the process (rough-turning
steel stock),

3. tasks required to complete an operation (select cutting tool and place in
holder),

4. variance from safe practices with the potential to cause hazards (incorrect
cutting tool used),

5. hazard that has the potential to cause an injury (worker in proximity to
lathe when incorrect cutting tool used),

6. triggering event causing hazards to result in incidents, brought about by
human error or situational, or environmental factors (starting the lathe),

V-6


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Because ignition and explosion characteristics vary widely depending on what
substance is present in an area, hazardous atmospheres are categorized into
seven groups. Class I locations fall into Groups A through D, and Groups E, F,
and G pertain to Class II hazardous locations. Table [1-3 provides example
materials for each group and industries where they might be found.

Table III-3. Example NEC Class and Group Distinctions

Class Group Description Examples Industries
I A Highly flammable Acetylene Welding fuel
gases and vapors generators
I B Highly Hydrogen Chemicals and
flammable gases Butadiene plastics
and some liquids
I c Highly Diethyl ether Hospitals,
flammable Acetyaldehyde chemical plants
chemicals
I D Flammable fuels, Gasoline Refineries,
chemicals Ammonia chemical plants,
Acetone paint spray areas
0 E Metal dusts Magnesium Chemical plants
I F Carbon,coke, Carbon black Mines, steel mills,
coal dust Charcoal power plants
te G Grain dust Flour, starch Grain mills and
dust elevators

Determining whether a hazardous area exists and then classifying it is generally
straightforward in most industries. Defining the limits of the of the hazardous
location; however, can be difficult. The NEC provides guidelines for certain
circumstances. For circumstances not covered by the NEC, a general rule can
be applied: the limits of a hazardous location are those mutually agreed upon by
the owner, the owner’s insurance carrier, and the authority enforcing the code.

The selection of electrical equipment that must be located in an area classified
as hazardous depends on the Class, Division, and Group. Often, equipment
manufacturers can aid in the selection of appropriate equipment. For example,
the selection of explosion-proof equipment (discussed below) depends on the
combustion characteristics of the specific atmosphere in which the fixture will
operate. In essence, equipment must be customized to the specific hazardous
environment where it will be used.

The more common techniques and equipment used in hazardous locations are
briefly described below. The NEC provides guidance for minimizing or elimi-
nating electrical ignition sources from hazardous areas. As always, keep in
mind that although these methods of control offer a necessary and significant
degree of protection, they are subject to failure and, therefore, do not ever
completely eliminate the risk of an adverse event.

Iil-14




Unit V
SYSTEMS SAFETY ANALYSIS

PURPOSE: To introduce the student to the importance of systems safety analysis in the design
of electrical systems
OBJECTIVE: To acquaint the student with:
1. Inductive methods of systems safety analysis
2. Deductive methods of systems safety analysis
SPECIAL TERMS: . Preliminary Hazard Analysis (PHA)

. Job Safety Analysis (JSA)

. Failure Mode and Effect Analysis FMEA)

. Systems Hazard Analysis (SHA)

. Fault Tree Analysis (FTA)

. Management Oversight and Risk Tree analysis (MORT)
. Sneak circuit analysis

V-1



AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Electrical stress effects

Temperature effects

Electrical. : Temperature : Chemical : Mechanical
Stress i ; and physical ; stress

7 2

reaction ‘

Arching a Chemical :
i

heat Sl byproducts
t

Friction

Environmental x environmental Pinching,
heat and cold | chemicals crushing
Process bending

F Temperature chemicals
fluctuations

: i Radiation

t t t t

Moisture facilitates deterioration
and increases the hazards
associated with failure

Figure II-2. Insulating materials are subject to deterioration or damage
from electric stress, temperature extremes and fluctuations,
chemical and physical agents, and mechanical stress. The
presence of moisture will facilitate failure.

For a more common example of an insulated wire that is subjected to potential
mechanical damage, check the wires supplying power to a car door (door light,
electric locks and windows, etc.). Can you see how it was designed to with-
stand the mechanical loads of opening and closing the door? Is the insulation
in good condition?

Insulation breakdown related to excessive electrical stress is a complicated mat-
ter. A complete understanding of this subject involves concepts beyond the
scope of this text, but in general, relatively high voltage is of concern. High volt-
ages can cause localized electrical discharges (sparks, arcs, or coronas) that can
create holes or chemical damage in an insulator. (Sparks, arcs, and coronas are
also a fire hazard and are discussed in detail in the next unit.) Sparking may
cause high, localized temperatures and produce ozone that, as discussed below,
can react with insulating materials. A corona will produce nitrous oxide that
reacts with moisture to form an acid that can damage an insulator.

Extreme temperatures and cyclical temperature changes can also contribute to
the breakdown of insulation materials. The flow of electric current always pro-
duces heat, and sometimes that heat production causes significant localized
temperature increases. Even at moderate temperatures, some polymers will go
through a slow but gradual breakdown.

IL-6


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

AN EXAMPLE
OF ELECTRIC
SHOCK

A man is building a storage shelf in his garage. He’s drilling holes in the wood
with an electric drill that has been designed without a third wire ground (we
will discuss grounding later). Or, as is often the case, the ground prong has
been removed because the electric socket in his garage does not have a three-
wire grounded receptacle. Inside the drill, an energized conductor comes into
contact with the external case of the drill, which in this particular drill is also a
conductor, It’s a hot, humid day, and he is working without a shirt. Preparing
to drill, he holds the tool with both hands and braces his shoulder against a
metal beam. The beam extends from the roof down to the floor, where it is
imbedded into the earth for structural stability. As soon as he energizes the
drill, his muscles freeze, and he is unable to let go of the tool. Within seconds,
he falls to the floor unconscious. An hour later, he is found dead. The autopsy
report identifies respiratory arrest due to electric shock as the cause of death.
The report also notes burns on his hands and back and a contusion on the left
side of his head. See Figure II-1.

Frayed-wire
Power contacts case

generator
fp

Figure II-1. Schematic showing the path of the current in the example. A
frayed wire touches the conducting case of the tool (short cir-
cuit), energizing it. The current passes from the energized
case, through the man’s arms, exiting at his shoulder. The cur-
rent loop is completed through the ground,

This is one hypothetical example of the many ways that a person can be
shocked by electricity. In this case, the man became part of the electrical cir-
cuit; as the current flowed from the energized drill through his body to the
ground, it caused his heart to stop and burned him at the points of current
entrance and exit. The contusion on his head was caused by his head striking
an object or the floor when he fell.

Fortunately, this type of incident can be avoided. In this course module, we
will discuss the hazard of electric shock and the effects of electric shock on the
human body. It will conclude with some methods to control the hazard through
design and safety procedures.

T-2


Unit V—Systems Safety Analysis

6. Probability Index—How likely is the event to occur under the circum-
stances described and given the required precursor events? These proba-
bilities are based on such factors as accident experience, test results from
component manufacturers, comparison with similar equipment, or engi-
neering data. Probability categories, which may be developed by individ-
ual companies or analysts, are sometimes classified as:

1—probable (likely to occur immediately or within a short period of
time),

2—1reasonably probable (probably will occur in time),
3—1emote (possible to occur in time), or

4—extremely remote (unlikely to occur).

7. Action or Modification—After the failure modes, causes and effects,
severity, and probability have been established, the system must be modi-
fied to prevent or control the failure.

The severity index, probability index, and a third index relating to personnel
exposure may be used to determine the overall risk. A review of the above
steps makes the objectives of FMEA clear. FMEA is intended to rank failures
by risk (severity and probability) so that potentially serious hazards can be
corrected.

When the analysis includes the severity, probability, and criticality indices, it
is sometimes called a Failure Mode, Effect, and Criticality Analysis
(FMECA). A failure modes effects (and criticality) analysis of a drill is shown
on Table V-2. In this case, the criticality index is the sum of the severity and
probability indices and represents a measure of the overall risk associated with
each combination of severity and probability.

Table V-2. Failure Mode and Effect Analysis* of an Electric Drill
‘Severity Probability Criticality ‘Action or

‘Component Failure Mode System Causes System Effects Index Index. Index. Modification
On-off switch Fails open. Normal wear Drill disabled 4 2 6 None
Fails closed Normal wear Drill continues torun 4 2 6 None
Reduction gear train Functional disability Normal wear Drill disabled 3 4 7 Preventive
overload, lack of maintenance, avoid
lubrication. overload
Commutator Functional disability Normal wear, Drill disabled 3 3 6 None
broken spring
Fault to case Carbon buildup Fatality, if operator. => 1 1 2 Ground the case or go
across insulating is grounded at a to double insulated
block remote extremity construction
Windings Burnout Overload, plus fuse Drill disabled 3 2 5 Check fuse, avoid
failure to open overload
Fault to case Insulation failure Fatality, if operator 1 2 3 Ground the case or go
is grounded at a to double insulated
remote extremity construction

**The analysis of this drill system includes a criticality index (jn this case, the sum of the severity and probability indices); therefore, it can be called a Failure Mode,
Effect and Criticality Analysis.

V-5


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

5to25mA

20 to 75 mA

70 to 300 mA

2.5 A or more

Current in this range is referred to as “let-go” current. Let-go current threshold
can be defined as the maximum current at which a person holding an electrode
in each hand can still let them go. Average let-go levels range from 9 to 16
milliamperes for men and 6 to 10.5 milliamperes for women. When a person’s
let-go threshold has been exceeded, the affected muscles “freeze,” and it is
impossible to voluntarily release the conductor. When this happens, the person
may be exposed to current for a longer period, and the potential for injury is
increased. The let-go threshold for direct currents occurs at 300 milliamperes
or greater.

Currents in this range can be very painful and injurious. Prolonged contact
may result in a fall (always a potential source of injury), unconsciousness, and
death as inhibition of the respiratory muscles stops breathing. Cessation of
breathing can be caused by prolonged contraction of the respiratory muscles or
by inhibition of the breathing control centers of the central nervous system.
When breathing stops because of respiratory muscle inhibition, it usually
resumes once the current is stopped or after first aid assistance is given. Death
from asphyxiation will occur unless resuscitation is performed within 3 or 4
minutes maximum. Respiratory arrest of a central nervous system origin is
rare and requires two things: a passage of current through the brain, and a
higher current. Resuscitating such victims is more difficult (Cabanes, 1985;
Hammer, 1989).

Current in this magnitude range lasting as little as a quarter of a second can be
fatal without immediate first aid. Currents in this range can cause ventricular
fibrillation of the heart muscle, which results in the blood not being pumped
through the body. The human heart is made up of four chambers that must
contract in a specific synchronized way to keep the blood circulating. An
external current applied to the heart muscle interferes with the synchronized
contraction pattern, which causes fibrillation. When the heart muscle fibril-
lates, blood is not pumped and vital nutrients cease to flow to the body tissues.
This is the most frequent cause of death in electrical contact events.
Alternating currents in this range lasting one heart cycle or more will cause
fibrillation in almost every case. Direct currents only cause fibrillation if
applied during a short instance of the recovery phase of the heart muscle’s
contraction cycle (Cabanes, 1985; Hammer, 1989). Dalziel and Lee (1968)
suggest that the threshold alternating current capable of causing fibrillation
can be approximated by the formula

116 to 185

ir mAmp mms

where T is the time of contact (in seconds) with an energized conductor and I
is the amount of alternating current in milliamperes.

Currents of this magnitude may stop the heart muscle all together, as opposed
to causing it to fibrillate. Respiratory arrest is also common, but immediate
resuscitation may succeed. Alternating currents at this level will also burn the
skin and internal tissues (Hammer, 1989).

Tl-14


NIDSH
ALERT JULY 1985

Request for Assistance in Preventing
Electrocutions from Contact Between

Cranes and Power Lines

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

PUBLIC HEALTH SERVICE

CENTERS FOR DISEASE CONTROL

NATIONAL INSTITUTE FOR OCCUPATIONAL SAFETY AND HEALTH


Page 4 - Request for Assistance in Preventing Grain Auger Electrocutions

transport. Manufacturers of grain augers are urged to consider
design modifications that will prevent grain augers from being
moved while in an elevated position.

3. Safety Signs

It is recommended that users and manufacturers of grain augers
affix safety signs onto the equipment that warn the user of the
potential hazards of moving the auger in its upright position. A
safety sign to draw attention to avoiding electrical hazards when
moving grain augers is provided with this Alert. This sign
should be placed on the grain auger in a conspicuous location so
that it will alert workers of life threatening hazards.

We are requesting editors of appropriate trade and farm journals,
members of farm extension associations, and those responsible for
safety and health (e.g. inspectors, managers, and agricultural
extension specialists) to bring these recommendations to the attention
of farm workers, managers, and owners.

Requests for additional information on control practices or questions
related to this announcement should be directed to Mr. John B. Moran,
Director, Division of Safety Research, National Institute for
Occupational Safety and Health, 944 Chestnut Ridge Road, Morgantown,
West Virginia 26505, Telephone (304) 291-4595.

We greatly appreciate your assistance.

VJ. Donald Millar, M.D., D.f.P.H. (Lond. )

Assistant Surgeon General |

Director, National Institute for
Occupational Safety and Health

Centers for Disease Control



AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Ignition

CHARACTERISTICS
OF COMBUSTION

An oxidizing agent is any substance that releases gaseous oxygen when exposed
to heat. Examples are hydrogen peroxide, sodium chlorate, nitric acid, and
organic peroxides. Ammonium nitrate is an example of a substance that is both
an oxidizing agent and a combustible, thus reacting with itself after ignition.

An ignition source is any supply of energy that will push the reactants over the
initiation energy barrier. Heat, a frequent energy source, may arise from a
flame (e.g., a match or pilot light), friction (e.g., machine parts such as dry
bearings or brake drums), or most important for our purposes, electrically pro-
duced energy. The energy produced, for instance, by a light bulb, an electrical
heating appliance, or an overloaded circuit may provide enough heat to ignite
combustible materials or flammable fuel/air mixtures under the right circum-
stances. Electrical discharges (e.g., arcs, sparks, coronas) may also provide
sufficient energy to ignite combustibles. These events may occur under normal
operating conditions, such as an arc produced when a switch breaks contact
between conductors or when the brush of an electric motor spins. Static elec-
tricity, lightning, or abnormal electrical discharge in circuits and electrical sys-
tems are also potential sources of ignition.

Three measures of initiation energy are used to characterize ignition energies
of substances: flash point, fire point, and spontaneous ignition temperature.

Flash point, which pertains to liquids only, is the minimum temperature at
which a liquid will release enough vapor to form a mixture with air that will
ignite under prescribed test conditions. The Able and Pensky-Martins tests are
commonly used to test and report flash points for different substances. Flash
points are sensitive to atmospheric pressure, decreasing with decreasing pres-
sure (Ridley, 1986).

Fire point, which also pertains only to liquids, is the lowest temperature at
which a burning vapor will produce sufficient heat to continue vaporizing the
liquid and sustain combustion (Ridley, 1986).

Spontaneous ignition temperature is the lowest temperature at which a sub-
stance in either liquid or gas phase will ignite spontaneously. Some organic
substances undergo slow partial oxidation at room temperature. The exother-
mic reactions cause the temperature to rise, and if the excess heat is not dissi-
pated quickly enough, the substance or reactants may spontaneously ignite
(Ridley, 1986). This condition is only reached when there is sufficient air for
oxidation but not enough ventilation to carry away the heat as fast as it is gen-
erated (National Safety Council, 1988).

Upper and lower flammable limits (explosive limits) are used to describe the
concentration of a gas or vapor mixture of substances that is capable of igni-
tion and subsequent flame-propagation when mixed with air under prescribed
test conditions.

The lower flammable (explosive) limit is defined as the smallest concentration

of gas or vapor capable of ignition and subsequent flame propagation under
test conditions.

Il-6


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS.

The application of sneak circuit analysis requires the thorough, systematic,
branch-by-branch inspection of a circuit with attention to the intended function
of each circuit and possible malfunctions.

Sneak circuit analysis can also be applied to systems involving hydraulic sys-
tems, pneumatic systems, mechanical systems, and computer software.

REFERENCES AND SUGGESTED READINDS

1. American Society of Safety Engineers, “Introduction to System Safety,” Des Plaines, IL,
1987.

2. EG&G, Idaho National Engineering Laboratory, “Glossary of Terms and Acronyms,”
Publication 76-45/28, EG&G, Idaho Falls, ID, 1984.

EG&G, Idaho National Engineering Laboratory, “Systems Safety Course Material.” EG&G,
Idaho Falls, ID.

Firenze, R.J., “The Process of Hazard Control,” Kendal! Hunt, Dubuque, IA, 1978.

Firenze, R.J., “Evaluation and Control of the Occupational Environment,” National Institute
for Occupational Safety and Health, Cincinnati, OH 1988.

Firenze, R.J., “The Impact of Safety on. High-Performance/High Involvement Production
Systems,” Creative Work Designs, Inc., 1991.

Hammer, W., “Occupational Safety Management and Engineering,” Fourth Edition,
Prentice Hall, Englewood Cliffs, NJ, 1989.

Smith, L.C., “The J Programs,” National Safety News, September 1980.

y

we

a

nm

e

SAMPLE QUIZ QUESTIONS
1. What is the difference between inductive and deductive systems safety
analysis techniques?
2. When is the appropriate time to apply the PHA technique to a design?

3. Apply the PHA technique to an office chair with coasters, adjustable
seat height and backrest, and swivel seat.

4. Perform a JSA on a household task of your choosing.
5. Perform a FMEA on a tricycle.

6. Apply fault tree analysis to the undesirable event of falling from the office
chair described in problem (3). Do not assign or calculate probabilities.

7. Assign probabilities (your best estimate) to each of the bottom events in
the fault tree you developed in problem (6), and calculate the probability
of falling from the chair.

8. Discuss the primary goals of MORT.

V-12


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

A large underground gasoline storage area used a carbon dioxide total flooding
system as a fire suppression system. During a system demonstration at the
opening ceremonies, a static discharge apparently caused by the flowing car-
bon dioxide ignited gasoline vapors. The resulting explosion killed a number

of people.

Similarly, a damaged tanker vessel that had carried a flammable liquid was
being prepared for cutting. Because of concern that the sparks and heat created
by the cutting tool could ignite vapors, it was decided to create an inert atmos-
phere inside the vessel by discharging a number of carbon dioxide extinguish-
ers. A static discharge caused an explosion, and a number of fatalities resulted.

In a hairdressing salon, a static discharge created by brushing a client’s hair
ignited the vapor of a flammable hair spray being used on another customer; a
fatality resulted.

In 1964, a discharge of static electricity while a plastic cover was being
removed from the third stage of a NASA Delta rocket ignited the fuel; two
people were killed and three others were critically injured (Hammer, 1989).

A man was cleaning a container with toluene. As he swirled the liquid around
the container, a static discharge ignited the toluene vapor.

Figure III-9 shows a hypothetical (but very potentially real occurrence) life
history of a bulk static electrical charge that could result in two separate fire
incidents. A flammable liquid becomes charged by flowing through a tap; an
equal and opposite charge is left behind, (a). The charged liquid induces equal
and opposite charges on the walls of the bucket in which it is collected, (b). The
bucket is grounded by placing it on the ground and loses its outer charge;
although both the bucket and the liquid are charged, they are now neutral as a
whole, (c). The liquid and its charge are poured into a basin, and the process of
the inducing charge is repeated; the exterior charge in the bucket is left behind,
(d). A man in rubber soled shoes shares the charge with the bucket and a spark
passes from his hand to a grounded fuel tap to cause a fire, (e). Another man
without rubber-soled shoes (and thus grounded) puts his hand near the surface
of the charged liquid and a spark passes to cause another fire, (f). This scenario
has, in fact, caused many such incidents, and grounding the basin makes very
little difference.

These examples illustrate that static electricity fire hazards exist across many
industries and tasks. They share three things in common, however: the genera-
tion of static charges; the storage of static charges and associated energy; and
the release of that energy to ignite a fuel. Using such information from past
events is very helpful in taking measures to avoid future disasters.

THl-24


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Moisture

Equipment failure
that results in an
open or short circuit

Short circuit

Environmental moisture and humidity can also dramatically affect the perfor-
mance of an insulator. Some insulators, such as nylon, absorb moisture. Since
water is a conductor, its presence may provide conductive paths through or
over the insulation. An insulator that has been damaged, such as by cracking
or tearing, may provide direct paths through the insulation to the conductor. If
moisture seeps through, the potential for shorting or shock is dangerously
increased. In fact, operating electric tools and equipment in damp environ-
ments commonly contributes to injuries and fatalities.

Even if the insulator is in perfect condition, the presence of moisture can pro-
vide conductive paths over or around it. Take, for instance, the case where a
young child playing on the floor in her home received severe electrical burns
in and about her mouth. This child, with a child’s propensity to chew and suck
on things, picked up an extension cord and inserted the junction (where a table
lamp was plugged into the cord receptacle) in her mouth. When saliva seeped
around the insulated cords and came into contact with the bare energized con-
ducting prongs, the girl’s tongue and cheek became part of the circuit linked
by the saliva. Severe burns and permanent disfigurement resulted.

Moisture may also provide a good home for biological agents. Even without
moisture, some insulators contain nutrients on which organisms feed. Whether
these biological factors alone can damage or reduce insulating properties is not
clearly understood, but the additional presence of moisture and possible degra-
dation from other factors can exacerbate potential failure or breakdown of
some insulators. Rodents such as rats and mice, as well as insects, may also
feed on or debilitate insulating materials.

Considering all of the ways that insulation might fail, you can see that selec-
tion of materials for a design is a complicated, but critical, matter. Fortunately,
past experience has taught us much and there are numerous sources to help
designers choose appropriate insulating materials for particular applications.
Even the best designed products will, however, require inspection and mainte-
nance throughout their operating life.

Any time a conducting element in a circuit comes in contact with another con-
ductor, whether it is part of the designed circuitry or not, a short circuit or very
low impedance connection can occur. If you’ve ever driven behind a car on
which the tail lights kept dimming or randomly blinking, you have likely wit-
nessed the results of a short circuit. A common scenario with vehicle electrical
systems as they age is the breakdown and deterioration of the insulation cover-
ing wires throughout the vehicle. If the conducting wire touches the frame or
another conductor, the current will flow in unexpected paths and reduce or
eliminate the current from the intended path, such as to the tail lights.
Similarly, if a conductor is severed, or if a path in a circuit is opened by other
means, the open circuit may cause current to increase in other parts of the cir-
cuit, and to overload components, which may cause failure. Although failed
vehicle tail lights certainly create a hazardous situation, we’re more interested
in the shock hazards associated with short or open circuits in this section.

As depicted in Figure II-3, a short circuit may occur between a circuit element
and a grounded conductor not intended to be part of a circuit (a) or between
conductors within the same circuit.

IL-8


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Occupational
Safety and Health
Act (OSHAct)

Occupational
Safety and Health
Administration
(OSHA)

Federal legislation during the 1960’s was aimed primarily at specific indus-
tries. The Construction Safety Act of 1969 required that all federal or federally
financed or assisted projects in excess of $2,000 comply with established safe-
ty and health standards enforced by the U.S. Secretary of Labor. The Federal
Metal and Nonmetallic Mine Safety Act of 1966 and the Federal Coal Mine
Health and Safety Act of 1969 also directed attention to occupational safety
and health. The Federal Mine Safety and Health Act, promulgated in 1977,
established a single mine safety and health law for all mining operations
enforced by the Mine Safety and Health Administration (MSHA) within the
Department of Labor.

On October 29, 1970, Richard Nixon signed Public Law 91-596. This law, the
Williams-Steiger Occupational Safety and Health Act of 1970 (OSHAct),
became effective on April 28, 1971. Its stated purpose is:

"To assure safe and healthful working conditions for working men and
women: by authorizing enforcement of the standards developed under the
Act; by assisting and encouraging the States in their efforts to assure safe
and healthful working conditions; by providing for research, information,
education, and training in the field of occupational safety and health; and
for other purposes.”

The Occupational Safety and Health Administration (OSHA) was established
to enforce the OSHAct. OSHA is located within the Department of Labor.
Under the provisions of the OSHAct, the National Institute for Occupational
Safety and Health (NIOSH) was established within the Department of Health,
Education and Welfare (currently the Department of Health and Human
Services). Whereas OSHA is primarily an enforcement agency, the primary
functions of NIOSH are to perform safety and health research, develop and
establish recommended standards, and facilitate the education of personnel
qualified to implement the provisions of the OSHAct.

The regulations relating to the OSHAct are included in Parts 1900-1927 of
Title 29 of the Code of Federal Regulations (CFR) and the operation of the
Occupational Safety and Health Review Commission (OSHRC) are included
in Parts 2200-2499. Unless specifically excluded, the OSHAct applies to every
employer who has one or more employees and who is engaged in a business
affecting interstate commerce. Major examples of specific exclusions of the
OSHAct are state and local government employees, self-employed persons,
farms at which only immediate members of the farm employer’s family are
employed, and workplaces already protected by other federal agencies under
other federal statutes (operators and miners covered by the Federal Mine
Safety Act of 1977, for example). Employers with fewer than 10 full or part-
time employees are excluded from the record keeping requirements of the
OSHAct.

Iv-8


Unit I—An Introduction to Safety and Some Basic Electrical Engineering Concepts

Accident

Safety

Economics and safety

An accident is an unexpected event that interrupts the work process and car-
ries the potential for injury or damage. Accidents may or may not result in
fatality, injury, or property damage, but they have the potential to do so. An
accident may be attributed to a human factor, a situational factor (operations,
tools, equipment, and/or materials), or an environmental factor.

In public health, practitioners are avoiding the term “accident” because it does
not convey the predictability and thus preventability of many fatalities and
injuries attributed to “accidents." The public health interest is to encourage
society to begin regarding fatalities and injuries as preventable events rather
than as unexpected, chance events, as conveyed by the term "accident."

Although it has been defined in many ways, safety can be thought of as the
absence of hazards or minimization of exposure to hazards. Firenze (1991)
also notes that safety is the control of hazards to an acceptable level.

This last statement, “safety is the control of hazards," is a theme that you will
see throughout this book. Each time we discuss a hazard, we will present some
methods by which the hazard can be controlled. The last part of the statement,
".., to an acceptable level," is most difficult to ascertain. This is where the real
conflict begins between the cost of methods to control a hazard and the likeli-
hood of an accident occurring. How much risk are you willing to take? How
much risk is your company willing to take? How much risk is acceptable to
the public?

Perception of the cost of providing safety varies over history and among indi-
viduals and organizations. In 1893, a railroad executive said that it would cost
less to bury a man killed in an accident than it would to increase safety by
putting air brakes on rail cars. At that time, in purely economic terms, that may
well have been true. But society has since realized that the economic costs of
injuries and deaths is significant. The National Safety Council (1989) notes for
1988:

¢ that 10,600 workers were killed on the job;

* that deaths and injuries (about 2 million annual injuries) in the work-
place cost an estimated $47.1 billion; and

« that each death costs approximately $550,000 and each disabling injury,
$16,800 with each worker having to produce goods or services in the
amount of $410 just to offset these accident costs.

These are only the economic costs for workplace injuries and deaths. These
numbers only include the economic costs incurred by industries; they do not
include financial losses by the injured parties or families of the deceased.
When the cost of civil lawsuits, medical charges, insurance, property damage,
and many other indirect expenses related to injuries and deaths outside the
workplace are considered, the economic liabilities to individuals and society
resulting from such incidents become astronomical.

When events outside the workplace are included, these figures increase to

more than 90 thousand deaths and about 9 million disabling injuries per year,
amounting to a total financial loss of more than $100 billion.

1-3


Unit 11—Ignition of Flammable and Combustible Materials and Explosives

Controlling static
electricity fire
hazards

oN

— +O +

+ +
+ +

Figure M11-9. A hypothetical life history of a bulk static
electrical charge (adapted from Cooper, 1986).

When determining appropriate controls for a hazard, first determine what con-
stitutes the hazard. Drawing on the above discussion, the fire hazard associated
with static charge involves two basic elements: (1) static electricity with suffi-
cient potential energy to ignite (2) a flammable atmosphere. The most effec-
tive hazard reduction strategy would involve controlling both of these ele-
ments.

Controlling the flammable atmosphere may include choosing alternative mate-
rials, or processes, or both that do not use flammable materials (or the use of
less hazardous materials); ventilation; or other means of minimizing flamma-
ble fuel-air mixtures, etc., as discussed in other sections of this course.

The most effective method of controlling the static charge would be to hinder
the generation, accumulation, and discharge of stored energy. It would be
wrong, however, to believe that generation of static electricity could be com-
pletely eliminated without eliminating all movement and the presence of other
charged bodies. We can, however, reduce the generation and increase the rate
of leakage to a level that does not constitute a hazard in a given environment
(Wright, 1985).

I-25


Unit I1I—Ignition of Flammable and Combustible Materials and Explosives

Controlling
lightning hazards

Lightning will follow the path of least resistance, often striking a conducting
structure that protrudes above the ground (Hammer, 1989). Lightning may also
travel from one cloud to another. When this happens, the sudden release of the
induced charge on the ground may cause hazardous sparking among structures
such as tanks if they are not properly bonded.

The voltage necessary to cause a lightning strike to ground exceeds 100 mega-
volts, with resulting current flows between 3 and 200 kiloamperes. Such high
current flows can cause appreciable current to flow in conductors that are par-
allel to a lightning strike. For low resistance conductors, the temperature
increase may be only about 1 degree Celsius because of the short duration of a
strike, perhaps only 0.1 millisecond. If lightning strikes objects of high resis-
tance, the temperature increase can be quite substantial. For example, when
lightning strikes a tree (trees are poor conductors), the water in the trunk can
flash into steam, which can create pressures that blow the tree apart, possibly
igniting fragments. In fact, many forest fires are started by lightning striking
trees.

The need for and the lengths taken in the design of lightning protection sys-
tems is a function of the frequency that lightning occurs in a given location
and the consequences of strike for a given structure, system, or process. If
lightning is rare in a location, and the hazards associated with a strike are low
as well, the lengths and expenses taken to provide protection probably do not
need to be overly elaborate. If, however, lightning strikes are of low probabili-
ty yet the consequences of a strike could be catastrophic, it is probably best to
design for the worst case. Company officials, insurance carriers, and local reg-
ulatory authorities should work together to determine the type and extent of
protections necessary for a given facility.

Probably the most familiar lightning protection is the lightning rod. A con-
ducting rod placed atop a structure that provides an effective conducting path
to ground “entices” any discharge that may occur in that vicinity to strike the
rod and shunts the current to ground. The structure is, we hope, protected
(although, power surges in parallel conductors may still create shock, fire, and
equipment hazards).

For high-hazard industries and structures, a variation of the lightning rod con-
cept may be used. For example, protecting an explosives manufacturing facili-
ty or storage area may, in part, be done by suspending a series of horizontal
conductors over the structure, suporting them with effectively grounded con-
ducting masts.

Since lightning strikes can cause power surges even when lightning rod con-
cepts are used, another measure of protection is surge arrestors located at the
service entrance to buildings and on individual pieces of equipment. These
devices act to minimize voltage spikes and, hence, protect circuits from over-
loading conditions and associated shock and fire hazards. Surge protectors are
also valuable for protecting sensitive electronic equipment, such as computers.
Be aware that the protective capability of these devices may be limited.

TI-31


Contents (continued)

Figure III-1. (a) The fire triangle; (b) the fire pyramid sesaeeneeeeee
Figure III-2. Picture-symbol labels Promulgated by the National Association

of Fire Equipment Distributors TiI-3
Figure IIJ-3. The portable fire extingu: her marking scheme defined i in

NFPA Standard No. 10 seseeseeeseeeseseenesesesees seteeeaeeeeseeseneees Il-4
Figure III-4. Typical curve relating ignition energy to concentration for a

gas/air mixture, where MIE is the minimum ignition energy and LEL is

lower explosive limit and UEL is the upper explosive limit... I-7

Figure IlI-5. Frequency distribution of the types of dust involved j in 357
dust explosions between 1965 and 1980 (adapted from Bartknecht, 1989) .......ssseecreee HES

Figure III-6. When sparks or arcs ignite flammable gases, vapors, or liquids,
explosion-proof equipment contains the explosion and vents only cool gases
into the surrounding hazardous atmosphere (adapted from U.S. Department
of Labor, 1981)...

Figure III-7. Static charged by induction. The shaded ai areas represent insulation
(adapted from Wright, 1985)

Figure II-8. (a) Equal and opposite charge separation between a a low ‘conduction
fluid and a pipe without flow; (b) when fluid flows in the pipe, the charges begin

to accumulate and move with the fluid (“streaming current”); (c) charge separation
in a filter (adapted from Hammer, 1989)
Figure III-9. A hypothetical life history of a bulk static electrical char
{adapted from Cooper, 1986) ae secseeseeesueseesenees
Figure III-10. Examples of tanks with interior ‘surface projections: (a) spark
discharges can occur between the fluid surface and tank walls or between an

object floating on the surface and the tank wall; (b) charged fluid entering a

tank; discharge can occur between fluid surface and projections inside the tank
(adapted from Hammer, 1989)... sesseeseseeseseeesseseeen

Figure II-11. When a cloud becomes ss charged duet to atmospheric flow, an equal
and opposite charge develops on the ground. The charge on the ground may
accumulate the most on projections (such as the house pictured) .........:cssessecseereesneesneesseeesnes
Figure V-1. Fault Tree Symbols... socsssceasesceseeseenesseesnecnennees
Figure V-2. Fault Tree Analysis of Flight bulbs ‘adapted from EG&G).

Figure V-3. Sneak circuit analysis (used by permission, ASSE, 1987)

. I-16

. 1-21

. W-23

. Ifl-25

. I-28

List of Tables

Table II-1. Summary of Effect of 60 Hz Alternating Currents Through the
Body of an Adult Human

Table II-2. Rough Estimates of Human Resistance to 5 Electrical Current,
Table II-1. The Main Groups of Organic Compounds, with Examples and Their Use......
Table III-2. Minimum Spark Ignition Energies for Various Fuels .
Table IlI-3. Example NEC Class and Group Distinctions
Table III-4. Examples of Static Charge Generation Between Flowing Materials............ss--++ Ti-22
Table V-1. Preliminary Hazard Analysis of Flight of Daedalus and Icaru:
Table V-2. Failure Mode and Effect Analysis of an Electric Drill

vil


Unit V—Systems Safety Analysis

Sneak circuit
analysis

Sneak circuit analysis is a type of systems safety analysis particularly applica-
ble to the analysis of electrical systems—a method of analyzing a circuit to
determine unintended and unexpected ways in which current might flow in a
circuit. It may be used to determine how equipment might operate undesirably
in the absence of component failure. This distinguishes sneak circuit analysis
from the other systems safety techniques discussed above such as FTA and
FMEA. Sneak circuit analysis is also intended to help determine where current
might be induced to flow when it shouldn’t because of the influence of outside
factors.

Sneak circuit analysis can be of several different types:
1. Sneak Path, determining if current may flow along an unexpected route.

2. Sneak Timing, determining if current may be caused to flow or prevented
from flowing to activate or prevent a particular function at an unexpected
time.

3. Sneak Indication, determining if an ambiguous or false display of the
operating conditions of the system may exist at a particular time.

4. Sneak Label, determining if an unexpected current flow may lead to an
incorrect, incomplete, or misleading /abel. The resulting misinterpretation
of the function of a control might lead to improper system performance.

An example of a sneak path is shown in Figure V-3. In this case, the ignition
switch of the automobile is intended to be between the battery and the radio.
When the ignition switch is off, the current path to the radio and tail lights is
intended to. be broken. This intended path is shown by the long arrows. In cer-
tain conditions, however, the radio will play intermittently with the ignition
switch off. This happens when the emergency flashers are turned on and the
brake pedal is depressed. In this case, when the flashers activate, intermittent
current is also sent to the radio even when the ignition is off. This unexpected
current path is shown with short arrows.

Expected ———>

poy
yf uae ale
on” | Switch

t ia

C2) wees |

} : Battery | _

Switch
(pressed)

Figure 5.3. Sneak circuit analysis (used by permission,
ASSE, 1987).

V-il


Unit I1]—Ignition of Flammable and Combustible Materials and Explosives

Humans and static
electricity

Where controls cannot satisfactorily reduce static accumulations and the haz-
ardous dust/air mixtures that are expected, making the atmosphere inert is an
effective means of reducing ignition hazards (Bartknecht, 1989).

A relatively new method of controlling dust explosions involves infrared detec-
tors to locate “hot spots” in product flowing through ducts. When particles
reach potentially hazardous temperatures, the device will trigger a suppression
system and stop the process before an explosion can occur (Field, 1989).

Humans can generate static charge by body movements, especially when
wearing nonconducting clothing containing synthetic fibers. Movement of the
body over other nonconducting surfaces, such as walking on synthetic carpet
in low humidity conditions, also generates static charges. When accumulated
charges cannot leak away because of nonconducting footwear or an insulated
floor, the resulting charge buildup can produce sufficient energy to ignite cer-
tain flammable fuel/air mixtures.

A person can also accept a charge from another charged body. The charge may
be transferred by current during contact or may be in the form of a spark dis-
charge of, potentially, enough energy to ignite flammable atmospheres. An
induced charge may develop as electrons move over the surface of the body if
a person is merely near a charged body. Again, once a body is charged, the
charge can be discharged either to another conducting body or to ground with
sufficient energy to ignite certain flammable atmospheres.

Only by eliminating people from an area can the static charge on their bodies
be avoided. Charge accumulation can, however, be minimized by providing
conductive floors (that are effectively grounded) and conductive paths to the
floor from the body. For example, looping a conductive material in contact
with clothing and the body around the bottom of the foot would provide a path
to a conductive floor. This is especially critical in flammable atmospheres
were people are expected to be present (Hirst, 1989).

Hospitals are an especially hazardous environment requiring extensive mea-
sures to minimize static hazards. Hospitals may have oxygen-rich atmospheres
(due to oxygen being administered to patients) in combination with flammable
gases used as anesthetics, and patients may have catheters or other invasive
devices that bypass the relatively high resistance of the skin or that may even
be in direct contact with the heart. During surgery, the body is directly open to
the environment and contact with the attending medical personnel. A static
discharge could result in explosion or may produce enough current to cause
heart fibrillation (irregular and ineffective heart beat). For these reasons, very
innovative methods aré used to bond and ground equipment and personnel in
the hospital environment.

Til-29


Unit V—Systems Safety Analysis

DEDUCTIVE
METHODS OF
SYSTEMS
SAFETY
ANALYSIS

Fault Tree
Analysis (FTA)

7. incident resulting from effect of triggering event on hazard (stock comes
off centers when lathe is running),

8 effect indicating type of injury or damage resulting from the incident (eye
injury),
9. hazard consequence classification (called the severity index in FMEA),
10. hazard probability (same as in FMEA),
11. procedural requirements to eliminate or reduce hazards in the workplace,

12. safety and personal protective equipment requirements to reduce the pos-
sibility of injuries and illnesses while performing operations and tasks,
and

13. instructions/recommendations to ensure safety and health in the work-
place.

Inductive methods of analysis analyze the components of the system and postu-
late the effects of their failure on total system performance. Deductive methods
of analysis move from the end event to try to determine the possible causes.
They determine how a given end event could have happened. One widespread
application of deductive systems safety analysis is Fault Tree Analysis.

Fault Tree Analysis postulates the possible failure of a system and then identi-
fies component states that contribute to the failure. It reasons backwards from
the undesired event to identify all of the ways in which such an event could
occur and, in doing so, identifies the contributory causes. The lowest levels of
a fault tree involve individual components or processes and their failure
modes. This level of the analysis generally corresponds to the starting point in
FMEA.

FTA uses Boolean logic and algebra to represent and quantify the interactions
between events. The primary Boolean operators are AND and OR gates. With
an AND gate, the output of the gate, the event that is at the top of the symbol,
occurs only if all of the conditions below the gate, and feeding into the gate,
coexist. With the OR gate, the output event occurs if any one of the input
events occur. Figure V-1 illustrates the basic gates used. Figure V-2 illustrates
a simple FTA. The “basic events” in Figure V-2 are represented as rectangles
rather than circles because they could possible be developed (reduced) further.

When the probabilities of initial events or conditions are known, the probabili-
ties of succeeding events can be determined through the application of
Boolean algebra. For an AND gate, the probability of the output event is the
intersection of the Boolean probabilities, or the product of the probabilities of
the input events, or:

Prob (output) = (Pr Input 1) x (Pr Input 2) x (Pr Input 3)

For an OR gate, the probability of the output event is the sum of the “union” of
the Boolean probabilities, or the sum of the probabilities of the input events
minus all of the products.


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

American National
Standards Institute
(ANSI)

The ANSI is an organization that issues standards (more than 8,000) on a wide
variety of topics, often in conjunction with other professional organizations.
For instance, the 1990 edition of the NEC, titled ANSI/NFPA 70-1990, indi-
cates joint sponsorship and approval between the ANSI and the NFPA. All
standards issued by the ANSI are consensus and thus, voluntary as evidenced
by the following passage contained in their standards:

“An American National Standard implies a consensus of those substan-
tially concerned with its scope and provisions. A National Standard is
intended as a guide to aid the manufacturer, the consumer, and the general
public. The existence of an American National Standard does not in any
Tespect preclude anyone, whether he has approved the standard or not,
from manufacturing, marketing, purchasing, or using products, processes,
or procedures not conforming to the standard. American National
Standards are subject to periodic review and users are cautioned to obtain
the latest edition.”

The ANSI coordinates and administers the federated voluntary standardization
system in the United States—a system that provides all segments of the econo-
my with national consensus standards dealing with operations and protection
of the consumer and industrial workers. It also represents the United States in
international standardization efforts through the International Organization for
Standardization (ISO), the International Electrotechnical Commission (IEC),
and the Pacific Area Standards Congress (PASC).

The ANSI coordinates the standard development efforts of about 1,200 nation-
al trade, labor, professional, and consumer organizations; government agen-
cies; and individual companies. It will approve a standard as an American
National Standard if its Board of Standards Review determines that the docu-
ment reflects a national consensus (National Safety Council, 1988).

In addition to joint sponsorship of the NEC with the NFPA, the ANSI also
sponsors such standards as:

Classification of Gases, Vapors and Dusts for Electrical Equipment in
Hazardous (Classified) Locations, ANSI/NFPA 497M.

Electrical Equipment for Use in Class I, Division 2 Hazardous
(Classified) Locations, ANSI/ISA-S12.12.

National Electrical Safety Code (NESC), ANSI C2.
Safe Current Limits for Electromedical Apparatus, ANSI/AAMI ES1.

Preparation of Test Procedures for the Thermal Evaluation for Insulation
Systems for Electrical Equipment, ANSI/IEEE 99.

Static Electricity, ANSI/NFPA 77.
These examples are but a few of the hundreds of standards listed in the

Catalog of American National Standards that cover procedures, practices,
designs, etc., that would be of interest to engineers.

IV-6


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

EXPLOSIVE
ATMOSPHERES

A deflagration consists of a rapid reaction that progressively transfers heat
from the reacting materials to neighboring materials that then react because of
the increased heat, thus propagating the reaction. Although the deflagration
rate is high, it is less than the speed of sound and will not create a shock wave
unless it is in a confined space, such as a room or building.

If the velocity of a reaction through the reacting materials is greater than the
speed of sound, the explosion is called a detonation. The reaction will create a
shock wave even if the reaction is not confined (Hammer, 1989).

Some materials are specifically classified as explosives because of their fast
reaction rates. Explosives, however, need not be present for an explosion to
occur. In fact, many explosion disasters have occurred in the absence of any
classified explosive materials.

In 1937, a school in New London, Texas, was destroyed by an explosion
that killed 413 students and 14 teachers. In 1944, in Cleveland, Ohio, a
tank containing 80 million cubic feet of natural gas failed, leaking its con-
tents over the surrounding area and into the storm sewers. When the vapors
ignited, the liquified gas vaporized rapidly and burned in the open air. The
gas that had leaked into the confined space of storm sewers, however,
exploded, causing widespread damage and 135 deaths. In 1947, the French
ship “SS Grandchamp” exploded at the pier in Texas City, Texas, destroy-
ing the city and killing 561 people. In 1974, a Flixborough, England,
chemical plant exploded, devastating the town and killing 29 people and
injuring 100. More recently, the U.S. space shuttle “Challenger” exploded
during its ascent, killing all aboard. These are only a few of the more
notable examples of catastrophic explosions, but all share a common
thread: no classified explosives were present. Under the right conditions,
seemingly benign substances and mixtures can explode when ignited
(Hammer, 1989).

Dust explosions may occur wherever material that will burn is present in pow-
der form. Examples are coal, wood, agricultural products, and certain synthetic
resins present in plastic industry. Figure III-5 shows the frequency distribution
of dust types involved in 357 dust explosions that occurred between 1965 and
1980.

Ml Wood

C1 Food & Foodstutt |
| EB Plastics

13% @ metals

ES coayPeat

i i
| El Miscellaneous |

13% | Paper

25%

Figure III-5. Frequency distribution of the types of dust involved
in 357 dust explosions between 1965 and 1980
(adapted from Bartknecht, 1989).

TH-8


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Fuel

Oxygen

ORDINARY
1. Extinguishers suitable for Class A fires should be identified
by a triangle containing the letter "A." If colored, the triangle
should be colored green.
COMBUSTIBLE
FLAMMABLE
2. Extinguishers suitable for Class B fires should be identified
by a square containing the letter "B.” If colored, the square
should be colored red.
LIQUIDS
ELECTRICAL
1. Extinguishers suitable for Class C fires should be identified
by a circle containing the letter °C." If colored, the triangle
should be colored blue.
EQUIPMENT
COMBUSTIBLE
1, Extinguishers suitable for fires involving metals should be
identified by a five-pointed star containing the letter "D." If
colored, the star should be colored yellow.
METALS:

Figure II-3. The portable fire extinguisher marking scheme
defined in NFPA Standard No. 10.

Any combustible substance can be considered a fuel. All organic chemicals or
compounds are combustibles. Examples include oil, natural gas, plastics, wood,
and even living tissue (see Table IlI-1 for additional examples of organic com-
pounds). Some combustibles will burn more easily than others, but all will burn
under the right conditions. Most, but not all, inorganic chemicals are noncom-
bustible. Examples of combustible inorganic chemicals include hydrogen, sulphur,
phosphorus, and some metals such as magnesium, aluminum, and titanium.

There are only three sources of oxygen for combustion. (1) the atmospheric
environment, which may be air (21% oxygen) or a controlled atmosphere
containing more or less oxygen, (2) substances known as oxidizing agents,
and (3) the combustible material itself.

Air is the most common source of oxygen for a fire, and its gaseous state is
most efficient for combustion. Certain industrial processes and medical envi-
ronments may involve oxygen-enriched atmospheres: these require special pre-
cautions to control fire hazards. Originally, carrying an atmosphere containing
79 percent of apparently useless nitrogen in space vehicles was believed uneco-
nomical and it was, therefore, replaced by pure oxygen. After three astronauts
were killed in their Apollo command module in 1967, the designers wisely
reconsidered the contents of the atmosphere (Hirst, 1989). The fatal fire was
ignited by a short circuit within an electronic component. Below are some
example settings where oxygen-enriched atmospheres are present (Hirst, 1989).

Ti-4


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

The amount of current flow in R, is dependent on the ratio of resistances of the
two branches, R, and R;,

if Rs) _ (0.0943 amp)(250 Q)
IWR AR; 77500

i, = 0.0314 amp

This same can be done for R,

i ( R, ) — (0.0943 amp)(500Q)
TAR AR: 750 Q)

i, = 0.0629 amps
Vay = i,Ry = (0.0943 amp)(1000 Q) = 94.3 V

Voa= Vea= te!

i,R, = (.0314 amp)(500 Q) = (.0629)(250 Q) = 15.7 V

This problem could have been solved in a more streamlined manner, but the
method used illustrates basic concepts you should be familiar with. If you need
additional review, please consult one of the many available circuit analysis
texts.

REFERENCES AND SUGGESTED READINGS

1 Bloswick, D.S., “Industrial Safety,” Chapter in Industrial Engineering Handbook, Edited by
Maynard, McGraw-Hill, New York, NY, 1992.

2. Firenze, R.J., “The Impact of Safety on High-Performance/High Involvement Production
System," Creative Work Designs, Inc., 1991.

3. Firenze, R.J., “Evaluation and Control of the Occupational Environment,” National
Institute for Occupational Safety and Health, Cincinnati, OH 1988.

4, Firenze, R.J., “The Process of Hazard Control,” Kendall Hunt, Dubuque, IA, 1978.

5. Hammer, W., “Occupational Safety Management and Engineering,” Fourth Edition,
Prentice-Hall, Englewood Cliffs, NJ, 1989.

6. Martin, M., Schinzinger, R., “Ethics in Engineering,,” Second Edition, McGraw-Hill, New
York, NY, 1989.

7. National Safety Council, “Accident Facts,” NSC, Chicago, IL, 1989.

8. National Safety Council, “Accident Prevention Manual for Industrial Operations,
Administration and Programs,” Ninth Edition, NSC, Chicago, IL, 1988.

9. National Safety Council, “Accident Prevention Manual for Industrial Operations,
Engineering and Technology.” Ninth Edition, , NSC, Chicago, IL, 1988.

10. Ridley, J., “Safety at Work, Second Edition,” Butterworths, Boston, MA, 1986.

1-12


Unit I]—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Double
insulation

Test
Button

Reset _/4
Button ~ |

Figure 11-13. Receptacle-type ground-fault circuit interrupter (GFCI)
(adapted from U.S. Department of Labor, 1981).

In previous ground fault examples, we have used a drill as an illustration. That
is, a fault (short circuit) within the drill resulted in an energized case, which then
was a potential source of shock to a person. Most new hand tools, however, are
double insulated. As the name implies, double insulating involves enclosing the
electrical components of equipment in a layer of insulation, and then enclosing
that within a case made of nonconducting material. Any shafts or components
that pass through the insulation must also be made of nonconducting materials.
When the insulation is intact, there is no path to the case or to a person and thus
no shock. See Figure II-14 for a schematic representation of a double-insulated
drill.

Brushes and commutator are
Inner noncondueting liner surrounded by high-impact
insulating material

Insulation around
armature shaft

Nylon grommet

LS Back cap offers protection
against possible loose or
frayed brush wires

Insulated switch
Insulated handle

Figure I-14. On a double-insulated, shock-proof electric tool, an internal
layer of protective insulation completely isolates the electri-
cal components from the outer metal housing.

TI-27


Unit 11—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

SAMPLE QUIZ QUESTIONS

1. a) In the case study at the beginning of this course module, the man held the drill
with both hands and leaned against a metal pole with his shoulder. Now assume
that he held the drill in his left hand and grasped the metal pole with his right hand
(no shoulder contact). Calculate the current that would have flowed through his
body if he had dry hands and if he had wet hands. Use the following data and
assume a 110 V source.

Dry Skin : Wet Skin

R(right hand) = 60K Q R(right hand) = 1K Q
R(left hand) = 20K Q R(eft hand) = 1K Q
R(internal tissues) = 400 Q R(internal tissues) = 400 Q.

b) Describe the potential consequences of the shock for each case. What effect does the
length of time he was exposed to the current have to do with the severity of his
shock (i.e, what if current only passed through him for 1 second as opposed to 10
seconds)?

2. What conditions are necessary for a person to be shocked? Briefly describe the princi-
pal ways people experience electric shock, and provide your own example for each.
For example, lightning strike is one way that people are exposed to electric shock. One
example might be that of a woman being severely shocked when she was struck by
lightning while stranded on a golf course during an electrical storm.

3. a) Briefly describe basic ways in which the insulation covering an energized conductor

might become damaged.

b) What effect might moisture have on insulation, particularly when the insulation is
already deteriorated to some extent?

4. Find a real world example of an insulated conductor that might be exposed to most or
all of the factors you discussed in problem (3), and hypothetically describe how and
why it might deteriorate over time.

. What is a short circuit? What can cause a short circuit?

. What is an open circuit? What can cause an open circuit?

. What are the potential hazards associated with: a) a short circuit; b) an open circuit?

. Describe two situations where a relatively minor shock from static electricity may
result in severe or even fatal injuries.

9, What is a “let-go” threshold current? What are the implications to the sevetity of a shock
when a person is exposed to a current greater than or equal to the let-go threshold?

10. If a person was exposed to a current of 50 milliamperes for 10 seconds, would the per-
son be in danger of having heart fibrillation? What is heart fibrillation?

11, Imagine your is job to control the hazards associated with a high-voltage device to be
located outdoors in a residential neighborhood. What methods would you use to mini-
mize the potential of someone being exposed to the hazard of electric shock?

12. Briefly describe the concept of “grounding.” How does grounding equipment protect
people from electric shock?

13. How would grounding have protected the man in the case study at the beginning of this
unit? Describe the grounding method you would recommend.

14. Describe situations in which an installation or wiring error, even with a three-wire
receptacle, can still result in an energized power tool (or any other electrical device)
and the potential for electric shock.

15. What is a GFCI? How does one work?

16. What is “double-insulation”? How might it have protected the man in the case study?

WIAA

I-31


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Control of static
hazards in
industries handling
or producing dusts
or powders

a)
») = =i =
—|[+ +4 2 +
+ + t+
teet +t 40 t+ etl
o> — ee er
Fuel flow a 7 - =
with charge

Figure III-10. Examples of tanks with interior surface
projections: (a) spark discharges can occur
between the fluid surface and tank walls
or between an object floating on the sur-
face and the tank wall; (b) charged fluid
entering a tank; discharge can occur
between fluid surface and projections
inside the tank (adapted from Hammer,
1989).

Electrostatic discharge was traced as the ignition source in 9 percent of 357
dust explosions studied between 1965 and 1980. Just as flowing fluids or
solids generate static charges, so do dusts or powders flowing through ducts or
shifting in piles and in storage tanks.

The first step in minimizing the explosive hazard is to eliminate or reduce dust
air mixtures to a level that is below the lower minimum ignition energy for the
specific mixture. When this is not possible, control the hazard by controlling
the generation, accumulation, and discharge of static electricity (this logic
should be quite familiar by now).

As with fluid flow through pipes, ducts handling moving dust should be con-
structed of conducting materials and all equipment should be effectively bond-
ed and grounded. Limiting flow velocities and feed rates into vessels such as
silos will also limit charge generation.

Til-28


Page 4 - Request for Assistance in Preventing Fatalities of Workers
Who Contact Electrical Energy

this case, CPR was begun within the 4-minute recommendation. An
ambulance, equipped and staffed to provide ACLS, arrived within 6
minutes. The standards and guidelines [5] for CPR within 4 minutes,
and ACLS within 8 minutes, were met and the worker did survive.

In Case #2, the worker's contact with electrical energy was prolonged
and a coworker who aided him received an electrical shock, because
coworkers did not know how to de-energize the circuit. The optimal
times for CPR and ACLS were exceeded, and the resuscitation was
unsuccessful. Providing appropriate medical care after an electrical
energy incident will not guarantee success. However, as has been
reported elsewhere [5] and supported in the NIOSH case reports, the
chance for successful resuscitation after cardiopulmonary arrest is
best when the criteria for providing emergency medical care are met.

RECOMMENDATIONS
1. PREVENTION

PREVENTION must be the primary goal of any occupational
safety program. However, since contact with electrical energy
occurs even in facilities which promote safety, safety
programs should provide for an appropriate emergency medical
response.

2. SAFE WORK PRACTICES

No one who works with electrical energy should work alone, and
in many instances, a "buddy system" should be established. It
may be advisable to have both members of the buddy system
trained in CPR, as one cannot predict which will contact
electrical energy.

Every individual who works with or around electrical energy
should be familiar with emergency procedures. This should
include knowing how to de-energize the electrical system
before rescuing or beginning resuscitation on a worker who
remains in contact with an electrical energy source.

All workers exposed to electrical hazards should be made aware
that even "low" voltage circuits can be fatal, and that prompt
emergency medical care can be |i fesaving.

3. CPR AND ACLS PROCEDURES

CARDIOPULMONARY RESUSCITATION (CPR) and first aid should be
immediately available at every worksite. This capability is
necessary to provide prompt (within 4 minutes) care for
victims of cardiac or respiratory arrest, from any cause.


Page 3 - Request for Assistance in Preventing Grain Auger Electrocutions

straight path to the truck, approximately 40 yards away, that
was to be loaded with grain. As the workers pushed the auger
forward, it contacted a 7200 volt electrical line which was
25 feet above the ground. The two workers and the farm owner
were electrocuted.

REGULATORY STATUS

OSHA estimates that over 90% of all farms in the United States are not
covered by OSHA regulations. OSHA regulations are not applicable to
most farms because they employ fewer than 10 employees. However, for
farms employing 11 or more workers (family members do not count in
this number), OSHA jurisdiction does apply and mandatory compliance is
required to 29 CFR 1928.57, Guarding of Farm Field Equipment, Farm
Machinery & Farmstead, Sub Part D.

CONCLUSIONS

Based on the information collected on the two cases cited, it can be
concluded that the five fatalities occurred as a result of the
fol lowing:

1. The lack of hazard recognition.

2. The failure to lower the grain augers to the horizontal
position before moving them to other locations.

RECOMMENDATIONS

NIOSH recommends that all farm owners/managers, farm/agricul tural
workers, and farm equipment manufacturers be made familiar with, and
reinforce the following steps:

1. Hazard Awareness

A survey of the farm should be conducted to identify hazards
posed by the locations of overhead electrical lines. When all
such hazards are identified and documented for future reference,
workers should be informed of their location and instructed in
the steps necessary to safely move grain augers.

2. Safe Movement of Grain Augers and Other Farm Equipment

Grain augers pose a life threatening hazard when moved in an
elevated position if they contact overhead electrical lines or if
they tip over. Therefore, it is essential that grain augers be
lowered to a horizontal position before being moved from one
location to another. In addition, all other equipment to be
moved should be evaluated in order to determine the most
appropriate method that will ensure worker safety during its


Unit I—An Introduction to Safety and Some Basic Electrical Engineering Concepts

HAZARD CONTROL
AS A DESIGN
CRITERIA

Design for people

REVIEW OF THE
FUNDAMENTALS OF
ELECTRICITY AND
CIRCUITS

An engineer's familiarity and interest in technology tend to make many of the
concepts and associated hazards second nature. We begin to think of any haz-
ards as obvious. What is obvious to an engineer may, however, be a mystery to
the user of an unfamiliar product or system. Moreover, knowledge of a hazard
does not preclude an accident. Knowledgeable workers are the most frequent
victims in electrical accidents. Is this because of worker carelessness? That’s
one way to look at the problem, but if we truly desire to "fix" the problem, as
an engineer should, then we must look at it as a system failure. Armed with the
knowledge that people make mistakes while operating or maintaining a prod-
uct, what can we do to minimize the probability of an accident in such situa-
tions? Many techniques are available; only some will be covered in this text.
Be advised that this is only a glance at safety theory and practices and at elec-
trical safety in particular. Throughout your career you must research specific
topics on your own and expand on the knowledge recorded by others.

In the past, engineers often believed the theory that dangers and unsafe condi-
tions would be learned once the product was designed and sold. By studying
the events that invariably occurred, they could learn the hazards and control
them in future designs. Although studying such incidents is very instructive
for future safety, we can no longer afford to let after-the-fact information be
our primary guide. Today, engineers must treat hazard control as any other
design criteria, from the day a product idea is first conceived through the day
the last remnants of the product exist. Fortunately, rigorous hazard analysis
techniques are available and will be discussed in the last course unit (Systems
Safety Analysis).

The second unit covers the hazard of electric shock and some general control
strategies. The third unit discusses the fire hazards associated with electrical
equipment and again describes some general abatement strategies to eliminate
or minimize the hazards. Standards and professional liabilities, topics covered
in the fourth unit of this course, also provide engineers with a minimum level
of safety information and features for many products and components.

Together with the vast amount of information now available to the engineer,
these pages provide a basic understanding of electrical safety. Safety consider-
ations (not a “necessary evil," as some may tend to believe) are the essence of
good engineering. We design technology for people; we do not attempt to
manipulate people to fit with or serve technology. A system that is difficult or
dangerous to operate isn't meeting its optimal potential and is contributing to
adverse events; it is—pure and simple—poorly designed.

Modern theory states that all matter is made up of atoms. The basic structure
of an atom consists of a nucleus of packed neutrons and protons surrounded by
orbiting electrons. Neutrons have no electric charge, protons are positively
charged, and electrons are negatively charged. In its normal state, an atom has
no overall charge. That is, the number of protons (+) is equal to the number of
electrons (-), leaving a net charge of zero for the atom. Applying energy to
atoms, however, can result in a loss of one or more electrons and cause the

1-7


NIOSH

ALERT

ALERT

ALERT

ALERT

ALERT
ALERT

y Request for Assistance in

Preventing Fatalities of Workers
Who Contact Electrical Energy

December 1986

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service

Centers for Disease Control

National Institute for Occupational Safety and Health


Unit I[]—Ignition of Flammable and Combustible Materials and Explosives

Fire is actually an example of a more general exothermic chemical reaction
called oxidation. Just as iron “rusts” because of chemical oxidation reactions,
combustion is an oxidation reaction between a fuel and oxygen. The obvious
difference between rusting iron and combustion is the speed and amount of
heat the respective oxidation reactions involve. The speed of the reaction
depends on many factors such as the mixing and the physical state of the reac-
tants. For exothermic reactions, though, increasing the temperature generally
speeds the reaction. A fire, then, is a fast, exothermic, chemical oxidation reac-
tion involving oxygen and a fuel. You'll note from the fire triangle and pyra-
mid figures that heat, or some form of initiation energy, is necessary to start
the reaction. In the case of combustion, once ignition has occurred, the reac-
tion will continue until at least one of the reactants has been controlled or
exhausted. If the combustion process is confined so that pressure can increase,
an explosion may result (National Safety Council, 1988).

When extinguishing a fire, one of the basic elements (heat, oxygen, fuel, or
chemical chain reactions) must be removed or controlled. For example, fire
fighters spray water on certain types of fires to lower the heat (evaporation is
an endothermic, or cooling, process) and quench the flames. Since water is
conductive, however, it isn’t appropriate for electrical equipment fires.
Electrical fires are best controlled by nonconducting chemical agents such as
carbon dioxide, which may act to smother the fire, (1) removing oxygen from
the reaction or (2) by chemically inhibiting the chain reactions. Class C
portable fire extinguishers contain nonconducting agents meant specifically
for use on fires involving electrical wiring and equipment. The type of agent
used to control a fire depends on the fuel being burned. Figure III-2 describes
the classification scheme for portable fire extinguishers set forth by the NFPA.

Figure III-2. Picture-symbol labels promulgated by the National
Association of Fire Equipment Distributors.

These symbols are used on a Class A extinguisher (for extinguishing fires in
trash, wood, or paper). The background symbol at left (a) is blue. Since a
Class A extinguisher is not recommended for use on Class B or C fires, the
background of (b) and (c) are in black, with a diagonal red line through each.
For use on a Class A/B extinguisher, illustrations (a) and (b) would be in blue,
and (c) would be in black with red diagonal. For use on Class B/C, the last two
({b] and [c]) would be in blue. On a Class A/B/C, all three would be in blue.

il-3


Contents ( ‘continued 1)

CONTROLLING THE SHOCK HAZARD... I-18
Insulation; Interlocks...... I-18
Isolation; Marking; Warning devices; Grounding. I-19
Bonding; Ground-fault circuit interrupter (GFCI) I-25
Double insulation ...........cessesssesceeeeeees woe II-27

CONCLUSIONS REGARDING SHOCKS... 11-28

INADVERTENT ACTIVATION OF EQUIPMEN’ IL-28
Case study; Lockout/tagout procedures ...... TI-28
Interlock .....sssessssecssesessessecssecsnecaveessseenee I-29

REFERENCES AND SUGGESTED READINGS I-30

SAMPLE QUIZ QUESTIONS ........scsesssesees I-31

UNIT IlI—IGNITION OF FLAMMABLE AND COMBUSTIBLE
MATERIALS AND EXPLOSIVES

Purpose; Objective; Special Terms .....cesscsesssssessessecssssseeneenessseneseess T-1

INTRODUCTION ........-.eesseesssessseceneeeseeeseeesee Til-2

MECHANICS OF FIRE. TII-2
Fuel; Oxygen Il-4
Ignition TII-6

CHARACTERISTICS OF COMBUSTION... 1-6

EXPLOSIONS ...0...scssssssssssesstsccseeneeneeeeseeenee IN-7

EXPLOSIVE ATMOSPHERES..........:ss:sssses000 I11-8

ELECTRICAL PHENOMENA AS AN IGNITION SOURCE 1-9
Electrical overloading T-9
Sparks, arcs, and coronas IH-10
Electrical heating equipment................ . W-11

METHODS TO CONTROL IGNITION HAZARDS. TIl-11
Defining hazardous locations....... Ti-11
Intrinsically safe designs; Encapsulation and embedment; Hermetic sealing;

Explosion-proof equipment. T-15
Pressurization THl-16
Isolation. ossosenncnscusesnssasessessosossnsnenesaceereseoseesees . U1-17

OVERHEATING, DAMAGE TO EQUIPMENT, ELECTRICAL EXPLOSIONS TI-17
Reasons for overheating... I-17
Effects of electrical overheating I-18
Electrical explosions; Methods to protect circuits from overheating; Fuses ...... THI-19
Circuit breakers; Controlling electrical equipment explosion hazards; Static electricity I-20
Controlling static electricity fire hazards I-25
Control of static hazards in hydrocarbon industrie: I-26
Control of static hazards in industries handling or producing dusts or powders I-28
Humans and static electricity . I-29
Lightning .. TH-30
Controlling lightning hazards. I-31

REFERENCES AND SUGGESTED READINGS .......scssssssssssssesessssecssseessveecanseeeessneesseenseesssseessssseeees T-32

SAMPLE QUIZ QUESTIONS .......cssssssssssesssssecssssseescessssececsssseeceneteesseeesaeesenmecssnmtsssseessssnessseseeeseee D338


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Circuit breakers

Controlling
electrical equipment
explosion hazards

Static electricity

Circuit breakers, located at specific points in an electrical system or circuit,
also react to overloads by breaking the flow of current. Three types of circuit
breakers are in general use: magnetic, thermal, or a combination of both. The
magnetic type utilizes a wire coiled around a plunger, which is attached to a
switch. When the current exceeds the designed level, the magnetic field moves
the plunger and opens the switch, stopping current flow. Magnetic breakers
will stop the flow of current any time the current reaches a preset limit.

Thermal breakers utilize material properties of bimetallic conductors. As the
conductor heats in proportion to electrically produced heat, it bends. When the
current exceeds the rated current of the breaker, the conductor bends to a point
where it will activate a switch, cutting the flow of current. A thermal breaker
reacts to prolonged overloads, but is generally not affected by minor short-
lived current surges.

Some circuit breakers are designed with magnetic and thermal elements placed
in series. The magnetic element protects the circuit from heavy overloads and
short circuits, and the thermal element protects against lighter, but sustained,
overloads.

When electrical equipment or components are located in flammable or explo-
sive environments, we must expect the unexpected. That is, even though the
circuitry has been designed to minimize the possibility of overload or electri-
cal discharges (sparks, arcs, coronas), we must assume that they will neverthe-
less occur. Through lessons from past accidents, designers have learned that
special precautions must be taken for electrical equipment operating in envi-
ronments where flammable gases or vapors are or may be expected. One such
design precaution is the use of explosion-proof equipment, which was prévi-
ously discussed. Refer to previous sections describing methods to control igni-
tion hazards.

We're all familiar with static electricity: a shock when you touch a door
knob—your hair standing on end after pulling a sweater over your head—the
“static cling” of your clothing. In every day life, static electricity manifests
itself more as an annoyance than as a hazard. Under some conditions, though,
the electrical discharge may be enough to ignite a flammable atmosphere.
Lightning, a more violent form of static discharge than we typically experi-
ence, is also a significant shock and fire ignition hazard. First we'll discuss the
nature of static electricity and present some methods to control its associated
hazards; a discussion of lightning and controlling its hazards will follow.

Any time two dissimilar materials are in contact, static charges are formed.
Loosely held electrons may drift from one material to another to create a
negative charge on one surface and a corresponding positive charge on the
other. Depending on the conductivity of the materials, separating them may
cause equal but opposite charges to remain. Before materials that are good
conductors are fully separated, the charge separation will neutralize as the
electrons flow freely between the surfaces. If the materials are poor conduc-

TII-20


Unit I—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Chemical and
physical reactions

The heat may also be part of the operating environment. For example, the
insulating materials used in or near equipment that produces high temperature
as a normal part of operation must be capable of withstanding the heat for the
designed life of the system. Methods to control potentially damaging tempera-
ture increases that result from overload will be discussed when we cover fire
hazards in the next unit. Essentially, circuit designs that require very low oper-
ating energies or design precautions that immediately cut power when an over-
load occurs are good preventive measures. Processes involving high tempera-
tures as a normal operating characteristic call for a cooling system to remove
heat.

Cold environments can also damage insulating materials. Some insulating
materials may become brittle when exposed to cold environments. If you live
in a cold climate, think of what happens when you leave a garden hose out for
the winter. It becomes brittle and cracked. Similarly, fluctuations in tempera-
ture may also affect an insulator. Temperature fluctuations can expand and
contract the insulating materials and possibly result in their mechanical failure
and breakdown of the insulating properties.

Thus, whether you are responsible for the design or the operation of electrical
equipment, temperature is an important consideration. The amount of heat pro-
duced by the insulating conductor, as well as by the environmental tempera-
tures, must be accounted for in design, and appropriate insulating materials
and temperature controlling systems must be chosen to minimize the potential
for failure throughout the life of the product or system.

Insulating materials are also susceptible to a wide range of chemical and phys-
ical reactions. Such reactions can be caused by electrical phenomena inherent
in the equipment or may result from environmental factors.

Take oxidation for example. Oxidators such as oxygen and ozone occur natu-
rally in the atmosphere. Sparks and arcs, typical occurrences when equipment
such as motors and generators are operated, produce significant amounts of
ozone. When the equipment is enclosed, as motors and generators often are,
the ozone is contained and may reach high levels. Ozone is a much more
unstable and reactive form of oxygen and may accelerate deterioration of some
insulators. Ozone is also naturally present in the environment; its concentra-
tion varies with altitude, air pollutants, weather and season, geographic loca-
tion, and solar radiation.

Solar and nuclear radiation, may also degrade insulation properties, especially
polymers. Photochemical processes initiated by solar radiation can affect a
variety of polymers, including natural or synthetic rubber, vinyl chloride, and
vinyledene chloride. The degradation process may release other reactive
agents that cause further damage to the insulator.

Chemical incompatibility between the insulating material and such things as
acids (including acid rain), salts, lubricants, and solvents can lead to chemical
breakdowns of insulators. Again, these chemical reactions can be caused by
agents, materials, and electrical properties inherent in the system or may come
from the environment.

II-7


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Effects of electrical
overheating

A circuit can receive over currents or excess voltage in many ways. Some may
seem rather unlikely; the fact is they do occur, and the consequences may be
catastrophic. Some of the more common causes of circuit overloading are
short circuits, current surges, transient surges during startups, and lightning
strikes (discussed in the next section). Circuit protection methods often will
minimize the associated hazards, but in some cases, they may not be able to
react quickly enough, or may not be designed to sense the particular overload
condition. We will briefly discuss situations that can overload a circuit; recog-
nize that some of the concepts may be beyond your current level of training in
electrical engineering.

When a switch is closed to energize a system, the sudden input of energy may
for a brief period produce a current or voltage spike larger than that expected.
An individual spike of this type is generally not a problem, but repeated expo-
sure to small spikes may-lead to failure over the long term. Such failure may
contribute to a short circuit, spark, arc, or corona, which would increase shock
and fire hazards.

A short circuit between elements in a circuit, or an unintended open circuit ina
parallel element, can cause greater current to flow in a conductor or circuit
component.

Lightning sometimes strikes power lines and sends voltage spikes through the
lines and to customer installations. Lightning can also induce current in con-
ductors that are parallel to its path. In either case, the resulting overload condi-
tions can be sufficient to severely damage equipment and cause fires or explo-
sions.

These more common situations lead to overloaded circuits and equipment. Be
assured that many other scenarios may unexpectedly increase the energy sup-
plied to a circuit, such as induced currents and resonance occurring in compli-
cated electronic systems.

Electrically produced heat can contribute to a fire hazard by:

— increasing the temperature of a flammable mixture to a point where it
easily ignites;

— raising a flammable mixture to its ignition temperature, and then ignit-
ing it;

— causing damage to insulating material, possibly igniting some;
— causing organic solids to melt, char, or burn;

— contributing to rapid evaporation of liquids to a point where flamma-
ble concentrations exist; and

— breaking down otherwise noncombustible materials to a combustible
form.

I-18


DISCLAIMER

Mention of the name of any company or product
does not constitute endorsement by the National
Institute for Occupational Safety and Health

DHHS (NIOSH) Publication No. 87-100

Copies of this and other NIOSH documents are available from

Publications Dissemination, DSDTT
National Institute for Occupational Safety and Health
4676 Columbia Parkway
Cincinnati, Ohio 45226
(513) 841-4287

q


Unit Hl—Ignition of Flammable and Combustible Materials and Explosives

Electrical heating
equipment

METHODS
TO CONTROL
IGNITION HAZARDS

Defining hazardous
locations

Electrical heating appliances are commonplace in the home and workplace.
Certainly, such equipment is not appropriate in areas where flammable materi-
als or atmospheres are expected. Generally, heating appliances should be
equipped with temperature limiting devices to ensure that the surface tempera-
tures of the heating elements or guards do not reach a level that may potentially
ignite combustibles or cause burns to persons that may contact the equipment.
Choosing the right equipment for an application is of the utmost importance,
but maintaining it and the environment is equally necessary. Too often poor
housekeeping or work practices contribute to fires.

For example, periodically bumping and kicking a space heater under a desk
can eventually cause an electrical failure. Another example is when papers,
rags, solvents, etc., are accidentally left near a heating appliance; a slow
buildup of heat can cause an eventual fire. So, as in all hazard control strate-
gies, a combination of design features, maintenance procedures, and work or
use practices all contribute to the ultimate goal of minimizing or eliminating a
hazard.

The best way to stop electrical energy from igniting is to stop using electric
equipment in hazardous areas. Unfortunately, this is not always possible.
Therefore, when electrical equipment must be located in hazardous areas, spe-
cial precautions must be taken to eliminate the hazard of fire or explosion.

Whenever and wherever possible, hazards should be completely eliminated. If,
however, hazardous areas and processes are necessary, great care must be
taken to control them. The cost of designing to eliminate or control a hazard is
usually far less than the cost (in both life and property) of an accident
(National Safety Council, 1988).

The National Electric Code (NEC) is one source that must be consulted during
the design or operation of any industrial plant or process. These codes should
become part of your library. The NEC regulations will be discussed in more
depth in a future unit; here we'll discuss Article 500, which classifies haz-
ardous locations.

Hazardous locations are areas in which explosive, or flammable gases, or
vapors; combustible dust; or ignitable fibers are present or likely to become
present. Hazardous locations are classified as Class I, Class II, or Class IIT,
with the classification depending on the physical properties of the combustible
substance that might be present. These classes are subdivided into Divisions 1
and 2, depending on the degree of likelihood that an ignitable atmosphere
might be present. The assessment of combustible substances into one of seven
groups (A through G) depends on the nature of the substance’s behavior on
contact with an ignition source. :

Class I locations are those in which flammable gases or vapors are or may be present
in the air in quantities sufficient to produce explosive or ignitible mixtures.

Class II locations are those that are hazardous because of the presence of com-
bustible dust.

Til-11




Unit I1—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

SHOCK HAZARDS
IN SPECIAL
ENVIRONMENTS

———_—>

Energized ‘
Drill Lk

R, = skin of hand resistance

R, = internal tissues resistance

R, = skin of back resistance

Dry skin Wet skin
R,=90K Q R, = 1000 Q
R, = 500 Q R, = 500 Q
R,=50K Q R, = 850 Q

Figure II-5. A schematic diagram representing the path of current and
associated resistances for the man described in the electric
shock case study on page TI-2.

Therefore, if the man had dry skin at the time of contact, he probably would
have experienced little more than a sensation of shock. Because it was a hot
and humid day and his skin was wet with sweat, the skin’s resistance was sig-
nificantly lowered. We note from Table 1-1 that 68.75 mAmps is within the
current range that may cause respiratory arrest, which is consistent with the
autopsy report.

Jn some environments, the hazards associated with even very low currents can
be devastating to equipment and life. For example, electrostatic discharge from
static electricity where silicon-based chips are manufactured or used can
severely damage equipment. Electrostatic discharge has also been suggested as
a cause of heart fibrillation in hospital patients with catheters or other direct
paths to the heart. In fact, a hospital is a very special environment where the
potential for shock must be highly controlled. Many patients have catheters that
bypass the skin’s relatively high resistance; some have catheters or monitoring

I-17


Unit Hi—Ignition of Flammable and Combustible Materials and Explosives

Table III-1. The Main Groups of Organic
Compounds, with Examples and Their Use*

Group Example Use
Aliphatic hydrocarbons Methane Natural gas petroleum gas
Butane
Aromatic hydrocarbons Benzene Solvent (toxic)
Toluene Solvent
Halocarbons Bromomethane Fumigant
Trichloroethane Solvent
Alcohols Ethanol “Alcohol”
Glycerol Glycerine
Carbonyl compounds Formaldehyde (methanol) Fumigant
Benzaldehyde acetone Manufacturing solvent
Ethers Ethyl ether Anaesthetic
Dioxan Solvent
Amines Methelamine Manufacturing
Aniline
Ethanoic acid Acetic acid manufacturing
Phthalic acid
Esters Ethyl acetate Solvent
Amides Acetamide Manufacturing
Urea By-product

Adapted from Ridley 1986.

Hyperbaric chambers, including decompression chambers, diving bells, and
underwater tunnels.

Hospitals, where oxygen-enriched atmospheres are needed for some medical
treatments and where oxygen supplies, which may leak, are located throughout
the building.

Cutting and welding, operations that use bottled oxygen, can be very haz-
ardous should the container begin to leak oxygen.

Aircraft with emergency oxygen supplies that can leak and create a hazardous
oxygen-enriched atmosphere should ignition occur.

Industry where oxygen is needed in a variety of industrial processes. For
example, huge quantities are required for the Bessemer process in steel mak-
ing. Even when a process does not use oxygen, a hazard may develop. For
example, liquid nitrogen has a boiling point of -196 degrees Celsius; that of
liquid oxygen is a bit higher at -183 degrees Celsius. When liquid nitrogen is
used to cool materials for processing, oxygen can condense from the air to cre-
ate an enriched area.

Transportation where large quantities of oxygen are transported to supply
industry. A spill can cause a wide area of oxygen-enriched atmosphere.

Tll-5


Unit Il—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

WHAT IS
ELECTRIC SHOCK

Contact with a
normally bare
energized conductor

Electric shock is a sudden stimulation of the body’s nervous system by an
electric current (Hammer, 1989), which can cause pain, injury, or death
(Bahill, 1981). Although it is usually painful, electric shock is not always asso-
ciated with actual damage to the body tissues. The most common sensation is
stabbing and numbing pain at the points of current entry and exit and some-
times along the path of the current through the body.

Any time a person comes in contact with an energized conductor there is a
potential for electrical shock. As in the example, current will flow through the
human body if it becomes part of an electric circuit when the potential differ-
ence (voltage) is adequate to overcome the body’s resistance (Hammer, 1989).
In the example, the man became part of the circuit because his body became a
path of least resistance to ground.

The five principal ways that people experience electrical shock will be dis-
cused in detail:

1. contact with a normally bare energized conductor,

2. contact with a normally insulated conductor on which the insulation has
deteriorated or been damaged so that it is no longer protective,

3. equipment failure that results in an open or short-circuit, which, in tum
causes the current to flow in an unexpected manner,

4, static electricity discharge, and

5. lightning strike.

One common situation and a frequent cause of electrical injuries, is contact
with live, overhead power lines. Examples include the boom of a crane, the
boom of a “cherry picker,” a raised dump truck bed, a metal ladder, the metal
mast of a sailboat, or any number of construction, maintenance, or recreational

* tools and vehicles inadvertently coming into contact with high-voltage, over-

head lines. When this happens, and a person is in contact with the tool or vehi-
cle, the high voltage often means severe injury or death.

NIOSH ALERT: Request for Assistance in Preventing Electrocutions
from Contact Between Cranes and Power Lines *

Contact between cranes and overhead power lines is a major cause of
fatal occupational injuries in the United States...approximately 2,300 lost
workday occupational injuries...These 2,300 injuries were extremely
severe, resulting in 115 fatalities and 200 permanent total disabilities [in
1981]...Comparable statistics...from 1964 to 1976 produced an estimated
annual average of 150 fatalities resulting from such incidents...

*This is an excerpt; see reference materials at the end of this book for a com-
plete copy of this Alert.


Page 6 - Request for Assistance in Preventing Fatalities of Workers

Who Contact Electrical Energy

REFERENCES

1.

Centers for Disease Control: Leading Work-Related Diseases and
Injuries--United States. MMWR 33:3-5 (1984).

National Fire Protection Association: National Electrical Code
1984, NFPA 70-1984. Quincy, MA: NFPA, 737 pp., (1983).

Wright RK, Davis JH. The investigation of electrical deaths: a
report of 220 fatalities. Journal of Forensic Sciences, JFSCA
25(3):514-521 (1980).

Straatsma Glen W. Electrical shock syndrome. Alaska Medicine,
pp. 129-130 (November 1973).

1985 National Conference on Cardiopulmonary Resuscitation (CPR)
and Emergency Cardiac Care (ECC). Standards and Guidelines for
Cardiopulmonary Resuscitation (CPR) and Emergency Cardiac Care
(ECC). JAMA 255(21):2905-2989 (June 1986).

Contributed by Mr. Edward J. Craren, Assistant Director, Emergency
Medical Services, Nebraska Department of Health, Lincoln, Nebraska
and Mr. Michael Dodge, Vice President, Eastern Ambulance Service,
Lincoln, Nebraska.

National Institute for Occupational Safety and Health: Alert:
Request for Assistance in Preventing Electrocutions of Workers in
Fast Food Restaurants. Cincinnati, OH: DHHS (NIOSH) Publication
No. 85-104, pp. 1-3 (December 1984).


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Specification versus
performance

A standard may also be classified as being a specification or a performance
standard. A specification standard is one that describes exact details of what
and how something must be done. For example, Article 200 of the NEC, “Use
and Identification of Grounded Conductors,” provides specific requirements
for (1) identification of terminals; (2) grounded conductors in premises wiring
systems; (3) identification of grounded conductors. Example specification lan-
guage from this section includes:

“200-6. Means of Identifying Grounded Conductors.

“(a) Sizes No. 6 or Smaller. An insulated grounded conductor of No. 6 or
smaller shall be identified by a continuous white or natural gray outer
finish along its entire length.” [Exceptions listed.]

Although specification standards may be difficult to develop, they are easy to
implement and enforce. Specification standards emphasize the methods neces-
sary to satisfy the given code.

Performance standards emphasize the results that should be realized to satis-
fy the code without specifically stating the methods to do so. Performance
standards allow the user more leeway and creativity in devising methods to
eliminate or reduce hazards. An example of a performance standard taken
from the NEC states the following.

“Article 230-41. Insulation of Service-Entrance Conductors.

“Service-entrance conductors shall normally withstand exposure to atmos-
pheric and other conditions of use without detrimental leakage of current.
Service-entrance conductors entering or on the exterior of buildings or
other structures shall be insulated.” [Exceptions listed.]

This passage states only that the specified conductors shall normally withstand
exposure to ... Shall be insulated [on exterior buildings or structures]. It does
not specify the materials, design, or methods to accomplish the goal; it is thus
a performance standard.

Performance standards are relatively easy to develop but more difficult to
enforce.

Classifying most standards as solely performance or specification is difficult,
but a rough distinction can usually be made as to one or the other. Industries
tend to prefer performance standards because they can then customize for their
particular needs. Performance standards also allow innovative hazard control
techniques to be implemented by incorporating new ideas and new technology.

Some performance standards, intentionally designed to stimulate new technol-
ogy, set performance goals that are currently not feasible. For example, setting
a pollution emission level for an industry below the currently feasible levels
will, in theory, stimulate research and product development which will meet
the new performance levels.

Iv-4


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

The polarity of a receptacle can be easily tested by measuring the voltage
between the three terminals (V;, V2, and V3), as shown in Figure I-11 (a).
There are six possible fault conditions: any of the three leads may be open, and
there are three ways of interchanging the connections (assuming only one fault
condition at a time).

Neutral Hot i
“ © 0)
(a

Ground

)

Receptacle where tests Receptacle where

are being performed power is consumed
N H N H
a O- ie}
G G

Neutral
Hot line !
U-ground ¢ ®- 2
Referencee¢——{V,)
ground

|____©

(b)

Figure II-11. Detecting improper wiring in receptacles: (a) measuring V1,
V2, and V3 can detect improper wiring conditions (H, N, or
G not connected or N-H or H-G reversed); (b) method used
to detect reversed N-G (adapted from Bahill, 1981).

T-24


Unit I1l—Ignition of Flammable and Combustible Materials and Explosives

SAMPLE QUIZ QUESTIONS

1. Briefly describe the mechanics of fire.

2. (a) What is a “Class C” fire extinguisher?

(b)What type of fire extinguisher should be used to extinguish an electrical equipment fire? Why?

3. (a) Discuss the sources of oxygen in a fire.

(b)Is there a way to control the source of oxygen as a method to minimize a fire hazard? Explain.

4. Two liquids, A and B, have the same boiling point (the temperature at which a liquid will begin to

boil). When both are spilled onto a surface of that temperature, however, only liquid A will ignite.

Why might this occur?

What distinguishes a fire from an explosion? Explain.

. What is a “short-circuit”? Why is a short-circuit an undesirable event?

. What is an “open circuit?” Why is an open circuit an undesirable event?

What is the difference between a “spark” and an “arc”?

The “corona effect” has been called “St. Elmo’s Fire.” Describe a corona, and postulate why it may

have acquired the nick-name of “St. Elmo’s Fire.”

10. What does “NEC” stand for?

11. What NEC Hazardous Class, Division, and Group would you assign to the following area: 50
pound bags of flour are stored in a room. The flour dust is normally not in suspension in the air,
except when a bag is accidently damaged. Ignitable dust concentrations are controlled by a reli-
able ventilation system.

12. What is the principle behind “intrinsically safe designs”?

13. What is the principle behind “explosion-proof equipment”?

14, What is the principle behind “pressurization”?

15. Describe three situations that.can lead to an overloaded circuit.

16. How could an overloaded circuit contribute to a fire hazard?

17. In addition to the fire hazards that an overloaded circuit presents, what other hazards to people
might an overloaded circuit present? Explain.

18. Describe a device that takes advantage of the fact that specific materials can be designed to rapidly
heat and melt. What are such devices used for?

19. If you were designing a capacitor, why might you design it so that it can be installed in only one
orientation with respect to polarity?

20. What does the conductivity of a material have to do with its ability to accumulate a static charge?

21. How and why does the presence of water affect a material’s ability to accumulate a static charge?

22. Describe how one charged body can induce a charge on another body.

23. Once a body has accumulated a static change, how can the charge be dissipated?

24. Assume your skin has a capacitance of about 200 pico-farads (200 x 10-12 farads). Now assume
that you have just walked across the office carpeting and entered an area that has a dangerous
fuel/air mixture of ethylene due to an accidental spill. As you walked across the carpet, you
developed a static charge resulting in a 1 kilovolt potential difference between your body and a
grounded appliance. As you reach for the appliance, the charge dissipates as a capacitor. Is the
energy released sufficient to ignite the ethylene/air mixture?

25. How and why could water settling through a tank of flammable liquid create a fire hazard?

26. What can be done to minimize the generation and accumulation of static charges on the human
body?

27. Why are special precautions taken to minimize the generation and accumulation of static charges
in a hospital environment? .

28. People who have survived lightning strikes often report a strong feeling of static charge immediate-
ly preceding the strike. What would you expect this to be caused by?

29. Describe the different ways that lightning can contribute to a fire hazard.

WwONAUW

I-33



AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS.

Sparks, arcs,
and coronas

Short circuits may be the result of overload or may cause overloads. Short cir-
cuits occur when an energized conductor contacts an unintended conductor not
in the circuit design. Generally, short circuits occur when insulation is dam-
aged and the energized conductor is exposed. Contact may occur with conduc-
tors that were not intended to be energized to create both a shock hazard and a
potential overload situation. Similarly contact may occur with other circuit
components to energize them beyond their design capabilities and cause fail-
ure or abnormal electrical discharge. This, in turn, may ignite flammable mate-
rials or fuel/air mixtures.

Sparks, arcs, or coronas are common precursors to fires caused by electrical sys-
tems, and they may ignite flammable solids, liquids, vapors, dusts, or powders.

A spark is a rapid, heavy discharge of electrons. A spark will occur any time an
ionized path develops in the dielectric medium separating two conductors or a
conductor and ground. A spark may be a single event that completely exhausts
the electrical energy in a system, or it may repeat each time the energy is
replenished.

An arc is a sustained stream of electrons flowing across a gap between con-
ductors that initially were in contact or in proximity but separate while current
is flowing. An arc will continue until the voltage is no longer adequate to sus-
tain it; this may occur by lowering the system voltage, or by further separating
the conducting surfaces, or by lengthening the arc until it can no longer sustain
itself. Unlike a spark, an arc will not reform unless the conductors are brought
back in contact, or the voltage across the gap exceeds the dielectric breakdown
potential.

A corona, or the corona effect, occurs in high-voltage systems when the
dielectric medium separating conductors partially breaks down. The potential
difference reaches a level necessary to ionize gas in the dielectric medium but
not enough to produce sparking. Ions form a cloud that drifts, typically from
the negative to positive electrode, to create a low-grade continuous flow. If the
medium breaks down further, a spark may occur. The corona effect may be
visible as a faint glow in the dark, and it may produce a hissing sound and
electromagnetic interference. The glow has been referred to as “St. Elmo’s
Fire.”

Arcs, sparks, and coronas can all cause insulating materials to degrade and
lose their nonconducting properties, thus creating additional hazards. They
may also cause electrical noise, which may interfere with the intended opera-
tion of electrical equipment. The worst effect, however, is the potential these
events have as ignition sources for flammable materials and atmospheres.
Even a spark caused by the discharge of static electricity can cause fires in cer-
tain environments (Hammer, 1989).

TIH-10




Page 5 - Request for Assistance in Preventing Electrocutions from Contact
Between Cranes and Power Lines

RECOMMENDATIONS BY NIOSH

The existing OSHA standard appears sufficient to prevent the crane-related
electrocutions described in this alert as well as all others. NIOSH urges all
employers who use cranes in the vicinity of overhead power lines to
familiarize themselves with and implement the existing OSHA standard, NIOSH
urges safety and trade associations, crane manufacturers, electric utility
companies, and OSHA state consultative services to bring this standard to the
attention of employers who use cranes. Implementation of the work practices
described by the CSA of Ontario can provide an additional margin of safety.

Suggestions, requests for additional information on safe work practices, or
questions related to this announcement should be directed to Mr. John Moran,
Director, Division of Safety Research, 944 Chestnut Ridge Road, Morgantown,
West Virginia 26505-2888, Telephone (304) 291-4595.

We greatly appreciate your assis ce.

Dodald Millar, M.D., ).T.P.H., (Lond.)
Assistant Surgeon Genera.
Director, National Institute for
Occupational Safety and Health
Centers for Disease Control

ee


DISCLAIMER
Mention of the name of any company or product does not constitute endorsement
by the National Institute for Occupational Safety and Health.
DHHS (NIOSH) Publication No. 89-110
w

Copies of this and other NIOSH documents are available from

Publications Dissemination, DSDTT
National Institute for Occupational Safety and Health
4676 Columbia Parkway
Cincinnati, Ohio 45226
(513) 533-8287

For further information on other occupational safety and health problems, call
1-800-35-NIOSH


_

Unit H—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Static electricity
and lightning

ss

(b)

Figure II-4. (b) An open circuit that results from a severed high-voltage
power line.

Static discharges and lightning strikes are significant fire/explosion hazards
and the nature of each will be covered in detail in Unit III.

These events are also shock hazards. We’ ve all experienced a shock from stat-
ic electricity. Usually, it is little more than a rude surprise. Such a surprise,
however, can and does cause people to suddenly move, possibly striking an
object or falling. Falls are the third leading cause of accidental death in the
United States, so this type of hazard is nothing to take lightly.

Shock from static electricity in the medical environment has been suggested as
a possible cause of heart fibrillation for patients that have catheters passing
through the skin and contacting the heart muscle. Significant efforts are taken
to minimize the generation, accumulation, and discharge of static energy
where patients may be at risk.

Lightning, which is a large discharge of static charge accumulations from the
atmosphere, has the potential to shock a person, and even result in death, if
they are directly hit or even nearby when it strikes. Lightning can also over-
load power lines and equipment, which has already been established as a
potential shock hazard.

TI-11


Unit |
AN INTRODUCTION TO SAFETY AND SOME
BASIC ELECTRICAL ENGINEERING CONCEPTS

PURPOSE: To introduce students to: 1) the importance of safety in dealing with electrical

systems, and 2) basic electrical engineering concepts.

To acquaint the student with:
1. Definitions of safety-related terms
2. Basic electricity concepts
3. Basic circuit laws and analysis

OBJECTIVE:

Hazard
. Danger
. Damage
Risk
|. Accident
Safety
. Electric current
. Conductors
. Insulators
10. Resistance
11. Impedance
12. Voltage
13. Direct current
14. Alternating current
15. Capacitance
. Ohms
. Hertz
. Amperes

SPECIAL TERMS:

CONAWAWHE

Tl



REQUEST FOR ASSISTANCE IN PREVENTING FATALITIES
OF WORKERS WHO CONTACT ELECTRICAL ENERGY

ATTENTION!

PROMPT EMERGENCY MEDICAL CARE CAN BE LIFESAVING FOR WORKERS WHO HAVE
CONTACTED EITHER LOW VOLTAGE OR HIGH VOLTAGE ELECTRICAL ENERGY.
IMMEDIATE CARDIOPULMONARY RESUSCITATION (CPR) FOLLOWED BY ADVANCED
CARDIAC LIFE SUPPORT (ACLS) HAS BEEN SHOWN TO SAVE LIVES.

SUMMARY

Recent incidents that have come to the attention of NIOSH have shown
that electrocution victims can be revived if immediate cardiopulmonary
resuscitation (CPR) or defibrillation is provided. While immediate
defibrillation would be ideal, CPR given within approximately
4 minutes of the electrocution, followed by advanced cardiac life
support (ACLS) measures within approximately 8 minutes, can be
lifesaving. This alert describes recommendations that can be used to
help save the lives of workers who contact electrical energy. Editors
of appropriate trade journals, safety and health officials, and
especially those who work with electrical equipment, are requested to
bring these recommendations to the attention of owners, managers, and
workers.

December 1986


REQUEST FOR ASSISTANCE IN PREVENTING
ELECTROCUTIONS DUE TO DAMAGED RECEPTACLES AND CONNECTORS

WARNING!

PERSONS USING ELECTRICAL EQUIPMENT ARE CAUTIONED THAT THE USE OF DAMAGED
RECEPTACLES AND CONNECTORS CAN BE EXTREMELY DANGEROUS.

SUMMARY

The National Institute for Occupational Safety and Health (NIOSH) is
requesting assistance in preventing the electrocution of workers due to
the use of damaged electrical receptacles and connectors. Two recent
incidents are described. Results of the investigations indicate that
periodic inspection, recognition of hazards, and proper use _ of
receptacles and connectors, and prompt repair of damaged connectors
and receptacles, could prevent such incidents. Editors of appropriate
trade journals, safety and health officials, and especially those who
work with electrical equipment, are requested to bring these
recommendations to the attention of owners, managers, and workers.

October 1986




AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Frustrated and hungry, a person attempts to free a piece of toast by inserting a
knife into the slot and contacts the energized elements. If that person offers a
path to ground (e.g., by being in contact with a grounded kitchen appliance), a
portion of the current will pass through the person and to the ground. The
GFCI will sense the difference in current levels between the hot and grounded
neutral wires—a difference indicating a ground fault (i.e., that current is flow-
ing in an unintended path to ground, a person in this case). When the current
differences reach some predetermined level (frequently 5 milliamps) the GFCI
will automatically cut power to the system, in as little as 25 milliseconds and
protect the person from further shock hazards. See Figure II-12.

Samperes_ Ground fault
=— 0.5 ampere
Hot wire \ 4 P

GFCI

| — Load
Q) Neutral WY
: wire

Ground wire \

—L 4.5 amperes

Figure I-12. A 0.5 milliampere ground fault causes a current flow imbalance
between the hot and neutral wires. Sensing the imbalance, the
ground-fault circuit interrupter (GFCT) breaks the circuit.

Figure I-13 illustrates a familiar receptacle-type GFCI commonly used in
bathrooms. Because of the expected presence of water in a bathroom and the
likelihood that water could provide a conductive path to ground, GFCI’s are an
important protection against shock in such environments. The National
Electric Code mandates the use of GFCI systems for all bathroom, some
kitchen, all basement and below-grade locations. Note also that monthly test-
ing is recommended for all GFCIs and that test buttons are provided for this

purpose.
Note also that fuses and circuit breakers do not protect people from shock. In

the home situation, for example, a person could be killed long before a 15
ampere fuse blows.

TI-26


ABSTRACT

The primary objective of this instructional module is to familiarize engineering faculty and students with
the occupational safety and health concerns which should be inherent to every electrical system. In
designing, operating, and maintaining electrical systems, the engineer must consider the occupational and
environmental hazards associated with electrical components as well as the safety of the surrounding
community from fires and explosions. This module can be studied separately or as a component of an
upper-level engineering design course.

Unit I provides background on basic safety and electrical engineering concepts which are expanded upon
in later modules. Unit II identifies the different types of electric shock hazards, their effects on the body
and methods to control these hazards as well as the inadvertent activation of equipment.

Unit IL is concerned with the electrical ignition of flammable and combustible materials, resulting in fires
or explosions, from overheating components, static electricity and other sources. Methods to control igni-
tion and explosion hazards are discussed.

Unit IV summarizes pertinent safety standards, such as the National Electric Code, and those organiza-
tions promulgating the standards, such as the National Fire Protection Association and the Occupational
Safety and Health Administration. Unit V provides examples of both inductive and deductive methods of
systems safety analysis and how these methods identify the hazards associated with the design, operation,
and maintenance of electrical systems.

vit


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Source Hot
/

4 yy

Neutral Tu

A short circuit inside the
+ drill will energize the case.

The person will potentially

become the short to ground.

Figure II-6. In a circuit without a third wire ground, a person may
become the path to ground in the event of a failure. Also
note that the current completes a loop through the ground.

The idea behind grounding is to protect people and equipment from receiving a
current due to a fault. Grounding provides an alternative, low-resistance, con-
ductive path to ground. With a third wire ground system (see Figure II-7), the
current resulting from a fault will flow through the ground wire rather than
through the person. In contrast, when there is no third wire ground (Figure II-6),
current may flow through a person (i.e., a shock) if that person provides a path to
ground.

a Les

J e_1Ground | e
a |

Figure II-7. Schematic of an electric drill equipped with a third-wire
ground and plugged into a third wire ground receptacle.

For the three-wire grounding method to be effective, the polarity of the recep-
tacle must be correct. That is, the “hot” wire must be connected to the “hot”
wire of the electrical equipment, and similarly for the neutral and ground
wires. Engineers have attempted to control this by designing a three-wire
receptacle so that the connector can be plugged in only one orientation. Figure
II-8 illustrates a variety of connector and receptacle designs configured so that
they can only be used with specific power systems. The 15-ampere plug and
receptacle is likely the most familiar because that power system is used in resi-
dential designs. Note that in all the designs the ground prong is longer than the

II-20


Page 4 - Request for Assistance in Preventing Electrocutions from Contact
Between Cranes and Power Lines

TABLE 1

Status of Compliance with OSHA Standards (or Use of CSA-Ontario
Recommended Work Practices) in Operations Which Resulted in
Six Crane-related Electrocutions

Relevant OSHA Standard (or

CSA-Ontario Recommended

Work Practice) Status of Compliance by Case
a 2k 68 & 6

1. Move, insulate, or de-energize No No No No No
power line before starting work (OSHA)

2. Maintain recommended absolute No No No No No
limit of approach (minimum
clearance) for specific voltage (OSHA)

3. Utilize a signal man (OSHA) No No No No No

4. Utilize nonconductive taglines, No No No No No
rather than direct contact, to
stabilize load (CSA-Ontario)

5. Do not store combustible materials No Yes No Yes Yes
directly beneath power lines
(OSHA & CSA-Ontario)

6. Use boom guards, insulating No No No No No
lines, or proximity warning
devices in addition to
other requirements (OSHA)

7. Use insulating boots and No No No No No
gloves when workers connect
loads or contact the crane
while in the vicinity of
overhead power lines (CSA-Ontario)

No = Data demonstrated lack of compliance with the OSHA standard (or lack of
use of CSA-Ontario recommended work practices).

Yes = Data demonstrated compliance with the OSHA standard (or use of
CSA-Ontario recommended work practices).


Unit I—An Introduction to Safety and Some Basic Electrical Engineering Concepts

10.

11.

12.

13.

SAMPLE QUIZ QUESTIONS

. Define “safety.”
. Is there any difference between “hazard,” "danger," and "risk?"

Explain.

. Discuss the difficulties faced when trying to determine what "an

acceptable degree of safety" is for a given product or process.

. Discuss the difficulties faced when trying to determine the "cost of

safety.” Do you think the "cost" refers to the upfront cost of safety fea-
tures, the costs incurred when an injury or fatality results from lack of
safety, or both?

. What are some of the hidden costs associated with adverse events?

Explain.

6. Briefly discuss some of the factors that affect a person's perception of risk.
. Describe a hazard that you voluntarily expose yourself to (e.g., a sport

or driving or riding in an automobile). Then compare that risk to one is
forced on you-one that actually presents less of a risk (e.g., something
at work, flying, etc.). Which activity actually presents the greater risk?
Which risk do you feel the most comfortable accepting?

. Review the risk perception study adapted from Tversky and Kahneman

(pg. 1-5). Discuss the implications of the results and comment on what
they say about human risk perception.

. Describe a risk that you are generally unwilling to take in your life.

Compare that risk with others that you do accept. Is there a rational
basis for the risks you have identified as acceptable versus those you
will not accept?

What would you do if:

(a) You were aware of a life threatening defect in a product that your
company was ready to put on the market?

(b) Assume you informed your superiors of the defect and the potential
consequences and their response was that you had better not men-
tion it to anyone else because it would be too costly to stop produc-
tion now.

Develop an argument against this statement: “Lawnmowers are inher-
ently dangerous. People know that, and if they are dumb enough to put
their hands and feet into the blades, then they deserve to lose them.”
Locate a newer model lawnmower, and describe the safety features that
have been incorporated into the design and operation to protect people
from being injured by the blades.

Refer to the circuit on page I-11 and solve for the currents i,, i, and i,,

and the voltages Vi, Veg» Veg at each resistor using the following val-
ues: R, = 2,000 Q, Ry = 1,000 Q, R, = 2,500 2,V = 220 volts.



AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Electric current

Conductors

Insulators

Resistance

atom to have a net positive charge. Likewise, an atom can gain electron(s), and
have a net negative charge. When a conductive material, such as a copper
wire, is charged (either positive or negative) at one end, electrons flow from
atom to atom in the direction necessary to balance the charge difference.

Benjamin Franklin (1706-1790), without the benefit of our modern atomic the-
ories, surmised that electricity traveled from positive to negative, setting a
convention that we still use today. Based on modern theories described above,
however, we note that the path of electron flow in a material is actually from
negative to positive. Therefore, conventional current, which we use in circuit
analysis, is the movement of positive charges. The movement of electrons in
the material (called electron current) is actually in the opposite direction. See
Figure 1.1.

i= conventional current

+
Oo— ; O

electron current

Figure L.1.In circuit analysis, the positive direction of a conven-
tional current moves from positive (+) to negative (-).
The actual flow of electrons, however, is from negative
to positive.

Basic electrical engineering concepts are briefly summarized below.

Very simply, a conductor is any material in which, due to its atomic make-up,
electrons are easily moved. Examples of conductors include metals and water.
Since humans consist mostly of water, we are also conductors.

Conversely, materials in which electrons are not easily moved about are called
insulators. Rubber, plastics, glass, and air are all examples of insulators.
Insulators are used to stop the flow of electric current or to keep a conductor
from being in contact with other conductors. For example, the coating over an
electrical cord keeps the current from passing to your body when you grasp it.

When currents flow in conductors, the electrons collide with the lattice of
atoms that make up the material. This impedes, or resists, the motion of the
atoms. The greater the interference with electron movement in a material, the
greater the resistance of that material. All conductors exhibit resistance to
some extent at room temperature (although researchers are busily trying to
devise "super conductors," that would have no resistance at room tempera-
ture). Resistance is measured in “Ohms,” named after Georg Simon Ohm
(1787-1854), a German physicist. The Greek letter omega (Q) is used to desig-
nate Ohms.

1-8


Contents (continued)

UNIT IV—SAFETY STANDARDS, ORGANIZATIONS PROMULGATING
STANDARDS, AND PROFESSIONAL LIABILITY

Purpose; Objective; Special Terms ... + soseaeeseeeee Iv-1
HAMMURABI’S CODE (approximately 1750 B.C.)... Iv-2
THE ROLE OF SAFETY STANDARDS Iv-2
A few definitions............ Iv-2
Voluntary versus mandatory; Consensus; Horizontal versus vertical Iv-3
Specification versus performance ... Iv-4
ORGANIZATIONS PROMULGATING STANDARDS... Iv-5
The National Fire Protection Association (NFPA) The National Electric Code (NEC) Iv-5
American National Standards Institute (ANSD IvV-6
American Society for Testing and Materials (ASTM); Underwriter’s Laboratories;
The Institute of Electrical and Electronics Engineers (IEEE);
State and Federal Government Regulations ...........:ssssesseeee IV-7
Occupational Safety and Health Act (OSHAct):
Occupational Safety ar and Health Administration (OSHA)... sessesesteatenceneeeeeteseeteneeseeeseeseee [WB
OSHA Standards... saseeceneeseeaneene sesesneee Iv-9
PROFESSIONAL LIABILITY............ Iv-9
CONCLUSIONS... Iv-11
REFERENCES AND SUGGESTED READINGS. . IV-12
SAMPLE QUIZ QUESTIONS .....c-esseeeeseeees . IV-13
UNIT V—SYSTEMS SAFETY ANALYSIS
Purpose; Objective; Special Terms .. sscseaconseses V-1
INDUCTIVE METHODS OF SYSTEMS SAFETY ANALYSIS ......--sesssecsesseessssstersees V-2
Preliminary Hazard Analysis (PHA)... V-2
Job Safety Analysis (JSA); Failure Mode and Effect. ‘Analysis (FMEA: V-4
Systems, Hazards Analysis (SHA) V-6
DEDUCTIVE METHODS OF SYSTEMS SAFETY ANALYSIS... V-7
Fault Tree Analysis (FTA)... V-7
Management Oversight and Risk Tree “analysis (MORT). V-10
Sneak circuit analysis. V-1l
REFERENCES AND SUGGESTED READING: V-12
SAMPLE QUIZ QUESTIONS ..... V-12

APPENDIX—NIOSH ALERTS

Publication No.
Request for Assistance in Preventing Electrocutions from Contact Between

Cranes and Power Lines .. aoe . 85-111
Request for Assistance in Preventing Electrocutions of Workers rs Using

Portable Metal Ladders Near Overhead Power Lines. eevee 89-110
Request for Assistance in Preventing Grain Auger Electrocution: 86-119

Request for Assistance in Preventing Fatalities of Workers Who Contact Electrical 1 Energy .. .-. 87-103

Request for Assistance in Preventing Electrocutions Due to Damaged
Receptacles and Connectors



DISCLAIMER

Mention of the name of any company or product
does not constitute endorsement by the National
Institute for Occupational Safety and Health

DHHS (NIOSH) Publication No. 86-119

Copies of this and other NIOSH documents are available from

Publications Dissemination, DSDTT
National Institute for Occupational Safety and Health
4676 Columbia Parkway
Cincinnati, Ohio 45226
(513) 841-4287


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Management
Oversight and
Risk Tree Analysis
(MORT)

For the example in Figure V-2, the end event will not occur if no events in
either of the following sets occur:

— the power supply fails, the switch is open, the fuse is blown, and light 1 is
burned out, or

— the power supply fails, the switch is open, the fuse is blown, and light 2 is
burned out.

Each of the two sets above represents a path set since, if none of the events in
either of the sets occur, the end event (room dark) cannot occur.

Management Oversight and Risk Tree Analysis is defined as a formalized, dis-
ciplined logic or decision tree to systematically relate and integrate a wide
variety of safety concepts. As an incident analysis technique, it focuses on
three main concerns: specific oversights and omissions, assumed risks, and
general management system weaknesses (EG&G, 1984).

It is essentially a series of fault trees with three basic subsets or branches:
1. abranch that deals with specific oversights and omissions at the worksite,

2. a branch that deals with the management system that establishes policies
and makes them work, and

3. an assumed risk branch that acknowledges that no activity is completely
free of risk and that risk management functions must exist in any well
managed organization. These assumed risks are those undesirable conse-
quences that have been quantitatively analyzed and formally accepted by
appropriate management levels within the organization.

MORT includes about 100 generic causes and thousands of criteria. The
MORT diagram terminates in some 1,500 basic safety program elements need-
ed for a successfully functioning safety program—elements that prevent the
undesirable consequences indicated at the top of the tree. MORT has three pri-
mary goals:

1. to reduce safety-related oversights, errors, and omissions;

2. to allow risk quantification and the referral of residual risk to proper orga-
nizational management levels for appropriate action; and

3. to optimize the allocation of resources to the safety program and to orga-
nizational hazard control efforts.

MORT programs and their associated training courses place emphasis on con-
structing trees for individual program needs and on a set of readymade MORT
trees that can be used for program design, program evaluation, or accident
investigation.

V-10


Unit I11—Ignition of Flammable and Combustible Materials and Explosives

Isolation

OVERHEATING,
DAMAGE TO
EQUIPMENT,
ELECTRICAL
EXPLOSIONS

- Reasons for
overheating

Isolation is a desirable approach to reduce fire and explosion hazards because
it completely removes either the flammable mixtures or the electrical ignition
source. Isolation is not fool-proof, however, because the enclosure or physical
partitions between the elements may fail.

Isolating an ignition source from a flammable or explosive atmosphere can be
done in two ways: all electrical ignition sources can be sealed off from the
hazardous environment; or the hazardous environment can be enclosed and
sealed from any electrical ignition sources.

Equipment failures, malfunctions, damage to equipment, or tampering with
intended protection devices can lead to hazardous situations for any electrical
component or system. The hazard may be an increased risk of shock, a fire or
explosion hazard, the hazard of equipment activating unexpectedly or operat-
ing in an unexpected way, or any number of dangerous scenarios. The results
may be injury or death to people, damage to equipment and facilities, or both.

In this section, we will focus on the hazardous effects of electrical equipment
overheating and the conditions that lead to it and some methods to control the
hazard. In previous sections we discussed the mechanics of fires and explo-
sions and accentuated the hazard of electrical discharge (sparks, arcs, and
coronas) as an ignition source. Here, we continue with electrical overheating
as an additional source of ignition and suggest you review the section on fire
mechanics where needed.

All energized conductors release energy in the form of heat. (The only excep-
tion to this rule would be a super conductor, which at the time of this writing is
an intriguing area of research but one not likely to reach widespread commer-
cial application in the near future. A super conductor offers virtually no resis-
tance to the flow of electrons and, therefore, loses no energy.) In general, the
electrical energy converted to heat is equal to the current (I) squared multiplied
by the resistance (R), or

Electrical energy converted to heat = PR
This expression is sometimes referred to as the Joule heat equation.

When designing a circuit, an engineer considers that heat will be generated in
the conductors. Appropriately specified materials are chosen so that the
expected operating energies, with a safety factor, will not generate too much
heat to damage the conductors or insulating materials. Without circuit protec-
tion (sometimes even with circuit protection, which we discuss below), a cir-
cuit may experience over currents or over voltages, generally referred to as
overloads, above those designed for. If this occurs, the conductors may heat to
temperatures sufficient to increase fire hazards or even be the ignition source
for a fire or explosion. Even if the generated heat is not a fire hazard in itself,
this condition can deteriorate properties of the conductor or insulator, which,
in turn, may cause immediate or eventual failure or malfunction. In some
cases, the temperature may rise so quickly that the component itself explodes.

W-17


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

For example, a rig was being positioned over a well so that a pump motor
could be lifted. One operator drove the truck, while another operated a 34-foot
boom. The boom came into contact with a 12-kilovolt overhead line; the boom
operator was killed instantly. The driver, hearing the noise of the contact,
stepped out of his cab to investigate. As one foot touched the ground, a large
current driven by the full 12-kilovolt charge shot through his body. His body
had become the path of least resistance to ground. He died a few hours later
from massive burns and internal bleeding.

NIOSH ALERT: Request for Assistance in Preventing Electrocutions
of Workers Using Portable Metal Ladders Near Overhead Power
Lines*

Contact between portable ladders and overhead power lines causes seri-
ous injuries...Data show that during the years 1980 through 1985, the con-
tact of metal ladders with overhead power lines accounted for approxi-
mately 4% of all work-related electrocutions in the United States (e.g., 17
out of 382 deaths for 1985).

NIOSH ALERT: Request for Assistance in Preventing Grain Auger
Electrocutions*

... The grain auger is an essential piece of farm equipment which is used to
move grain from one location to another. However, every year accidents
occur when. this piece of equipment is improperly moved in the elevated
Position and it comes in contact with high voltage power lines. This has
resulted in one or more fatalities per incident...

*These are excerpts; see reference materials at the end of this book for com-
plete copies of these Alerts.

Other common situations involving contact with high-voltage overhead power
lines affect workers whose duties require them to be in proximity to energized
lines. People who work on roofs, TV antenna and cable installers, and power
industry employees are all at risk of contacting bare conductors. Of course,
unskilled people are also at risk when they attempt to perform home repairs,
etc., near high-voltage lines.

w-4


Page 5 - Request for Assistance in Preventing Fatalities of Workers
Who Contact Electrical Energy

Employers may contact the local office of the American Heart
Association, the American Red Cross, or equivalent groups or
agencies, to set up a course for employees.

Provision should be worked out at each worksite to provide
ADVANCED CARDIAC LIFE SUPPORT (ACLS) within 8 minutes (if
possible), usually by calling an ambulance staffed by
paramedics. Signs on or near phones should give the correct
emergency number for the area, and workers should be educated
regarding the information to give when the call is made. For
large facilities, a prearranged place should be established
for company personnel to meet paramedics in an emergency.

We are requesting that employers, worker representatives, editors of
appropriate trade journals, and safety and health professionals assist
in disseminating these recommendations to those individuals and
organizations responsible for providing a safe workplace. Suggestions
or questions related to this announcement should be directed to
Mr. John Moran, Director, Division of Safety Research, National
Institute for Occupational! Safety and Health, 944 Chestnut Ridge Road,
Morgantown, West Virginia 26505-2888, telephone (304) 291-4595.

We greatly appreciate your assistance.

\ Doni Tar, WO., D.T.P.H\ (Lond. )
sskatant Surgeon General
Directér, National Institute for

Occupational! Safety and Health

Centers for Disease Control


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

More than $

Moral obligations

Perception of risk

Exposure to risk

Voluntary

Of course, there is more to injuries, and particularly deaths, than economics.
The emotional costs are more difficult to measure but are equally important in
the aftermath of an injury or death. In fact, the financial liabilities are often
easier to recover from than are the emotional losses. This may be on an indi-
vidual level, where family, friends, fellow workers, etc., suffer emotionally
from an injury or loss of friend or loved one (which may undermine the pro-
ductivity of all involved). The emotional consequences may affect society or
the reputation of an entire organization. When the Space Shuttle Challenger
exploded in mid-air, the Nation watched in disbelief. Shuttle launches were
put on hold for about 2 years, and NASA may never fully recover from the
effects of that event. When an oil tanker spilled its cargo in the pristine wilder-
ness of Prince William Sound, Alaska, the Nation was outraged. You can be
sure that the oil company will feel the effects of the clean-up and damaged
reputation for many years. On a more personal level for engineers, imagine
being responsible for a system that failed, especially if the failure resulted in
death or injury. Proper attention to safety can save society, organizations, indi-
viduals, and your own conscience from the emotional trauma and the econom-
ic consequences of such incidents.

As engineers, we have a moral obligation to provide safe products, environ-
ments, and workplaces. This is where the tough decisions must be made—the
cost of incorporating hazard controls versus the cost of an injury or fatality
versus ethical concerns. People want safe products and services, but they may
not be willing to pay for the features that protect them or the persons providing
them from harm. Further complicating the issue, what's safe for one individual
may not be safe another individual because of different perceptions of safety,
or our different abilities, or different predisposition to certain types of harm.
For example, a high voltage device with exposed energized conductors may be
safe in the presence of an electrical engineer because of knowledge and train-
ing but would never be safe in the home environment.

People's perception of risk, and what risks are acceptable to them individually
or as a society, complicates the ability to arrive at an "acceptable level of safe-
ty." Although we are not always rational in our decision making processes, our
determination of an acceptable risk is affected by, among other factors,
whether the risk exposure is voluntary or not, whether it is job related, and
how the risk is presented to us.

Consider the risks you voluntarily expose yourself to and those that you
have no control over. For example, you may voluntarily ride three-wheel, all-
terrain sport vehicles. During the 5 years before they were taken off the market
in the United States, nearly 900 people died (about half of them under 16 years
old) and 300,000 were injured while using them (Martin and Schinzinger,
1989). As much fun as they may be, a user voluntarily exposes him/herself to
a significant risk of injury or death. This same person, however, may be
adamantly opposed to being exposed to the relatively minor risks associated
with a high-voltage, overhead powerline running near his home.

We are loath to let others do unto us what we happily do to ourselves.

Chauncey Starr

1-4


Page 3 - Request for Assistance in Preventing Fatalities of Workers
Who Contact Electrical Energy

NIOSH CASE REPORTS
Case #1 - SUCCESSFUL RESUSCITATION

A 30-year-old construction worker was working on a fire escape in a
building being renovated. Another worker handed the victim a metal
pipe, and he was holding it with both hands when it contacted a
nearby high voltage line, completing a path-to-ground. The worker
instantly collapsed from this contact with electrical energy.
Approximately 4 minutes after he collapsed, the fire department
rescue squad arrived and began CPR. Within 6 minutes, a paramedic
unit was on the scene providing defibrillation and other ACLS
measures. They were able to establish a heartbeat and pulse, but
the individual continued to require respiratory support during
transport to the hospital. He regained consciousness and was
discharged within two weeks. He did have to return for further
medical care for burns he received on his hands (current entrance)
and buttocks (current exit) [6].

Case #2 - UNSUCCESSFUL RESUSCITATION

An 18-year-old male restaurant worker contacted electrical energy
when he kneeled to plug a portable electric toaster into a
110-120 V/20 amp floor outlet. After a scream was heard, the victim
was found convulsing on the damp floor, with one hand on the plug
and the other on the receptacle box. The assistant manager went to
the electrical panel, but was unable to locate the appropriate
circuit breaker. A coworker attempting to take the victim's pulse
received an electrical shock, but was not injured. After
telephoning the emergency medical service, the assistant manager
returned to the panel and de-energized all of the circuits (3 to 8
minutes after the worker contacted electrical energy). The injured
worker was covered with a coat to "keep him warm." After about 5
minutes, another call was placed to the emergency squad, and the
assistant manager "yelled" for an off-duty employee who lived in an
apartment across the lot, who came and began CPR. The emergency
service was on the scene 10 minutes after receiving the first call.
ACLS measures were available, but the resuscitation was unsuccessful
and the worker was pronounced “dead on arrival" at the local
hospital. The exact time span between the worker contacting
electrical energy and the beginning of CPR is unknown, but it is
reasonable to assume that it was longer than 4 to 6 minutes.
Paramedics with ACLS capability arrived 10 minutes after receiving
the call, but more than 10 minutes after the accident occurred [7].

CONCLUSIONS

in Case #1, basic life support was begun within 4 minutes by the fire
department rescue squad who happened to be stationed nearby. They
were experienced and had up-to-date knowledge in CPR techniques. In


Unit Il—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

— = —Intended current path
= wee Altemate current path

— Intended current path
+ «Alternate current path

(b)

Figure II-3. A short circuit can occur (a) between a conducting element
and a grounding conduction body, or (b) between conduct-
ing elements in the same circuit.

1-9


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

REFERENCES AND SUGGESTED READINGS

1,

4, Earley, M.W., Murray, R.H., Caloggero, J.

i.

12.

Bass, L., “Products Liability, Design and Manufacturing Defects,” Shepard’ s/McGraw-Hill,
Inc., Colorado Springs, CO, 1986.

. Bloswick, D.S., “Industrial Safety,” Chapter in Industrial Engineering Handbook, Edited by

Maynard, McGraw-Hill, New York, NY, 1992.
Code of Federal Regulations, Title 29, U.S. Government Printing Office, Washington, DC, 1988.

., “The National Electrical Code Handbook, With
Complete Text of 1990 Code,” National Fire Protection Association, Quincy, MA, 1990.

. Hammer, W., “Occupational Safety Management and Engineering,” Fourth Edition,

Prentice-Hall, Englewood Cliffs, NJ, 1989.

. Institute of Electrical and Electronics Engineers, Inc., “Introduction to the World of IEEE

Student Memebership,” a brochure, IEEE, New York, NY, 1991.

. Kolb, J., Ross, $.S., “Product Safety and Liability, A Desk Reference,” McGraw-Hill,

New York, NY, 1980.

. Martin, M., Schinzinger, R., “Ethics in Engineering,” Second Edition, McGraw-Hill, New

York, NY, 1989.

. National Fire Protection Association, “The National Electrical Code Handbook,” Sixth

Edition, NFPA, Boston, MA 1993.

). National Safety Council, “Accident Prevention Manual for Industrial Operations,

Administration and Programs,” Ninth Edition, NSC, Chicago, IL, 1988.

Occupational Safety and Health Administration, Publication 2201, “General Industry
Digest,” U.S. Department of Labor, Washington, DC, 1991a.

Occupational Safety and Health Administration, Publication 2202, “Construction Industry
Digest,” U.S, Department of Labor, Washington, DC, 1991b.

IV-12


Page 2 - Request for Assistance in Preventing Fatalities of Workers
Who Contact Electrical Energy

BACKGROUND

It has been estimated that at least 700 occupational electrocutions
occur each year [1]. Therefore, a primary goal of occupational safety
programs must be to prevent workers from contacting electrical
energy. Effective measures include safe work practices, job training,
proper tools, protective equipment, and lockout/tag-out procedures.

Investigations by NIOSH, as part of its Fatal Accident Circumstances
and Epidemiology (FACE) Project, also have revealed that once an
electrical energy incident occurs, emergency response plans are often
lacking, even in organizations which promote safety. Hence, a
secondary goal of safety programs must be to provide appropriate
emergency medical care to workers who contact electrical energy.

The National Electrical Code divides voltages into two categories:
greater than 600 volts (high voltage) and less than or equal to 600
volts (low voltage) [2]. Momentary contact with low voltages produces
no thermal injury, but may cause ventricular fibrillation (very rapid,
ineffective, heartbeat) [3].

In contacts with high voltage, massive current flows may stop the
heart completely. When the circuit breaks, the heart may start
beating normally [3]. Supporting respiration by immediate
mouth-to-mouth techniques may be required, even if heartbeat and pulse
are present. If extensive burns are present, death may result from
subsequent complications [4].

APPROPRIATE STANDARDS AND GUIDELINES

The revised "Standards and Guidelines for Cardiopulmonary
Resuscitation (CPR) and Emergency Cardiac Care (ECC)" published in
June 1986, is a product of the 1985 National Conference on CPR and
ECC. There are two parts: basic cardiopulmonary resuscitation (CPR)
and advanced cardiac life support (ACLS). A lay person can be trained
in CPR to support circulation and ventilation of the victim of cardiac
or respiratory arrest, until ACLS (provided by medical professionals
using epaclal equipment) can restore normal heart and ventilatory
action [5].

Speed has been found to be critical to resuscitation: immediate
defibrillation would be ideal. The highest success rate has been
achieved in those patients for whom CPR followed cardiac arrest within
approximately 4 minutes, and ACLS was begun within approximately
8 minutes of the arrest [5]. CPR often must be initiated immediately
by lay individuals at the scene of the incident. It should be noted
that CPR skills can be gained in 4-hour courses similar to those
taught by the American Heart Association or the American Red Cross.


Unit IV—Safety Standards, Organizations Promulgating Standards, and Professional Liability

ORGANIZATIONS
PROMULGATING
STANDARDS

The National Fire
Protection
Association (NFPA)

The National
Electric Code (NEC)

Throughout your career you will probably sift through thousands of pages of
standards from a variety of organizations and agencies. Some organizations
that you are or should become familiar with, include the Occupational Safety
and Health Administration (OSHA), the American National Standards Institute
(ANSD, the National Fire Protection Association (NFPA), Underwriter’s
Laboratories (UL), the Institute of Electrical and Electronics Engineers
(IEEE), and the professional societies in your field. A brief description of each
of these organizations and their purpose follows.

The NFPA is the clearinghouse for information regarding fire protection, fire
prevention, and fire fighting. It is a nonprofit technical and educational organi-
zation with a membership of approximately 32,000 companies and individuals.

The NFPA is the administrative sponsor of the National Electric Code (NEC).
The 1993 NEC, formally known as ANSI/NFPA 70-93SB, should become part
of your personal library, and you should update it with each new edition. A
brief description and history of this important document follows (National
Safety Council, 1988).

The NEC is the most widely adopted code in the world. It is, in fact, the basis
for all electrical codes used in the United States and is also used extensively
by other countries, especially where American-made equipment is installed.

The basis for the NEC originated from a proposal drafted at an 1881 meeting
of the National Association of Fire Engineers. It covered such things as insu-
lated conduit, the use of single disconnect units, and the identification of the
white wire. The first nationally recommended electric code was published in
1895 by the American Insurance Association, known then as the National
Board of Fire Underwriters. By combining the efforts of architectural, electri-
cal, insurance, and allied interests, the National Electric Code was drafted in
1887.

In 1911, the National Fire Protection Association (NFPA) assumed sponsor-
ship of the NEC. Since 1920, the American National Standards Institute
(ANSJ) (formerly known as the United States of America Standards Institute
and the American Standards Association) has officially endorsed the NEC, and
the NFPA has maintained its capacity as administrative sponsor.

In 1923, the NEC was rearranged and rewritten. In 1937 and 1959, it was edi-
torially revised. Since then, new additions have been adopted, the most recent
(at the time of this writing) being the 1993 edition, which superseded all previ-
ous editions. As is typical for consensus standards, each revision is the culmi-
nation of proposed amendments by members and interested citizens—amend-
ments that must pass through committees until final language is agreed on
(thus, a “consensus” is reached).

The 1993 NEC, “...is purely advisory as far as the NFPA and ANSI are con-
cerned but is offered for use in law and for regulatory purposes in the interest
of life and property protection” (National Fire Protection Association, 1993).

Iv-5


Unit I—An Introduction to Safety and Some Basic Electrical Engineering Concepts

Risk on the job

Assessment of risk

Skewed perceptions

On the other hand, many people accept greater risks at work than they would
in other environments. The reasons for this are complex, but to a certain
degree, people will accept the risks because they like their job, or they need
their job, or they may fear dismissal if they were to complain.

Another factor that affects’ the type and degree of risk people will accept
depends on their ability to accurately assess a risk based on the available infor-
mation. The information people use to make their decisions greatly influences
how risks are perceived. Take for example the following information (Tversky
and Kahneman, 1981, as reported by Martin and Schinzinger, 1989).

Imagine that the United States is preparing for an outbreak of an unusual dis-
ease, which is expected to kill 600 people. Four alternative programs to com-
bat the disease have been proposed. Assume the exact scientific estimate of the
consequences of the programs are as follows:

— If program A is adopted, 200 people will be saved.

— If program B is adopted, there is 1/3 probability that 600 people will
be saved, and 2/3 probability that no people will be saved.

— If program C is adopted, 400 people will die.

— If program D is adopted, there is 1/3 probability that nobody will die,
and 2/3 probability that 600 people will die.

Which of the four programs would you choose?

If you haven't already noticed, programs A and C are identical, as are pro-
grams B and D. And statistically, each program has the same expected value:
200 people will survive and 400 will not. Only the way the information is pre-
sented is different.

In an experiment, programs A and B were presented to one group of people,
and programs C and D were presented to another group (about 150 people in
each group). In the first group, 72 percent of the people chose program A, and
28 percent chose program B. In the second group, however, only 22 percent
chose program C (which is identical to program A), and 78 percent chose pro-
gram D (which is identical to program B). Apparently, people will choose
options that provide definite gains over options that appear risky or only prob-
able. In addition, options whose chances of success are perceived as probable
will be favored over options that emphasize definite losses. Such decision
making influences are certainly not rational, but they are real.

Nor is our reaction to large-scale accidents a rational one, nor to those that
happen to someone we know. For example, whenever a large-scale airplane
crash occurs, we are deluged with grizzly scenes and descriptions of the disas-
ter by the media. The emotional trauma suffered by the friends and loved ones
of the victims leaves an impression on us. Some people refuse to fly, whereas
others fly in fear. Of course, flying is a safer mode of transportation than is
traveling by car. If you need to travel an appreciable distance, you are far safer

1-5


Unit 1V—Safety Standards, Organizations Promulgating Standards, and Professional Liability

CONCLUSIONS

Under the theory of strict liability the standard of responsibility is to provide
a product free of defects. A product may be deemed defective if a reasonably
prudent manufacturer or seller, knowing of the risk which the product present-
ed, would have put the product into the stream of commerce. Under strict lia-
bility theory, even if a manufacturer did not know of a defect, it can be held
liable if expert testimony reveals a defect.

A warranty is a promise made by a seller that a product possesses certain per-
formance, safety, or quality characteristics. Under warranty, the standard of
responsibility is whether the capabilities of the actual product differ from the
expressed capabilities. Within this theory, a seller may be found liable under
express warranty or implied warranty. An express warranty is breached when
the product cannot safely be used for purposes portrayed in statements, pic-
tures, or advertisements. Implied warranty may be defined as a promise that
the goods are fit for the ordinary purposes for which such goods are used.

Any party along the stream of commerce (design through sales) may be liable
for misrepresentation of fact, intentional or not, if the public relies on such
information and consequently suffers harm. That is, even if a product is manu-

_ factured with the highest standards and is free of defects, any misrepresenta-

tion concerning the product that contributed to an injury may make the defen-
dant(s) liable.

In many product liability lawsuits, more than one of these theories will be tested,
and most or all of the individuals, or corporations, or both involved in the
design, manufacturing, and distribution of the product will be named.

The engineers that passed before us laid groundwork upon which we can
expand, Our predecessors built foundations for greater and more complicated
technologies; by analyzing their successes and unfortunate failures, we can build
for those that follow us. To protect the safety, health, and welfare of the public
and to speak out against abuses in those areas affecting the public interest, we
must maintain [our] professional skills at a level of the state of the art and recog-
nize the importance of current events in our work (excerpted from the IEEE
Code of Ethics).

Our responsibilities include being familiar with the standards promulgated by
our professional societies, standard organizations, trade associations, and regu-
latory agencies. Wherever necessary, we must adhere to those standards and,
whenever possible, expand on them, as they usually serve as the minimum
means of protection. These standards, which are the compilation of the lessons
previously learned, become increasingly necessary to study as we push tech-
nology to even more complicated and potentially hazardous limits.

If we fail to uphold our professional responsibilities, we have failed as engi-
neers and as members of society. Individuals that experience injury or hard-
ship because of our failures have legal recourse through the civil court system
under products liability theory. Although none of us are completely immune
from becoming involved in such lawsuits, our best protection is to keep our
education up to date and to keep the safety, health, and welfare of the public
paramount in the performance of our professional duties.

Iv-11


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

INDUCTIVE
METHODS OF
SYSTEMS SAFETY
ANALYSIS

Preliminary
Hazard Analysis
(PHA)

The general area of system safety analysis may be defined as a directed or sys-
tematic process for the acquisition, review, and analysis of specific informa-
tion relevant to a particular system. This process is methodical, careful, and
purposeful. The purpose is to provide information for informed management
decisions. System safety analysis techniques can be categorized as either
inductive or deductive.

Inductive methods use observable data to predict what can happen. These
techniques consider systems from the standpoint of the component parts and
determine how a particular mode of failure of component parts will affect the
performance of the system. Major inductive methods are Preliminary Hazard
Analysis (PHA), Job Safety Analysis (JSA), Failure Modes and Effects
Analysis (FMEA), and Systems Hazard Analysis (SHA).

Preliminary Hazard Analysis is a qualitative study conducted during the con-
ceptual or early developmental phases of a system’s life. Its objectives are to:

1. identify known hazardous conditions and potential failures,
2. determine the cause(s) of these conditions and potential failures,

3. determine the potential effect of these conditions and potential failures on
personnel, equipment, facilities, and operations,

4. determine the probability of each failure, and

5. establish initial design and procedural requirements to eliminate or control
these hazardous conditions and potential failures.

In some cases an additional step, the estimation of the probability of an
adverse event due to the hazard, is performed between steps 3 and 4 above.

Table V-1 illustrates an amusing but instructive PHA of the feathers, flax, and
beeswax wings that Daedalus and Icarus, in Greek mythology, used.

The PHA is often based on a limited number of hazards determined as soon as
initial facts about the system are known. These basic hazards must be dealt
with even though many different circumstances might lead to the different
hazards. The design process may be monitored to determine whether these
hazards have been reduced or eliminated and, if not, whether the effects can be
controlled.


Page 2 - Request for Assistance in Preventing Electrocutions from Contact
Between Cranes and Power Lines

Case #3:

Roof materials for an addition to a commercial building were
stored outside the building directly beneath a 7,200 volt
power line. While hooking a load (joist angle bracing) to the
crane, a worker was electrocuted when the cable came into
contact with the power line as the boom swung.

Case #4:

A construction company was in the process of laying concrete
water pipe with a crane. As workmen were placing support
timbers beneath the crane's outrigger pads, the operator began
extending the crane boom for the next lift when the boom came
into contact with a 3 phase 13,800 volt overhead power Line.
One worker touching an outrigger of the crane was electrocuted.

Case #5:

At a highway construction site, a carpenter attached a 4' x 8'
wood and metal form to a crane, While holding on to the form
in attempting to guide it into place, the carpenter was
electrocuted when the boom or cable came into contact with a
34,000 volt power Line.

APPROPRIATE STANDARDS AND RECOMMENDED WORK PRACTICES

The Occupational Safety and Health Administration (OSHA) Safety and Health
Regulations for Construction, Subpart N--Cranes, Derricks, Hoists, Elevators,
and Conveyors (29 CFR 1926.550(a)(15)) contains specific requirements for the
safe use of cranes proximate to overhead power lines. Electrical distribution
and transmission lines are required to be de-energized and visibly grounded,
moved, or separated from cranes with independent insulating barriers. The
regulation states that when it is not possible to meet these requirements,
cranes may operate proximate to power lines only if:

a) minimum clearance (absolute limit of approach) is maintained
between the crane and the lines (10 feet for <50 kV and 10 feet
plus 0.4 inch for each 1 kV over 50 kV, or twice the length of
the line insulator but never less than 10 feet); or,

b) in transit with no load and boom lowered, minimum clearance
(absolute limit of approach) is maintained (4 feet for <50 kV,
10 feet for 50 kV to 345 kV, or 16 feet for up to and including
750 kV).


Unit I1l—Ignition of Flammable and Combustible Materials and Explosives

—Is the zone below the surrounding elevation or grade such that the flam-
mable liquids or vapors may accumulate?

Class I, Division 1:

—Is a combustible dust likely to exist in suspension in air, under normal
operating conditions, in sufficient quantities to produce explosive or
ignitable mixtures?

—Is combustible dust likely to exist in suspension in the air because of
maintenance or repair operations, in sufficient quantities to cause explo-
sive or ignitable mixtures?

—Would failure of equipment be likely to cause an electrical system failure
coinciding with the release of combustible dust in the air?

—Is combustible dust of an electrically conductive nature likely to be present?

Class il, Division 1:
—Are easily ignitable fibers or materials producing combustible flyings
handled, manufactured, or used?

Or, should the atmosphere be considered that of a Division 2—one where no
ignitable atmosphere exists but where abnormal circumstances might create a
hazardous environment? A “yes” answer indicates a Division 2 classification.

Class I, Division 2:

—Is the flammable liquid, vapor, or gas piping system in an inadequately
ventilated location but one not likely to leak?

—lIs the flammable liquid, vapor, or gas handled in an adequately ventilated
location and can the flammable substance escape only in the course of
some abnormality, such as failure of a gasket or packing?

—Is the location adjacent to a Division 1 location or can the flammable sub-
stance be conducted to the location through trenches, pipes, or ducts?

—If positive mechanical ventilation is used, could failure or improper oper-
ation of. ventilating equipment permit mixtures to build up to flammable
concentrations?

Class I, Division 2:

—Is the combustible dust likely to exist in suspension in air only under
abnormal conditions but can accumulations of dust be ignited by heat
developed by electrical equipment or by arcs, sparks, or burning materials
expelled from electrical equipment?

—Are dangerous concentrations of ignitable dusts normally prevented by
reliable dust-control equipment such as fans or filters?

—Is the location adjacent to a Division 1 location and not separated by a
fire wall?

—Are dust-producing materials stored or handled only in bags or containers
and only stored—not used—in the area?

Class III, Division 2:
—Are easily ignitable fibers or flyings only handled and stored and not
processed?
—Is the location adjacent to a Class II, Division 1 location?

TH-13


Unit IlJ—Ignition of Flammable and Combustible Materials and Explosives

Electrical
explosions

Methods to protect
circuits from
overheating

Fuses

Overheating can also cause equipment to burn out by raising its temperature so
high that it fails or ignites itself. The damage to equipment depends to some
extent on how fast the overheating occurs. There are forms of circuit protec-
tion designed to deenergize a circuit if the current increases at a certain rate.
Conditions exist, however, where the circuit can be overloaded at a rate faster
than the protection device can react and result in equipment failure. Systems
can also overheat when operating at normal energies, such as when a cooling
system fails.

Overheated equipment can also be a burn hazard by heating surfaces to tem-
peratures capable of causing. skin burns if a person touches them. Operating
temperatures should be determined in the design of a system, and appropriate
measures should be taken to cool or isolate equipment as a protective measure.

A conductor may experience rapid electrical loading and, due to its size and/or
composition, explode. When exposed to overcurrents, rapid overheating may
occur in switches, fuses, or circuit breakers and cause them to melt and vapor-
ize very quickly. This effect is sometimes used purposefully (see discussion of
fuse below), but when it occurs unexpectedly, it can be very hazardous, espe-
cially if it occurs in an explosive or flammable atmosphere. We will discuss
methods to control such hazards at the end of this unit.

Electrical explosions can also occur when contaminants (water, for example)
are present in equipment such as oil-filled breakers or transformers. Electrical
heating can cause the contaminants to vaporize and thus create high pressure
in the sealed enclosure, that would cause it to rupture.

Capacitors may explode if exposed to excessive currents. Some capacitors, as
well as batteries, are sensitive to polarity and may explode if the they are sub-
jected to the wrong polarity of current and voltage. This can occur if the com-
ponent is installed incorrectly or if the leads are reversed in a direct current
system.

Guarding circuits from overload and its potential overheating and explosive
effects is done by devices that sense specific overload conditions and deener-
gize the circuit by interrupting the flow of current. We will briefly describe the
most common of these devices.

Fuses are devices placed at strategic locations in electrical circuits and sys-
tems; they conduct rated currents without overheating but melt when exposed
to currents in excess of designed loads. You are probably familiar with fuses if
you own a stereo system or an automobile, for example. Occassionally a fuse
will “blow;” this may seem like an inconvenience until you realize that that
blown fuse may have protected your equipment from major damage due to
overcurrents. If a system repeatedly blows fuses, it indicates that a more
severe problem exists and repair is in order. People sometimes short circuit a
fuse when this happens, which is, almost literally, “playing with fire.”

1-19


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

Review of basic
circuit analysis

It is assumed that you have already had experience solving electric circuit
problems using Kirchhoff's current law (KCL) and Kirchhoff's voltage law
(KVL). As a refresher, we present some of the very basic concepts.

For a resistor, the current (i) and the voltage (v) are related such that the cur-

rent in a resistive element is directly proportional to the voltage. The required
voltage equals the current multiplied by the resistance (R) of the element, or

i
>

Ot
(e)

V=iR

So, if the resistance is 1000 Q and the current is 0.5 ampere (amp), the voltage
in the element is (1000 Q)*(0.5 amp) = 500 volts.

When resistors are in series (one after another in succession), the values can be
added together and treated as one element. The current remains constant, and

O+
oO

Ry Ro
V=iR,+iR, = i(R, +R.) =iRy
Reg =R,+R,
where Rog is the equivalent resistance.

When resistors are arranged in parallel, the current arriving at the junction
must divide in proportion to the resistance of each branch. The following rela-
tionships can be proven to hold for resistors in parallel.



Unit 1V—Safety Standards, Organizations Promulgating Standards, and Professional Liability

SAMPLE QUIZ QUESTIONS

. How do a standard, a code, and a recommended practice differ? Explain.

2. When an engineer is working on a new or unique design or process, reviewing
existing standards that deal with conceptually similar products or systems is
very helpful. Assume you are working on a new product idea and you correctly
determine that some initial research into similar types of products is necessary.
Describe your new product idea, and explain what types of similar products
you would study to help guide you in your design.

3. In the library, locate some or all of the standards you can that relate to your
product design in problem (2). Explain how they would help you in your new
product design, particularly concerning safety.

4, Do safety standards, codes, etc., represent the “state-of-the-art” in safety for a

given product or process? Discuss.

. Briefly discuss how consensus standards are developed.

. Briefly discuss how mandatory standards are developed.

. Discuss the difference between mandatory and consensus standards.

. Discuss the difference between a horizontal standard and a vertical standard.

. Discuss the difference between a specification standard and a performance standard.

10. In the library, find a safety standard pertaining to a subject that interests you.
Classify the standard as horizontal or vertical, specification or performance.
You may find that different parts of a given standard can be classified differ-
ently. Justify your classifications.

11. What is the NFPA, and what do they have to do with electrical safety?

12. What is ANSI, and what does ANSI do?

13. What role does UL play in product safety?

14. Why is it in an engineer’s best interest to belong to a professional society?
What benefits do professional societies offer their members?

15. What is the “General Duty Clause?” Discuss.

16. What is the BLS? How could information compiled by the BLS help you in
making a product or process safer?

17. The Three Mile Island nuclear power plant accident, the space shuttle
Challenger explosion, and the Ford Pinto rear-end collision explosions are
examples of well-publicized cases involving safety issues and engineering
ethics. Each has been written on extensively. Choose one of these, and do a
case study of the event and the aftermath. Note the errors that led to the acci-
dent(s) and how they might have been avoided. Discuss the ethical dilemmas
faced by the engineers involved in the case.

-

oC aorntnvnn

IV-13



Unit Il
ELECTRIC SHOCK HAZARDS AND CONTROLS,
AND INADVERTENT ACTIVATION OF EQUIPMENT

To introduce the student to the concepts of electric shock and inadvertent activation
of equipment

PURPOSE:

To acquaint the student with:

. Catises of electric shock

. Effects of shock on the body

. Shock hazards in special environments

. Methods to control the shock hazard

. Consequences of inadvertent activation of equipment
. Methods to prevent inadvertent activation

OBJECTIVE:

AnwPwne

Electric shock

Short circuit

Open circuit

Insulation

. Interlocks

. Isolation

. Grounding

Bonding

. Ground-Fault Circuit Interrupter (GFCI)
. Lockout/tagout

SPECIAL TERMS:

)
a

SEE

Seer avayyr

=

A
oe

U-1



AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

REFERENCES AND SUGGESTED READINGS

1.

Bahill, A.T., “Bioengineering, Biomedical, Medical, and Clinical Engineering,” Prentice-
Hall, Inc, Englewood Cliffs, NJ, 1981.

.. Cabanes, J., “Physiological Effects of Electric Currents on Living Organisms, More

Particularly Humans,” in “Electrical Shock Safety Criteria, Proceedings of the First
International Symposium on Electrical Shock Safety Criteria,” Pergamon Press, Elmsford,
NY, 1985.

. Cooper, F., “Electrial Safety Engineering,” Second Edition, Butterworths, London,

England, 1986.

4. Daziel, C.F., “Electric Shock Hazard,” IEEE Spectrum, February, pp. 41-50, 1972.
5. Daziel, C.F., Lee, W.R., “Reevaluation of Lethal Electric Currents,” IEEE Transactions on

Industry and General Applications, 4(5), September/October:467-476, 1968.

. Hammer, W., “Occupational Safety Management and Engineering,” Fourth Edition,

Prentice-Hall, Englewood Cliffs, NJ, 1989.

. Firenze, R.J., Walters, J.B., “Safety and Health for Industrial/Vocational Education, for

Supervisors and Instructors,” Division of Training and Manpower Development, NIOSH,
Cincinnati, OH, 1981.

. National Safety Council, “Accident Facts” 1986 Edition, NSC, Chicago, IL, 1986.
. U.S. Department of Labor, “An Mlustrated Guide to Electrical Safety,” Occupational Safety

and Health Administration, Washington, DC, 1981.

). U.S. Department of Labor, “Ground-Fault Protection on Construction Sites,” Occupational

Safety and Health Administration, Washington, DC, 1987.

I-30


Unit IV—Safety Standards, Organizations Promulgating Standards, and Professional Liability

OSHA Standards There are three separate sets of OSHA standards: General Industry (29 CFR
1910), Construction (29 CFR 1926), and Maritime Employment (29 CFR
1915-1919). OSHA has summarizedof major portions of the OSHA standards,
and these are available in digest form. OSHA publication 2201 is a summary
of the OSHA General Industry standards (OSHA, 1991a) and OSHA publica-
tion 2202 is a summary of the OSHA Construction standards (OSHA, 1991b).

“To consider hazards not specifically included in OSHA standards, OSHA has
turned to the provisions of Section 5(a) of the OSHAct or the “General Duty
Clause” that states:

“Each employer—

(1) shall furnish to each of his employees employment and a place of
employment which are free from recognized hazards that are causing or
are likely to cause death or serious physical harm to his employees;

(2) shall comply with occupational safety and health standards promul-
gated under this Act.”

In summary, four governmental units have the primary responsibility to carry
out the act:

1. The Occupational Safety and Health Administration (OSHA) is concerned
with national, regional, and administrative programs for developing and
ensuring compliance with safety and health standards. It also trains OSHA
personnel. U.S. Department of Labor, Department of Labor Building, 200
Constitution Avenue, NW., Washington, D.C. 20210; (202) 523-8148.

2. The Occupational Safety and Health Review Commission OSHRC),
reviews citations and proposed penalties in enforcement actions contested
by employers or employees. 1825 K Street, NW., Washington D.C.,
20006; (202) 634-7970.

3. The National Institute for Occupational Safety and Health (NIOSH) provides
research, training, and education. U.S. Department of Health and Human
Services, 1600 Clifton Road NE., Atlanta GA,30333; (404) 329-3061.

4. The Bureau of Labor Statistics (BLS) conducts statistical surveys and
establishes methods for acquiring injury and illness data. U.S.
Department of Labor, 200 Constitution Avenue, NW., Washington D.C.
20210; (202) 523-1092.

PROFESSIONAL “Engineers shall hold paramount the safety, health and welfare of the public
LIABILITY in the performance of their professional duties.”

This statement is the first principle listed in “The Fundamental Canons” of the
“Code of Ethics of Engineers.”


Unit Il—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

Generalized effect of
60 Hz alternating
currents

1.4mA

The human body is controlled by a vast network of nerves that communicate
by passing small electrical signals. The nervous system is responsible for orga-
nizing all of the functions necessary to keep us alive, such as the muscle con-
tractions that expand and contract the rib cage to make us breathe and the heart
muscle contractions that keep our blood flowing. Electric shock can inhibit
normal physiological function by interfering with the body’s own “electrical
communication” network of nerves; breathing and/or normal heart function, in
the most severe cases, can be arrested. It is interesting to note that 50 to 60 Hz,
which includes the alternating current frequency of most residences and indus-
tries, is the frequency at which human muscles are most likely to be excited to
the point of involuntary contraction and, therefore, is quite hazardous.

The magnitude of a current flow in the body is the most damaging factor. The
effect of alternating current levels at 60 Hertz are outlined below (Table II-1
summarizes the current ranges and the general hazards). Understand that these
are average values and that the method of current application and individual
human differences may affect these values.

Table I-1
Summary of Effect of 60 Hz Alternating Currents
Through the Body of an Adult Human

AC Current Level Effects on Human Body

1.1 toSmA Threshold of perception

Sto 25mA Let-go threshold

201075 mA Very painful and injurious; potential respiratory arrest

70 to 300 mA Potentially fatal without immediate first-aid; may cause

ventricular fibrillation and respiratory arrest

2.5 A or more Potentially fatal without immediate first-aid; may stop heart
muscle completely; may cause respiratory arrest; causes burns
to tissue

The average person will perceive a shock, including sensations of tingling or
heat. Some people may feel a current as low as 0.5 milliampere, whereas others
may not feel it until a current level of 2 milliamperes. Women are believed to
have a lower threshold of shock perception than men. Perception of shock at
these current levels results mostly in inadvertent muscle reflexes. Such sudden
motions can cause a person to jerk violently and possibly result in a fall or other
event with a potential for injury (Cabanes, 1985).

I-13


REQUEST FOR ASSISTANCE IN PREVENTING
GRAIN AUGER ELECTROCUTIONS

WARNING!

MOVING GRAIN AUGERS IN THEIR ELEVATED POSITION MAY RESULT IN
ELECTROCUTION IF THEY CONTACT OVERHEAD POWER LINES WHILE BEING MOVED.
FARM OWNERS AND MANAGERS SHOULD ENSURE THAT AUGERS ARE IN THE LOWERED
POSITION PRIOR TO MOVING THEM.

SUMMARY

This Alert requests the assistance of farm owners/managers,
farm/agricultural workers, and farm equipment manufacturers in the
prevention of electrocutions which may occur while moving metal grain
augers. The grain auger is an essential piece of farm equipment which
is used to move grain from one location to another. However, every
year accidents occur when this piece of equipment is improperly moved
in the elevated position and it comes into contact with high voltage
power lines. This has resulted in one or more fatalities per
incident. This Alert describes two separate incidents that resulted
in five fatalities, and occurred within the same week (150 miles
apart). Neither of the incidents fell under OSHA jurisdiction because
both farms were family operations employing fewer than 10 workers.

July 1986


Unit I—An Introduction to Safety and Some Basic Electrical Engineering Concepts

_RR,
ea “RR,

(Coc)

(ie
Goa )

Now lets apply these concepts to a single circuit.

Where R, = 1000 Q, R, = 500 Q, R, = 250 Q, and V = 110 volts. Let V,, be
the voltage between points a and b, V,4 the voltage between points b and d,
and V,, the voltage between points c and d. We know that

ip =i +i,
Vay =i Ry
Via = Ro = Vea=isRy
and we can calculate the equivalent resistance as follows
RRs (500. 2)(250 2)
Reg = Rit Rar, 7 1002+ ‘59.0 +2502
Ryg = 1166.67 2

Then

V=i,R,,= 110 volts = i,(1166.67 2)

_ _ 10 volts.
11 = 7166.67 Q = 9.0943 amp

0.0943 amp

Hhtls

J-11


Unit 111—Ignition of Flammable and Combustible Materials and Explosives

ELECTRICAL
PHENOMENA AS
AN IGNITION
SOURCE

Electrical
overloading

Industries take special precautions to minimize dust concentration, such as
ventilating, enclosing dust producing processes, or applying dust arresting sub-
stances to keep the dust from mixing to dangerous concentrations in the air. As
a last resort, some high-risk industries design buildings or enclosures with
“blow out” walls and vents to minimize the devastating pressure should an
explosion occur.

Gases and vapors that can produce flammable mixtures with air are commonly
found in industrial settings. Precautions are needed to prevent explosions.
Example gases include hydrogen, acetylene, propane, LP-gas, methane, and
natural gas.

In industry, some commonly used liquids that emit flammable vapors include
gasoline, benzene, naphtha, and methyl alcohol.

From a safety standpoint, every effort must be made to eliminate a hazard at
its source. Refer to the fire triangle or pyramid; controlling a fire hazard could
involve controlling the fuel, heat or ignition energy, oxygen, or chain reac-
tions. Controlling the availability of oxygen can be done by making the atmos-
phere inert in certain situations. Controlling the availability of a fuel and heat
or initiation energy can be done in a number of ways. Although removing and
controlling flammable materials from and in an environment by ventilation,
enclosure, choosing nonflammable materials for processes, etc., are necessary
defenses, such controls are rarely sufficient to control hazards to an acceptable
level. Even if a hazard has been controlled (which is often not possible), a fail-
ure of the controlling safety system may create a dangerous situation.
Therefore, the hazard must also be controlled by eliminating potential ignition
sources.

Electrical phenomena are a common source of ignition, and we now turn our
attention to electrical events that create a fire or explosion hazard and to meth-
ods to control those hazards.

The three principal ways in which electrical installations may cause fires are
electrical overloading; electrical discharges in the form of sparks, arcs, or
coronas; and electrical heating appliances. All are potential ignition sources
for combustible materials and atmospheres. Lightning and static electricity
(forms of electrical discharge) will be discussed separately (Hammer, 1989;
Cooper, 1986).

Electrical overloading exists where the electrical power supplied to a circuit
exceeds its designed levels and this results in excess heat and/or in damage to
components. The increased temperatures may ignite flammable material in the
vicinity, including some electrical insulating materials. A severe overload may
result in a sudden and immediate fire hazard as the overloaded components
release heat or experience failure causing electrical discharges. Less severe or
intermittent overloading may cause material changes and degrade insulation
properties to a point where failure does occur. The property changes and insu-
lation failure may result in excess heat and undesirable electrical discharge
events.

Il-9






DISCLAIMER

Mention of the name of any company or product
does not constitute endorsement by the National
Institute for Occupational Safety and Health.

DHHS (NIOSH) Publication No. 85-111

Additional copies of this alert are available from

Publications Dissemination, DSDIT
National Institute for Occupational Safety and Health
4676 Columbia Parkway
Cincinnati, Ohio 45226
(513) 841-4287

ii


Contents (continued)

List of Figures Page

Figure I-1. In circuit analysis, the positive direction of a conventional current
moves from positive (+) to negative (-). The actual flow of electrons, however,
is from negative to positive............ soesteeeneesneeanennes:

Figure I-2. (a) Direct current is constant over time; (b) ateating current
fluctuates over time... soeeenene
Figure II-1. Schematic ‘showing the path of of the e current in the example. ‘A frayed

wire touches the conducting case of the tool (short circuit), energizing it. The
current passes from the energized case, through the man's arms, exiting at his
shoulder. The current loop is completed through the ground....

1-8

1-9

-2

Figure II-2. Insulating materials are subject to deterioration or ‘damage from electric
stress, temperature extremes and fluctuations, chemical and physical agents, and
mechanical stress. The presence of moisture will facilitate failure. -
Figure II-3. A short circuit can occur (a) between a conducting element and

a grounding conduction body, or (b) between conducting elements in the same circuit ............. 1-9
I-10
I-11

II-6

Figure II-4. (a) An unexpected open circuit may overload parallel components..............

Figure II-4. (b) An open circuit that results from a severed high-voltage power line ......

Figure II-5. A schematic diagram representing the path of current and associated
resistances for the man described in the electric shock case study on page II-2..........0.0.

Figure II-6. In a circuit without a third wire ground, a person may become the
path to ground in the event of a failure. Also note that the current completes a
loop through the ground...

Figure II-7. Schematic of an electric drill coined with a third-wire ground and
plugged into a third wire ground receptacle...
Figure II-8. Receptacles and plugs with NEMA configurations

(adapted from U.S. Department of Labor, “An Illustrated Guide to Electrical Safety,”
Occupational Safety and Health Administration, 1981) .........ssesssssssssssesseeennes

Figure II-9. Examples of incorrect wiring of receptacles
(a) hot and neutral reversed; (b) ground and neutral reversed;
(c) hot and ground reversed (adapted from U.S. Department of Labor, 1981)...

Figure II-10. (a) Duplex receptacle correctly wired to designated terminals
(adapted from U.S. Department of Labor, 1981); (b) schematic of the entire,
correctly wired, three-wire receptacle and circuit sosesseene
Figure I-11. Detecting improper wiring in receptacles: (a) measuring

Vj, V2, and V3 can detect improper wiring conditions (H, N, or G not
connected or N-H or H-G reversed); (b) method used to detect reversed

N-G (adapted from Bahill, 1981)...

Figure II-12. A 0.5 milliampere ground fault causes a current nt flow imbalance
between the hot and neutral wires. Sensing the imbalance, the ground-
circuit interrupter (GFCI) breaks the circuit ...
Figure I-13. Receptacle-type ground-fault circuit interrupter (GFCI)
(adapted from U.S. Department of Labor, 1981).......
Figure I-14. On a double-insulated, shock-proof electric tool, an internal

layer of protective insulation completely isolates the electrical components

from the outer metal housing ..........ccsssssssscsssssssessecssessseesssesssseeesseessssessseessnesnsertecsseesseessecsseees I-27

I-17

11-20

11-20

H-21

1-22

TL-23

11-24

wilt

11-26

U-27

vi


Unit I1V—Safety Standards, Organizations Promulgating Standards, and Professional Liability

American Society
for Testing and
Materials (ASTM)

Underwriter’s
Laboratories

The Institute of
Electrical and
Electronics
Engineers (IEEE)

State and Federal
Government
Regulations

ASTM is the world’s largest source (currently over 8,000 standards) of volun-
tary consensus standards for materials, testing, products, systems, and ser-
vices. The roughly 30,000 members draw on a broad spectrum of interests and
include engineers, scientists, researchers, educators, testing experts, compa-
nies, associations, institutes, government agencies, consumers, and even
libraries.

ASTM standards cover many topics of interest to electrical engineers, includ-
ing, for example, material specifications and testing procedures regarding elec-
tronic equipment and protective equipment for electrical workers (National
Safety Council, 1988).

Underwriter’s Laboratories (UL), a not-for-profit organization, maintains labo-
ratories where the compliance of a product, device, system, or material with
specific safety standards is tested. It is not an enforcement agency, but it does
issue labels certifying that UL criteria have been met.

UL publishes annual directories listing the manufacturers whose products have
met the appropriate safety standard criteria and whose products are covered
under the UL’s Follow-Up Services program. UL inspectors make at least four
unannounced follow-up site visits to companies making UL-Listed products to
evaluate the effectiveness of a manufacturer’s quality control for continued
compliance with appropriate safety standards.

It is important to realize that UL-listed products are only tested under specific
conditions set forth in particular standards. Satisfactory performance under the
prescribed conditions may have no bearing on performance or other factors
outside the scope of the UL investigation. That is, any use of the product out-
side the conditions prescribed by the criteria set forth in the particular standard
may present a hazard (National Safety Council, 1988).

The IEEE is the world’s largest technical professional society. Founded in
1884 by a handful of practitioners of the new electrical engineering discipline,
today’s Institute is composed of over 52,000 students and 320,000 members
who conduct and participate in its activities in more than 130 countries.

In addition to its standard activities, the IEEE keeps its members abreast with
their field through numerous conferences, journal publications, and education-
al services. The IEEE has 35 specialized technical Societies with interests that
span the many disciplines that encompass electrical, electronics, computer
engineering, and computer science. IEEE is not just for electrical engineers—
its publications and services span a wide variety of topics of interest and
importance to engineers in general.

During the first half of the 20th century, the federal government’s involvement
in safety legislation was largely limited to setting safety and health standards for
its contractors. The Walsh-Healy Public Contract Act of 1936 provided that con-
tracts in excess of $10,000 entered into by an agency of the United States pro-
hibit the use of materials “manufactured in working conditions which are unsan-
itary or dangerous to the health and safety ‘of the employees” (Bloswick, 1992).

IV-7


AN INTRODUCTION TO ELECTRICAL SAFETY FOR ENGINEERS

CONTROLLING
THE SHOCK
HAZARD

Insulation

Interlocks

equipment directly in contact with the heart muscle. A shock hazard that
would pose little danger in a normal environment can be lethal in these situa-
tions. In certain industries or situations, therefore, special precautions, through
design and procedure, are needed to minimize shock hazards.

Up to this point we’ve discussed the hazards that electrical shock present to
personnel and equipment. Fortunately, engineers and designers have learned
from past failures and applied their expertise to develop methods that elimi-
nate or significantly reduce specific hazards. As an engineer, your responsibili-
ty will be to understand and apply known hazard controls and to develop new
methods as your experience and expertise grows.

Here are some of the more common methods used to protect people from elec-
trical shock. A system is usually designed with some combination of these fea-
tures so that a failure of one method will not immediately result in a shock
hazard.

We know that a person can be fatally injured by contacting a bare energized
conductor or that equipment can fail or be damaged when energized conduc-
tors contact in unintended ways. Such hazards can be significantly reduced or
eliminated by insulating the conductors in a system. The hazard can be further
reduced by insulating all parts and equipment that a person will contact rou-
tinely or by accident during operation or maintenance of a system. Control
switches, buttons and knobs, meters, and handles are typical examples of items
that should be insulated. Also recall that insulation can and does break down
with time, use, system, and environmental factors. Therefore, selecting an
insulation material that will retain its insulating properties during foreseeable
use is critical.

Insulated conductors are marked by the manufacturer to show the maximum
voltage, American Wire Gage size, the type letter of the insulation, and the
manufacturer’s name and location. Equipment grounding conductors are gen-
erally colored green or a green with yellow stripes. Insulation on neutral wire
is generally white or natural gray color, and hot wires can be any color other
than green, gray, or white (often they are colored red or black).

Personnel can also be insulated from electrical systems through the use of non-
conductive floor mats (e.g., rubber) and nonconductive gloves and shoes. This
is especially critical during maintenance procedures that may bypass insulation
designs intended to protect an operator.

An interlock automatically deenergizes a system when a protective enclosure
is opened. Just as opening the door to your clothes washer stops it when it is in
the spin cycle, an interlock will deactivate the power to a system when some-
one opens the door or cover of an enclosure. Because enclosures are often
used to keep personnel from accidently contacting the energized components
within, an interlock still protects people if the enclosure is opened.

I-18


Request for Assistance In

NIOSH
ALERT

Preventing Electrocutions of Workers Using Portable
Metal Ladders Near Overhead Power Lines

The National Institute for Occupational
Safety and Health (NIOSH) is requesting
assistance in preventing electrocutions
that occur when portable metal ladders
(including aluminum ladders) contact
overhead power lines. Portable metal lad-
ders are used widely in many industries.
This Alert describes six deaths that oc-
curred because portable aluminum lad-
ders, which are electrical conductors,
came in contact with energized overhead
power lines. If nonconductive ladders had
been used instead, or if safe working
clearances had been maintained, these
deaths might have been prevented.

Specific Occupational Safety and Health
Administration (OSHA) regulations govern
the use of portable metal ladders. These
regulations should be implemented and
enforced by every employer, manager, su-
pervisor, and worker in operations that use
portable metal ladders. Editors of ap-
propriate trade journals, safety and health
Officials, and other persons (especially
those in the building trades) are requested
to bring the recommendations in this Alert
to the attention of contractors and
workers.

BACKGROUND

Contact between portable metal ladders and
overhead power lines causes serious and
often fatal injuries to workers in the United
States. Data show that during the years 1980
through 1985, the contact of metal ladders
with overhead power lines accounted for ap-
proximately 4% of all work-related electrocu-
tions in the United States (e.g., 17 out of 382
deaths for 1985) [NIOSH 1988].

REGULATIONS

Safety regulations promulgated by the Oc-
cupational Safety and Health Administration
(OSHA) establish specific requirements in-
tended to prevent workers from positioning
portable metal ladders where they might con-
tact electrical conductors [29 CFR*
1926.450(a)(11) and 1926.951(c)(1)]. These
regulations stipulate that "portable metal or
conductive ladders shall not be used for
electrical work or where they may contact

*Code of Federal Regulations. See CFR in references.




Unit II—Electric Shock Hazards and Controls, and Inadvertent Activation of Equipment

hot and neutral prongs. This feature ensures that the ground will always becon-
nected before the hot and neutral energize the equipment. Thus, if there is a
fault in the equipment, the ground will already be connected to safely carry the
fault current to earth. Similarly, when the plug is being removed, the ground is
the last connection to open.

The utility of these design features is only effective if the receptacle is correctly
wired to the power system. A potential and unfortunately all too common prob-
lem is reversing wires during installation. If a fault (i.e., a short energizing the
casing) occurs within a drill, the ground wire of a correctly wired system carries
the current safely away from the person (Figure II-7). When wires are reversed
at the receptacle, however, unexpected and possibly deadly situations may
arise. Figure II-9 illustrates three possible miswired cases: reversed polarity
(reversing the hot and neutral or hot and ground wires); reversing the ground
and neutral; and reversing the hot and ground.

15-Ampere plug,
15 Ampere
Either,
20 Amy
Gy
a IY
@& a) SP

20-Ampere plug,
125-Volt receptacles
and plugs

240-Volt, 30-ampere
receptacle and plug

y a's +207240-Volt, S0-ampere TM C5 OS
receptacie and plug

10-50R 10-50P

A= Receptacle
P= Plug

Figure II-8. Receptacles and plugs with NEMA. configurations (adapted
from U.S. Department of Labor, “An Illustrated Guide to
Electrical Safety,” Occupational Safety and Health
Administration, 1981).

Ti-21