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Working: The Faulty Core Is Looped With Sound Core of The Same Cross Sectional Area

This document describes several methods for locating faults in electrical cables: 1. A DC test circuit uses a slide wire resistance box to locate faults by balancing resistance measurements. The fault location is calculated from the resistances. 2. A DC charge and discharge test locates open circuit faults by charging and discharging cable sections and measuring the distance to a fault. 3. A phase-to-phase test uses an insulation tester to identify short circuits between phases by measuring lower resistance to ground.

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Hassan Abdo
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109 views3 pages

Working: The Faulty Core Is Looped With Sound Core of The Same Cross Sectional Area

This document describes several methods for locating faults in electrical cables: 1. A DC test circuit uses a slide wire resistance box to locate faults by balancing resistance measurements. The fault location is calculated from the resistances. 2. A DC charge and discharge test locates open circuit faults by charging and discharging cable sections and measuring the distance to a fault. 3. A phase-to-phase test uses an insulation tester to identify short circuits between phases by measuring lower resistance to ground.

Uploaded by

Hassan Abdo
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as DOC, PDF, TXT or read online on Scribd
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ground fault, one or more cables are earthed.

if the fault current is more than 10 mA


when battery voltage is 100V, the fault resistance may be of the order of 10K . A
high gain dc amplifier can be used for high sensitivity.
Working: the faulty core is looped with sound core of the same cross sectional area
and a slide wire or resistance box with 02 sets of coils are connected across the open
end of the loop. A Galvanometer is also joined across the open end of the loop and a
dc hand operated generator supplies the current for the test. Balance is obtained by
adjusting the slide or resistance. The fault position is given by the formula;
d = a/(a+b)
Where

a = resistance connected to faulty cable


b = resistance connected to sound cable
Loop length = x + y i.e. 2 times the route length
2. DC charge and discharge test for open circuit fault location

This test is used to locate discontinuity in the core of cable, with high resistance to
earth. Preparing for the test , charge the cable with a battery for a very short time say
for 15 sec and then discharge it through a moving coil galvanometer. Test is repeated
at the other end for the similar reading. The distance of the faulty point from end A is
given by

In this test it is necessary to earth all the broken cores at far end and also other cores
except the core to be tested to take correct readings.
In these days, electronic cable faults locators are available which give the reading
directly on scale. The principle used in such instruments is impressing voltage
impulse on the cable under test. These impulses get reflected from the fault location.
Then reflections are projected on CRO (cathode ray oscilloscope) in the image
format. From image type and distance are determined.
3. Phase to phase fault test for short circuit fault location
The cable is tested with the help of insulation tester (megger). Testing between two
cables, if short

Circuited, will indicate zero. If the conductor is earthed then the testing between
conductors to earth will show less resistance in comparison to sound conductor. If two
phases are short circuited, then the faulty point can be located by the formula
4. Fall of potential test for earth fault location

Ammeter, voltmeter battery and variable resistance are connected as shown in


diagram. Let the reading taken across the faulty cable be V 1and across the sound cable
be V2. Then the fault point distance can be given as

Where = total equivalent length of cable.


During the performance of test the same value of current should be maintained in the
circuit. There are many deferent circuit arrangements but accuracy is not as good as
Murray loop test.
5. Capacity test
It is adopted to locate open circuit fault in a cable when insulation resistance of the
faulty core is hire. The principle of this method is to compare the capacity of the
faulty core with one which is sound or with a standard condenser. The faulty core is
charged to a certain voltage and the charge is released by discharging through a
moving coil galvanometer. The deflection of instrument is noted carefully. In the
similar manner the sound core of the cable is charged and discharged. The duration of
charging is however maintained same in both the tests. The distance of break can be
determined with the help of the following formula
Distance of break = ( a/b) x length of cable
a=deflection of the galvanometer of the faulty core.

b= deflection of the galvanometer of the sound core.

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