Figure 6. Fired cartridge following test in air oven.

appropriateness for mine duty. These tests, in accordance with
MIL Std 810C (Military Standard 810C, 1984), include high and
low temperature which approximate the cyclic temperature
extremes (-29 to +38 °C) that may be experienced during storage
and operation. In addition, a thermal shock test should be
conducted to determine the effect of sudden changes in these
temperature extremes. To measure the effect of warm humid air,
the indicator should be subjected to 38 °C at 95% humidity. A
mechanical shock test would gauge the ability of the device to
withstand drops of up to 91.5 cm that may occur during shipment.
A vibration test of 1.5 g up to 200 Hz would simulate the motion
of common carrier shipping. The device should be exposed to
dust concentrations of 10 mg per m? at both 91.4 and 533.4 m per
min and their effects on operation noted. A corrosion test per
ASTM Standard B117 (The American Society for Testing and
Materials, 1995) with an acidified spray would complete the
evaluation of the assembly. Following exposure, the assembly
operation should be tested through exposure to heat. Once
installed on an insulator, a dielectric strength test (American
National Standards Institute (ANSI) C59.48, 1994; Institute of
Electrical and Electronic Engineers (IEEE), Standard No. 4, 1995)
should be conducted to determine if the addition of the metallic
clamp and assembly affects the original insulation qualities of the
insulator,

SUMMARY AND CONCLUSIONS.

The NIOSH, PRL has devised a passive means to detect
overheating insulators on mine trolley/track haulageways. The
detector consists of a spring-loaded cartridge that ejects a
reflective streamer of white Teflon tape when subjected to
elevated temperatures. The adjustable activation temperature of
the cartridge is determined by the melting of a wax seal. The
cartridge assembly is attached to a stainless steel clamp that can
be easily installed over the outer metallic shell of an existing
wolley line insulator. When an insulator overheats due to ground.
leakage currents, the visible streamer alerts mine personnel

traveling on the haulageway. Laboratory tests showed that when
installed on an insulator, the device can effectively warn of the
presence of heat from above and below. Air-oven tests
established the reliability of a 100-sample lot of the spring-loaded
cartridges at approximately 97%. However, the Teflon streamer
may not remain intact should the insulator be subject to arcing.
Such faults on mine trolley systems are better detected by a neural
network-based algorithm recently devised at PRL (Peterson and
Cole, 1997). Nevertheless, implementation of the research
documented in this report has the potential to minimize the
incidence of overheating insulators and related fires on mine de
trolley systems.

REFERENCES

American National Standards Institute (ANSI) C59.48, 1994,
“Dielectric Breakdown Voltage and Dielectric Strength of
Electrical Insulating Materials at Commercial Power
Frequencies," 26 pp.

Crest Manufacturing Company Applications Manual, 1995,
Lincoln, RI, p. 9.

Dubina, S. F., 1981, Standard for Direct Current Trolley
Insulators. MSHA Investigative Report No. L-120781-3, p. 13.

Gillenwater, B., McCoy J., 1981, “Innovative Techniques in
Trolley Line Power Distribution” (contract J0188083, Foster-
Miller Associates, Inc.), BuMines OFR 49-82, 76 pp.

Hudson, A. J, 1996, Heat-Sensitive Warming Device. Patent
Application MIN/PGH-9604, 17 pp.

Institute of Electrical and Electronic Engineers (IEEE), Standard
No. 4, 1995, "IEEE Standard Techniques for High Voltage
Testing,” 144 pp.

Military Standard 810C, 1984, Environmental Test Methods and
Engineering Guidelines, 145 pp

Peterson, J. S., Cole, G.P., 1997, "Detection on Downed Trolley
Lines Using Arc Signature Analysis," NIOSH, PRL Report of
Investigation No. 9639, 10 pp.

The American Society for Testing and Materials, 195, Standard
BI17-95, Practice for Operating Salt Spray (Fog) Apparatus.
Section 3-Metal Test Methods, Vol. 03.02, Wear and Corrosion,
Metal Corrosion, p. 1-8.

Trelewicz, K., 1981, "Environmental Test Criteria for the
Acceptability of Mine Instrumentation" (contract J0100040,
Dayton T. Brown, Inc.), BuMines OFR, 135 pp.

U.S. Code of Federal Regulations, 1993, Title 39—Mineral
Resources; Part 18~Electric Motor-Driven Mine Equipment and
Accessories, Subpart B--Construction and Design Requirements,
Section 18,.23-Limitation of External Surface Temperatures,
p. 104


Assembled unit:

‘Stainless steel clamp

Wox Spool / tope Cartridge

© -— i) =—— oe ©
wax

‘Machine screw Spring

Figure 4. Hot insulator detector prototype.

to retum 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 (U.S. Code of Federal Regulations, 1993). In
addition, some dc trolley insulator manufacturers specify a
maximum operating temperature of 121 °C (Dubina, 1981). 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 were planned to quantify this temperature
gradient and facilitate selection of the temperature rating for
device activation. These tests would also demonstrate whether
the thermal indicator was able to sense heat from below as well
as above where it is mounted on the insulator.

A thermal indicator was installed on an insulator, suspended on
a test stand. A 2-ohm, 250-W, wire-wound resistor was placed
above the insulator to serve as a heat source. Type T, #24 AWG
thermocouples were fixed at key locations on the insulator and
detector (Figure 5). The thermocouples were connected to a 32-
channel data logger that was programmed to read and transmit
temperature data to a personal computer. Commercial data
acquisition software was used to collect the data, display them in
real time, and store them on a disk. With 30 V applied to the
Tesistor, heat was generated above the insulator and, through
conduction via the mounting bolt, was gradually transferred to the
insulator and detector. Plots of temperature and time are shown
in figure 6. It can be seen that with a heat source from above, the
difference in temperature between the insulator housing under the
clamp and the cartridge was approximately 35 °C. Similar results
were obtained with the resistive heat source below the insulator.
For maximum sensitivity, the temperature rating of the wax pellet
was chosen at 65 °C. Thus, the streamer should activate before
maximum rated insulator temperatures are reached regardless of
the location of the heat source.

Additional tests were planned to gauge the reliability of the
design. One hundred wax-sealed cartridges were constructed and
mounted on a 1-m by 1.5-m aluminum panel (Figure 6). This
panel was inserted into a 2.7-cu m air oven and the temperature
in the oven was gradually increased from ambient at the rate of
25 °C per hr. 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. A 600-V direct current supply in PRL's Mine Electrical
Laboratory was placed in series with 150 mH of inductance.
Consisting of large air-core windings, this inductance simulated
the electrical characteristics of a mine trolley/track haulageway.
Its presence facilitates electrical arcs on these systems. In the
laboratory, the arc was initiated by placing a #18 AWG fuse wire
between the insulator outer housing and center threaded stud.
Upon energization, the fuse wire quickly melted leaving an
ionized path for the electrical arc. This arc was maintained for 4
to 5 sec. During this time there was significant erosion of the
insulator stud and housing. The rapidly escalating heat 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 cannot be
effective in the case of an arcing trolley insulator.

ADDITIONAL TEST RECOMMENDATIONS

It is recommended that the prototype clamp and cartridge
assembly undergo additional tests (Trelewicz, 1981) to determine
Key
Thermocouple locations
———= Top of insulator

-——- Under SST clamp

eH Tip of cartridge
——— Bottom of insulator

—s-— Ambient oir

TEMPERATURE , °C.

TIME, min

Figure 5. Temperature gradients with heat source above insulator.


A PASSIVE MEANS TO DETECT HOT TROLLEY INSULATORS

M. R. Yenchek
A. J. Hudson

NIOSH
Pittsburgh, PA


ABSTRACT

Faulty insulators on mine trolley/track haulageways may allow
the flow of leakage currents into the mine roof and ultimately
result in combustion of the local roof material. The National
Institute for Occupational Safety and Health (NIOSH), Pittsburgh
Research Laboratory (PRL), has devised a passive means to detect
overheating insulators on direct current systems. The detector
consists of a spring-loaded cartridge that ejects a reflective
streamer of white Teflon tape when subjected to elevated
temperatures. The cartridge assembly can be easily installed over
the outer metallic shell of an existing trolley line insulator. If an
insulator overheats due to ground leakage currents, the visible
streamer alerts mine personnel traveling on the haulageway.

INTRODUCTION

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 direct
current, 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 six meters
from insulators which are anchored into the roof by bolts. The
purpose of the insulators is to prevent current from flowing to the
grounded retum track via the overlying strata.

Trolley insulators are expected to maintain dielectric strength
in a dusty and wet environment. 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. The likelihood of such fires on de trolley
systems can be minimized if deteriorating insulators can be
promptly detected and replaced.

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. But 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 can not 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. Such a device using audio and
visual techniques was conceptualized in prior research
(Gilienwater and McCoy, 1981). However, the design effort
never proceeded to the prototype stage due to product cost
concems.

This report documents the accomplishments of a research
project supporting the NIOSH, PRL goal of enhancing the safety
of the Nation's underground miners. The specific objective of this
project was to devise a passive means to detect overheating

insulators on mine trolley/track haulagewuys. ‘The results of this
‘work have the potential to minimize the incidence of overheating
insulators and related fires on mine dc trolley systems.

GENERAL DESIGN CRITERIA

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. The same
environmental contaminants that contribute to leakage currents
must not interfere with indicator function. Finally, installation of
the indicator on the insulator must not introduce new hazards.

APPROACHES,

A number of thermal indicating were considered at the
outset. These included paint-filled capsules, an encapsulated
liquid, a deflective polymer, a bimetallic strip, and a spring-
loaded cartridge held in place with wax. Each was evaluated and
critiqued for worthiness prior to construction of the prototype.

A preliminary idea involved mounting clear fluoropolymer
shroud around the insulator body (Figure 1), Paint-filled capsules
would be contained within the shroud adjacent to the insulator
housing. An overheating insulator would cause the shroud to
shrink around the capsules, The capsules would be pierced by
adjacent sharp protrusions, releasing paint over the exterior of the
insulator. This passive technique had several advantages. It

Cleor Fecrpetyme,
‘wend

Figure 1. Thermal detection using point-filled capsules.


Tequired no extemal power to activate and the shroud material
was flame retardant. However, the capsule and shroud assembly
would be difficult to install without removing the insulator from
service. In addition, the paint might be difficult to detect once the
shroud was covered with dust.

Another concept involved placing antifreeze inside a plastic
tube which would be wrapped around the insulator body
(Figure 2). Insulator leakage current would heat the antifreeze
inside the tube. With sufficient internal pressure, a retaining cap
‘would be ejected and a spool of Teflon tape would unroll. ‘The
weighted tape would unwind by gravity. This white reflective
streamer flapping in the ventilating air would be easily visible to
personnel traveling in vehicles. In addition, the antifreeze would
remain liquid in areas of the haulage near the portal where
temperatures may drop below freezing. But, no easy way could be
found to retrofit the tubing on an insulator without losing fluid.

A third possible method featured a polymer disk which would
be installed over the top of the insulator (figure 3). A cartridge
containing a Teflon streamer would be mounted on top of the
disk. When heated, this disk would deform and a wax seal inside
the cartridge would melt, The seal is intended to keep dirt from
fouling the streamer. The streamer would unroll by gravity. This
simple concept required no external power. However, after
careful consideration, it was felt that the wax would not reliably
melt as the polymer disk was a poor heat conductor.

A fourth mechanism relied upon the movement of a bimetallic
strip and gravity to activate a streamer. For maximum deflection,
 spirally-wound strip was needed. An engineering analysis of
this design showed that to produce an angular deflection in excess
of 90° required a strip so large as to be impractical when mounted
‘on an existing insulator (Crest Manufacturing Company
Applications Manual, 1995).

The final concept envisioned, used a cartridge containing a
spring-loaded streamer which was brazed to an adjustable hose
clamp (figure 4). The clamp would be installed around the
insulator housing. When leakage currents caused the insulator to
become elevated in temperature, heat would be transferred via the
clamp to the cartridge. Temperature-sensitive wax seals would
hold the spooled streamer in place, At the melt temperature
specified for the wax, the spring would eject the spooled streamer

Presie be

Ready Activates 1

u
Figure 2. Thermal detection using pressurized liquid.

Figure 3. Thermal detection using deflective polymer disc.

out of the cartridge. Gravity would pull the nonconductive
streamer downward where air currents along the haulageway
would cause it to flutter noticeably. This design was simple in
nature and was easy to install on insulators in service. It had the
potential to be inexpensive, while providing a recognizable
warning signal to vehicles traveling the haulageway.
Consequently, it was the design of choice for the project.
Reliability would subsequently be evaluated through prototype
tests.

PROTOTYPE CONSTRUCTION

The assembly of the prototype is summarized here to facilitate
understanding of the concept. A hose clamp with a suitable
adjustment range was selected for the design. Made of stainless
steel to prevent corrosion, this clamp should easily fit around
commonly-used insulators, For economy, a 44-magnum brass
cartridge was chosen to house the spring-loaded streamer. This
cartridge was modified by enlarging the primer opening. Taking
care not to deform its circumference, the brass cartridge was
brazed to the clamp. A brass rod was then machined as a spool
to fit within the cartridge. Teflon tape was carefully wound
around the spool, This assembly was inserted into the cartridge
along with a compressed stainless steel spring. A stainless steel
machine screw and nut temporarily secured the spool and spring
within the cartridge. A heat gun was used to affix wax in two
locations. On the inside of the cartridge, a temperature-sensitive
wax pellet was used as the trigger mechanism for the spring to
eject the spool wound with the Teflon streamer. An exterior wax
seal excluded dirt from inside the cartridge. The design was fully
documented in a patent application (Hudson, 1996).

LABORATORY TESTS.

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