NATIONAL BUREAU OF STANDARDS REPORT

10 008

FIELD EVALUATION OF A CABLE FAULT LOCATOR

DEVELOPED BY NBS

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. Today, in addition to serving as the Nation’s central measurement laboratory,

the Bureau is a principal focal point in the Federal Government for assuring maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. To this end the Bureau conducts research and

provides central national services in three broad program areas and provides central national services in a fourth. These are: (1) basic measurements and standards,

(2) materials measurements and standards, (3) technological measurements and

standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, and the Center for Radiation Research.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the

United States of a complete and consistent system of physical measurement, coor- dinates that system with the measurement systems of other nations, and furnishes

essential services leading to accurate and uniform physical measurements throughout

the Nation’s scientific community, industry, and commerce. The Institute consists

of an Office of Standard Reference Data and a group of divisions organized by the

following areas of science and engineering:

Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic Physics—Cryogenics2—Radio Physics2—Radio Engineering2—Astrophysics2—Time

and Frequency.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to methods, standards of measurement, and data needed by industry, commerce,

educational institutions, and government. The Institute also provides advisory and

research services to other government agencies. The Institute consists of an Office of

Standard Reference Materials and a group of divisions organized by the following areas of materials research:

Analytical Chemistry—Polymers—Metallurgy — Inorganic Materials — Physical

Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides for the creation of appropriate opportunities for the use and application of technology within the Federal Government and within the civilian sector of American industry. The primary functions

of the Institute may be broadly classified as programs relating to technological meas- urements and standards and techniques for the transfer of technology. The Institute

consists of a Clearinghouse for Scientific and Technical Information,3 a Center for

Computer Sciences and Technology, and a group of technical divisions and offices organized by the following fields of technology:

Building Research—Electronic Instrumentation — Technical Analysis — Product

Evaluation—Invention and Innovation— Weights and Measures —Engineering

Standards—Vehicle Systems Research.

THE CENTER FOR RADIATION RESEARCH engages in research, measurement,

and application of radiation to the solution of Bureau mission problems and the

problems of other agencies and institutions. The Center for Radiation Research consists of the following divisions:

Reactor Radiation—Linac Radiation—Applied Radiation—Nuclear Radiation.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted ; mailing: address Washington, D. C. 20234.

2 Located at Boulder, Colorado 80302.

3 Located at 5285 Port Royal Road, Springrfield. Virginia 22151.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

2120414 March 196a

NBS REPORT

10 008

FIELD EVALUATION OF A CABLE FAULT LOCATOR

DEVELOPED AT NBS

Prepared for Naval Air Systems Command

Department of the N avy

Washington, D. C. 20360

By

Visual Landing Aids Field Laboratory

Photometry Section

Metrology Division

IMPORTANT NOTICE NATIONAL BUREAU OF S'

for use within the Government,

and review. For this reason, tl whole or in part, is not authc Bureau of Standards, Washing!

the Report has been speciticall;

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 2015.

:ss accounting documents intended

subjected to additional evaluation

! listing of this Report, either in e Office of the Director, National

by the Government agency for which

:opies for its own use. <^BS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

FIELD EVALUATION OF A CABLE FAULT LOCATOR DEVELOPED BY NBS

By

Visual Landing Aids Field Laboratory

Photometry Section

Metrology Division

ABSTRACT

A prototype of a Cable Fault Locator (CFL) was designed

and constructed by the National Bureau of Standards. This

report covers the field testing and evaluation of this equipment. Much of the evaluation was a comparison of performance

of the CFL with the performance of the AN/TSM-11, the equipment

currently used for cable tracing and fault location. For the

conditions tested, the capabilities of the CFL were equivalent

or definitely superior to those of the AN/TSM-11. The depth

of buried cables, which could not be measured reliably before, can usually be determined satisfactorily with the CFL. Although there were many definite improvements of the CFL over

the earlier equipment, several areas of possible improvements

are pointed out which should be considered if this equipment

is to be produced in quantity.

1 . INTRODUCTION

Modern airfields use a large number of electrical circuits for the

airfield lighting, visual landing aids, and power circuits. These circuits are virtually all installed underground either by direct burial or

in ducts of some type. The cables for these circuits are not readily

accessible for inspection and often the route of the cable is unknown.

A means of accurately locating the position of a fault for repair or the

route of a cable to avoid damage from construction is frequently needed*

Several types of equipment have been developed to answer this need but

none of these devices have been fully satisfactory.

The cable test-detecting set AN/TSM-11— developed in 1955, is

the equipment currently used by the military services for tracing

cable and for locating faults in these circuits. The AN/TSM-11 was designed for use in tracing direct-burial cable and in locating lowresistance-to-ground faults. It has been found suitable for these purposes except for some conditions when interference from power circuits

or other signals are encountered. The AN/TSM-11 is easily portable and

convenient to use but experience with this equipment revealed the need

for a number of improvements. Some of these deficiencies are: The

batteries for the receiver are not readily available) the response of
.
2

the indicating meter is not linear with signal field strength; the

receiver amplifier is easily saturated; discrimination is inadequate

in the receiver between the generated signal and harmonics of power

frequencies and other signals used on airfields; and the vibrator

does not adequately restrict the signal frequency. To improve

capability in the location of high-resistance-to-ground faults and

in the location of ungrounded opens and to eliminate the deficiencies

listed above, a new design by the National Bureau of Standards (NBS) was undertaken rather than modification of the AN/TSM-11

.

The cable fault locator (CFL) developed by NBS consists of two

major units; a test signal oscillator (TSO) and a detector unit (DU).

Both units are easily portable and can be used at any desired location

on the airfield. The equipment is described in o. 9886

2.1. Test Signal Oscillator (TSO) Unit . The test signal oscillator

is designed to be energized from either llO to 130-volt, 55- to 60-hertz

alternating current or 10- to 16-volt direct current. The oscillator

can generate a square-wave signal of either 150, 270, or 570 hertz.

The circuit for generating the signal is solid state and does not use

a mechanical vibrator. Frequency shifts of the basic generated signal

for 20 or 200 milliseconds at a repetition rate of 130 and 95 times per

minute, respectively, can be selected to aid in identification of the

test signal when interference is encountered. The output signal is

isolated from the oscillator circuits by a transformer with output taps

for approximately 6, 13, 28, 60, 130, 280, and 600 volts. An ammeter

is provided to measure the power-amplifier collector current.

2.2. Detector Unit (DU) . The detector unit is a tuned solid-state

amplifier for detecting, amplifying, and indicating the radiation from

the test signal in the cable being tested. It is a hand-held unit

powered by four 1 1/2-volt pen-light cells. A ferrite-cored coil on a telescoping tube is used as the detecting element or probe. This probe

is identical to that used with the AN/TSM-11. A switch for selecting

the frequency used in the test and for testing the battery condition is

provided. The gain can be adjusted by a logarithmic-response potentiometer.

3.1. General . The field tests were made at the Areata Airport on previously installed cables. Some of the cables used in the tests were installed by direct burial in areas which were isolated from other

circuits and others were near other circuits and conductors. Some of

the cables were installed in metal pipe, frequently with other cables.

Where suitable installations were available, the effects of interfering

2. DESCRIPTION OF THE EQUIPMENT

",'Circuit Description of a Prototype Cable Fault

3. FIELD TESTS AND RESULTS

3

power- and radio-frequency signals were evaluated. Each of the three

TSO frequencies and the 20- and 200-millisecond Frequency Shift

signals were evaluated for many of the test conditions.

3.2. Cable Tracing . Except for the differences of the CFL from the

AN/TSM-11, the procedures used for tracing cable were similar to those

given in section 2 of NBS Report No. 8596 "Guide to Use of AN/TSM-11

Cable Test-Detecting Set. ".3/ The major differences in using the two

devices were that for the CFL one of the three signal frequencies had

to be selected on the TSO and DU, where the AN/TSM-11 had only a single

frequency, and the matching of the signal generator output to the test

circuit was determined in a different manner. For the AN/TSM-11 the

generator output was matched to the circuit by selecting the position

of the three Output Impedances which gave the highest voltage as determined by a supplemental voltmeter connected across the Output terminals.

For the TSO, the Output Voltage tap-selector switch was placed on the

lowest voltage position which would produce a reading on the panel meter

above 0.7 amperes. Thus the over-current breaker would not trip. The

impedance matching of the signal generators to the test circuit might

differ appreciably. For the CFL, either of the test frequencies and

either 0- or 20-millisecond Frequency Shift could be used on most circuits, but the 570-hertz signal and the 20-millisecond Frequency Shift were preferred unless difficulties indicated the need for another signal

frequency. Both devices were usually satisfactory for tracing circuits,

but for circuits with the end not connected to the signal generator open

circuited, the CFL could trace closer to the end of the circuit.

3.2.1. Tracing Directly Buried Cables . Tracing directly buried cables

which were isolated from other circuits was easy and accurate with both

the CFL and the AN/TSM-11. For buried cables that were not isolated

only two problems were encountered with the use of instruments of this

type.

1. In some instances through leakage or inductive coupling,

the signal was transferred to other cables or conductors

such as pipes and the operator could follow the wrong

path. Only by thoroughly investigating changes of signal

strength could misleading indications be avoided.

2. In some areas there were strong interfering signals,

especially at power frequencies. In such areas, using

the Frequency Shift or interrupted signal was very

valuable in identifying the test signal. The Frequency

Shift of the CFL was preferred to the interrupted signal

of the AN/TSM-11 because the meter indication was still

useful and also the identification was better.

- 4 -

With both problems, the availability of more than one test frequency

on the CFL was helpful. The 570-hertz signal was superior to the

other frequencies when interference from power frequencies caused

trouble; however, this signal was most likely to give a false lead

onto other conductors. The 150-hertz signal was more likely to be

bothered by power “frequency interference but in some cases the interference was worse with the 270-hertz than with the 150-hertz test

signal. The 20-millisecond Frequency Shift of test signal was very

helpful in many cases and its use did not present any special problems.

The 200-millisecond Frequency Shift was not of particular improvement

over the 20-millisecond Frequency Shift for any conditions tested.

3.2.2. Tracing Cable in Metal Duct . Much of the airfield lighting

at the Areata Airport is installed in steel pipe. (Not many airfields

have cable installed in long lengths of steel pipe, but frequently

magnetic shielding of cable in one form or another is encountered.)

Usually either the CFL or the AN/TSM-11 could be used to trace cable

in the presence of magnetic shielding, but much more often than for

directly buried cable, the operator encountered problems and confusing

indications. For either the CFL or the AN/TSM-11, the gain of the

detecting unit had to be increased considerably to obtain meter indications comparable to that of a similar directly buried circuit. The use

of increased gain made radiated signal interference problems more

serious. Also the test signal heard in the headphones was not so easily identified, especially for the 150-, 250-, and 270-hertz signals.

The interrupted or frequency shifted signals were frequently necessary

for identification. The additional power from the TSO, the availability

of the 570-hertz signal, and the improved discrimination of the DU made

the CFL better for tracing circuits installed in steel pipe; however,

the extra power and sensitivity and perhaps greater coupling of the

570-hertz signal would lead the operator farther astray with the CFL

than the AN/TSM-11 if he lost the circuit or continued beyond its

termination and followed some other conductor.

3.2.3. Tracing Energized Power Cables . The radiation from power circuits,

which in some cases can be a hindrance when tracing cables, may be used

to trace power cables without de-energizing the circuit. Both the CFL

and the AN/TSM-11 were used successfully for tracing these circuits. The

signal generator was not used. A high Gain setting of the detecting unit was needed. Starting from a known position over the circuit, if radiation

could be detected, the circuit was traced in a manner similar to normal

cable tracing using the meter indications or the tone in the headphones.

Power signals gave a stronger indication with the detector-amplifier of

the AN/TSM-11 than with the DU. Usually the strongest indication from

power currents with the DU were obtained when the 150-hertz position was selected but occasionally the 270-hertz position gave a higher reading.
' -

'
5

3.3. Locating Ground Faults in Cable . Ground faults can be

classified as low-resistance-to-ground, high-resistance-to-ground,

and multiple grounds. The basic procedure for locating any type

of ground ..fault was similar to that given in Section 3 of NBS Report

No. 8596.— By following these procedures carefully, both the CFL and

the AN/TSM-11 were used successfully to locate all three types of

ground faults, but neither unit was satisfactory for locating certain

ground faults in the presence of interference and shielding problems

or for locating very high resistance-to-ground faults.

3.3.1. Ground Faults in Buried Cable . The CFL was more effective in

locating ground faults in unshielded, directly buried cable than was

the AN/TSM-11. Both types of equipment could locate low-resistance

faults in these circuits to within two feet and usually to within less

than six inches. The greater power output of the TSO and the increased

gain and more linear response of the DU permitted detection of somewhat

higher-resistance faults with the CFL than with the AN/TSM-11. Generally,

any of the three test frequencies could be used to locate ground faults

satisfactorily, but for certain conditions, one frequency was better than

the others. The 570-hertz frequency had more power and greater sensitivity, especially in the headphones, but it sometimes had a stronger

tendency to radiate into other conductors. The response from the 150-

and 270-hertz signals was sometimes affected by radiation from power

circuits which made the 570-hertz signal preferable. In seeking ground

faults the continuous signal was slightly preferred, except that in cases of interference from extraneous sources the 20-millisecond Frequency

Shift was desirable. The use of the Frequency Shift did not appreciably

reduce the accuracy of locating ground faults.

3.3.2. Locating Ground Faults in Cable in Metal Pipe . Sometimes the

CFL and AN/TSM-11 could be useful for locating ground faults on circuits

installed in metal pipe, but results were not dependable. Tests on some circuits in pipe gave indications of changes in signal which probably

were faults, but the cables could not be examined for the faults since

they were inside of the pipe. In other cases, neither instrument was of

value in locating ground faults in metal pipe. In a cable with insulation

resistance to ground of 70 megohms, a short to ground was installed. Although the operator was aware that the intentional ground was installed,

this ground was overlooked, but a definite fault indication was noted 400

feet beyond the actual ground and end of the cable. In this case, not

only was there a failure to find the fault but misleading indications of

faults were obtained. None of the test signal frequencies were satisfactory. When the cable was in steel pipe, any changes in signal strength

seemed to be more gradual than occurred in directly buried cable.

6

3.4. Locating Open-Circuit Faults . The general procedure for using

the CFL to locate open-circuit faults is similar to that for the

AN/TSM-11. These procedures are given in Section 4 of NBS Report

No . 8596. The location of grounds in connection with open-circuit

faults is discussed in this reference and the comments in Paragraphs

3.3.1 and 3.3.2 above apply for locating the ground. This discussion

will be limited to ungrounded open-circuit faults.

3.4.1. Locating Ungrounded Open-Circuit Faults in Directly Buried

Cable . The CFL as compared to the AN/TSM-11 could be used to more accurately locate ungrounded open-circuit faults in directly buried

cable. With the TSO connected between ground and one side of the

circuit, the DU Gain setting had to be increased appreciably over that

required for grounds on the circuit to obtain a full-scale meter reading.

Within a few hundred feet of the open fault, the signal strength decreased rapidly and usually became unusable before reaching the open

fault. The Gain setting of the DU had to be at or near maximum when

within the final 200 or 300 feet of the open fault, and any interfering

radiation caused serious trouble at this sensitivity.

The CFL at each of the test frequencies was equally or more

effective than the AN/TSM-11. For an open fault in a cable in an area with very little interference the AN/TSM-11 could be used to follow the

cable to within 60 and 100 feet of the fault using the meter and to

within 40 and 75 feet using the audible signal. (The distances quoted

are respectively those obtained when the open was approached from the

shorter and the longer run of cable.) With this same fault and using

the CFL, the 150- and 270-hertz signals could be used to approach 5 to 10

feet nearer the fault than with the AN/TSM-11. The 570-hertz signal

could be followed easily and accurately to within 25 feet of the fault

and could be detected directly over the fault. The 570-hertz test signal was the most effective in approaching an open fault, but in some cases,

could actually lead the operator across the fault without his recognizing

it. The detectable signal had a tendency to spread over the ground as the

open fault was approached. The capability of locating open faults with

the CFL was also tested by connecting the TSO between the terminals of a series circuit which had an ungrounded open. This method of connection

permitted the fault to be approached somewhat closer from both directions

and the operator was not required to return to the TSO to change connections. However, with this method of connection, the fault might be passed

over without recognition, especially when the 570-hertz signal was used.

3.4.2. Locating Ungrounded Open Faults in Cable in Metal Duct . Neither

the CFL nor the AN/TSM-11 could be used to satisfactorily locate open

faults in cable installed in steel pipe, nor was this capability expected.

The shielding effect and concentration of the return signal made it too

easy to pass over the fault without recognition. In one case with this

type of fault, the operator proceeded several hundred feet beyond the

open before finding any indication of a fault. Even when the location

of this fault was known, indications of the open fault could not be detected.

7

3.5. Determining the Depth of Buried Cable . The AN/TSM-11 was of

very little value in determining the depth of buried cable. The

depths of cables in many conditions were accurately determined with

the CFL, but there were some conditions when the depths obtained with

the CFL were in error. The method used for determining the cable

depth was to place the probe on the ground directly above the cable,

adjust the DU Gain for a full scale or marked increment on the meter,

then carefully raise the probe vertically until the meter reading was

one-half the former value. The depth of the cable approximately

equaled the height to which the probe was raised. This method of depth

determination was very accurate for isolated direct-burial cable at

depths of three feet or less. Some of the tests appeared to have about

10 percent error for depths of four feet and greater. If there was any

radiated interference, the depth indication was likely to be in error. Each of the three test frequencies worked well for determining depth

under good conditions, but the 570-hertz signal was less likely to be

affected by radiated interference from power circuits. In some cases,

the depth of cable in steel pipe was determined satisfactorily but

in other cases the depth indications were seriously in error. Depth

measurements near where the cable terminated or made a sudden change in

direction were not accurate, as would be expected.

3.6. Miscellaneous . 3.6.1. Radio Frequency Interference . When using the DU in the vicinity

of the airfield-lighting vault, the meter on the DU responded to some radio transmissions. The meter reading would sudden ly increase or go

off-scale, depending on the Gain setting, and the increased meter reading

would be maintained for the duration of the radio transmission, but the

audible signal was not affected except perhaps as a transient when the

radio was keyed. The radio transmissions to which the DU meter responded

were in the very high frequency (VHF) range, but other ranges might have

a similar effect under the right conditions. Apparently the radio interference was rectified in the metering circuit and was not a signal in the

audio range. The DU meter was more sensitive to radio frequency interference when on the 570-hertz position than on the 150- or 270-hertz

position. The response to radio transmissions was never noted when

working in the field away from the vault area. There have been no known

instances of response to radio transmissions when using the AN/TSM-11,

but this condition was not thoroughly investigated.

3.6.2. Mechanical Noise in DU . The DU was not noticeably microphonic

(microphonics have been a problem with the AN/TSM-11), but there was

one type noise which was bothersome. When the DU Gain was high and the

probe struck weeds and grass or other objects, a very sharp sound was

noted in the headphones. This noise was particularly bothersome when

the signal strength was low and the path was being followed carefully

under confusing conditions. This mechanical noise was observed on all

three test frequencies but was unnoticed until the DU Gain was high. A

method of reducing the effect of this mechanical noise was not determined,

but using a shock absorbing covering over the probe surface might be

helpful

8

4. PHYSICAL EVALUATION

4.1. Physical Design . The physical design of the CFL is generally

satisfactory but some improvements which are considered desirable

are listed below.

1. DU is too large and heavy and its shape makes it

uncomfortable to carry. This unit was much more tiring

to use than was the AN/TSM-11 detector-amplifier. (In

checking some long circuits, this unit may be carried

for miles and held for a couple hours, hence a comfortable unit is required.)

2. The balance of the DU with the ferrite detecting element

attached makes it awkward and its use tiring. The

balance of the AN/TSM-11 detector-amplifier is better.

3. The cord from the headphones to the phone jack is too

long and a shorter cord is needed. However, an extension section for special use may be desirable at times.

(The cord and headphones are the same as those used

with the AN/TSM-11.)

4. The headphones are heavy and interfere with hearing

external noises. A single, lightweight earphone

would be adequate. This would improve safety by

permitting the operator to better hear outside noises,

such as aircraft, when he is working along runways.

In some cases the elimination of outside noises is

useful, but most of the time this noise can be tolerated.

5. The DU, including the probe connector and phone jack,

should be waterproof or as watertight as possible. This

protection will be especially important when the unit is

used in tracing underwater circuits.

5.

EVALUATION OF ELECTRICAL CIRCUITS AND COMPONENTS

5.1. General . Both the CFL and the AN/TSM-11 operate on the same basic principles. As a result of more than a year of testing in the

field, the effectiveness of the design changes in the CFL and areas

of possible improvements can be reported.

5.2. Circuit Components . The CFL has been used in the test and

evaluation program for nearly 18 months without a malfunction. The

operating time during this period probably exceeds that which would be

expected on most airfields in normal use. The improvement in quality

of components, as well as the particular design, aid in the reduction

of malfunctions. Near the end of this period the original set of

9

batteries in the DU were replaced. The type AA l! penlite" batteries

used in the DU are more readily available than are the batteries

required for the detector -amplifier of the AN/TSM-11, and the life of

the batteries in the DU is much longer. Also the battery test circuit

on the DU was very useful.

5.3. Frequency Shift of TSO . The 20-millisecond Frequency Shift of

the TSO test signal was preferred over the interrupted signal of the

AN/TSM-11. This Frequency Shift was more comfortable to follow using

the headphones than was the interrupted signal of the AN/TSM-11, but

the major improvement was that the meter indication was usable with

the 20-millisecond Frequency Shift. The 20-millisecond Frequency Shift

could be used for all test conditions without seriously hampering performance. The 200-millisecond Frequency Shift did not appear to be an improvement over the 20-millisecond Frequency Shift for the conditions

tested.

5.4. Test Signal Frequencies . The provision of the TSO for selection

of any one of three test signal frequencies was an improvement. The

570-hertz signal was usually the best to use for this equipment, but on occasions one of the other frequencies was preferred. The use of

solid-state components for signal generation provided a more stable

frequency than the mechanical vibrator of the AN/TSM-11; however, the

570-hertz signal was actually 563 hertz.

5.5. Output Voltage Adjustment . The additional voltage taps and the

increased maximum output voltage of the TSO as compared to the AN/TSM-11

signal generator were useful improvements. More voltage taps, or a con- tinuously variable output, are desired. Often the maximum voltage that

could be obtained from the TSO was approximately one-half that which

might be expected because the next step, which may more than double the vo Itage, could not be used as it just tripped the circuit breakers. For

open-circuit or high-resistance-to-ground faults and for tracing open

circuits to the termination, the 600-volts output signal was helpful

and in some cases even higher voltages would be useful.

5.6. Radio Frequency Interference . Some radio transmissions affected

the meter indication of the DU but did not seriously affect the audio

signal. A radio frequency interference filter to eliminate the effects

of radio transmissions on the meter reading would be a desirable improvement .

5.7. Power Frequency Detection . In some instances, the DU was used

conveniently to trace energized power circuits by using the 150-hertz

input position. In normal use of the CFL for tracing cables and locating

faults, this detection of power frequencies and resulting interference was a serious problem for which improved filtering is needed. If the

filtering is improved, a 60-hertz position on the DU for detection of

energized power circuits would be desirable.

10

5.8. Test Signal Distortion . The test signal from the signal

generator was affected by the circuit to which the signal was

applied. The waveform of the output voltage from the TSO and the

signal generator of the AN/TSM-11 and output current to the circuit were determined for several test circuits. The approximately square

wave voltage output from the signal generators was often seriously

distorted at the output terminals. The output current waveforms

often had little resemblance to the output voltage waveforms. (A

sine-wave signal was also applied to these same circuits and usually

there was less distortion of the sine wave than of the square wave.)

The waveforms of the detected signal at the input to the DU and at the

headphones jack were determined. These waveforms were also distorted

and in some cases appeared as damped oscillations; however, frequently

the signal at the headphone jack was approximately a sine wave. (In

most cases the sine-wave voltage test signal was less distorted than was the signal from the TSO.)

5.9. Detecting Element . As noted earlier the same detecting element

or probe was used with both the CFL and the AN/TSM-11 units. The

sensitivity of the probe depended on the circuit to which it was

connected. Some tests indicated that the probe was poorly matched to

the DU. These tests are discussed further in Paragraph 6.5.

6. DISCUSSION AND ANALYSIS OF RESULTS

6.1. Location and Installations . The field tests were made at the

Areata Airport where the soil has poor conductivity, often thousands

of ohms resistance between ground rods three feet apart. There is

also a vast network of steel pipe and abandoned cables buried throughout

the airport. These conditions may not be typical of most airfields, and

results at other locations may vary from those attained in these tests.

The signal leakage to ground often concentrates on the buried pipes and

cables at Areata rather than dispersing through the ground. The con- centration of return signal into the pipes and old cables serves as a false signal path to confuse the operator. The poor soil conductivity

can effectively increase the impedance of high-resistance ground faults,

and the change in signal strength may be reduced.

Much of the cable installed at Areata is in steel pipe used as ducts. Not many airfields have similar installations, but the results

from tests on cables in steel pipe indicate the type of performance that can be expected from other cases of magnetic shielding. The magnetic

shielding effect occurs with armored cables, cable in short lengths of

pipe, cable in raceways, and perhaps cables below counterpoises. (No

tests were made on armored cables or cables near counterpoises as such

cables were not available.) Any magnetic shielding reduces the detected

signal strength but usually the signal can be followed. Frequently

ground-type faults can be located in the presence of magnetic shielding,

but for some conditions even low-resistance grounds cannot be detected.

The presence of other current-carrying cables in the same or adjacent

- 11 -

pipes, the concentration of return signal into the pipe, and the induction

of test signal into other cables often add to the confusion in locating

faults or lead the operator astray. The tests of cables in steel pipe

indicate the extreme effects of magnetic shielding. Tests of circuits

in armored cables and in conjunction with counterpoises are needed to

determine better approaches to fault location in these conditions.

6.2. Use and Comparison of Test Frequencies . For most conditions when

using the CFL, the 570-hertz signal with the 20-millisecond Frequency

Shift was satisfactory or preferred,, As a single frequency for this

equipment, this frequency would be preferred, but the other frequencies

were not evaluated on an equal basis with the 570-hertz frequency. With

a given test circuit the output voltage of the TSO varied only slightly

with test-signal frequency. However, the changes in the current in the

test circuit will be a function of the type of impedance of the circuit.

Because of the characteristics of the sensing element and the input of

the DU, the DU favored the 570-hertz signal by a factor of approximately

three. When using the headphones, the human ear also favors this signal

appreciably. Some of these factors favoring the 570-hertz signal are inherent and cannot be made comparable by design changes. The higher

frequency usually will be detectable to a position nearer an open fault

or open termination of a circuit; however, this signal can lead the

operator farther astray if he is misled onto a wrong circuit or conductor.

6.3. Signal Waveforms . The waveforms of test signals were studied with

an oscilloscope. The basic output voltage signals from both the TSO and

the AN/TSM-11 signal generator are square waves, but the TSO signal normally

has a sharp peak at the leading edge of the voltage wave. These output

voltage waveforms are often seriously distorted by the circuit to which

the signal generators are connected. The voltage waveform of the TSO is

also affected by the signal frequency and the Output-Voltage switch

position with the distortion increased as the frequency or Output Voltage

are increased. These test signals and a sine-wave voltage of comparable

frequency were applied to several circuits of various types, both open

circuits and grounded circuits, and the effects on the output voltages, on

the current into test circuit , on Undetected signals from the probe, and

at other points were observed. The sine-wave signals were least distorted and the TSO signal was more likely to be seriously affected. The

impedances of the generators were not equal nor were they matched to the

loads. This difference in impedance was responsible for the greater distortion of the TSO signal. Increasing the output impedance of the TSO

would reduce the distortion of the test signal current waveforms but

would also reduce the current into the test circuit.

The detected signal is dependent on the test current in the circuit

not the voltage applied to the circuit. The amplitude and wave shape of

the current in the test circuit are functions of the circuit impedance

and the applied voltage waveform, amplitude, and frequency. Since these

functions are interrelated, tests were made to compare the effectiveness

of the TSO signal and the sine-wave signal on some installed circuits.

In these tests, the sine-wave voltage applied to the circuit was at the

12

same amplitude and frequency as the TSO signal, and the DU was used to

detect and measure both signals. The sine-wave signal gave meter

readings 10 to 20 percent greater than those from the TSO signal. The

audible signal from the sine-wave signal was preferred at 570 hertz,

but was less satisfactory at 150 hertz. Neither signal showed any

marked advantage in locating either ground or open faults. For the

present DU, any improvement by changing the test signal to a sine wave

would be minor.

6.4. Matching Signal Generator to Test Circuit . The output power and

voltage of the TSO are increased by a factor of two over the AN/TSM-11

signal generator and the TSO has many more voltage taps for selection

but this additional voltage is not always usable. The operation of the

circuit breakers to deenergize the equipment when the collector current

exceeds a given value may reduce the voltage by a factor of two because

the next tap will just trip the circuit breaker. A continuously variable

output voltage or smaller voltage steps would be convenient. This over- current

protection usually functions before the voltage applied to the test circuit reaches a peak. The voltage from the TSO probably can equal or exceed that from the AN/TSM-11 signal generator in nearly all conditions.

However, for open circuit tracing or fault location or in cases of

radiation interference, a still higher voltage would be useful.

6.5. Sensing Element . Better filtering in the DU is needed for some conditions. This filtering could be aided by proper matching of the

sensing element or probe to the DU. A frequencyresponse test was made

with the probe connected to the DU and to the AN/TSM-11 detector-amplifier.

The test was made by applying a variable -frequency, constant-voltage, sine- wave signal to a circuit with the lead to the circuit wound around the

ferrite core of the probe and measuring the signal amplitude at the

connector of the probe. When the test signal was connected to the DU,

maximum response occurs at approximately 1550 hertz. The response is

more than four times that obtained at 570 hertz and over 10 times that

at 150 hertz. With the probe connected to the AN/TSM-11 detector-amplifier,

the peak response occurs at approximately 270 hertz and was down by a factor of two at 120 and 740 hertz. Better matching of the probe to the

DU for the test frequencies, especially at 570 hertz, should be a worthwhile improvement.

Some earlier tests with the AN/TSM-11 using an 8-inch loop as the

sensing element instead of the ferrite-core probe indicated that the

loop was more critical in directional alinement than was the probe.

This limiting of direction might be helpful where radiated interference

or stray paths of signal cause problems." Such a loop could be provided

for use as a substitute for the probe in problem areas.

13

6.6. Cable Tracing Problems . The CFL and AN/TSM-11 are both

satisfactory for tracing most cables, and the CFL was often useful

beyond the capabilities of the AN/TSM-11. Still, conditions occur when circuits cannot be followed satisfactorily with the CFL. The

problems in tracing cable are either from radiation interference from

other current-carrying circuits or from the operator following stray

test signal onto other conductors. Improvements in the CFL might be

helpful for these problem areas. Better filtering of the DU to further

reduce the effects of power frequencies and harmonics may be possible.

As compared to the response at any selected test signal frequency, the

DU response at any harmonic of the power frequency should be down at

least 10, and preferably 20, decibels. Matching the DU to the probe

may aid in this filtering. Increased power of the test signal would

help in cases of power signal interference and in approaching the

termination of open circuits. An auxiliary unit to increase the power

might be a possibility.

6.7. Ground Fault Location . Locating low-resistance ground faults

present few problems with this type equipment except in the presence of

signal interference or magnetic shielding. Results on high-resistance

faults, single or multiple, depend on conditions accompanying the faults.

A very high-resistance single fault can be detected under favorable

conditions but a fairly low-resistance fault will be hard to locate

under difficult conditions. Many ground faults, especially multiple

grounds, require "the try-and-see if you find it" approach. The

probability of finding such faults is improved by clearing all faults

which can be located; testing from both ends of the circuit, if practical;

and evaluating all possible fault indications carefully.

6.8. Open-Circuit Faults . Precise location of ungrounded open-circuit

faults can seldom be accomplished with equipment of this type, but the

CFL produces better accuracy than does the AN/TSM-11. The higher

frequency and increased power in the test signal are the major reasons

for this improvement. An increase of power in the signal would be one method of improving capability of locating open faults. An ungrounded

open cable appears basically as a capacitor to the TSO. The square-wave

test signal is differentiated by the cable capacitance and the current

into the cable is a sharp pulse. Tests comparing a sine-wave signal of

equal amplitude and frequency to the square wave from the TSO into open

circuits did show some increase in response of the DU to the sine wave*

but the accuracy in locating ungrounded open faults was not markedly

improved. A sine-wave signal might be beneficial if it can be attained

without too much extra cost.

6.9. Depth Determination . A method of determining the depth of buried

cable is needed for maintenance of underground circuits. In using this

type equipment for locating ground faults, a depth measurement is often

needed because an increase in cable depth gives a decrease in response
,
14

similar to the change at a fault. The AN/TSM-11 is not satisfactory

for measuring cable deptho The CFL can usually be used to satisfactorily determine the depth of a cable. The principle of the method

used is based on the fact that the magnetic field strength around a long, straight conductor carrying a current is inversely proportional

to the distance from the conductor. Thus, if the response of the

detector is linear with the magnetic field strength, a decrease in

response of one-half would accompany a doubling of distance from the

conductor. Since the response of the DU is linear with the strength

of the detected signal, by making a reading at the ground surface

directly over a cable with the test signal and then raising the probe

until the meter reading is reduced to one-half the former reading, the

depth of the buried cable should equal the distance the probe is raised.

The response of the AN/TSM-11 detector-amplifier is not linear, thus it

is not satisfactory for depth measurements.

Depth measurements in the vicinity of turns in the cable path are often in error. To measure depth accurately, it is necessary to stay 10

to 20 feet from turns in the cable. Some depth measurements of cable in

steel pipe give satisfactory results but others do not. Depth measurements in the presence of detectable signals from another source or of

the test signal in another conductor are likely to be in error. The

presence of other signals causes most of the error in the unsatisfactory

measurements of cable in pipe. To avoid the effects of- magnetic shielding

and of changes in direction of the cable, depth measurements should be at

least 10 feet from light fixtures and handholes.

7. CONCLUSIONS AND RECOMMENDATIONS

A person with little experience and training can use the CFL

successfully in many situations, but in cases of radiation interference,

magnetic shielding, leakage and stray signals, and certain types of faults,

experience and careful use are required. The CFL is a definite improvement over the AN/TSM-11 in many respects; however, there are still con- ditions where satisfactory results cannot be obtained. From the results

of these field tests, some improvements are recommended.

7.1. Recommendations for Improvements . Certain features of the CFL

should be changed before production in quantity. These needed improvements are as follows:

1. Redesign the DU for better balance, shape, and size. The

balance should consider the operating position with the

probe attached. Eliminate square corners and edges,

especially where the unit rests in the hand.

2. Provide a type of headphones that will be convenient to wear

and not interfere with hearing traffic or aircraft. A single earphone should be satisfactory. Also shorten the
»

'

‘

.
15

length of the cord from the headphones to the DU.

An extension cord arrangement of short sections is

desirable

3. Make the DU, including the connector for the probe

and the headphone jack, waterproof or as watertight

as possible.

4. Investigate the savings that may result from

eliminating the 270-hertz test signal and the

200-millisecond Frequency Shift. If the savings

are appreciable, remove these features.

5. Improve the filtering of the DU, especially for the

570-hertz frequency. The response at all harmonics

of the power frequency should be down at least 10

decibels and preferably down 20 decibels.

6. If the filtering of the DU is improved significantly,

provide a circuit for detecting 60-hertz signal for use in tracing energized power circuits.

7. Provide better matching of the sensing element to the

DU circuits to improve filtering.

16

REFERENCES

1. Handbook of Operation and Service Instructions,

Cable Test-Detecting Set AN/TSM-11, NAVAER

08-20-501, February 1955.

2 . Circuit Description of a Prototype Cable Fault

Locator , David Brenner, National Bureau of

Standards Report 9886, July 1968.

3 . Guide to Use of the AN/TSM-11 Cable Test-Detecting

SetJames E. Davis and J. W. Simeroth, National

Bureau of Standards Report 8596, December 1964.

USCOMM-NBS-DC '

I