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Murray Loop Test and Locating Faults

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Q.1
(a) Using the Murray Loop test, determine the distance to an earth fault on one of the cores of a uniform three-core underground cable. Ra = 2 ohms, Rb = 1 ohm and the cable length is 300 m.

(b) If the cable measurement is accurate to ±1% what length of excavation would be required to locate the fault?

Q.2
A cable run consists of 100 m of 120 mm2 three-core cable jointed to 100 m of
240 mm2 cable. The ratio of the potentiometer resistances in a Murray Loop test,
Rb/(Rb + Ra), is 1/3

For an earth on one core of the cables, determine the location of the fault.

Solution Summary

The expert examines Murray loop tests and locating faults. A cable measurement accuracy is determined.

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Q.1(a) Using the Murray Loop test, determine the distance to an earth fault on one of the cores of a uniform three-core underground cable. Ra = 2 ohms, Rb = 1 ohm and the cable length is 300 m.

(b) If the cable measurement is accurate to ±1% what length of excavation would be required to locate the fault?

Solution: (a) Murray loop test is based on the Wheatstone bridge principle.
Wheatstone bridge:

Ra Rc
Ammeter

Rb Rd

V

Wheatstone principle: When current through the ammeter is zero (balanced Wheatstone bridge), various resistors are related as follows: Ra/Rb = Rc/Rd
Murray loop test:
Length L = 300m Three core cable
A B
C Rc D
Ra E E Rd G F
x m (300-x) m
Earth fault
...