No. 474 August 1998

U.S. Department of Health and Human Services

Public Health Service

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

A Passive Means To Detect Hot Trolley Insulators 

Objective

To devise a passive means to detect overheating insulators on

mine trolley/track haulageways.

Background

Electrical trolley systems are used for both mineral haulage and

movement of personnel and supplies in nearly 50 U.S. mines,

mostly in southwestern Pennsylvania and West Virginia. These

systems use a trolley wire, energized at 300- or 600-V dc, to power

vehicles constrained to run on a network of permanently installed

steel track. The bare copper trolley wire along with a feeder wire

are suspended about every 6 m from insulators that are anchored

into the roof by bolts. The insulators prevent current from flowing

to the grounded return track via the overlying strata.

Coal and rock dust accumulations, as well as acidic drainage

and condensation, may jeopardize insulator integrity. Leakage

currents from the trolley wire into the roof strata can heat the

insulator, as well as the immediate area in which the suspended

bolt is anchored. If not detected and corrected, this may result in

ignition of roof coal and a catastrophic mine fire.

An insulator and surrounding strata that are subject to leakage

currents may be discolored, have an odor, or exhibit no physical

evidence of deterioration. Leakage can be confirmed through

voltage measurements across the insulator. However, this can be

a time-consuming, tedious task considering the thousands of

insulators distributed over miles of haulageway. A portable

infrared detector can be used to scan for heat on the insulators and

in the roof from a slow-moving vehicle. This can be effective

when done regularly, but this method cannot detect impending

failures between examinations. An insulator, integrally designed

to give some indication of the presence of leakage currents, could

be detected and replaced promptly.

Approach

Trolley systems in underground coal mines are diminishing in

number as new mines opt for belt haulage and diesel power.

Consequently, those mines still employing trolley and track for

haulage tend to be older with a limited operational life. The

haulageways in these mines must still be maintained, but there is

little incentive from mine management's view for improvement.

Wholesale replacement of system components such as insulators

is simply not justified economically. Accordingly, any thermal

indicating means for trolley insulators must be easily retrofitted on

existing insulators. Also, it must be inexpensive compared to the

cost of a new insulator.

To be effective in preventing fires, a thermal indicator must

activate in the presence of leakage currents at the lowest practical

temperatures. However, it must not react to other sources of heat,

such as idling mine locomotives. Reliability dictates that it be

simple in both design and function. Ideally, it should be a passive

device that requires no external power to operate.

How It Works

The National Institute for Occupational Safety and Health

(NIOSH), Pittsburgh Research Laboratory, designed a passive

device to detect overheating insulators on mine trolley/track

haulageways (figure 1). The hot insulator detector features a

cartridge containing a spring-loaded streamer, which is brazed to

an adjustable hose clamp. The clamp is installed around the

insulator housing. When leakage currents cause the insulator to

become elevated in temperature, heat is transferred via the clamp

to the cartridge. Temperature-sensitive wax seals hold the spooled

streamer in place. At the melt temperature specified for the wax,

the spring ejects the spooled streamer out of the cartridge. Gravity

pulls the nonconductive streamer downward, where air currents

along the haulageway cause it to flutter noticeably. This design is
 Figure 1.CActivated detector installed on trolley

insulator.

simple and is easy to install on insulators in service. It has the

potential to be inexpensive, while providing a recognizable

warning signal to vehicles traveling the haulageway.

Test Results

The thermal indicator must react before heat generated by

leakage currents through resistive paths can ignite nearby

combustibles. These paths may be present on the surface of the

insulator in the form of moisture and dirt. In the case of a cracked

insulator, the resistive path for leakage current may be internal.

In addition, heat may be generated in the roof as the current seeks

to return to the grounded rail. To preclude ignition of coal dust

accumulations on external surfaces of mechanical or electrical

components, Title 30 of the Code of Federal Regulations imposes

a 150 °C limitation. In addition, some dc trolley insulator manufacturers specify a maximum operating temperature of 121 °C.

Due to thermal resistance, an indicator brazed to a clamp wrapped

around the insulator housing will lag in temperature rise.

Consequently, the temperature at which the device activates must

be less than limitations imposed on the insulator housing. 

Laboratory tests with heat sources above and below the insulator

quantified this temperature gradient as 35 °C. For maximum

sensitivity, the device activation temperature (the rating of the wax

pellet) was chosen at 65 °C.

Additional tests gauged the reliability of the design. One

hundred wax-sealed cartridges were mounted on an aluminum

panel and inserted into an air oven. The temperature in the oven

was gradually increased from ambient. Ninety-seven of the

cartridges successfully ejected the Teflon streamer within ±5 °C

of the target temperature of the wax. An examination of the three

that failed to activate revealed deformities in the brass cartridges

that hindered spool ejection. Careful packaging and handling of

the device should preclude this damage in storage and transit.

Condensation may form on underground surfaces during the

summer months when warm, humid air is drawn into the mine.

This moisture may cause sudden arcing across trolley insulators,

especially on 600-V systems. Laboratory tests were conducted to

determine if the thermal indicator would activate in the presence

of an electrical arc and if the indicator would be damaged by the

arc. The rapidly escalating heat of the test arcs activated the

insulator detector nearly instantaneously. In some cases, the

Teflon streamer remained intact after arc interruption. However,

most of the time the arc energy severed the streamer and it fell to

the ground. Consequently, the insulator detector is not effective

in the case of an arcing trolley insulator.

Patent Status

An application for a patent on this device has been filed.

For More Information

To obtain a free copy of a technical paper on this topic or for

additional information, contact Michael R. Yenchek, NIOSH

Pittsburgh Research Laboratory, Cochrans Mill Rd., P.O. Box

18070, Pittsburgh, PA 15236-0070, phone: (412) 892-6099, fax:

(412) 892-4384, e-mail: mby5@cdc.gov

To receive additional information about mining issues or

other occupational safety and health problems, call 1-800-

35-NIOSH (1-800-356-4674), or visit the NIOSH

Home Page on the World Wide Web at http://www.cdc.

gov/niosh 

Mention of any company name or product does not

constitute endorsement by the National Institute for

Occupational Safety and Health.