thermal characteristics of the trailing cable under dynamic test
conditions.

DISCUSSION

A distributed temperature measuring system based on fiber-
optic technology allows researchers to safely monitor
temperatures along an energized mine trailing cable under
dynamic test conditions. A fiber-optic-embedded test cable

ires only one access point to measure temperatures to within
+ 1 °C at I-m intervals along the entire cable. As with most
sophisticated instruments, a basic understanding of the
underlying technology is necessary to correctly interpret
generated data. Special precautions are necessary to protect the
fiber-optic sensor embedded in the trailing cable. An OTDR
proved to be a valuable quality assurance tool during the test
cable manufacturing process. Initial shuttle car tests indicate the
fiber-optic approach is viable. Subsequent research will allow
quantitative assessment of thermal characteristics of reeled
trailing cables under dynamic conditions.

REFERENCES

Insulated Cable Engineers Association, 1985, “Ethylene-
propylene-rubber-insulated wire and cable for the transmission
and distribution of electrical energy,” ICEA Publ. S-68-516.
Available from Natl. Electr. Manuf. Assoc. [WC8-
1976(R1982)]. Part 8, p. 7.

Kovalchik, P. G., Cole, G. P., and Duda, F. T., 1994, "Thermal
characteristics of reeled trailing cables for shuttle cars," Bureau
of Mines Information Circular (IC 9414), 14 pp.

U.S. Code of Federal Regulations, 1994, Title 30-Mineral
Resources; Chapter I-Mine Safety and Health Administration,
Department of Labor, Subchapter B-Testing Evaluation, and
‘Approval of Mine Products, Part 18-Electric Motor-Driven Mine
Equipment and Accessories; pp. 115-150; Part 75-Mandatory
Safety Standards-Underground Coal Mines; pp. 501-625.


TEMPERAURE, °C

n 1 1 1 1 1 1 L 1 Lig —
'5.000dB7DIV 60,000 dbm (0400 knvbIv
Marker information: A O.768km

B 0.950km
Distance = O.187km
2Pt Loss 2.14748

Figure 3. OTDR display.

46,

w
o

~
o

2)

o

14 —__ ae cn |
30 «Sf = 152,213, 274 335) 396 457
DISTANCE, m

Figure 4, DTS 80 display which shows 7 layers of cable.


DYNAMIC TEMPERATURE MEASUREMENT OF OVERHEATED
SHUTTLE CAR TRAILING CABLES IN UNDERGROUND COAL MINES

P. G. Kovaichik
L. W. Scott
T. H. Dubaniewicz
F. T. Duda

NIOSH
Pittsburgh, PA


ABSTRACT

Recent research conducted by NIOSH, Pittsburgh Research
Laboratory showed that electrical current levels permitted under
Present regulations may not limit cable temperatures to less than
90 °C in reeled trailing cables, Reeled cables, tightly wrapped
with many tums, all carrying current in an enclosed space, can
become overheated due to poor heat transfer. This overheating
can cause the cable insulation to soften and become easily
damaged. The damaged insulation reduces the life of the cable
and can lead to shock, electrocution, and mine fires.

This paper describes a method of measuring the conductor
temperature of mine trailing cables during normal operation. The
temperature is measured using a 50/125 micrometer Graded Index
Multimode Optical Fiber located in the center of the metallic
conductors, Distributed temperature measurements are made
using an Optical Time Domain Reflectometer (OTDR). This
system is capable of measuring temperature with an accuracy of
plus or minus one degree centigrade over a length of 4 km at }-m
intervals. With this unique way of measuring temperature, tests
can be conducted simulating actual mine conditions and provide
important data to allow MSHA's Approval and Certification
Center to develop approval criteria for cyclically rated reeled
machines.

INTRODUCTION

The safe electrical operation of shuttle cars depends upor
maintaining temperatures at or below 90°C for the trailing cable
Present electrical requirements for trailing cables used in
‘underground coal mining are contained in 30 CFR Parts 18 and 75
(U.S. Code of Federal Regulations, 1994). These Federal
regulations require trailing cables to be rated according to the
standards set by the Insulated Cable Engineers Association
(ICEA) (Insulated Cable Engineers Association, 1985).

Overheating of electric cables on shuttle cars has long been
recognized as a cause of premature insulation failure leading to
shock and electrocution. Use of cable reels on shuttle cars can
cause excessive heat build up which, in tum, causes the cable
insulation to soften and become easily damaged. This
heat-softened insulation reduces the life of the cable. Repeated
cycling of a cable in this manner can cause premature aging of the
insulation. The insulation becomes brittle, cracks, and allows
electrical leakage paths to form, These leakage paths provide the
opportunity for shock end electrocution to occur if miners come in
contact with the damaged section of cable. It is imperative that
cable operating temperatures be maintained at safe thermal limits.

A previous study by the former U.S. Bureau of Mines (USBM)
supported ICEA efforts to establish appropriate derating factors
for reeled mine trailing cables (Kovalchik et al., 1994). Empirical
and theoretical models were established to simulate a variety of
test conditions, including those that cannot be conducted in the
laboratory. Results showed, under static test conditions,
excessive heating can occur for round trailing cables operated
using presently accepted derating factors. Results for the flat
cables showed the derating factors to be on the conservative side.
‘The success of this effort prompted the ICEA to request the study

of flat and round cables be extended to include dynamic loads to
provide a complete picture of realistic trailing cable usage. Phase
1 of the current study involves round trailing cables, Flat cables
will be studied at a later date.

TECHNICAL APPROACH

Monitoring cable temperatures under dynamic conditions
requires a different approach than previously used. Under static
test conditions, researchers could place thermocouples under
conductor insulation at many locations simultaneously along the
reeled cable. This approach becomes unworkable when the cable
is constantly reeled in and payed out, as is done in practice. One
example is a shuttle car operated in a room-and-pillar scenario.
Constant movement of the trailing cable would entangle the
thermocouple leads and increase the risk of electrocution by the
energized conductor, A new approach, using a distributed fiber-
optic sensor embedded within conductors along the entire length
of the trailing cable, can overcome these obstacles.

The test setup is shown in Figure 1. A trailing cable with an
embedded optical fiber is reeled onto a shuttle car drum and
connected to a 550-V, three-phase, ac power source, The shuttle
car drum diameter is 25cm. Connections to the optical fibers are
made near the ac power source with fiber-optic jumper cables.
The exposed conductors at the sensor breakout locations are
reinsulated. The reinsulation procedure and the electrical
isolation provided by the fiber-optic cables minimize risk of
electrocution. The optical signal is processed by a York DTS 80
distributed temperature measuring system. Temperature and
distance data are then downloaded via an ARC Net link to a
personal computer for logging and visual display. With this
setup, the shuttle car can move freely without interfering with the
data acquisition process.

The DTS 80 is capable of measuring temperatures at 1-m
intervals along the entire length of the optical fiber. The sensing
technique is based on temperature-dependent Raman scatter of
light pulses launched into the optical fiber. The standard DTS 80
is configured for communication grade 50-sm core silica optical
fiber, although other fiber options are available. Distance
‘measurements to localized hot spots along the fiber are calculated
after tracking the time required for scattered light to reach a photo
detector. Although temperature measurements can be made from

PC
“Cable

FO

ac
Power Coble 80

Figure 1. Test setup.


a single end connection, maximum performance requires
connection of both ends to the instrument. For the shuttle car test
setup, the "loop" was completed by installing a short fiber-optic
jumper cable inside the drum. One of three separate loops
embedded in the trailing cable is typically monitored during
shuttle car tests. With this loop configuration, temperature
resolution at each I-m interval is 1 °C.

Fiber .

The fiber-optic sensors must be protected in accordance with
the intended application. In this case, the fiber-optic sensors were
embedded within a Tiger Brand #4 AWG, 3-conductor, G-GC
trailing cable during the manufacturing process. A fiber-optic
cable containing two optical fibers replaced the center copper
element of the #4 AWG conductors as shown in Figure 2. Outer
copper elements were wrapped around the fiber-optic cable as it
was pulled through the cabling machine. The cross-sectional
areas of the copper elements were adjusted to maintain the
equivalent cross-sectional area of a #4 AWG conductor.
Insulation was extruded over the fiber-optic embedded #4 AWG
conductor. The insulated conductor was then cut into thirds and
extruded again to complete the final product. These
manufacturing processes dictate that the sensor jacket must be
abrasion resistant, able to tolerate high extrusion temperatures,
and withstand stresses associated with pulling of the cable. To
minimize special setup procedures, the overall diameter of the
fiber sensor jacket should be compatible with standard cable
manufacturing dimensions. Operating conditions such as the
drum diameter, temperature, and number of test cycles the cable
is expected to endure over a 6-month test period, must also be
considered. Some of these fiber jacket design parameters are:

Initial extrusion temperature 220 °C
Final 2-day cure temperature 150 °C
Operating temperature 90°C
Bend radius 10cm
‘Approximate flex cycles 10,000

Optical fiber protection options were proposed by various
vendors. One proposal consisted of two polyimide-coated, 50-um
core optical fibers contained within a polyvinylidene fluoride
(PVDF) loose tube buffer. A sample of this cable was
incorporated into a production run of a #4 AWG conductor. An
optical time domain reflectometer (OTDR) monitored the
attenuation characteristics of one of the optical fibers as the
sensor cable was pulled through the machine. The OTDR display
showed intensity of back-scattered light as a function of fiber
length (Figure 3), As the test run started, it soon became
apparent the fiber was undergoing excessive stress as indicated by
the signal attenuation between markers "A" and "B". The
problem was caused by excessive pulling tension between the
main capstan and a gear driven take-up reel. Although a clutch
adjustment to the take-up reel eliminated further problems, this
experience suggested more substantial protection was needed. An
altemative design including stress-bearing kevlar strands was
subsequently chosen. In this configuration, an inner tefzel loose
tube isolates two polyimide-coated, 50-jzm core optical fibers
from the kevlar. An outer tefzel tube protects the kevlar strands
from abrasion.

FO EMBEDDED “/, ROUND G-GC CABLE

Figure 2. Cross section of fiber-optic embedded test cable.

RESULTS.

Researchers compared DTS 80 measurements with
thermocouple measurements using a simple laboratory setup.
First, an 8-m length of standard 3-conductor #44 AWG G-GC
trailing cable was suspended in the air. A length of fiber-optic
cable was tie wrapped to the trailing cable and connected to the
DTS 80. Eight thermocouples were then attached to the fiber-
optic cable at locations roughly corresponding to the midpoint of
the 1-m intervals defined by the DTS 80. The trailing cable was
then energized until thermocouple readings indicated the cable
had reached thermal equilibrium. DTS 80 measurements at
thermal equilibrium were within + 1 °C of the thermocouple
measurements, verifying the specified accuracy of the DTS 80
over the expected temperature range of the shuttle car tests.

Researchers also conducted preli shuttle car tests with
the fiber-optic-embedded trailing cable. The shuttle car was
loaded with coal and operated in a manner similar to a room-and-
pillar scenario. A typical DTS 80 display shows temperature
readings as a function of distance from the instrument (Figure 4).
The drum-terminated end of the trailing cable corresponds with
the sharp downward peak in the center of the figure. This
downward peak is caused by jumper cable connector reflections
in the center of the drum and should not be misinterpreted as
actual temperature. Reflections effectively blind the DTS 80 over
short distances, commonly referred to as dead zones. Dead zones
can be eliminated by replacing connector interfaces with splices.
‘However, space limitations inside the drum make reliable splice
installation difficult. Dead zones are an example of concepts that
need to be understood when interpreting DTS 80 generated data.

The symmetrical side lobes around the central dead zone in
Figure 4 represent temperature readings from two fiber-optic
sensors. These jumper-connected fiber-optic sensors form one
sensor loop. The periodic downward peaks indicating lower
temperature are consistent with sections of cable closest to the
outside of the reel. These sections of cable are most susceptible
to convective cooling effects from the surrounding atmosphere.
Further testing is required to make quantitative assessments of