Distributed Measurement of Conductor

Temperatures in Mine Trailing Cables

Using Fiber-Optic Technology

Thomas H. Dubaniewicz, Jr., Peter G. Kovalchik, Lawrence W. Scott, and Mark A. Fuller, Member, IEEE

Abstract— Mine trailing cables operated above safe thermal

limits can cause premature insulation failure, increasing electrocution and fire hazards. Previous U.S. Bureau of Mines Pittsburgh

Research Center research showed that, under static test conditions, electrical current levels permitted under present regulations

may not limit cable temperatures to less than the 90 C rating

of reeled trailing cable. Continuing research under the National

Institute for Occupational Safety and Health (NIOSH) addresses

thermal characteristics of reeled trailing cable under dynamic

test conditions more representative of field conditions, where

operators constantly reel in and pay out cable. This research is in

support of efforts by industry associations and the Mine Safety

and Health Administration to establish safety guidelines for

cyclically rated reeled machines. This paper describes a unique

approach to measuring temperatures within reeled cable under

dynamic test conditions. Fiber-optic sensors embedded within

the metallic conductors measure temperatures at 1-m intervals

along the entire length of cable. Temperature measurements are

reported to be accurate to within 1 C. The test setup requires

access to only one end of the trailing cable, allowing researchers to

freely reel in and pay out cable while temperature measurements

are made, simulating field conditions. Manufacture of a fiberoptic-embedded trailing cable is described, along with initial test

results that indicate the fiber-optic approach is viable.

Index Terms—Fiber optics, mine trailing cable, temperature.

I. INTRODUCTION

THE safe electrical operation of shuttle car trailing cables

depends upon maintaining electrical conductor temperatures below 90 C. Present electrical requirements for trailing

cables used in underground coal mining are contained in Title

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].

A previous study conducted by the U.S. Bureau of Mines

(USBM) supported ICEA efforts to establish appropriate derating factors for reeled coal mine trailing cables [3]. Empirical

and theoretical models were established to simulate a variety of 

 

 

T. H. Dubaniewicz, Jr., P. G. Kovalchik, and L. W. Scott are with

the National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA 15236 USA (e-mail: TCD5@cdc.gov;

PDK0@cdc.gov; LNS5@cdc.gov).

M. A. Fuller is with AmerCable, Division of Associated Materials, Inc.,

Sewickley, PA 15143 USA.

.

Fig. 1. Test setup.

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 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 one of the current study involves round

trailing cables. Flat cables will be studied at a later date.

II. 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, e.g., a shuttle car operated in a manner

similar to 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 distributed fiber-optic sensors embedded

within conductors along the entire length of the trailing cable,

can overcome these obstacles.

The new test setup is shown in Fig. 1. A fiber-opticembedded trailing cable is reeled onto a shuttle car drum

and connected to a 550-V three-phase ac power source. The

shuttle car drum diameter is about 22 cm. Connections to

the fiber-optic sensors are made near the ac power source
Fig. 2. Cross section of fiber-optic-embedded test cable.

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 fiberoptic cables minimizes risk of electrocution. The optical

signal is processed by a York DTS 80 distributed temperature

monitor. 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 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- m 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 it takes for

scattered light to reach a photodetector. 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

is 1 C.

III. OPTICAL FIBER INSTALLATION

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 three-conductor

ground–ground check (G–GC) trailing cable during the trailing

cable manufacturing process. A fiber-optic cable containing

two optical fibers replaced the center copper element of the 4-

AWG conductors, as shown in Fig. 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,

Fig. 3. OTDR display.

able to tolerate high extrusion temperatures, and capable of

withstanding 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 six-month test period, must

also be considered.

Some of these fiber jacket design parameters are as follows:

initial extrusion temperature 182 C;

final 3-1/4 hour cure temperature 132 C;

operating temperature 90 C;

bend radius 10 cm;

approximate flex cycles 10 000.

Optical fiber protection options were proposed by various

vendors. One proposal consisted of two polyimide-coated,

50- m core optical fibers contained within a polyvinylidene

fluoride (PVDF) loose tube buffer. A sample of this cable

was incorporated in 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 (Fig. 3). As the test run started, it soon became

apparent that the fiber was undergoing excessive stress, as

indicated by the signal attenuation between markers “ ” and

“ .” The problem was caused by excessive pulling tension

between the main capstan and a gear-driven takeup reel.

Although a clutch adjustment to the takeup reel eliminated

further problems, this experience suggested more substantial protection was needed. An alternative design including

stress-bearing Kevlar strands was subsequently chosen. In

this configuration, an inner tefzel loose tube isolates two

polyimide-coated 50- m core optical fibers from the Kevlar.

An outer tefzel tube protects the Kevlar strands from abrasion.
Fig. 4. DTS 80 display.

IV. RESULTS

Researchers compared DTS 80 measurements with thermocouple measurements using a simple laboratory setup. First, an

8-m length of standard three-conductor 4-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 preliminary 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 (Fig. 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 Fig. 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

thermal characteristics of the trailing cable under dynamic test

conditions.

V. DISCUSSION

A distributed temperature measuring system based on fiberoptic technology allows researchers to safely monitor temperatures along an energized mine trailing cable under dynamic

test conditions. A fiber-optic-embedded test cable requires only

one access point to measure temperatures to within 1 C at

1-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

[1] 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; 1994,

pp. 501–625.

[2] “Ethylene-propylene-rubber-insulated wire and cable for the transmission and distribution of electrical energy,” Insulated Cable Engineers

Association, ICEA Pub. S-68-516, 1985, pt. 8, p. 7.

[3] P. G. Kovalchik , G. P. Cole, and F. T. Duda, “Thermal characteristics

of reeled trailing cables for shuttle cars,” U.S. Bureau of Mines,

Washington, D.C., USBM IC 9414, 1994, p. 14.

Thomas H. Dubaniewicz, Jr. received the B.S.E.E.

degree from the University of Pittsburgh, Pittsburgh,

PA, in 1988.

Following graduation, he joined the Pittsburgh

Research Center, U.S. Bureau of Mines, Pittburgh,

PA, which is currently under the direction of the National Institute for Occupational Safety and Health.

His work involves mine safety instrumentation, with

a specialty in fiber optics. He has developed fiberoptic sensors for methane and strain and systems

for optical powering of remote instrumentation. He

is currently working with the International Electrotechnical Commission and

the Instrument Society of America to develop intrinsic safety guidelines for

lasers used in underground coal mines.

Mr. Dubaniewicz is a Registered Professional Engineer in the State of

Pennsylvania and a member of the Instrument Society of America and Eta

Kappa Nu.

Peter G. Kovalchik received the B.S. degree from

the University of Pittsburgh, Pittsburgh, PA, in 1983.

He joined the U.S. Bureau of Mines, Pittburgh,

PA, in March 1978 as an Electronic Technician in

the Electrical and Electronic Systems Group. He is

currently an Electrical Engineer with the Mining

Injury Prevention Branch of the Pittsburgh Research

Laboratory. He has been responsible for programs

concerning trailing cables and hoisting.

Mr. Kovalchik is a member of the IEEE Industry

Applications Society.
Lawrence W. Scott received the B.S. degree in

physics from Tennessee State University, Nashville,

in 1964.

Following graduation, he joined the Pittsburgh

Research Center, U.S. Bureau of Mines, Pittburgh,

PA, which is currently under the direction of

the National Institute for Occupational Safety

and Health. His work involves explosion-proof

enclosures, safety instrumentation, and fiber optics.

He has been instrumental in developing design

criteria for high-voltage explosion-proof enclosures,

high-voltage explosion protection techniques, and intrinsic safety. He is

currently working with the Insulated Cable Engineers Association (ICEA)

and the Mine Safety and Health Administration (MSHA) to develop improved

ampacity derating factors for reeled shuttle car trailing cables.

Mark A. Fuller (M’87) received the B.S. degree

in education from Manchester College, North

Manchester, IN, in 1973 and the B.S. degree in

mathematics from Indiana Wesleyan University,

Marion, IN, in 1988.

Since 1977, he has worked in the electrical power

cable industry for various companies, including

Anaconda Wire and Cable, Cablec Industrial Cables,

and, currently, AmerCable, Division of Associated

Materials, Inc., Sewickley, PA. During his career,

he has been engaged in research and development

and new product development, holding such positions as Portable Cables

Engineer, Manager of Sales Engineering, and, most recently, Applications

Engineer for Mining Products, in which his responsibilities include cable

design, specification writing, overseeing manufacture of special composite

cables, and technical representation in the field.