TABLE I

CALCULATED RMS CURRENTS
Test Number Maximum layers on, during test Calculated RMS current
1 2 34.4 amps
2 3 31.5 amps
3 4 33.0 amps
4 5 32.5 amps
TABLE I
‘TEST RESULTS
‘Test Number Maximum layers on, ‘Dynamic Tests, Static Tests
uring test Hottest temperature Hottest temperature
1 2 49°C 48°C
2 3 60°C 61°C
3 4 78°C 80°C
4 5 86°C 84°C

measurements were made using a DTS 80 Distributed
Temperature Measuring System. Static tests were compared
to dynamic tests. Results showed the static and dynamic tests
were within 1 or 2 °C of each other. Each test had similar
temperature profiles, with the maximum temperature
occurring in the center wrap of cach layer. The hottest layer
was found to be a function of the number of layers and
independent of the shuttle car’s speed, duty cycle, and current.
Results indicated that a calculated RMS current value is a
viable solution for determining ampacity ratings for reeled
shuttle car cables and maintaining operating temperatures at

safe thermal limits. This method of determining the ampacity
rating for mine machinery using reeled cables will provide a
complete picture of realistic trailing cable usage.

a)

22)

GB)

VI. REFERENCES

U.S. Code of Federal Regulations. 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; 1994.

Insulated Cable Engineers Association.  Ethylene-
propylene-rubber-insulated wire and cable for the
transmission and distribution of electrical energy. ICEA
Publ. $-68-516, 1985, Available from Natl. Electr. manuf.
Assoc. [WC8-1976(RI 1982)]. Part 8, p. 7.

P.G. Kovalchik, G.P. Cole, and F.T. Duda. "Thermal
characteristics of reeled trailing cables for shuttle cars,”
USBM IC 9414, 1994, 14 pp.


Investigation of Ampacity Derating Factors for
Shuttle Cars Using Fiber Optics Technology

Peter G. Kovalchik
National Institute for Occupational Safety and Health
Pittsburgh, PA 15236-0070 USA
Phone (412) 892-6098 Fax (412) 892-4384 E-Mail: pdkO@cdc.gov WWW: http:/Avww.usbm.gov

Lawrence W. Scott
National Institute for Occupational Safety and Health
Pittsburgh, PA 15236-0070 USA
Phone (412) 892-6607 Fax (412) 892-4384 E-Mail: InsS@cdc.gov WWW: http://www.usbm.gov

Frank T. Duda
National Institute for Occupational Safety and Health
Pittsburgh, PA 15236-0070 USA
Phone (412) 892-4624 Fax (412) 892-4384 E-Mail: fod2@cdc.gov WWW: http://www.usbm.gov

Thomas H. Dubaniewicz
National Institute for Occupational Safety and Health
Pittsburgh, PA 15236-0070 USA
Phone (412) 892-6596 Fax (412) 892-6764 E-Mail: tedS@cdc.gov WWW: http://www.usbm.gov

Abstract - 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
turn, 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; it becomes brittle,
cracks and allows electrical leakage paths to form. These
leakage paths provide the opportunity for shock and
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.

This paper discusses a research project conducted by the
National Institute for Occupational Safety and Health,
Pittsburgh Research Center to determine dynamically the
conductor temperature of reeled shuttle car cables using
fiber optics technology. The research portrays typical shuttle
car loading cycles (both current and time cycles). Root-
mean-square current loading is calculated based on the
results. The thrust of this effort is to provide the scientific
basis for ampacity derating factors specifically for mine
machinery using reeled cables. This information will reduce
the probability of electrical hazards from reeled cables.

I. INTRODUCTION

The safe electrical operation of shuttle cars depends upon
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 [1]. These Federal regulations require trailing cables to
be rated according to the standards set by the Insulated Cable
Engineers Association (ICEA) [2].

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 turn, 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; it becomes brittle, cracks and allows electrical
leakage paths to form. These leakage paths provide the
opportunity for shock and 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
supported ICEA efforts to established appropriate derating
factors for reeled mine trailing cables [3]. Empirical and
theoretical models were established to simulate a variety of test
conditions, including those that cannot be conducted in the
laboratory. Results showed that, 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
that 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.

Il. TECHNICAL APPROACH

Monitoring cable temperatures under dynamic conditions
Tequires 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 sctup 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 25 cm. 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
bya York DTS 80 distributed temperature measuring system.
Temperature and distance data are then downloaded via an
ARCnet link toa 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 a fiber-optic sensor. 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-
um core silica optical fiber, although other fiber options are
available. Distance measurements to localized hot spots along

PC

ad i 7
FO ARCnet
jumper | OTS

cables 80

Figure 1. Test setup.

the fiber are calculated after tracking the time required for
‘scattered light to reach a photo detector. Although temperature
measurements can be made from 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 1-m interval
is1°C.

Il. TEST PLAN

Four separate dynamic tests were conducted. For each test, the
shuttle car was driven back and forth over a 27.69 m test track
for a period of 8 hours or until temperature stabilization, The
shuttle car was loaded with 6.25 tons of material. A 139 m
length of No. 4 AWG round 3 conductor G-GC trailing cable
instrumented with fiber optics embedded in the center of each
conductor was used to conduct power to the shuttle car, as well
as provide a temperature profile. A 74 m length of this cable
spanning the distance from the load center to the area in the
vicinity of the tie-off point always remained in contact with the
ground. For the first test, one layer was manually wrapped onto
the cable reel and a second layer was spooled on and off the reel
as the shuttle car was driven over the test track. As the car was
driven from the tie point, the cable was spooled off to the
ground and as the car was driven back to the tie point the cable
‘was spooled back onto the reel. For the second test, two layers
were manually wrapped onto the cable reel and a third layer
was spooled off and on as the car was driven, The third and
fourth tests were conducted with four and five layers
permanently on the reel and the fifth and sixth layers
respectively spooled off and on, Each layer consisted of 15
wraps of cable with layer 1 having 12.9 m of cable, layer 2
having 14.8 m of cable, layer 3 having 17.3 m of cable, and
layer 4 having 20 m of cable.


The DTS-80 fiber-optic instrumentation provided the
capability to monitor the conductor temperature as a function
of time over the test period and as a function of distance over
the entire 139 m length of cable. A data logger in conjunction
with a shunt provided the capability to monitor the shuttle car
load currents as a function of time. A typical DTS 80 display
shows temperature readings as a function of distance from the
instrument (figure 2). 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 2 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.

To characterize the changing currents obtained by the data
Jogger over an eight-hour test due to the dynamic operation of
the shuttle car, a generalized ampacity equation based on an n-
segment duty cycle developed by John Mesina of MSHA’s
Approval and Certification Center was used to calculate an
RMS current. The data logger sampled current values at a rate
of one sample every two seconds. Therefore, the test cycle of
one 8 hours was divided up into approximately 14,400

46

26

TEMPERAURE, °C
o 3

4

agb 1 bo L

30 a 152, 213 274 «89335 «6396 «457
DISTANCE, m

Figure 2. DTS 80 display.

segments, The following equation for an n-segment duty cycle
was used to obtain an RMS value:

2
5, Is2? Is]? IsIIs2
it (ts2— ts]
a ”
duty cycle m2 th
where s = segment number - general notation,

n= total number of segments in one duty cycle,

1 = designates values at the start of segment s,

2 = designates values at the end of segment s,
tn2 = time at the end of segment n,

tll = time at the start of segment 1 (usually zero)

An RMS value was obtained for cach of the four tests.

A second series of four static tests was conducted to determine
if the RMS characterizations of the currents were suitable. The
‘same 139 m length of cable was used for these static tests. For
the first test, two layers were manually wrapped on the spool
and the cable was energized with a d.c. current equivalent to the
calculated RMS value obtained from the first dynamic test.
Again the conductor temperature was monitored with the DTS
80/fiber optic instrumentation as a function of distance and
time. The test was considered complete when the hottest
temperature was reached and stabilized for 1 hour. The other
three tests were conducted in a similar manner using the RMS
values obtained from the dynamic test. Table I lists the
calculated RMS values from all four dynamic tests.

IV. TEST RESULTS

Static tests were compared to the dynamic tests with the results
shown in Table II. Results show the static and dynamic tests
within 1 or 2 degrees C of each other. Each layer tests had
similar temperature profiles with the maximum temperature
occurring in the center wrap of each layer. The hottest overall
layer was found to be a function of the number of layers on the
reel and independent of the shuttle car speed, duty cycle, and
current. These initial tests indicate that the calculated RMS
currents are viable solutions for determining ampacity ratings
for reeled cables. However, these are only a handful of test
results and further testing must be conducted not only for round
cable, but also flat cables.

V. SUMMARY

The conductor temperatures of reeled shuttle car cables were
determined dynamically using fiber optics technology. The
‘sensor was a 50 micrometer core silica optical fiber embedded
in the conductors at the time of manufacture. Temperature