26-TESTING-OF-CABLES

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Transmission and Distribution
10EE53
LOCATION OF FAULTS IN UNDERGROUND CABLES
There arc various methods for Locating faults in underground cables. The method
used for locating any particular fault depends on the nature of the fault and the
extent of the experience of the testing engineer. Cable faults can be categorized as
1. Conductor failures or
2. Insulation failures.
In general, conductor failures are located by comparing the capacity of the insulated
conductors. On the other hand, insulation failures are located by fault tests that
compare the resistance of the conductors.
In short cables, the fault is usually located by inspection, that is, looking for
smoking manholes or listening for cracking sound when the kenetron is applied to
the faulty cable.
The location of ground faults on cables of known length can be determined by
means of the balanced-bridge principle.
Kenetron is a two-electrode high-vacuum tube. They arc used as power rectifiers tor
applications requiring low currents at high dc voltages, such as for electronic dust
precipitation and high-voltage test equipment.
Fault Location by Using Murray Loop Test
It is the simplest of the bridge methods for locating cable failures between
conductors and ground in any cable where there is a second conductor of the same
size as the one with the fault. It is one of the best methods of locating highresistance faults in low-conductor-resistance circuits. Figure 13 shows a Murray
loop.
The faulty conductor is looped to an un faulted conductor of the same crosssectional area, and a slide-wire resistance box with two sets of coils is connected
across the open ends of the loop. Obviously, the Murray loop cannot be established
if the faulty conductor is broken at any point. Therefore, the continuity of the loop
should he tested before applying the bridge principle. In order to avoid the effects of
earth currents, the galvanometer is connected as shown in the figure. A battery
energizes the bridge between the sliding contact or resistance box center and the
point at which the faulty line is grounded. Balance is obtained by adjustment of the
sliding contact or resistance. If the non-grounded (un-faulted) line and the grounded
(faulted) line have the same resistance per unit length and if the slide wire is of
uniform cross-sectional area,
Transmission and Distribution
10EE53
WHERE X=DISTANCE FROM MEASURING END TO FAULT POINT
L= length of each looped conductor
A=resistance of top left-hand side bridge arm in balance
B=resistance of bottom left-hand side bridge arm in balance
Therefore, the distance X from the measuring end to the fault can be found directly
in terms of the units used to measure the distance L.
Fault Location by Using Varley Loop Test
It can be used for faults to ground where there is a second conductor of the same
size as the one with the fault. ft is particularly applicable in Locating faults in
relatively high-resistance circuits. Figure 4.35 shows a Varley Loop.
The resistance per unit length of the un-faulted conductor and the faulted conductor
must be known. Therefore, if the conductors have equal resistances per unit length
(e.g., rc), the resistance (2L—X)rc constitutes one arm of the bridge and the
resistance
Transmission and Distribution
Figure 14. Varley loop.
10EE53
Transmission and Distribution
10EE53
If the conductor resistance is not known, it can easily be found by changing the switch to
the r position and measuring the resistance of the conductor 2L by using the Wheatstone
bridge method.
Distribution Cable Checks
Newly installed cables should be subjected to a nondestructive test at higher than
normal use values. Megger testing is a common practice The word
Meggeris the trade name of a line of ohmmeters manufactured by the James G. Diddle
Company. Certain important information regarding the quality condition of insulation can
be determined from regular Megger readings that is a form of preventive maintenance.
For example, Figure 13 shows a portable high-resistance bridge for cable-fault-locating
work. Faults can be between two conductors or between a conductor and its conducting
sheath, concentric neutral, or ground. Figure 14 shows a heavy-duty cable test and faultlocating system, which can be used for either grounded or ungrounded neutral 15-kV
cables. The full 100 mA output current allows rapid reduction of high-resistance faults on
cables rated 35 kV ac or higher to the lcvcl of 25 kV or lower for fault- locating purposes.
Figure 15 shows a lightweight battery-
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