An Alarm to Warn of Overhead Power Line Contact by Mobile Equipment

G. T. Homce, J. C. Cawley, Senior Member IEEE,

M. R. Yenchek, Senior Member IEEE, and H. K. Sacks, Member IEEE

National Institute for Occupational Safety and Health

Pittsburgh Research Laboratory

P.O. Box 18060

Pittsburgh, PA 15236 USA

Abstract- As inmanyotherindustries,suchas construction and

agriculture, overhead electric powerlines are a serious hazard at

mining and mining-related operations. Even when excluding

injuries that occur during electrical maintenance work, over one

fourth of electrical fatalities in the mining industry are due to

accidental overhead line contacts, and for eachfatalitynearly two

serious non-fatal injuries occurdue to such contacts. In incidents

involving high-reaching mobile equipment, many of the victims

touched the equipment after the fact, unaware that the machine

frame had become energized by the line contact. MSHA data for

accidents involving overhead power line contacts in the mining

industry between1980 and 1997 reveal that in 57% of the cases

personnel were unaware of the accidental line contact until after

one or more workers touched the equipment or a hoisted load

and were injured. The situation is similar in the construction

industry, where according to the National Institute for

Occupational Safety and Health (NIOSH) data at least 20% of

such victims are unaware of the line contact. This suggests that

a device that alerts workers when a power line has been

contacted could help prevent many of these injuries. Such a

device would not prevent power line contacts, yet if widely

employed could yield a significant reduction in the number of

resulting injuries. Researchers at the NIOSH Pittsburgh

Research Laboratory are attempting to develop such a device.

The approach being investigated is based on measuring electric

current flow to ground through a machine during a line contact.

The specific technique being tested involves the diversion of

some part ofthis current througha shunt cable mountedon board

the machine, to provide a point at which to install a current

sensor. Experiments thus far indicate that this approach is

feasible. Ongoing research is better defining electric current flow

throughmobile equipment, refining techniquesformeasuring this

current, and identifying factors that could limit the effectiveness

of the proposed alarm.

INTRODUCTION

Common overhead electric power line contact incidents

involve operators and nearby workers around cranes, dump

trucks, drill rigs, and other highreachingmobile equipment. In

typical cases, workers lifting and placing aerial loads with

cranes, maneuvering trucks with raised beds, and placing

drilling equipment into position fail to recognize their

proximity to power lines of which they may or may not be

aware. When a machine contacts a bare overhead power line

conductor, the frame becomes energized to approximately

line-to-groundvoltage,often rangingfromafewthousandvolts

to more than 10 kV. Three possible scenarios can lead to

injury under this circumstance. In one, workers guiding a

suspended load or otherwise in direct contact with both the

machine and ground immediately become a path for electric

current, as illustrated in Fig. 1. In another, equipment

operators are not aware of the line contact, or may perceive

themselvesto be in immediate danger and attempt to dismount

the vehicle, simultaneously bridging the high voltage between

the equipment and ground. Finally, nearby workers who may

not realize a serious electrical hazard exists, try to help those

involved in the incident, and in doing so contact energized

equipment or victims. Analyses of line contact incidents in the

mining industry however, suggest that many of the resulting

injuries could have been prevented by a device that simply

alerts operators and other workers that a line has been

contacted. Researchers at the National Institute for

Occupational Safety and Health Pittsburgh Research

Laboratory(NIOSH, PRL) are attempting to developa reliable

and practical alarm that will alert workers on or near high

reaching mobile equipment when an overhead electric power

line has been contacted.

BACKGROUND

The U.S. Department of Labor’s Bureau of Labor Statistics

(BLS) compiles the Census of Fatal Occupational Injuries

(CFOI).1 BLS also compiles the Survey of Occupational Injury

and Illnesses (SOII), a stratified sample of nonfatal

occupational injury data in the U.S. Table 1 shows BLS

electrical injury data for all industries from 1992-1998. A

total of 32,309 “days lost” nonfatal electrical injuries and

2,268 electrical fatalities occurred during this period. This

table also shows that 1,220 (3.8%) of the nonfatal electrical

injuries, and 933 (41%) ofthe fatal electrical injuriesinvolved

contact withoverhead power lines. This indicates that although

relativelyfewoccupational electrical injuries involve contact

withoverheadpowerlines,injuriesresultingfromline contacts

are muchmore likely to be fatal than is the case for other types

of electrical accidents. Three industry sectors, construction,

agriculture andminingaccountedfor 1,323 (58%) ofthe 2,267

occupational electrical fatalities from 1992-1998, with 46%

1

 Data for the Census of Fatal Occupational Injuries are compiled from various

federal, state, and local administrative sources, including death certificates,

workers' compensation reports and claims, reports to various regulatory

agencies, medical examiner reports, and police reports, as well as news

reports.
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 TABLE 1

 ELECTRICAL INJURY DATA, ALL INDUSTRIES (1992-1998)

Electrical Injuries

 Year

1992 1993 1994 1995 1996 1997 1998 Total

Nonfatal electrical injuries (total) 14,806 4,995 6,018 4,744 4,126 3,710 3,910 32,309

Overhead power line contact 1174 133 273 155 92 79 314 1,220

Percent of Nonfatal Electrical 3.6% 2.7% 4.5% 3.3% 2.2% 2.1% 8.0% 3.8%

Injuries

Fatal electrical injuries (total) 334 325 348 348 281 298 334 2,268

Overhead power line contact 140 115 132 139 116 138 153 933

Percent of Fatal Injuries 42% 35% 38% 40% 41% 46% 46% 41%

1. Nonfatal injuries involving days away from work.

Source: BLS - CFOI, SOII, 1992-1998.

Figure 1. Typical electric current paths during

an overhead power line contact incident.

of these due to contact with overhead power lines. This is an

average of 87 overhead power line contact deaths per year in

these three sectors.

Although most overhead line related injuries were due to

direct contact or contact through a handheld object such as a

ladder, a significant number involved mobile equipment. For

example, 316 (33.8%) of the 933 overhead power line injury

narratives in CFOI (cited above) mention a truck or crane.

Similarly,datafromthe Mine SafetyandHealthAdministration

(MSHA) indicate that 13 (17%) of the 75 electrical fatalities

reportedin the U.S. mining industry for the period 1990-1999

involved a crane, derrick, cherry-picker, or boom hoist.

CFOI data also show that 19 different job titlesreported 10

or more overhead power line contact fatalities (705 of 933

reported fatalities) between 1992-1998. Based on 1998 BLS

employment figures for those job titles, more than 6 million

U.S. workers are exposed to overhead power line hazards.

EXISTING TECHNOLOGY

Most recommendations regarding safe working distances

from energized overhead power lines are based on those

contained in the National Electrical Safety Code (NESC).

MSHAmandatesseveral preventiontechniquesfor the mining

industry in Title 30, Code of Federal Regulations, Parts

56.12071 and 77.807-1 through 77.807-3. OSHAhas similar

recommendations also based on the NESC. A NIOSH Alert

“Preventing Electrocutions of Crane Operators and Crew

Members Working Near Overhead Power Lines” suggests

techniques to use when working near overhead power lines.

These include de-energizing lines, maintaining appropriate

distancesfromenergizedlines, use of an observer to warnthe

operator of impendingcontact,andbarriersto prevent physical

contact withan energizedline. Like most overhead power line

contact prevention techniques, all of these rely on the active

participation and heightened awareness of the machine

operator and crew. 

Rossignol points out that,while training solutions are often

suggestedfor electrical hazards,interventioneffortsmustshift

toward engineering control solutions “to reduce the hazard at
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its source” [1]. Manuele suggests that a change in the attitude

of behavioral scientists is slowly occurring, placing greater

emphasis on engineering controlsolutions. He attributes 60%

ofsafetyproblemsto “facilityandequipment”deficiencies[2].

While training cannot be overlooked, it should be one part of

an intervention strategy that includes appropriate engineering

controls. Indeed, techniques that require little or no

participation by personnel often afford the greatest benefits.

At least one commercial device is currently available which

is advertised to detect proximity to an energized overhead

power line [3]. Using electric field sensors mounted on the

protected machine, the device is designed to sense electric

fields that surround an overhead power line and to warn when

protected machine projections are less than a preset distance

from the power line. It is reported to be effective in many

applications,but the manufacturerstressesthat personnel must

fully understand the device’s operation and limitations. The

manufacturer also points out that the device should not be

relied upon as the primary means of power line contact

prevention, but should supplement a comprehensive safety

strategy. Ultimately, despite being available for decades,

power line proximity warning devices have not found

widespread acceptance,due to their technical and operational

limitations.

Another protection technique, the use of an insulating load

link in the hoisting line of a crane,is intendedto prevent injury

to workersin contact with a hoisted load in the event of a line

contact. Analysis of past line contacts suggests that

widespread use of such links could reduce injuries. However,

surface contamination and moisture can reduce a load link’s

insulationresistance. In addition, workers in contact with parts

of the crane other than the isolated load would be unprotected

by a load link [4]. The relatively high cost of load links also

limits their acceptance by industry.

CFOI narratives for 1992 to 1998 indicate that 83 boom

truck (a small crane or hoisting device mounted on a flatbed

truck) operators or support personnel were involved in fatal

overhead power line accidents. For this type of equipment,

operators often stand on the ground next to the bed and

manually operate the boom controls. Should a power line be

contacted, the flow of current through the operator at the

controls can be immediate. One effective solution sometimes

employed on this type of equipment is replacing truck-side

mountedmanual controls witha radio link remote control that

electrically isolates the operator from the truck.

POWER LINE CONTACT ALARM CONCEPT AND APPROACH

While existing proximity warning and insulating load link

technology have not found widespreadacceptance for a variety

of performance- and cost-related reasons, closer study of

overhead line contactincidentssuggeststhatmany line contact

injuries and fatalities couldbe avoidedsimplyby knowing that

mobile equipment has become energized by contacting an

overhead power line. A review of NIOSH FatalityAssessment

Control and Evaluation (FACE) reports for overhead power

line contactfatalitiesinthe construction industryindicatesthat

at least 20% of fatalities could have been prevented by

knowing that the machine chassis had become electrically

energized, allowing workersto avoid contact (about 92 fewer

fatalitiesfrom1992-1998). Similarly, an unpublishedNIOSH

studyexamined1980-1997MSHAdatafor accidentsinvolving

overhead power line contacts in the mining industry [5].

MSHA accident narratives revealed that in 57% of the cases,

personnel were unaware of the accidental line contact until

after one or more workerstouched the equipment or a hoisted

load and were injured. Based on these findings, NIOSH

researchers are attempting to develop an alarm that warns

mobile equipment operators and nearby workers whena power

line has been contacted.

For a line contact alarm to be effective and reliable, it

should be suitable for use on the types of mobile equipment

most frequentlyinvolvedinsuchincidents,workreliably under

a wide range of conditions, function with minimal attention

from personnel, be relatively inexpensive, and be easily

retrofitted to existing equipment. Focusing on these

characteristics, the approach being investigated is based on

measuring electric current flow to ground through a machine

during a line contact. The specific technique being tested

involves the diversion of some part of this current through a

shunt cable mounted on board the machine, to provide a point

at which to install a current sensor.

Most high reaching mobile equipment employ at least one

weight bearing joint that rotates in a single plane, such as the

pivot on a dump truck bed or the base of a crane boom. Such

a joint can be bridged with a heavy, flexible electrical

conductor monitored by a specially-constructed current

sensing transformer (CT). Current passing through a machine

from an overhead power line contact will flow to ground

through its tires and/or stabilizer jacks. Since the pivot joint

has some finite resistance, part of this current flows through

the shunt cable and is sensed by the CT. The signal from the

CTis processedbyelectronicsin the alarmdevice to trigger an

audible and visual alarm, warning both the equipment operator

and nearby workers of the potentially lethal electrical hazard

presented by simultaneously touching the machine frame and

ground. U.S. and Canadian patent applications have been filed

for the current sensing overhead power line contact alarm.

One critical issue for a line contact alarm isreliability, and

an important aspect of reliability is the ability to withstand the

high machine frame voltage and current flowpossible during a

power line contact. The current-sensing system proposed
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however, is inherently resistant to damage from such

conditions. Specifically, points on board mobile equipment to

which alarm system components will be mounted may be at a

high potential relative to ground, but there will likely be

relatively little voltage difference among them. Since the

proposed alarm system has no components that must be

maintainedat ground potential, exposure to highvoltage should

not pose a problem. In addition, the use of a window type CT

protects downstream system circuitry from high currents, by

limiting the amount of energy that can be transferred. This

leaves only the proposed pivot joint shunt vulnerable to

possible high currents, requiring low resistance and rugged

construction.

NIOSH RESEARCH

A. Initial Feasibility Study

A 1998 NIOSH feasibility study first examined this

approach [6]. This work looked at electric current flow to

ground through a small crane and dump truck parked on grass,

a gravel road, and a limestone quarry floor, and produced two

important findings. When these pieces of mobile equipment

were energized at voltages up to 950 Vac (a level muchlower

than typical overhead lines), measurable currents flowed

through the tires and/orstabilizer jacks to ground. Also, some

portion of the current flowing could be divertedaround a pivot

joint on the machine, using a shunt cable. In general, test

results suggest that the concept and approachare feasible, but

experiments also pointed out that the road surface

conductivity,the contactresistance betweenthe vehicle andthe

road surface, and the bulk resistivity of the earth as a return

pathfor electric current are all critical parametersthat need to

be better understood. Grass-covered earth for example, is a

good conductor (i.e., has lowresistivity) andequipment parked

on grass had the highest current flow to earth. In the case of

the test crane parked on grass, only 105 volts was required to

achieve 2 amperesof current flowto ground. A gravel road and

a quarry floor (a massive horizontal limestone formation) had

progressively higher resistivities and therefore limited

currents to successively lower magnitudes. A limiting case

was observedwiththe crane parked onthe quarryfloor withits

stabilizer jacks deployed. The small contact area of the jack

pads onthe hard,high resistivity limestone surface reduced the

current flow to low levels, as little as 23 milliamperes at 950

Vac.

B. Ongoing Experimental Program

While initial researchsuggestedthat an equipment-mounted

line contact alarm is feasible, further work is underway to

study the concept in more detail. Specifically, ongoing

research is better defining electric current flow through

mobile equipment, refining techniques for measuring this

current, and identifying factors that could limit the

effectiveness of an alarm based on detecting on-boardcurrent

flow. Should results warrant, a prototype alarm will be

designed and built to aid in promoting this concept for

commercial development. The experimental program under

this research has been designed to address several key issues.

1) In a mobile equipment-power line contact, immediate

electric current flow through the machine frame to ground

must be sufficient to allow detection and activate an alarm.

This must be true for the types and sizes of mobile equipment

most at risk for line contact, and for the types of roadways and

work surface conditions likely to be encountered.

2) It must be possible to detect machine frame current flow

in a simple and reliable manner. This research proposes

monitoring current through a conductor shunting a pivoting

joint (e.g. the pivot of a dump truck bed). The shunt must

therefore,under normal operating conditions,divertsufficient

current from the joint to activate an alarm.

3) The above approachmust be implemented in a design and

package that is reliable, inexpensive, operates with minimal

attention from personnel, and can be retrofitted to a wide

variety of equipment.

Another issue being examined is the possible presence of

dangerous voltage drops on-board equipment during a line

contact. This information is not critical to the development of

the proposed line contact alarm, but does relate to worker

safety under conditions where such an alarm is intended for

use.

C. Total Machine Current Experiments

Aspecializedtest site has been constructed at the Pittsburgh

Research Laboratory to study the total electric current flow

through equipment during line contacts. The experiments are

designed to measure total current while varying the specific

mobile equipment involved, the roadway or work surface

involved, and the voltage applied to the equipment frame. The

facility and test plan are described below.

1) The test site is a remote grass-covered hilltop, several

hundred feet from the nearest roadway or actively used

structure. The soil has a measured resistivity of approximately

61 Ohm m (200 Ohm ft), and there are no known buriedpipes,

conduit, or debris in the area. Based on the soil resistivity, a

driven-rodground bedwas designedand installed to supplya 5

Ohm (maximum) path to remote earth.

2) Test “pads” have been constructed at the site to replicate

commonroadway or workarea surfaces. These are a minimum

of 69 m (226 ft) from the ground bed, and include an
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Fig. 3. 20 tonne (22 short ton) rough terrain crane used in experiments.

Fig. 2. 20.9 tonne (23 short ton) tractor-trailer

dump truck used in experiments.

undisturbed grass covered surface, bare earth, compacted

crushed stone, and asphalt. The pads are 9.1 m (30 ft) by

15.2 m (50 ft) and level, with the crushed stone and asphalt

surfaces constructed to meet Pennsylvania Dept. of

Transportation standards for road construction. The addition

of a concrete pad is scheduled for early 2001.

3)Mobile equipment for these experiments will be selected

based on the types and sizes most frequently cited in

descriptions of past overhead line contacts. They include

mobile cranes, with emphasis on rough terrain cranes and

boom trucks, tandem and tractor trailer dump bed trucks, and

drill rigs on truck chasses. The variety and number of

machines tested will depend on time available and funding.

4)The highvoltage powersource for these experimentsuses

a 6,500 W portable generator to supply an adjustable

transformer which in turn powers a custom built high voltage

step up transformer. The final 60 Hz ac output is adjustable

from 0 to 9,000 V. Digital multimeters and a high voltage

probe are used for current and voltage measurements.2

Due to test site construction delays, this series of

experimentshas notyet been conducted. The test plan calls for

attachingthe high voltage source acrossthe ground bed and the

frame of a machine parked on one of the test pads, then

graduallyincreasingthe potential impressed across this circuit.

The applied voltage and the resulting current will be

documented. This procedure will be repeatedfor each specific

piece of mobile equipment on each test pad. In the case of

equipment with stabilizers, such as cranes or drill rigs, tests

will be conducted both with and without stabilizers deployed.

Themaximumvoltage andcurrent levelsto whichequipment

is subjected in these tests are critical. Ideally, the proposed

line contact alarm should activate if 2,300 Vor less is applied

to amachine frame,withthisvalue correspondingto the line to

ground voltage for a nominal 4,160 V overhead power line.

Minimum required total current flow however, will be

determined by the design and limitations of the contact alarm.

Preliminary resultsso far suggest a total machine ground fault

current lower limit of approximately600 mA, based primarily

on the amount of current a shunt can divert from a pivoting

joint (discussed in the next section). These factors will

influence the maximum voltage used for each individual test,

but that level may, inmany cases, be dictated by the power that

can be dissipated safely across mobile equipment tires.

D. Shunt Current Experiments

Aseries of tests has been completed examining the carried

performance of shunts bridging pivoting joints on mobile

equipment. Experiments measured the amount of current by

such a shunt when electric current was passed through the

frame of a piece of equipment. These tests were conducted as

follows.

1) Mobile equipment used included a 10.9 tonne (12 short

ton) tandem axle dump bed truck, a 20.9 tonne (23 short ton)

tractor trailer dump bed truck shown in Fig. 2, a 20 tonne (22

short ton)roughterrain crane shown in Fig. 3, and a 31.8 tonne

(35 short ton) industrial crane. All are government owned

equipment available at PRL.

2) On the trucks, a shunt was placed from the dump bed to

the truck frame, and on the cranes, betweenthe base section of

the boom and the main boom pivot supports on the turntable.

Except as noted in results, the shunt was a 91cm(36 in) piece

of #2/0 AWG stranded copper cable, with clean, tight

mechanical connections using C-clamps.

2

 All measurement instrumentation is annually calibrated to NIST traceable

standards.
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Fig. 4. Shunt cable and current sensor on boom pivot of 20 tonne

(22 short ton) rough terrain crane used in experiments.

3) The sensor used to monitor current through the shunt was

awindowtype current transformer (CT) with a ratio of 100/1,

designed to measure currents as low as a few milliamperes.

The CT output was terminated across a nominal 100 ohm

resistor,andthe voltage drop acrossthe resistormeasuredwith

a digital multimeter. The shunt cable and CT in place on the 20

tonne (22 short ton) crane are shown in Fig. 4.

4) An adjustable 60 Hz ac powersource was used to flowup

to 10 A through the equipment under test. The “hot” lead was

attachedto the top front edge of the bed (cab protector) on the

trucks, and on the tip of the boom on the cranes, while the

powersupplyreturnwas attachedto the frame onthe trucks and

the outrigger support framework on the cranes. Total current

was monitored directly on a digital multimeter.

5) In general,tests were conducted by passing 10 A nominal

total current through eachmachine andmeasuringflowthrough

the pivot joint shunt. Parameters that were varied for the boom

or bedineach test includedheight (vertical angle),motion, and

loading. Other factorstestedwere shunt size (resistance) and

the effect of wet weather.

Results from the dump truck tests are summarizedin Table

2. The test loads for the 20.9 tonne (23 short ton) and 10.9

tonne (12 short ton) trucks respectively were approximately

14.5 tonne (16 short ton) and 9 tonne (10 short ton). Bed

angles 

ranged from horizontal to approximately 50 degrees. Beds

were raised and lowered several times prior to data collection,

and readings were recorded at 10 degree intervals over 3

raise/lower cycles.

Although the amount of current through the shunt varied

widely with changes in truck load and bed position, the level

for any given set of conditions during testing was generally

stable,and current level changesfromone test conditionto the

next were typicallygradual and uniform. A total current of 10

A gave a voltage drop of less than 2 V for the complete test

circuit.

Resultsfromthe crane tests are summarizedinTable 3. The

20 tonne (22 short ton) rough terrain crane is aGrove RT-522,

and for the tests done under load, was configured for lifts at

38% and 73% of maximum loadchart capacity. The 31.8 tonne

(35 short ton) industrial crane is a Grove IND-2535, and was

tested unloaded only, due to pending hydraulic systemrepairs.

Crane load chart limits and overhead clearance in the test

building allowed40 to 50 degrees vertical boommovementfor

unloadedand 38% load tests, but only 5 degreesmovementfor

the 73 % load. Similar to the dump truck tests, booms were

raised and lowered several times prior to data collection.

Readings were taken at approximately 2 degree intervals for

73% load tests, and 10 degree intervals for all others, over a

minimum of 2 raise/lower cycles. 

The shunt current for any given set of conditions during

testing was generally stable, and current level changes from

one reading to the next were typically gradual and uniform. A

total current of 10 A gave a voltage drop of less than 2 V for

 TABLE 2

 RESULTS OF DUMP TRUCK BED PIVOT SHUNT CURRENT TESTS

 Truck Percent of Total Current Flow

 Through Bed Pivot Shunt

 Bed loaded Bed empty

Maximum Minimum Maximum Minimum

20.9 tonne tractor 89.3 % 9.4 % 90.0 % 51.6 %

trailer (23 short ton)

10.9 tonne tandem 88.3 % 7.8 % 87.0 % 9.7 %

(12 short ton)

 TABLE 3

 RESULTS OF CRANE BOOM PIVOT SHUNT CURRENT TESTS

 Crane Percent of Total Current Flow

 Through Crane Boom Pivot Shunt

Under load No load

 Maximum Minimum Maximum Minimum

RT-522 1.9 % 0.9 % 1.6 % 0.8 %

IND-2535 (no test) (no test) 2.1 % 0.9 %
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the complete test circuit. Two other experiments were

conducted to assess shunt performance. One test was run on

the 31.8 tonne (35 short ton) crane, where the 91 cm (36 in)

#2/0 AWG shunt was replaced by three 46 cm (18 in) #4/0

AWG cables connected in parallel (10 A total current as in

previous tests). This reduction in shunt resistance had no

significant effect onthe amount of current carried by the shunt.

Another test, on the 20 tonne (22 short ton) crane, measured

the effect ofsimulated wet weather conditions on the current

diverted by the shunt. In this experiment, the boom pivot area

and lift cylinders on the turntable were subjected to a heavy

water spray while monitoring current through the shunt (10 A

total current and #2/0 AWG shunt as in previous tests). The

boom was also moved through a 20 degree elevation change

during the test. No reduction in shunt current resulted from

the application of water.

E. Shock Hazards On Board Equipment

This research has also investigated the presence of

unexpected hazardous voltages on board mobile equipment

during a line contact. It is generally accepted that, in the

absence of a fire hazard, personnel should stay on board

equipment in the event of a line contact until the

line/equipment frame is de-energize. However, the possibility

of personnelbeinginjuredwhileremainingonboardequipment

in contact with a power line prompted researchers on this

project to investigate whether possible dangerous voltages are

present on crane and truck chasses whenelectric current flows

through them. These experiments used the 20 tonne (22 short

ton) and 31.8 tonne (35 short ton) cranes and the 20.9 tonne

(23 short ton) dump truck described earlier. A power source

was connected to flow10 Adc fromthe boom tip or top front

of the dump bed to a connection on the equipment chassis.

Resultingvoltagesweremeasuredfromthe chassis connection

to key points on the equipment, such as operator controls,

handles, steps, and other surfaces that a worker could contact

while operating or attempting to exit the machine. The results

were extrapolated for a current flow of 1,000 A, and suggest

that, for properly maintained equipment, it is unlikely that an

operatorwillbe exposed to a dangerous voltage during a power

line contact, unless touching the machine and ground

simultaneously. Table 4 summarizes the results.

One exception to these results is the possibility of high

voltage betweenthe tractor and trailer on a dump truck. Using

measurements from the 10 A dc dump truck tests described

above, a 1,000 A extrapolation produces approximately 24 V

betweenaccessible points on the tractor and trailer. This drop

is due to the combinedresistance of the trailer wiring harness

connections and the fifth wheel, although the wiring harness

would likely open under high current. Resistance of the fifth

wheel joint however, is highly variable. In an earlier separate

series of tests, up to 3,500 V ac was placed across the same

truck (including tires) producing 1 Atotal current flow. Under

these conditions, voltage measured between the tractor and

trailer variedwidely,ranging from less than a volt to over 500

V, changing due to physical movement and position ofthe fifth

wheel.

F. Conclusions

The following summarizes the significance of results from

experiments conducted to date.

1) Under conditions where electric current flows through

the framework of a dump truck or crane, a shunt bridging the

dump bed pivot or crane boom pivot (a #2/0 AWG stranded

conductor was used in experimentation) can divert part of this

current, and provides a convenient point at which to mount a

current sensor. 

2) Current flowing through such a shunt is generally stable

for a given boom/bed position and loading, and seems

unaffected by moisture on the pivot joint (e.g. wet weather).

3) The current carried by the shunt however, can be as little

as 0.8 % of the total flow on the machine frame. The current

sensor and downstream electronics anticipated for the

proposed power line contact alarm will likely require a

minimum of 5 mAshunt current. As a result, total current flow

to earth in a line contact, for the mobile equipment and

conditions tested so far, would have to be over 600 mA for

reliable alarm operation.

4) When a line contact occurs, hazardous voltages are

unlikely to develop on board mobile equipment chasses or

structures, with practically all of the voltage drop occurring

 TABLE 4

RESULTS OF MOBILE EQUIPMENT ON BOARD HAZARDOUS VOLTAGE TESTS

Values for 1,000 A dc current through crane and dump truck

chasses, extrapolated from 10 A dc measurements

Mobile Equipment Maximum Voltage Across

 Likely Touch and Step Points

Operator Operator

control area egress path

RT-522 crane 0.7 V 0.4 V

IND-2535 crane 1.2 V 0.0 V

20.9 tonne dump truck 3.4 V 2.2 V

(23 short ton)
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across equipment tires and inthe adjacent earth. Consequently,

notwithstanding an on board fire, the primary hazard is

contacting the equipment and ground simultaneously, and

personnel should not dismount or approach energized

equipment. One exception to the absence of dangerous

voltagesonboardequipment,is the possible hazardous voltage

developedbetweenseparate structures on the equipment,such

as the tractor and trailer of a semi-trailer type truck.

SUMMARY

Researchers at the NIOSH Pittsburgh Research Laboratory

are investigating whether an overheadpowerline contact alarm

for mobile high reaching equipment, based on current

measurement, can be reliable,simple to retrofit, and relatively

inexpensive. If such a device is feasible, this work will then

promote its use in the workplace. Specific tasks to achieve

these goals include accurately defining the electrical

characteristics of cranes, dump trucks,boomtrucks, drill rigs

and other high-reaching equipment that can contact overhead

power lines, construction and testing of a prototype overhead

power line contact alarm, promoting the commercialization

of such a device, and conducting technology transfer to the

private sector through labor organizations, equipment

manufacturers, and publications.

REFERENCES

[1] M. Rossignol, M. Pineault, “Classification of fatal occupational

electrocutions,” Canadian Journal of Public Health, vol. 85, no. 5,

pp. 322-325, Sept-Oct 1994.

[2] F.A. Manuele, “Behavioral safety: looking beyond the worker,”

Occupational Hazards, pp. 86-89, Oct 2000.

[3] SIGALARM TM, www.sigalarminc.com., SigAlarm, Inc., 5224-T W.

State Rd. 46, Sanford, FL 32771, 2001, (Reference to this product

and/or company does not signify endorsement for any purpose by

the authors, NIOSH, or the Federal Government).

[4] G.G. Karady,“Efficiency ofinsulating linkforthe protection of crane

workers,” IEEE Transactions on Power Delivery, vol. 6, no. 1, pp.

316-323, Jan 1991.

[5] G.T. Homce, “An analysis of overhead electric power line contact

accidents in the U.S. mining industry, with recommendations for

prevention,” submitted to the Journal of Safety Research in 2001.

[6] H.K.Sacks,J.C.Cawley,G.T.Homce,and M.R.Yenchek, “Feasibility

study to reduce injuries and fatalities caused by contact of cranes,

drillrigs,andhaultrucks with high tension lines,” Proceedings, IEEE

Industry Applications Society AnnualMeeting, Phoenix, AZ, 8 pp.,

1999.