IEEE Guide - 1234 - For Fault-Locating On Shielded Power Cable Systems - 2007 PDF
IEEE Guide - 1234 - For Fault-Locating On Shielded Power Cable Systems - 2007 PDF
IEEE Guide - 1234 - For Fault-Locating On Shielded Power Cable Systems - 2007 PDF
Techniques on Shielded
Power Cable Systems
1234
IEEE
3 Park Avenue IEEE Std 1234™-2007
New York, NY 10016-5997, USA
16 November 2007
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IEEE Std 1234™-2007
Sponsor
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Abstract: Tests and measurements that are performed on shielded power cables to identify the
location of a fault are described. Whenever possible, the limitations of a particular test and
measurement to locate a fault are provided and recommendations are made regarding
specialized fault-locating techniques. A fault characterization chart is included as an aid to select
a fault-locating technique.
Keywords: arc reflection, cable fault locating, cable testing, grounding, safety, sectionalizing,
thumping, time domain reflectometry (TDR)
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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 to
generate 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:
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Users are encouraged to check this URL for errata periodically.
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Patents
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covered by patent rights. By publication of this guide, no position is taken with respect to the existence or
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conducting inquiries into the legal validity or scope of those patents that are brought to its attention.
iv
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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:
†
deceased
The following members of the individual balloting committee voted on this guide. Balloters may have
voted for approval, disapproval, or abstention.
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
*Member Emeritus
v
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Also included are the following nonvoting IEEE-SA Standards Board liaisons:
Lorraine Patsco
IEEE Standards Program Manager, Document Development
Matthew J. Ceglia
IEEE Standards Program Manager, Technical Program Development
vi
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Contents
1. Overview .................................................................................................................................................... 1
1.1 General ................................................................................................................................................ 1
1.2 Scope ................................................................................................................................................... 1
1.3 Purpose ................................................................................................................................................ 1
2. Normative references.................................................................................................................................. 1
4. Safety.......................................................................................................................................................... 4
4.1 Safety practices.................................................................................................................................... 4
4.2 Responsibility ...................................................................................................................................... 4
4.3 Precautions .......................................................................................................................................... 5
4.4 Grounding............................................................................................................................................ 5
Annex B (informative) First response cable system fault location in URD ................................................. 22
Annex D (informative) Fault location on cable systems with concentric neutral corrosion......................... 26
vii
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IEEE Guide for Fault-Locating
Techniques on Shielded
Power Cable Systems
1. Overview
1.1 General
This guide has been developed as a guide for cable fault-locating techniques on shielded power cable
systems. 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.
1.2 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 dielectric
insulation may overstress insulation, creating defects that become faults after the cables are returned to
service.
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 failures
during the transient voltage surge episodes while in service.
1.3 Purpose
This guide is intended to provide trouble-shooting and testing personnel with information to quickly
identify a faulted cable section and/or locate a cable fault with minimum risk of further damaging
serviceable cables, terminations, and equipment.
2. 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.
1
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IEEE Std 1234™-2007
IEEE 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
3.1 Definitions
For the purposes of this guide, the following terms and definitions apply. The Authoritative Dictionary of
IEEE 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 Ω.
3.1.3 branch circuits: A cable system in which independent cables branch out radially from a common
source of supply. (See also: radial feed)
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.
3.1.9 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 liquid
insulation, 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.
2
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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.
3.1.17 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.
3.1.21 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, splice
transformer, 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 insulation
material 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
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
TDR: time domain reflectometer, frequently referred to as cable radar in the power industry.
4. Safety 5
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:
At the conclusion of high-voltage (HV) testing, attention should be given to the following:
4.2 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 understanding
of 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]
4
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
⎯ Knowledge of cable and equipment specifications and the ability to select cable fault-locating
techniques, 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 ) as
part 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 planned
conditions, 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. A
safety 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 concentric
shield; 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.
6
The numbers in brackets preceded by the letter B correspond to those of the bibliography in Annex A.
5
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
Cable fault-locating
operating environment
Insulation type
Radial, single conductor
cable systems with a
few branches Fault resistance
Three conductor
submarine, armored, Transformer primary
and pipe-type cables connection
Figure 2 —Radial distribution and network distribution categories, which determine cable fault-
locating operating procedures.
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 locator’s ultimate goal to
efficiently restore customer service while maintaining cable and equipment integrity. Procedures for cable
fault locating are listed as follows, and techniques for determining the location of the fault are described in
Clause 6.
6
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
3) If R < 5 Ω, the distance to the fault can be measured with a TDR, the fault’s location
pinpointed with audio frequency (tone) tracing equipment.
4) If R > 500 Ω, 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 fault’s location.
5) If 5 Ω < R < 500 Ω, method d or e (Table 1) may be selected. Preference should be given to
the method with the lower test or thump voltage.
b) Cable loop with transformers, surge arrester, etc. connected
1) With a TDR, the cable length and cable landmarks, such as cable start, splices,
transformers, cable transitions, cable end, etc. should be verified.
2) A thumper, HV coupler, and TDR combination should be used to determine the location of
the fault and measure its distance from cable landmarks.
3) The precise location of the fault can be verified with acoustic and/or electromagnetic
detectors.
c) Radial single conductor cable system with a few branches
1) With a TDR, cable length and cable landmarks, such as cable start, splices, Y (T)-splices,
branch ends, cable end, etc. should be verified.
2) A thumper, HV coupler, and TDR combination should be used to determine the location of
the fault and its distance from cable landmarks.
3) Branches and the precise location of the fault can be verified with acoustic and/or
electromagnetic detectors.
d) Three conductor submarine, armored, and pipe-type cables
T
1) Standard techniques
i) With a TDR, cable length and cable landmarks, such as cable start, splices, cable
transitions, cable end, etc. should be verified.
ii) With an insulation resistance tester/ohmmeter, the fault resistance, R, should be
measured.
iii) If R < 5 Ω, the distance to the fault should be measured with a TDR. The fault
location can be verified with audio frequency fault-locating instrumentation, ac or dc
current tracers.
iv) If R > 500 Ω, 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 Ω < R < 500 Ω, the fault-locating technique d or e (Table 1) may be used.
Preference should be given to the technique that permits locating the fault at a lower
test thump or burn voltage.
2) Alternative techniques
i) Using a TDR with two or three inputs or a digital TDR will permit comparison of a
faulted phase with good phases and thus simplify fault locating.
ii) Low- or high-voltage bridges may be used in combination with a TDR to determine
the distance to a fault.
iii) Low- or high-voltage bridges can be used to determine the distance to a fault on all
cables with fault current interrupters in the sheath.
7
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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, a
primary 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.
In network distribution, fault-locating efforts often will require more than one fault-locating
method. 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 will
determine 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 arc
reflection, 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 of
maintaining an arc current of 4 A to 5 A is also very effective in locating a faulted cable section
with a TDR.
d) The type of cable insulation restricts the use of burning and dc test voltages. Oil-paper
insulated cables often are subjected to burning in order to reduce the fault resistance for ease of
identification. 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 are
tied 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 resistance
tester/ohmmeter combination.
8
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
NOTE—TDR direct, comparison and difference, surge and burn arc reflection, surge pulse, and decay methods are
available in the majority of power utility TDRs and HV couplers. These techniques are not available on
telecommunication TDRs. 7
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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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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.
Open circuit
Cable will often hold a dc voltage greater than the conductor-to-
ground voltage.
a) Mechanical damage, open termination, or separated
splice.
b) Through re-closing, conductor is blown apart and the
conductor end is electrically sealed off.
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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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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 of
cable 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 in
damage 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 the
cable 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 currents
comparable 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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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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 been
used 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.
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.
Concentric neutral corrosion. With a TDR, the exact problem shall be determined
Corroded termination or splice. and the appropriate fault-locating procedure and
Corroded or burnt conductor. technique selected.
Water soaked, burnt cable section.
R > 1 MΩ Sealed off conductor. With a TDR, the exact problem shall be determined
Separated splice or termination. and the appropriate fault-locating procedure and
Missing concentric or sheath. technique selected.
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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 produced
by 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 between
cable start and any discontinuity or irregularity can be determined from the reflection-time display. A
TDR’s digital readout provides distance to the fault, as well as cable length measurements.
The magnitude of the pulse reflections produced by deviations from the homogeneous structure of the cable
are determined by the reflection coefficient shown in Equation (1):
R = (Z - Zo ) / (Z + Zo ) (1)
R is resistance; Zo is the cable’s characteristic impedance and Z an impedance value electrically describing
cable start, joints, splices, faults, changes in cable type, as well as cable end. For shunt cable faults on
concentric 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.
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 with
resistance 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.
For multi-conductor cable systems, differential high-voltage cable radar techniques are also available.
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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 for
shielded 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=½C×V² (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 be
verified by tracing the electro-magnetic signal generated by the arcing and /or by listening for the acoustical
signal—the thump—associated 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 when
fault 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.
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 thump’s loudness, the
fault location can easily be missed. When concentric neutral corrosion exists, finding the fault location is
haphazard at best.
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.
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.
The change from lead-paper to solid dielectric-type cables and modern splicing technology has imposed
limitations 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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IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
While burning was originally used to permanently change high-resistance and intermittent cable faults into
short 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, today’s applications for
burning should be limited as follows (taking appropriate safety precautions):
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 comparison
of 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 only
as 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.
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 state
of the fault cannot be observed. Surge arc reflection cannot be used on cables with fault current interrupters
in the sheath (sheath gaps).
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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IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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-soaked
cables, 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 TDR
techniques. Current tracing is usually used to verify the location of the fault.
The burn set must be capable of ionizing the fault and maintaining a burn current of at least 4 A to 5 A.
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.
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 the
cable. The spacing of the reflections displayed on a screen is a measurement of the distance to the fault.
It should be understood that the surge pulse technique has nothing in common with the TDR technique. A
TDR 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 method’s 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 neutral
corrosion exists, or when fault current interrupters are installed in the cable’s sheath.
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), and
on long and highly-attenuating cable runs where a TDR pulse has insufficient energy to produce a
reflection-time display.
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IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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 cable
capacitance approaches or exceeds a thumper’s capacitance.
A dc test set or burner will continuously charge the cable until the fault arcs over. At each arc-over, a
traveling 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 a
measurement of the distance to the fault.
The fault breakdown voltage and cable capacitance must be sufficiently high to produce a good flashover at
the fault.
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 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 or
used 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.
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.
NOTE—In the past, a capacitance bridge may have been used for open circuit faults instead of a TDR.
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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IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
Even though modern fault-locating bridges often are microprocessor-based and calculate and display the
distance 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, unless
the 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 conductor
resistance.
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.
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 and
overcome the high-resistance value of the fault, or b) a high voltage is used to convert the fault to low-
resistance state.
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 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 is
restricted in order to minimize contamination.
The faulted phase is identified with an insulation resistance tester. A current is injected into the faulted
phase 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 by
the total cable length, yields the distance to the fault.
6.12 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
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IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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 most
often depends on the operator’s skill. The principles of the major methods are described in 6.12.1 through
6.12.5.
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 parallel
combination 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 current’s direction frames the fault.
The tracing current methods are very often used for long feeder circuits with multiple branches, and when
transformers cannot be isolated, the ac or dc current sources are usually quite large, and the sensing devices
specialized.
Audio (tone) and radio frequency tracing methods are very similar to ac or dc current tracing methods. A
frequency 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 the
location of faults that will not “thump.”
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, which
can 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.
Turning the thumper on and listening for the thump in the ground is the most popular pinpointing
technique. 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.
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
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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.
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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 Power
Distribution Conference, 10 24 1989, Austin, TX.
[B3] Bascom, III, E. C., Von Dollen, D. W., Ng, H.W., “Computerized Underground Cable Fault
Location 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 Testing
Training Course. Bethlehem, PA: Gnerlich, Inc., 1993.
[B6] Gnerlich, H. R., “Underground Distribution & Transmission: Time Domain Reflectometer (Cable
Radar).” 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 Industrial
Plants. (IEEE Red Book™).
[B11] IEEE Std 400-1999, IEEE Guide for Making High-Direct-Voltage Tests on Power Cable Systems in
the 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/).
21
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
Annex B
(informative)
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 of
transformers 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 5
is 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.
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.1—Example of a TDR display of a faulted URD power cable loop section
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IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
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 of a short
circuit is a negative pulse deflection. A comparison of low voltage and high voltage traces indicates the
location of the fault where the two signatures depart from each other. The distance to the fault is displayed
by the TDR as 549.55 m (1803 ft).
On the primary side, transformers act as very large shunt impedance with respect to the HV surge, and do
not interfere with the measurement. On the secondary side, the transformed surge voltage will be small in
comparison to the nominal voltage.
In first-response cable fault location, customer service is restored in a short time with minimum man-hours
and the least amount of stress on customer and utility equipment. Checking fault indicators, isolating
transformers, and disconnecting lightning arrestors is unnecessary. A faulted cable section or the fault can
be identified with one or two thumps. Cable section replacement or fault repair can be performed when
work schedules permit.
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IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
Annex C
(informative)
Key:
2374 ft = 723.6 m 10 547 ft = 3214.73 m 258 ft = 78.64 m
2910 ft = 886.97 m 5717 ft = 742.54 m
Figure C.1—TDR signature of a feeder circuit
24
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IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
A good impedance match at the hook-up point allows the TDR pulse to enter and exit the cable to be tested.
Four splice signatures are clearly visible. One Y (T)-splice is identified 723.6 m (2374 ft) from the hook-up
point, and a second Y (T)-splice is identified at 886.97 m (2910 ft). The furthest distance on the feeder,
1 742.54 m (5717 ft), is verified by opening and short-circuiting a cable end. Should a failure occur on this
feeder section, it can be reasonably assumed that the fault can be located using a TDR technique. The pre-
recorded cable signature can be made available to the fault locator as an addition to the feeder map.
NOTE—Training courses, manufacturers’ application notes, and TDR operating manuals may be consulted for
impedance matching when fault-locating shielded power cable systems.
In Figure C.2, an open cable end is visible at 2666.09 m (8747 ft). A 12 kV thumper discharge into the
cable causes a fault flashover at approximately 1554.18 m (5099 ft) from the hook-up point. Since the
location of a Y (T)-splice at 1328.01 m (4357 ft) from the hook-up site is known, the distance to the fault is
measured as 224.94 m (738 ft) from the Y (T)-splice location. The exact fault location will be verified with
electromagnetic, acoustic, or coincidence locators.
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
Annex D
(informative)
When concentric neutral corrosion is suspected in a cable system, a TDR signature of the cable will help
establish the severity and extent of the corrosion problem.
Key:
307 ft = 93.57 m 183 ft = 55.78 m 1247 ft = 380.09 m
69 ft = 21.03 m 266 ft = 81.08 m 573 ft = 174.65 m
244 ft = 74.37 m
In Figure D.1, two areas of concentric neutral corrosion can be seen. In the cable section between
transformer 1 and transformer 2, the corrosion seems negligible; a splice at 93.57 m (307 ft) and
transformer 2 at 174.65 m (573 ft) can be identified. Between transformer 1 and transformer 2, a cable fault
could quickly be located with a thumper/TDR technique. This is not so in the cable section between
transformer 2 and transformer 3. Concentric neutral corrosion is severe. Transformer 3 is not visible in the
reflectogram. A thumper/TDR technique may work if the fault is within 55.78 m (183 ft) from transformer
2, but most probably not if the fault is beyond 81.08 m (266 ft) from transformer 2. Therefore, the fault-
locating equipment should now be moved to the transformer 3 location to establish the extent of corrosion
from transformer 3 towards transformer 2.
If no flashover fault can be recorded between conductor and remaining concentric, it must be assumed that
the fault discharges into the surrounding earth. Audio (tone) frequency tracing and step voltage techniques
may be required to locate the fault. Fault excavation based solely on acoustic measurements is not
recommended.
26
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IEEE Std 1234™-2007
IEEE Guide for Fault-Locating Techniques on Shielded Power Cable Systems
Annex E
(informative)
It is recommended that, at a minimum, personnel should have the following fault-locating tools:
Measuring wheel
27
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