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Simplifying cable fault locating

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
the closed loop to be used for fault-locating mea-
substantial quantities of cable insulation have been
surements and comparing this measurement to
destroyed by the fault. Gas concentrations in struc-
known circuit constants. Conductor continuity gen-
tures can be dispersed by a thorough purging with a
erally will have no effect on the operation of tracer-
positive-pressure blower. Gas testers and their ap-
plication are discussed in section II.
type fault-locating equipment. Faults exhibiting
both high series resistance (open conductor) and
5-15. Simplifying cable fault locating.
high parallel resistance (ungrounded conductor) can
be located by using a capacitance-type terminal
Locating faults in cables can be a complicated pro-
measurement device.
cess. The following paragraphs provide some helpful
hints which can simplify the process and increase
5-16. Cable fault locating equipment.
fault-locating accuracy.
a. Fault reduction. In cases where the parallel
Cable fault locating equipment is available from
resistance of a fault is too high to allow effective
test equipment rental companies. Member compa-
application of either tracer or terminal measure-
nies of the InterNational Electrical Testing Associa-
tion (NETA) can be hired to test and to provide the
ment devices, the fault resistance must be reduced,
test equipment. As with all techniques used infre-
that is it should be carbonized or "burned down".
Direct-current high-potential test sets, as described
quently, the skill of trained outside personnel may
well be worth the additional cost,. "Electrical Equip-
in section VII can be used for this purpose. The fault
reduction is accomplished by applying a continuous
ment Testing and Maintenance" covers terminal
and tracer cable-fault locating methods in more de-
potential between the faulted conductor and
ground. The voltage level is adjusted to give the
tail for those who wish an explanation of testing
technique principles. Three of the methods using
maximum current allowed by the rating of the test
set. As the fault carbonizes, a continually decreas-
less complex methods of measuring some electrical
characteristics of faulted cable are shown in figure
ing voltage will be required to sustain this current.
The fault reduction has been accomplished when
5-1. Another method uses a time domain reflec-
tometer tester.
virtually no voltage results in a steady flow of cur-
rent and fault-locating operations can then proceed.
a. Murray loop resistance bridge method. To use
b. Conductors grounded. If one conductor of the
this method, the grounded conductor must be con-
faulted cable remains ungrounded, terminal mea-
tinuous at the fault and a continuous ungrounded
surement devices can be used. If the fault grounds
conductor in the faulted cable must be available.
all conductors and low parallel resistance results,
The accuracy of this method is directly related to
only tracer methods can be effectively applied. The
the accuracy of the plans showing cable routing.
ohmic value of the fault may be used in some cases
The fault is located in terms of its distance from its
to anticipate the effectiveness of the various tracer
cable terminal by measuring and comparing electri-
methods that could be applied.
cal characteristics of the cable's faulted and
unfaulted conductors. It is essentially a Wheatstone
c. Conductor-to-conductor resistance. When a
bridge of the slide-wire type. When the bridge is
fault results in a low-resistance short circuit be-
balanced, the fault distance is found as indicated in
tween two conductors and the resistance to ground
figure 5-1. A number of slide-wire bridges designed
is high, reflection methods may be made effective if
for fault location are available commercially. They
the single phase fault can be reduced to a ground
range from inexpensive units with limited accuracy
fault before attempting to locate the fault. If the
to more expensive units which can locate a fault
single-phase fault cannot be reduced to a ground
within one foot per mile (0.2 meters per kilometer)
fault, one of the conductors involved may be
of cable length. Instructions for use, including appli-
grounded at a termination. Depending upon the
cable mathematical formulas, should be supplied
relative location of the grounded termination, the
with the instrument.
signal pattern and its level and direction may be
quite different from that obtained when locating an
b. Capacitance bridge measurement method. The
ordinary grounded fault.
capacitance bridge measurement method is effec-
d. Conductor continuity. Use of a bridge-type ter-
tive where both the parallel and series fault resis-
tances are high enough to treat an unfaulted and
minal measurement device depends upon the avail-
ability of a continuous ungrounded conductor in the
the faulted conductor as capacitances to a metallic
shield or sheath. This technique is simply the mea-
faulted cable, which can be looped to the faulted
conductor at the far end of the circuit. The faulted
surement of capacitance from one end of the faulted
conductor, though grounded, must also be continu-
cable to ground and comparing it in terms of dis-
ous. The required continuity is best checked by
tance with the capacitance of an unfaulted conduc-
making a bridge measurement of the resistance of
tor in the same cable. Almost any alternating-
5-7


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