Share
Explore BrainMass

Murray Loop Test and Locating Faults

Hi there,
Can anyone answer the following questions please?

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.

Many thanks in advance.

Solution Preview

Please refer to the attachment.

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
...

Solution Summary

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

$2.19