2007 - ieee guide for fault locating techniques on shield

8/2/2019 2007 - IEEE Guide for Fault Locating Techniques on Shield

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IEEE Std 1234-2007

IEEE Guide for Fault-Locating

Techniques on ShieldedPower Cable Systems

IEEE3 Park AvenueNew York, NY 10016-5997, USA

16 November 2007

IEEE Power Engineering Society

Sponsored by theInsulated Conductors Committee

12

34T

M

Authorized licensed use limited to: UNIV ESTADUAL PAULISTA JULIO DE MESQUITA FILHO. Downloaded on June 14,2011 at 13:14:35 UTC from IEEE Xplore. Restrictions ap


2/37Authorized licensed use limited to: UNIV ESTADUAL PAULISTA JULIO DE MESQUITA FILHO. Downloaded on June 14,2011 at 13:14:35 UTC from IEEE Xplore. Restrictions ap


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IEEE Std 1234-2007

IEEE Guide for Fault-LocatingTechniques on ShieldedPower Cable Systems

Sponsor

Insulated Conductors Committeeof theIEEE Power Engineering Society

Approved 17 May 2007

IEEE-SA Standards Board



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Abstract: Tests and measurements that are performed on shielded power cables to identify thelocation of a fault are described. Whenever possible, the limitations of a particular test andmeasurement to locate a fault are provided and recommendations are made regardingspecialized fault-locating techniques. A fault characterization chart is included as an aid to selecta fault-locating technique.Keywords: arc reflection, cable fault locating, cable testing, grounding, safety, sectionalizing,thumping, time domain reflectometry (TDR)

_________________________

The Institute of Electrical and Electronics Engineers, Inc.3 Park Avenue, New York, NY 10016-5997, USA

Copyright 2007 by The Institute of Electrical and Electronics Engineers, Inc.All rights reserved. Published 16 November 2007. Printed in the United States of America.

IEEE is a registered trademark in the U.S. Patent & Trademark Office, owned by The Institute of Electrical and ElectronicsEngineers, Inc.

National Electrical Safety Code and NESC are both registered trademarks and service marks of the Institute of Electricaland Electronics Engineers, Inc.

Print: ISBN 0-7381-5631-0 SH95696PDF: ISBN 0-7381-5632-9 SS95696

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the priorwritten permission of the publisher.



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Introduction

This introduction is not part of IEEE Std 1234-2007, IEEE Guide for Fault-Locating Techniques on

Shielded Power Cable Systems

Many fault-locator personnel are experienced in locating short and open circuits on shielded power cables.Proper locating of high-resistance or intermittent cable faults, which are the majority of the faults on cables

with extruded dielectric insulation, is considered tedious, inconsistent, and time-consuming. Therefore,

re-closing, re-fusing, burning, and thumping at unnecessarily high voltage and energy levels, in order togenerate an open or short circuit, are frequently used without consideration of cable and equipment

properties. The danger of activating dormant faults, generating new faults, or damaging utility and

customer equipment by improper locating methods is not always recognized.

By establishing cable fault-locating guidelines and training programs that incorporate recommended cable

fault-locating measurements and techniques, cable owners can realize substantial savings in manpower and

cable and equipment replacement, and minimize losses from customer outages.

Some information and figures in Clause 4, Clause 5, Clause 6, and Annex B, Annex C, and Annex D are

copyrighted by Gnerlich, Inc. and used with permission.

Notice to users

Errata

Errata, if any, for this and all other standards can be accessed at the following URL:http:// standards.ieee.org/reading/ieee/updates/errata/index.html.

Users are encouraged to check this URL for errata periodically.

Interpretations

Current interpretations can be accessed at the following URL:

http://standards.ieee.org/reading/ieee/interp/ index.html.

Patents

Attention is called to the possibility that implementation of this guide may require use of subject matter

covered by patent rights. By publication of this guide, no position is taken with respect to the existence orvalidity of any patent rights in connection therewith. The IEEE shall not be responsible for identifyingpatents or patent applications for which a license may be required to implement an IEEE standard or for

conducting inquiries into the legal validity or scope of those patents that are brought to its attention.

ivCopyright 2007 IEEE. All rights reserved.



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Participants

At the time this guide was submitted to the IEEE-SA Standards Board for approval, the C3TF1 Working

Group had the following membership:

Hans R. Gnerlich,Chair

Wolfgang B. Haverkamp,Vice Chair

Ted. A. Balaska

Earle C. Bascom

Vern BuchholzTom C. ChampionJack E. CherryFrank DiGuglielmo

Bill LarzelereMatthew S. Mashikian

James D. MedekDale T. MetzingerJohn T. Nierenberg

John S. RectorEwell T. Robeson

Lawrence W. SalbergNagu N. SrinivasGordon W. WhittenT. Shayne Wright

deceased

The following members of the individual balloting committee voted on this guide. Balloters may have

voted for approval, disapproval, or abstention.

James Fitzgerald

Arthur R. FitzpatrickMarcel FortinRobert B. GearHans R. Gnerlich

Richard L. HarpWolfgang B. HaverkampStanley V. HeyerLauri Hiivala

Richard A. HuberLawrence J. KellyAlbert KongCarl Landinger

Gabor Ludasi

Gregory Luri

Glenn LuzziMatthew S. MashikianSpiro G. MastorasL. Bruce McClung

J. D. MedekJohn E. Merando JrGary L. MichelDaleep C. Mohla

Shantanu NandiJames J. PachotArthur V. Pack Jr

Neal K. Parker

Gary PolhillDennis C. Pratt

Radhakrishna V. Rebbapragada

Robert A. ResualiJoseph H. Snow

Nagu N. SrinivasFrank Stepniak

John TanakaWilliam A. ThueStephen E. TurnerGerald L. Vaughn

Donald A. VoltzDaniel J. WardCarl WentzelWilliam D. Wilkens

Joe Zimnoch

When the IEEE-SA Standards Board approved this Standard on 17 May 2007, it had the following

membership:

Steve M. Mills, Chair

Robert M. Grow, Vice Chair

Don F. Wright,Past Chair

Judith Gorman,Secretary

Richard DeBlasioAlexander D. Gelman

William R. GoldbachArnold M. GreenspanJoanna N. GueninJulian Forster*Kenneth S. Hanus

William B. Hopf

Richard H. HulettHermann Koch

Joseph L. Koepfinger*John D. KulickDavid J. LawGlenn ParsonsRonald C. Petersen

Tom A. Prevost

Narayanan RamachandranGreg Ratta

Robby RobsonAnne-Marie SahazizianVirginia C. SulzbergerMalcolm V. ThadenRichard L. Townsend

Howard L. Wolfman

*Member Emeritus

vCopyright 2007 IEEE. All rights reserved.



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Also included are the following nonvoting IEEE-SA Standards Board liaisons:

Satish K. Aggarwal,NRC Representative Alan H. Cookson, NIST Representative

Lorraine PatscoIEEE Standards Program Manager, Document Development

Matthew J. CegliaIEEE Standards Program Manager, Technical Program Development

viCopyright 2007 IEEE. All rights reserved.



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Contents

1. Overview .................................................................................................................................................... 1

1.1 General ................................................................................................................................................ 11.2 Scope ................................................................................................................................................... 1

1.3 Purpose ................................................................................................................................................ 12. Normative references.................................................................................................................................. 1

3. Definitions, acronyms, and abbreviations .................................................................................................. 2

3.1 Definitions ........................................................................................................................................... 23.2 Acronyms and abbreviations ............................................................................................................... 3

4. Safety.......................................................................................................................................................... 4

4.1 Safety practices.................................................................................................................................... 44.2 Responsibility ...................................................................................................................................... 44.3 Precautions .......................................................................................................................................... 54.4 Grounding............................................................................................................................................ 5

5. Cable system fault characteristics............................................................................................................... 6

5.1 Radial distribution ............................................................................................................................... 65.2 Network distribution............................................................................................................................ 85.3 Cable system faults.............................................................................................................................. 9

6. Cable system fault locating....................................................................................................................... 10

6.1 Fault-locating preferences chart......................................................................................................... 106.2 Sectionalizing .................................................................................................................................... 116.3 Insulation Resistance ......................................................................................................................... 126.4 Time domain reflectometry ............................................................................................................... 136.5 Capacitive discharge (thumping)....................................................................................................... 14

6.6 Burning (fault conditioning).............................................................................................................. 146.7 Surge arc reflection............................................................................................................................ 156.8 Burn arc reflection............................................................................................................................. 166.9 Surge pulse reflection ........................................................................................................................ 166.10 Decay method.................................................................................................................................. 176.11 Bridge techniques ............................................................................................................................ 176.12 Tracing/locating/pinpointing ........................................................................................................... 18

Annex A (informative) Bibliography ........................................................................................................... 21

Annex B (informative) First response cable system fault location in URD ................................................. 22

Annex C (informative) Fault location in network feeders ............................................................................ 24

C.1 Fault tracing ...................................................................................................................................... 24C.2 TDR Assisted fault location.............................................................................................................. 24

Annex D (informative) Fault location on cable systems with concentric neutral corrosion......................... 26

Annex E (informative) Recommended minimum of fault-locating tools ..................................................... 27

viiCopyright 2007 IEEE. All rights reserved.



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IEEE Guide for Fault-LocatingTechniques on ShieldedPower Cable Systems

1.

1.1

1.2

1.3

2.

Overview

General

This guide has been developed as a guide for cable fault-locating techniques on shielded power cablesystems. It is intended to emphasize those fault-locating techniques that maintain cable integrity, reduce

customer outage time, and consider customer equipment sensitivity and safety. This guide applies to all

insulated, shielded power cable systems.

Scope

The introduction of cables with extruded dielectric insulation and of modern splicing technology has

imposed new conditions and restrictions on cable fault locating. The use of excessive high voltages and

energies during ac, dc, and surge testing of service-aged power cable systems with extruded dielectricinsulation may overstress insulation, creating defects that become faults after the cables are returned to

service.

This guide is intended to be applied to medium-voltage distribution cables. Medium-voltage distribution

systems generally operate at system voltages above 1 kV and up to 34.5 kV nominal.

The end user of the cable circuit should evaluate the necessity for verifying the integrity of extruded

dielectric insulated cables, and, if they are in critical service, proceed to perform the high-voltage/energies

testing. If not detected during dielectric tests, defects in dielectric materials may result in cable failuresduring the transient voltage surge episodes while in service.

Purpose

This guide is intended to provide trouble-shooting and testing personnel with information to quicklyidentify a faulted cable section and/or locate a cable fault with minimum risk of further damaging

serviceable cables, terminations, and equipment.

Normative references

The following referenced documents are indispensable for the application of this document. For dated

references, only the edition cited applies. For undated references, the latest edition of the referenced

document (including any amendments or corrigenda) applies.

1Copyright 2007 IEEE. All rights reserved.



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IEEE Std 1234-2007IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems

IEEE Std 510-1983, IEEE Recommended Practices for Safety in High-Voltage and High-Power Testing

(Reaff 1992).1, ,2 3

3.

3.1

Definitions, acronyms, and abbreviations

Definitions

For the purposes of this guide, the following terms and definitions apply. The Authoritative Dictionary ofIEEE Standards, Seventh Edition [B9]4, should be referenced for terms not defined in this clause.

3.1.1 aerial installation type: An assembly of insulated conductors installed on a pole or similar overhead

structure; it may be self-supporting or installed on a supporting messenger cable.

3.1.2 bolted fault: A cable fault having a resistance value of less than 5 .

3.1.3 branch circuits: A cable system in which independent cables branch out radially from a commonsource of supply. (See also: radial feed)

3.1.4 breakdown: A disruptive discharge through insulation.

3.1.5 cable tray installation type: A structure of ladders, troughs, channels, solid bottom, and other

similar devices through which cables systems may be routed.

3.1.6 characteristic impedance: The driving impedance of the forward-traveling transverse electro-

magnetic wave. In cable fault locating, an incident wave on a cable (time domain reflectometer [TDR],

thumper, etc.) is reflected back to the source positively, negatively, or not at all by discontinuities and

inhomogenities in the cable where impedance values differ from the characteristic cable impedance,

respectively.

3.1.7 concentric neutral shield (metallic shield type): Wires helically applied over the semi-conducting

insulation shield to carry charging, fault, and neutral currents.

3.1.8 conduit installation type: A structure containing one or more ducts.

NOTEConduit may be designated as iron pipe conduit, tile conduit, etc.

3.1.9 direct buried installation type: Cable laid in a trench or pre-cast trough and covered with sand,specially prepared backfill material, and/or excavated material; or, cable plowed directly into the earth or

installed into the earth with guided boring techniques.

3.1.10 direct distribution: A primary feeder or cable that supplies energy directly to a consumer.

3.1.11 drain wires shield (metallic shield type): Wires helically applied over the semi-conducting

insulation shield to carry charging currents only.

3.1.12 extruded dielectrics: Insulation like polyethylene (PE), crosslinked polyethylene (XLPE), tree

retardant crosslinked polyethylene (TR XLPE), ethylene propylene rubber (EPR), etc.

3.1.13 flashover: A disruptive discharge through air around or over the surface of a solid or liquidinsulation, between parts at different potential, produced by the application of voltage wherein the

breakdown path becomes sufficiently ionized to maintain an electric arc.

1 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854,

USA (http://standards.ieee.org/).2 The IEEE standards or products referred to in this clause are trademarks of the Institute of Electrical and Electronics Engineers, Inc.3 IEEE Std 510-1983 has been withdrawn; however, copies can be obtained from Global Engineering, 15 Inverness Way East,Englewood, CO 80112-5704, USA, tel. (303) 792-2181 (http://global.ihs.com/).4 The numbers in brackets correspond to those of the bibliography in Annex A.




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3.1.14 installation types:See: aerial installation type, cable tray installation type, conduit installation

type, direct buried installation type, and submarine installation type.

3.1.15 laminated dielectrics: Insulation like paper used in PILC cable design.

3.1.16 LC (longitudinally corrugated) shield (metallic shield type): A longitudinally-applied,

corrugated shield of copper or aluminum. LC shields are typically designed to carry both charging and faultcurrents, and sometimes neutral currents.

3.1.17 lead sheath shield (metallic shield type): An extruded layer of lead that serves as a metallic shield

and also as a hermetic moisture barrier.

3.1.18 loop feed: A number of tie feeders in series, forming a closed circuit.

3.1.19 metal tape shield (metallic shield type): A tape helically applied over the semi-conducting

insulation shield. Tape shields are typically designed to carry charging currents and limited fault currents.

3.1.20 network distribution:See: network feeder.

3.1.21 network feeder: A primary feeder that supplies energy to a secondary network.

3.1.22 pinpoint: To locate exactly the fault site for excavation and repair.

3.1.23 pre-locate: Locating the general area of a fault as a distance from cable start, end, splicetransformer, change in cable type, etc. Identifying a faulted section of cable between two transformers,

junction boxes, manholes, etc.

3.1.24 propagation velocity: The velocity at which an electric signal travels through a cable. Propagation

velocity is usually expressed in feet, yards, or meters per microsecond or as a percentage of the speed of

light. The value of the propagation velocity depends on the (relative) dielectric constant of the insulationmaterial used, the characteristic of the semicon shields, and the cable construction; it is assumed constant

for all practical purposes.

3.1.25 radial feed: A cable system in which independent feeders branch out radially from a common

source of supply.

3.1.26 reflection coefficient: A measure of how much of an incident wave is reflected back to the source.

3.1.27 shield (metallic shield types):See: concentric neutral shield (metallic shield type), drain wires

shield (metallic shield type), LC shield (metallic shield type), metal tape shield (metallic shield type),

and lead sheath shield (metallic shield type).

3.1.28 shield interrupt: An insulated break installed in a cable shield so as to interrupt the flow of induced

current in the metallic shield.

3.1.29 shielded cable: A cable in which each insulated conductor or conductors is/are enclosed in a

conducting envelope(s).

3.1.30 submarine installation type: A cable designed for service under water.

3.2 Acronyms and abbreviations

EPR: ethylene propylene rubber




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HV: high voltage

LC: longitudinally corrugated

PE, XLPE: polyethylene, crosslinked polyethylene, cable insulation.

PILC: paper insulated lead covereda cable design.

TDR: time domain reflectometer, frequently referred to as cable radar in the power industry.

URD: underground residential distribution

4.

4.1

4.2

Safety5

Safety practices

When testing, personnel safety and service reliability of the electrical systems are of utmost importance. All cable and

equipment tests must be performed on isolated and de-energized systems, except where otherwise specifically required

and authorized. The safety practices must include, but are not limited to, the following requirements:

a) Applicable user safety operating procedures

b) IEEE Std 510-1983

c) Applicable state and local safety operating procedures

d) Protection of utility and customer property

While testing, one or more cable ends will be remote from the testing site, therefore:

Cable ends must be cleared and guarded

Cables must be de-energized and grounded before testing is begun

At the conclusion of high-voltage (HV) testing, attention should be given to the following:

Special techniques required for discharging cables and cable systems

Grounding requirements for cables to eliminate the aftereffects of the cables dielectric absorption andcapacitance characteristics

Responsibility

Training requirements for cable fault-locating and trouble-shooting personnel will vary with cable type,

installation, system, environment, and the equipment and instruments used. Operations and cable fault-locating departments should establish initial and continuing education training programs to qualify their

cable fault-locating and trouble-shooting personnel.

The minimum qualification for the responsible, on-site cable fault locator or trouble-shooter should

include, but is not limited to the following:

Initial training in the use of cable fault-locating instruments and devices with thorough understandingof their advantages and limitations

Familiarity with all applicable user, state, and local safety operating procedures

5 Some of the material appearing in this document is adapted with permission from Gnerlich, Inc. training courses. [B5], [B6]




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Knowledge of cable and equipment specifications and the ability to select cable fault-locatingtechniques, instruments, and devices that minimize the risk of damaging cable, joints, terminations,and equipment

4.3 Precautions

Many cable fault locators and trouble-shooters use a high-voltage dc test (see IEEE Std 400 [B11]6) aspart of their standard fault-locating procedure. In the late seventies, it became apparent that dc testing may

exacerbate cable defects in service-aged extruded dielectric insulation lacking tree-retardant properties.

Such cables may ultimately fail sooner than they would have if dc testing had not been performed.

Therefore, proof testing of service-aged cables with extruded dielectric insulation lacking tree-retardant

properties is not recommended.

If dc proof testing of service-aged cables should become necessary for a justifiable reason, the cable

manufacturer should be consulted for the maximum dc maintenance test value. For example, testing of

cables by qualified cable fault-locating and trouble-shooting personnel, in critical service areas such as

hospitals, continuous-process industries, and cold-storage units, still provides the advantage of identifying

deteriorated cables prior to their failure, and enabling repairs/replacement to be done, under plannedconditions, without sudden interruption of service.

4.4

Grounding

Cables can only be considered de-energized and grounded when the conductor and the concentric shield are

connected to the system ground at the test site, and if possible at the far end of the cable.

When fault-locating on a defective cable, installation, or system, a single system ground at the test site is

recommended (see Figure 1). The shield or concentric conductor of the faulted cable is connected to system

ground. If this connection is missing, deteriorated, or has been removed, it must be replaced at this time. Asafety ground cable must connect the instrument case with system ground. If the test instrument is an

HV device, the safety ground cable should be at least a braided or stranded #2 copper cable. Only after the

safety ground cable is in place should the test cable be connected to the center conductor and concentricshield; the center conductor-to-ground connection can then be removed.

Should a local ground be advisable or required for the test equipment, the case ground must remain

connected to the system ground in order to maintain an acceptable single ground potential.

All ground connections must be screw-type connections, which cannot accidentally be disconnected.

Copyright Gnerlich, Inc. Used with permission.

Figure 1 Single system ground at test site

6 The numbers in brackets preceded by the letter B correspond to those of the bibliography in Annex A.




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5. Cable system fault characteristics

Cable faults vary, and even similar faults may exhibit different symptoms in different environments, cable

systems, cable types, or applications. To be able to more readily diagnose a fault and select the proper

operating procedure, cable fault locating can be divided into direct radial distribution and network

distribution categories. Cable fault-locating procedures in radial distribution can be tailored to a particular

problem and are easily controlled (see Figure 2). In Network Distribution, fault-locating procedures dependon many interrelated parameters that make solving a particular problem more complex.

Cable fault-locating

operating environment

Radial distribution

Cables are isolated

(sectionalized)

Cable loop systems with

transformers, lightning

arrestors, etc. connected

Radial, single conductor

cable systems with a

few branches

Three conductorsubmarine, armored,

and pipe-type cables

Overall circuit length

and number of branches

Lumped cable system

capacitance

Insulation type

Fault resistance

Transformer primary

connection

Network distribution

Figure 2 Radial distribution and network distribution categories, which determine cable fault-locating operating procedures.

5.1 Radial distribution

In radial distribution, verifying cable length, presence of transformers in a loop, short or open circuits, and

concentric neutral corrosion with a TDR are the recommended diagnostic procedures on which to base the

selection of fault-locating tools and methods. It should be the cable fault locators ultimate goal to

efficiently restore customer service while maintaining cable and equipment integrity. Procedures for cablefault locating are listed as follows, and techniques for determining the location of the fault are described in

Clause 6.

a) Cables are isolated (sectionalized)

1) With a TDR, the cable length and the location of splices should be verified.

2) With an insulation resistance tester/ohmmeter, the fault resistance,R, may be measured.




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3) IfR < 5 , the distance to the fault can be measured with a TDR, the faults location

pinpointed with audio frequency (tone) tracing equipment.

4) IfR > 500 , a thumper, HV coupler, and TDR combination may be used to measure the

distance to the fault. Acoustic and/or electromagnetic detectors will facilitate the

verification of the faults location.

5) If 5

500 , a thumper and/or burner, HV coupler, and TDR combination should be

used to measure the distance to the fault. The precise fault location can be verified

with acoustic and/or electromagnetic instrumentation.

v) If 5


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5.2

5.2.1

5.2.2

Network distribution

Network cable systems form the backbone of most three-phase underground distribution systems in areasof high load density and maximum reliability requirements. A network cable system is characterized by

circuits with multiple branches and service taps. It is installed within a duct/manhole system. Transformer

primaries are connected either directly to the cable via oil-filled termination boxes and preformed elbows,

or through sets of disconnect switches. The secondaries of network transformers, fed from multiple primary

circuits, are paralleled. Each transformer secondary can be isolated via a network protector.

Safety consideration

Network cable systems require mention of several additional safety issues, since the secondaries of

transformers are tied to a common bus. With transformer primaries connected in a delta configuration, aprimary cable could be energized via a closed network protector due to a faulty master relay within the

protector. To avoid backfeeding of transformer primaries and cable, all network protectors must be locked

in the open position before connecting fault-locating equipment. After verifying the status of all protectors,

the primary cables must be checked for voltage and must be grounded.

Fault-locating parameters

In network distribution, fault-locating efforts often will require more than one fault-locatingmethod. In a specific network, to select the right tool, the following factors should be considered

and weighed:

a) The overall circuit length, number of branches, and the number of connected transformers willdetermine the effectiveness of a fault-locating method. For efficient fault locating with TDR

techniques, more than one access point should be available in each network circuit. As a general

rule, one access point for every three to four branches is desirable.

b) Direct access to the defective cable is necessary for effective use of TDR, surge and burn arcreflection, surge (current) pulse, and voltage decay techniques. An impedance mismatch

between test equipment and test object will limit or prevent the use of TDR techniques.

c) The total lumped capacitance of the cable system limits the effective use of surge generators.When using a surge arc reflection method, a surge generator with internal capacitor of 10 times

the cable capacitance is necessary. Burn arc reflection with an ac or dc burn set capable ofmaintaining an arc current of 4 A to 5 A is also very effective in locating a faulted cable sectionwith a TDR.

d) The type of cable insulation restricts the use of burning and dc test voltages. Oil-paperinsulated cables often are subjected to burning in order to reduce the fault resistance for ease ofidentification. Burning of solid dielectrics usually does not result in a reduced fault resistance.

More importantly, burning of cables with solid dielectric insulation for relatively short periods

of time may lead to explosions; if the insulation ignites, manhole or duct fires can destroy

unfaulted and energized cables in the vicinity of the fault. In general, burning should only be

applied to paper insulated cables or cables submerged in water. Burning of cable faults should

always be monitored with a TDR, thus minimizing burning time and possible damage.

e) Transformer primary connections must be considered when selecting a cable fault-locating

method in situations where the cables cannot be isolated. Many network circuits utilize delta-connected transformer primaries, which are permanently connected to the cables. All phases aretied together, causing unwanted paths and reflection points for TDR-type fault-locating

equipment. A grounded, unfaulted phase will eliminate the use of fault-locating methods using

dc equipment. Grounded wye-connected transformer primaries also will preclude the use of dc

fault-locating equipment.

f) Whenever possible, the fault resistance should be measured using an insulation resistancetester/ohmmeter combination.




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5.2.2.1

5.2.2.2

Table 1

Tracer methods

The lack of direct access to a faulted cable, a large number of branches, and transformers that cannot be

disconnected will not permit the selection of TDR or bridge-based fault-locating methods. Therefore, ac,

dc, or pulse (surge) tracing methods are used to identify a faulted cable section. In tracer-type cable fault

locations, Walking the Route and entering manholes to locate an audible or electromagnetic signal arenecessary. Tracing methods are very popular since they require minimal training. They are, however,

manpower- and time-intensive.

Terminal methods

When direct access to a faulted cable exists and TDR and bridge-type fault-locating techniques are

possible, the measured fault resistance value will suggest which fault-locating method(s) to attempt first.

Table 1 may be used as a guide for preferred techniques.

Locating methods for various fault resistance values

Insulation resistance test

R < 0.1 M R > 0.1 M

Ohmmeter test

R < 5 5 1 k R < 500 M R >> 500 M

a. TDR Direct

b. Comparison anddifference methods

c. N/A

d. N/A

e. N/A

f. Bridge techniques

g. N/A

a. N/A


c. Surge arc reflection

d. Burn arc reflection

e. Surge pulse method


g. N/A

a. N/A



d. N/A



g. N/A

a. N/A

b. N/A


d. N/A


f. N/A

g. N/A

a. N/A



d. N/A


f. N/A

g. Decay method

NOTETDR direct, comparison and difference, surge and burn arc reflection, surge pulse, and decay methods areavailable in the majority of power utility TDRs and HV couplers. These techniques are not available on

telecommunication TDRs.7

5.3

Cable system faults

A fault can be described as a sparkgap in parallel with a nonlinear resistance. The sparkgap-nonlinear

resistance equivalent circuit can be thought of in shunt or in series with a cable section (see Figure 3).

Actual faults may be a combination of shunt and series faults.

Adapted from figure copyright Gnerlich, Inc. Used with permission.

Figure 3 Models of shunt (left) and series (right) cable faults

7 Notes in text, tables, and figures of a standard are given for information only and do not contain requirements needed to implement

the standard.




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Cable faults may be categorized as series or shunt, short or open circuit, phase-to-ground or phase-to-phase,

and nonlinear voltage dependent or nonlinear current dependent. Table 2 lists possible fault types based on

their electrical characteristics.

Table 2 Cable fault categories based on their electrical characteristics

Shunt faults

To detect a shunt fault, an insulation resistance test isperformed. The cable end remains open-circuited.

Series faults

To detect a series fault, a continuity test isperformed. The cable end remains short-

circuited.

Short circuita) Mechanical damage has forced center conductor and

concentric into contact.

b) Burnt cable insulation; a low resistance,R < 5 ,

carbon-metal bridge exists between conductor and

concentric.c) Evaporated insulation permits a low-resistance path

between conductor and concentric.

Nonlinear (voltage dependent)a) Most faults on cables with extruded dielectricinsulation fall into this category; at low voltage,

V < 500 volts, cable exhibits characteristics of an

unfaulted cable; at a voltage, V > 500 volts, the cablefault flashes over, or the cable fault exhibits the

characteristics of a nonlinear voltage-dependent

resistance.b) In submerged cable faults, the shunt resistance changes

with applied voltage.

Open circuitCable will often hold a dc voltage greater than the conductor-to-

ground voltage.

a) Mechanical damage, open termination, or separatedsplice.

b) Through re-closing, conductor is blown apart and theconductor end is electrically sealed off.

Open circuit

a) Mechanical damage. Concentric,sheath or conductor is severed;

separated splice.

b) Electrical damage. Cable, joints, orterminations are blown apart.

NOTEThis kind of cable system damage is

often caused when sectionalizing via re-closingor re-fusing is practiced.

Nonlinear (current dependent)a) Concentric neutral corrosion

b) Deteriorating splice or terminationc) Burnt conductord) Water-soaked blown-out fault

6.

6.1

Cable system fault locating

By eliminating re-closing, re-fusing, and unnecessary or excessive thumping, cost savings will be realized

due to reduced stress on cable insulation, cable accessories, transformers, and customer and utility

equipment. The following paragraphs describe various cable fault-locating devices and techniques. Variouscable fault-locating devices and techniques are described in 6.1 through 6.12.

Fault-locating preferences chart

To reduce the HV stress on service-aged cables, faults should be diagnosed. Fault-locating techniques that

enable fault locating at the lowest possible voltage in the shortest amount of time should be selected. Table

3 lists preferred pre-locating techniques for the most common types of cable faults. Re-closing or re-fusing

are not acceptable fault-locating methods.




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Table 3 Preferred pre-locating techniques

Bolted fault,short circuit

faults

Openconductors,

concentricneutral

corrosion

Oil- orwater-

soaked faults

HV flashoverfaults, high-

resistancefaults

Conductor-to-ground

faults

Very high HVflashover

faults,intermittent

faults

ConventionalTDRs

Comparison anddifference TDRs

Surge arc reflection

method

Surge pulse reflectionmethod

Burn arc reflection

method

Decay methodvoltage coupled

Bridge methods

TDR and bridge methods permit fault locating with the highest benefit /cost ratio. However, fault locating

with TDR and bridge techniques is not possible for all cable installations. Powerful thumpers and ac or dc

burnsets inject fault currents into the defective cable system. AC, DC, or pulse (surge) tracing instruments

are used to follow the fault current signal to the fault.

Fault conditioning, a euphemism for burning the cable fault for hours or days into a low-resistance state, is

often required when using current-tracing techniques. Current-tracing methods are quite destructive and

may result in cable system fires in the vicinity of the fault.

6.2 Sectionalizing

The cut and try method involves actual cutting or separation of a length of cable. The cut sections are

individually tested using a dc hipot or other tests. The method is repeated until a small enough section ofcable containing the fault is identified and removed. This is a very crude and costly method.

The sectionalizing by re-fusing method presently used on URD loops is very similar to the cut and try

method in that fuses and cable are sacrificed. Portions of a cable loop are isolated, and system line-to-

ground voltage is used for testing the remaining cable system section. This method typically results indamage to customer and utility equipment due to switching surges and fault currents. Therefore, this is not

a recommended fault-locating method.

Rather than closing in on a section of cable in order to determine if it is good or bad, dc testing of thecable section may be performed in which portable dc test sets with several mA of current are used.

Also popular is the use of rectified system line-to-ground voltages. In this method, the rectified voltage is

applied to the cable to be tested. While the cable charges, a current will flow. The current will stop flowing

when the cable has charged. If the cable has a fault, the current continues to flow. However, using rectified

system line-to-ground voltages has drawbacks. For example, cable systems with leakage currentscomparable to the available current from the rectifier may appear to have a fault when none exists.

Furthermore, the method is very time consuming. Since the dc resistance of a transformer is only a few

ohms, all transformers have to be disconnected before the test on a piece of cable can be performed. Even




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though the method is time-consuming and not always reliable, it has been very popular since little

personnel training is required.

Fault indicators are devices that sense the magnetic field produced by the fault current. They have beenused by utilities for many years and can be a great help in pre-locating the section of cable with the fault. A

reason that fault indicators are quite popular is again that very little personnel training is required. In

addition, fault locating is at a minimum if the section of cable is in a conduit and will categorically be

replaced when it is suspected to be defective. A major drawback to the use of fault indicators is cost; not

just the installation and maintenance cost, but also the man-hours required interrogating the devices.

6.3

Table 4

Insulation resistance

An insulation resistance tester/ohmmeter may be used as a diagnostic tool for locating cable faults. At

insulation resistance test voltage levels of 500 V to 2500 V, and ohmmeter test voltage levels of 1.5 V to

9 V, a cable fault can be categorized and the effectiveness of a cable fault-locating technique can thus be

predicted. In Table 4 and Table 5, cable faults are diagnosed from series resistance (continuity) and shunt

resistance measurements.

Fault diagnosis from series resistance (continuity) measurements

R Problem Solution

R < 5 High-resistance shunt fault. Measure fault shunt resistance.

5 1 M Sealed off conductor.

Separated splice or termination.Missing concentric or sheath.

With a TDR, the exact problem shall be determined

and the appropriate fault-locating procedure andtechnique selected.

Table 5 Fault diagnosis from shunt resistance measurements

R Problem Solution

R > 1 M High-resistance shunt fault.

Disintegrated concentric.

Separated splice.Open conductor.

A HV fault-locating technique such as arc reflection,

surge pulse, or voltage decay must be used.

With a TDR, the exact problem shall be determined

and the appropriate fault-locating procedure andtechnique selected.

R < 1 M With an ohmmeter the fault resistance,R, shall be measured.

R > 500 Solid shunt fault. A HV fault-locating technique or a bridge technique

for three conductor cables shall be used.

5


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6.4

6.4.1

6.4.2

Time domain reflectometry

Time domain reflectometers (TDRs) transmit short-time-duration pulses into the cable to be tested. The

elapsed time of a transmitted pulse traveling the entire length of a cable and the pulse reflections producedby deviations from the homogenous structure of the cable are displayed on a display screen. Any reflecting

surfaces, cable start, joints, splices, transformers, faults, changes in cable type, as well as cable end, are

shown in time sequence.

When the propagation velocity of a pulse through a cable is programmed into a TDR, the distance betweencable start and any discontinuity or irregularity can be determined from the reflection-time display. A

TDRs digital readout provides distance to the fault, as well as cable length measurements.

Limitations to time domain reflectometry

The magnitude of the pulse reflections produced by deviations from the homogeneous structure of the cableare determined by the reflection coefficient shown in Equation (1):

R = (Z-Zo) / (Z+Zo) (1)

R is resistance;Zois the cables characteristic impedance and Zan impedance value electrically describing

cable start, joints, splices, faults, changes in cable type, as well as cable end. For shunt cable faults onconcentric cables where the fault impedance Z is in parallel to the characteristic impedance, Zo, the

reflection coefficient derives from Equation (1) to be as follows in Equation (2):

R = (-Zo) / (2Z+Zo) (2)

Shunt cable faults between center conductor and concentric with resistance values much greater than the

characteristic cable impedance have small reflections [see Equation (2)], and cannot be distinguished from

reflections of naturally-occurring cable irregularities.

Recommendations for time domain reflectometry

TDRs make it possible to see into a cable to locate cable faults and identify cable landmarks such as

splices, transformers, joints, and cable transitions, in addition to locating the cable start and the cable end.

TDRs are well-suited to locate series cable faults such as broken conductors, concentric neutral corrosion,

separated splices, sealed off cable ends, etc. TDRs may also be used to locate shunt cable faults withresistance values of less than ten times the characteristic impedance of the cable to be tested.

With a TDR alone, it is not possible to locate faults with resistance values greater than ten times the

characteristic impedance, or high-voltage and intermittent cable faults. Auxiliary equipment and techniques

must be used to convert high resistance and intermittent shunt cable faults temporarily into low-resistance(flash over) faults, which can be located with a TDR or digital oscilloscope.

The techniques, often referred to as high-voltage radar, are as follows:

a) Surge arc reflection

b) Burn arc reflection

c) Surge pulse (current coupled) method

d) Decay (voltage coupled) method

For multi-conductor cable systems, differential high-voltage cable radar techniques are also available.




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6.5

6.5.1

6.5.2

6.6

6.6.1

Capacitive discharge (thumping)

Thumper, capacitive discharge device, and HV surge generator are alternate terms for an HV device

generating an audible thump at the location of a cable fault. The most frequently used fault-locating tool forshielded power cables has been the thumper. A HV capacitor is charged to an HV dc voltage. The energy

stored in the capacitor [as defined in Equation (3)] is discharged periodically via an electronically-operated

or manually-set spark gap into the faulty cable.

W= C V (3)

where:

W= energy

C= capacitor

V= voltage

This capacitive discharge generates a traveling voltage surge between center and concentric conductor.

When the voltage surge exceeds the fault breakdown voltage, a flashover occurs. The fault location may beverified by tracing the electro-magnetic signal generated by the arcing and/or by listening for the acoustical

signalthe thumpassociated with every flashover. Thumpers come with a wide variety of features. For

cable fault-locating, thumpers should be selected by operating voltage range and available energy at a

particular operating voltage. To reduce the use of unnecessary high voltage and excessive energy whenfault locating, preference should be given to controlled energy thumpers. These devices feature a variable

HV capacitance so that the thumping voltage can be set to within 2 kV to 3 kV of the fault flashover

voltage without loss of energy at the fault.

Limitations to capacitive discharge

A thumper does not give the location of a fault. To find it, the entire cable length has to be searched. Since

cable fault characteristics, cable construction, and soil condition greatly influence the thumps loudness, the

fault location can easily be missed. When concentric neutral corrosion exists, finding the fault location is

haphazard at best.

Recommendations for capacitive discharge

A thumper should rarely be used as a stand-alone cable fault-locating device. It is recommended to pre-locate the fault location with a thumper-TDR combination. Pinpointing is then accomplished quickly and

efficiently with acoustic and/or electromagnetic instruments.

Burning (fault conditioning)

Using an ac or dc burn set of sufficient voltage and current output, a high-resistance or intermittent fault

can temporarily or permanently be converted into a low-resistance fault. First, arcing is induced at the fault

point, then current flow is maintained, until through charring or metal fusion, a permanent low-resistance

path at the fault location, exists. If burning is continued, the cable may finally burn apart.

Limitations to burning

The change from lead-paper to solid dielectric-type cables and modern splicing technology has imposedlimitations on burning. Space charge build-up and multiple flashover during burning may activate dormant

faults or generate new defects. The benefits and disadvantages associated with burning in order to generate

a short or open circuit should be carefully weighed. There is a high risk of fire damage to the cable and

equipment, and appropriate safety precautions must be taken.




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6.6.2

6.7

6.7.1

6.7.2

Recommendations for burning

While burning was originally used to permanently change high-resistance and intermittent cable faults intoshort or open circuits that could then be pre-located with TDRs or bridges, and pinpointed with acoustical,

coincidence, electro-magnetic, or current or voltage gradient-type pinpointers, todays applications for

burning should be limited as follows (taking appropriate safety precautions):

a) When burning is used on lead-paper, pipe-type, water-soaked, or submarine cables, a TDRshould be connected across the cable (see 6.8). Monitoring the burning of the cable with a TDR

will pre-locate the distance to the fault at the instant the fault breaks down, thus minimizingburning time and current.

b) On large capacitance cables, burn sets may be used to quickly charge the cable until it arcs overand the cable fault can be pre-located with the Decay or Surge Pulse methods.

c) On all cable types, burning may be used to reduce the breakdown voltage of a fault to within therange of a thumper.

Additional applications for burn sets are as follows: Ground fault detection of pressurized oil-filled cablesor pipes, cable identification, and tracing.

Surge arc reflectionSurge arc reflection permits locating of faults in power cables at lowest possible HV levels with minimum

risk to serviceable cable. With a surge generator, high-resistance or intermittent cable faults can

temporarily be converted into faults having resistance values much less than the characteristic impedance.

Combining a TDR with the surge generator permits locating of the temporarily low-resistance faults. Acoupler isolates the TDR from the HV pulses and ensures that the high-frequency test pulses sent into the

cable by the TDR are not short-circuited by the surge generator.

During the first phase of the measurement, the TDR pulses are not reflected by the high resistance or

intermittent fault, and only cable start, joints, splices, transformers, irregularities, and cable end are visible.In the second phase, the surge generator is switched on. The surge pulse amplitude is made just high

enough to break down the fault and generate arcing at the fault location. The TDR pulse will be reflected

by the arc and an image of the temporary low-resistance fault, a negative deflection, will indicate the fault

location on the display. Once arcing ceases, the fault reverts back to its high-resistance state. A comparisonof the cable with and without HV applied is observed. During the intervals between arcing, when the surge

generator is in the charge mode, the reflected image of the cable, start to end, is displayed with all inherent

cable landmarks. During arcing, the high-resistance fault is converted to a low-resistance state and the

negative deflection is overlaid on the low voltage display. The fault location is easily determined, not onlyas a distance in feet, yards. or meters from the beginning or the end of the cable, but also in relation to the

other landmark reflection points.

Limitations to surge arc reflection

Arc reflection cannot be used where a flashover between conductors cannot be established (conductor to

ground faults). Cable faults on PILC cables and faults under water may have intermittent fault breakdowns,

and that may be difficult to capture with a TDR. Long cables with very lossy insulation, and radial cable

systems with many branches, may absorb the reflected TDR pulses and the temporary low-resistance stateof the fault cannot be observed. Surge arc reflection cannot be used on cables with fault current interrupters

in the sheath (sheath gaps).

Recommendations for surge arc reflection

For cables with extruded dielectric insulation, the application of surge arc reflection is, in general, not

limited by cable length or type, the number of transformers in a loop, or by cable operating voltage range.Since surge arc reflection is the simplest and quickest of the HV TDR techniques, it should be tried first.




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6.8

6.8.1

6.8.2

6.9

6.9.1

6.9.2

Burn arc reflection

Burn arc reflection is frequently used on HV cable faults where a surge-generated flashover cannot be

observed with a TDR. These faults frequently occur on lead-paper, submarine, and water- or oil-soakedcables, and pressurized oil-filled pipes. Arcing is induced at the fault point and a sufficient current flow,

usually 4 A to 5 A, is then maintained to sustain arcing. The arcing is monitored with a TDR, which is

connected to the cable through a HV coupler. The distance to the fault is measured using standard TDRtechniques. Current tracing is usually used to verify the location of the fault.

Limitations to burn arc reflection

The burn set must be capable of ionizing the fault and maintaining a burn current of at least 4 A to 5 A.

Recommendations for burn arc reflection

The application of burn arc reflection is an excellent adjunct to surge arc reflection. Conditioning of a cable

fault may be monitored and the distance to the fault recorded when the fault reaches a low-resistance state.

The time required to identify an approximate fault location is in general less than five minutes.

Surge pulse reflection

The surge pulse method effectuates the location of high-resistance and intermittent cable faults. It is a surge

generator technique and not a TDR technique, even though TDRs are frequently used as reflection-time

display terminals.

The surge generator sends a HV pulse into the faulty cable where it produces arcing at the fault location.

Part of the HV pulse energy is reflected to the cable start where it is partially reflected back into the cable

by a choke. The signal bounces back and forth until all its energy is dissipated. This process can be

observed by coupling a synchronized monitoring instrument, such as a digital oscilloscope or TDR, to thecable. The spacing of the reflections displayed on a screen is a measurement of the distance to the fault.

Limitations to surge pulse reflection

It should be understood that the surge pulse technique has nothing in common with the TDRtechnique. ATDR pulse width may be as narrow as 10 ns, providing excellent resolution and accuracy of the

measurement. Surge pulse widths are determined by the following:

a) the surge generator

b) the characteristic impedance of the cable, and

c) the fault

The accuracy of the measurement often depends on the skill of the operator.

A major limitation of the surge pulse method lies in the methods inability to distinguish between naturally

occurring reflection points such as Y-splices, cable transitions, etc., and faults. Furthermore, reflection

points such as splices, joints, transformers, and cable transitions, which could assist in the identification of

the fault location, are lost. Further complicating factors: Surge pulse is inadequate when concentric neutralcorrosion exists, or when fault current interrupters are installed in the cables sheath.

Recommendations for surge pulse reflection

The surge pulse method is a good back-up technique for surge arc reflection. It should be used on cable

faults where an arc between conductor and concentric cannot be established (splice to ground faults), andon long and highly-attenuating cable runs where a TDR pulse has insufficient energy to produce a

reflection-time display.




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6.10

6.10.1

6.10.2

6.11

6.11.1

Decay method

The decay method permits locating of high-resistance and intermittent cable faults where the fault

breakdown voltage is greater than the maximum available surge generator voltage, or where the cablecapacitance approaches or exceeds a thumpers capacitance.

A dc test set or burner will continuously charge the cable until the fault arcs over. At each arc-over, atraveling wave is generated, which reflects back and forth between cable start and fault until its energy is

dissipated. This process can be observed by coupling a synchronized monitoring instrument, such as a

digital oscilloscope or a TDR, to the cable. The spacing of the reflections displayed on a screen is ameasurement of the distance to the fault.

Limitations to decay method

The fault breakdown voltage and cable capacitance must be sufficiently high to produce a good flashover atthe fault.

Recommendations for decay method

The decay method should be used on cable faults where an arc between conductor and concentric cannot be

established with a thumper. When the energy released at the arc-over is sufficiently high (400 J to 1000 J),the cable fault can also be pinpointed acoustically. On three conductor cables, all three phases may be

connected in parallel to increase the total fault-locating capacitance.

Bridge techniques

Bridge techniques are one of the earliest forms of cable fault location. They have been very successful in

locating faults on PILC cables where faults had been conditioned to be either an open or a bolted fault.

Various bridges are in use today. A bridge is usually known by the name of the person who invented it orused it first. For example, one well-known fault-locating bridge is the Murray Loop.

In order to use a bridge fault-locating technique, fault resistance and continuity must be measured.

a) With an insulation resistance tester, the conductor-to-sheath (ground) or the conductor-to-conductor resistance is measured. If this resistance is in the hundreds of megohms, the fault

must be conditioned with a burn-set to lower the fault resistance value, preferably in the ohm or

low K-ohm range.

b) With an ohmmeter, the resistance of the loop created by the faulty conductor and a goodconductor connected together at the far end is measured.

If cable continuity and a low fault resistance exist, a bridge can be used to measure the distance to the fault.

If the continuity test shows an open circuit, a TDR shall be used to locate the fault.

NOTEIn the past, a capacitance bridge may have been used for open circuit faults instead of a TDR.

DC bridge techniques

The Murray Loop measures the distance to a low-resistance fault by joining one or two good conductors

with a faulted conductor, applying a dc voltage to the conductors, and adjusting two variable resistors until

a galvanometer placed across the joined conductors is nulled. From the known cable lengths and a ratio of

adjusted variable resistors, the distance to the fault can be calculated.




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6.11.2

6.11.3

6.11.4

6.11.5

6.12

Limitations to bridge techniques

Even though modern fault-locating bridges often are microprocessor-based and calculate and display thedistance to the fault in feet or as a percentage of the total cable length, it should be understood that the

measurements are often time-consuming. Special attention must be given to the following factors:

a) All bridge methods require at least one good conductor in addition to the faulted cable, unlessthe measurement can be performed on both ends simultaneously.

b) Access to both cable ends is required.

c) Contact resistances and connecting wire resistance must be much less than the conductorresistance.

d) Variations in resistance of the faulted conductor must be considered.

e) Stray dc and ac currents in the ground and on the cable will affect the measurement.

f) An unstable fault resistance will affect the measurement.

g) The total conductor length, not the above ground cable length, must be known.

h) Multiple faults on the faulted core will distort the measurement.

An effective pre-locating method optimizes the amount of time and work required to locate a fault or

isolate a faulted cable span.

Recommendations for bridge techniques

Bridge techniques are excellent fault-locating tools after TDR-based techniques have been exhausted. If the

fault is a bolted fault or the fault resistance is low, a low-voltage bridge should be used. If the fault has a

high-resistance value to ground, then a) a high-voltage bridge can be used to establish current flow andovercome the high-resistance value of the fault, or b) a high voltage is used to convert the fault to low-

resistance state.

Capacitance ratio techniques

The capacitance ratio method can be used to locate an open conductor fault when a TDR method is not

available. The capacitance of the cable from one terminal to the fault is measured. The ratio of faulted cable

capacitance to the capacitance of an identical unfaulted cable, multiplied by the total cable length,

determines the distance to the fault. Making a second measurement from the second end fences the fault

in and improves the accuracy of the measurement.

Ratiometric voltage division techniques

Ratiometric voltage division is used on three conductor cables with sheath current interrupter gaps or on

low- or high-pressure oil filled cables, where the use of high-voltage thump and burn equipment isrestricted in order to minimize contamination.

The faulted phase is identified with an insulation resistance tester. A current is injected into the faultedphase via one of the good conductors. The second good conductor is the voltage-sensing lead, connected to

the far end. The ratio of the voltages measured at the near and far ends of the faulted cable, multiplied bythe total cable length, yields the distance to the fault.

Tracing/locating/pinpointing

When tracing or locating a faulted cable, or pinpointing a fault, a transmitter sends a signal into the cable.

A receiver senses the amplitude, frequency, changes in magnitude, or response of the transmitted signal. A




29/37


skilled person can interpret the measurements and identify cables, locate cable routes and depth of cables,

and pinpoint cable fault locations. Many different signals are used. They are classified as high voltage or

low voltage and audio frequency (tone), radio frequency or the signals can be continuous, pulsed, or high-

voltage surges. The detection methods can be grouped into galvanically and magnetically coupled, and

acoustic methods, as well as their combinations. Many methods are available and their successful use mostoften depends on the operators skill. The principles of the major methods are described in 6.12.1 through

6.12.5.

6.12.1

6.12.2

6.12.3

6.12.4

6.12.5

AC and DC current tracing

Tracing methods using ac or pulsed dc currents may well be the oldest cable fault-locating techniques. A

low or high voltage, ac, dc, or surge voltage source is connected between the faulted cable and earth

ground. Current will flow through the conductor, the fault, and back to the source through the parallelcombination of outer cable conductor and ground. An antenna placed directly above the cable will sense a

magnetic field, which is proportional to the magnitude of the current flowing toward the fault. Once the

fault point is passed, a drop in conductor current is detected. In a duct/manhole system, the method is

excellent for verifying a faulted cable span.

A variation is the sheath pick method. A sensitive instrument (galvanometer) is used to measure the

direction and magnitude of the sheath current. A reversal of the sheath currents direction frames the fault.

The tracing current methods are very often used for long feeder circuits with multiple branches, and whentransformers cannot be isolated, the ac or dc current sources are usually quite large, and the sensing devices

specialized.

Audio and radio frequency methods

Audio (tone) and radio frequency tracing methods are very similar to ac or dc current tracing methods. Afrequency generator, typically in the range of 60 Hz to 200 kHz, is connected between cable conductor and

concentric. A current path for the signal is provided by the conductor, fault, and concentric. Additional

paths exist through the earth. The magnetic field generated by the injected current is detected with a tuned,

directional antenna. Depending on the polarization of the antenna with respect to the cable route and cable,

either a null or peak signal is detected directly above the cable. The measurements of signal changes,

especially in the null reading, are used for splice locating, concentric neutral corrosion detection, and thelocation of faults that will not thump.

Sheath fault location

Sheath fault location, earth gradient, and voltage gradient methods of fault locating can only be used on

direct buried cables. A dc source, often a thumper, hipot, or burner, forces a current through the fault and

surrounding ground back to the source. The current through the ground establishes an earth potential, whichcan be measured with a voltmeter. The voltmeter indication changes polarity when one walks beyond the

fault. When the voltmeter probes are positioned at equal distances from the fault, the indication is zero.

Acoustic methods

Turning the thumper on and listening for the thump in the ground is the most popular pinpointingtechnique. Traffic cones, shovel handles, stethoscopes, etc. have been used when searching for the elusive

pop in the ground. Geophones and directional acoustic detectors facilitate fault pinpointing and are

preferred listening devices.

Coincidence methods

A capacitor (thumper) is discharged into a faulted cable. An electromagnetic detector traces the thumper

pulse down the cable. An acoustic detector detects the thump caused by the flashover. In the vicinity of the

fault, the flashover is used to start a timer, and the thump to stop it. The measured elapsed time is an




30/37


indication of the distance to the fault. The operator is directly above the fault when the elapsed time

between flashover and thump is at minimum.




31/37


Annex A

(informative)

Bibliography

[B1] Accredited Standards Committee C2-2007, National Electrical Safety Code (NESC).8

[B2] Almonte, R. L., URD Cable Fault Locating for the 1990s, Forty-Second Annual PowerDistribution Conference, 10 24 1989, Austin, TX.

[B3] Bascom, III, E. C., Von Dollen, D. W., Ng, H.W., Computerized Underground Cable FaultLocation Expertise, Transactions of the T&D Conference, Chicago, IL, April 1994.

[B4] EPRI TR-105502, Underground Cable Fault Location Reference Manual, Project 7913-03, 1995.

[B5] Gnerlich, H. R., Underground Distribution & Transmission: Cable System Diagnostic Services and

Thumper (Capacitive Discharge Device) Training. Cable Fault Locating, Cable & Cable System TestingTraining Course. Bethlehem, PA: Gnerlich, Inc., 1993.

[B6] Gnerlich, H. R., Underground Distribution & Transmission: Time Domain Reflectometer (CableRadar). Cable Fault Locating, Cable & Cable System Testing Training Course. Bethlehem, PA: Gnerlich,

Inc., 1993.

[B7] IEEE Std 4-1995, IEEE Standard Techniques for High-Voltage Testing.9

[B8] IEEE Std 80-2000, IEEE Guide for Safety in AC Substation Grounding.

[B9] IEEE 100, The Authoritative Dictionary of IEEE Standards Terms, Seventh Edition, New York,Institute of Electrical and Electronics Engineers, Inc.

[B10]IEEE Std 141-1986, IEEE Recommended Practice for Electric Power Distribution for IndustrialPlants. (IEEE Red Book).

[B11]IEEE Std 400-1999, IEEE Guide for Making High-Direct-Voltage Tests on Power Cable Systems inthe Field.

[B12]Kuffel, E., Zaengl, W. S.,High Voltage Engineering:Fundamentals, Pergamon Press, 1988.

8 The NESC is available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08855-1331, USA

(http://standards.ieee.org/).9 IEEE publications are available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, Piscataway, NJ 08854,

USA (http://standards.ieee.org/).




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Annex B

(informative)

First-response cable system fault location in URD

First-response cable fault location is a new concept for trouble-shooting URD loops. It uses a self-

contained, portable, battery operated TDR/thumper device which enables technicians to respond to a

reported outage, isolate a faulted cable span, or locate a fault with one or two capacitive discharge surges,

and quickly restore electrical service.

In a typical URD power outage, part of a development is without electrical service. Any number oftransformers may be affected by the outage.

To explain the method of first-response cable fault location see Figure B.1. Assume transformer 1 is the

most convenient access point at which the test equipment can be connected. The cable end at transformer 5is parked. Assume that a cable fault exists at the cable end either below the transformer or in the elbow.

Transformers and lightning arrestors need not be disconnected in the cable system to be tested.

Copyright Gnerlich, Inc. Used with permission.

Key:552 ft = 168.25 m 295 ft = 89.92 m 1803 ft = 549.55 m

578 ft = 176.17 m 1330 ft = 405.38 m 2698 ft = 822.35 m

378 ft = 115.21 m 1508 ft = 459.64 m 264 ft = 80.47 m

Figure B.1Example of a TDR display of a faulted URD power cable loop section




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Using an arc reflection technique, the cable system signature is recorded; the open cable end appears as a

positive pulse deflection. A single HV pulse is now discharged into the cable. When the fault flashes over,

the TDR will record the flashover as a temporary short circuit to ground; the typical signature

2007 - ieee guide for fault locating techniques on shield

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