Document ID: OSHA-2010-0001-0075
Agency: osha
Document Type: Supporting & Related Material
Title: 
Posted Date: 2010-11-12T05:00Z

The following information is provided to OSHA as a result of an agency
request to MACOSH for information on electrical arc flash hazards
including safety abatement methods and case histories. The following
information in full or part is intended to be used in a SHIPS (Safety
and Health Injury Prevention Sheets) document on maritime electrical
safety. 

Submitted by MACOSH on July 14, 2010

Arc Flash

Electrical shock is not the only hazard associated with working on
energized equipment or circuits. Even though electric shock may be the
most recognized hazard among workers, arc flash can cause serious burns
and even death. Arc flash and arc blast are two hazards that are
generated when what is commonly referred to as an electrical explosion
occurs. There are several causes for electrical explosions. Among them
are: 

A short (the inadvertent connection between two or more conductors such
as a wrench falling across the buss work in an energized panel)

A fault (the inadvertent connection between an energized conductor and
ground) 

Making or breaking a connection under a load condition (such as hooking
up shore power with energized shore power leads while the main
switchboard breakers are in the on or closed position). 

The flash is generated by an arc or flashover. The temperature of the
arc can exceed 35,000° F. This is four times hotter than the surface of
the sun. No material on earth can survive this temperature. In fact, not
only do materials melt at this temperature, they vaporize.

Water expands 1,670 times when it vaporizes to become steam. Steam is
well known as a very powerful energy source. Copper expands 67,000 times
as it vaporizes. This vaporization happens instantly during an arc flash
event. Other examples of a solid instantly vaporizing are explosives
such as gun powder or C4. This is where the term “Electrical
Explosion” comes from. This explosion results from the expulsion of
near-vaporized droplets of molten metal from the arc. It also generates
plasma (ionized vapor) outward from the arc for distances proportional
to the arc energy and can create extreme pressures that can actually
knock a person across the room. The magnitude of the explosion is
determined by several factors such as voltage and duration (the amount
of time it takes for the circuit breaker or fuse to clear the circuit).
It is actually possible to generate a greater arc flash at 480 volts
than with higher voltages.

The magnitude of an arc flash is measured in calories per square
centimeter (cm). One calorie is equivalent to the flame of a butane
lighter exposed to one square centimeter the skin for one second.
However, 100 of those flames exposed to 100 square centimeters
(approximately 4”x4”) for one second is still just one calorie. When
you take into account the entire front of a person’s torso, we are
then talking about over 1,000 of those flames. Unfortunately, very few
arc flash events are rated at just one calorie. Flammable clothing is
instantly ignited and the person is dazed, temporarily blinded by the
bright flash, or knocked unconscious by the arc blast. Therefore not
even realizing they are on fire. This greatly worsens the extent of the
injury. Clothing made with synthetic fabric can become permanently
bonded to the skin through burning and melting of the material.

The most effective way to protect from the hazards of electric arc flash
is to de-energize electrical systems utilizing compliant (1910.147,
1910.333, 1915.181) lockout/tag out procedure and verifying the
de-energized condition with approved test instruments.   Until the
de-energized condition is verified or when work must be performed
energized, the appropriate PPE must be utilized.  

Selecting protective equipment for electric arc flash protection while
simple in concept is a complex process, several references can aid in
the selection of the proper equipment.  NFPA 70E, Electrical Safety in
the Workplaces should be consulted to assist in the calculations to
determine if an arc flash hazard is present for a given electrical
circuit  and the selection of PPE necessary to protect against the
hazard.

Properly worn and maintained Fire Resistant (FR) Clothing can be
instrumental in reducing the extent of injury during an arc flash event.
For proper maintenance of FR Clothing, see the manufacturer’s
instructions. There is FR clothing specifically designed for protection
from arc flash. FR rated face shields and gloves can greatly enhance the
protection provided by the FR clothing. DO NOT USE STANDARD PLASTIC FACE
SHIELDS! They will burn and melt causing greater injury. In some cases,
it would be best to utilize a flash suit, which will provide even
greater protection. The tremendous pressure and noise created during an
arc flash event can also cause ear damage. Therefore, it is also
advisable to use hearing protection while working on energized
equipment.

Since arc flash and blast can reach distances of ten feet or greater, it
is advisable to barricade the area and use warning signs around where
the work on energized equipment is taking place in order to prevent
injury to personnel in the area.

Please remember, the only time personnel are permitted to work on
energized shipboard equipment or circuits is when testing or adjusting.
For more information, see 29CFR Part 1915.181(b).

ARC FLASH SCENARIOS

Scenario One

An electrician was tasked to replace a 480 volt motor controller and
locked out the upstream circuit breaker per the company's energy control
policy.   As the electrician removed the power leads to the motor
controller an electric arc flash occurred.  The momentary release of
energy ignited the electrician’s clothing and caused second and third
degree burns to his face, arms, and hands.   The pressure wave created
from the blast threw the electrician against a bulkhead, injuring his
leg. Crew members and coworkers extinguished the fire and provided
assistance until medical aid arrived.  The electrician was diagnosed
with burns over twenty five percent of his body, a broken femur, and
internal injuries.

Although the electrician had locked out the upstream electrical breaker,
he did not to perform a voltage check on each phase at the motor
controller.  The upstream breaker had failed to disconnect one phase,
leaving the work area energized.  While the electrician did not receive
an electrical shock, as he removed the power leads the energized phase
passed closed enough to an equipment ground or grounded structure to
draw an electric arc.  Within milliseconds, thermal and radiant energy
engulfed the electrician and work area.   

Proper selection and use of arc flash rated personal protective
equipment (PPE) would have prevented or reduced the extent of the
electrician’s injuries. Testing to ensure the circuit was de-energized
should also have been done.   

Scenario Two

An electrical worker was assigned to pull additional wiring leads for
new equipment in an existing electrical panel.  While the leads the
worker is installing are de-energized, the supervisor and worker decided
not to de-energize other 240 volt leads in the panel.    Later in the
shift, a mechanic working in the overhead accidently dropped a spanner
wrench that fell into the open panel shorting across the exposed
energized components.  An arc flash created a fire ball momentarily
engulfing the area and ignited a fire requiring a fire response team. 
The worker received second degree burns to his hands and face as a
result of the arc flash.  The mechanic dropping the spanner was also
treated for burns.

Energized electrical panels, cabinets, or enclosures open during repairs
pose obvious shock hazards.  Less obvious arc flash hazards are also
present and can be controlled by either lockout/tag out or by the
selection and use of arc flash rated PPE for hazards present. Electrical
cabinets which are open and energized should be protected from items or
untrained personnel coming in contact with the electrical energy.