We have seen the types of faults in underground cables and how to detect them using a megger in the previous article. This article explains **how to locate the exact place where the fault has occurred**.

There are various

**methods for locating the faults in underground cables**. Following are some popular methods explained.## Murray loop test for location of faults in underground cables

Murray loop test is the most common and accurate method for locating earth faults and short-circuit faults. However, to perform the Murray loop test, it is necessary that a sound (good) cable runs along the faulty cable.This test employs the principle of Wheatstone bridge for fault location.**To perform the Murray loop test**, the alongside sound cable and the faulty cable are shorted with a jumper conductor at the far end. The test side end is connected through a pair of resistors to a voltage source. Also, a null detector or galvanometer is connected between the two conductors at the test end. The circuit diagram is as shown in the image below.

_{1}and R

_{2}so the null detector/galvanometer shows zero reading. That is, bring the bridge to the balance. Now, in the balanced position of Wheatstone bridge, we have,

Now, if

**r**is the resistance of each cable,

then,

**R**

_{x}+ R_{y}+ R_{g}= 2rPutting this in the above equation,

We know, the value of resistance is proportional to the length of the cable. Therefore the value of R

_{x}is proportional to the length L

_{x}. Therefore,

Where L is the total length of the cable under test. (The value of L is proportional to the value of R

_{g}.)

## Varley loop test

Varley loop test is also for**locating short-circuit and earth faults in underground cables**. This test also employs the principle of the wheatstone bridge. However, the

**difference between Murray loop test and Varley loop test**is that, in Varley loop test resistances R

_{1}and R

_{2}are fixed, and a variable resistor is inserted in the faulted leg. If the fault resistance is high, the sensitivity of Murray loop test is reduced and Varley loop test may be more suitable.

**To perform Varley loop test**, connections are done as shown in the circuit diagram above. Resistors, R

_{1}and R

_{2}are fixed and the resistor S is variable. In this test, the switch K if first thrown to the position 1. Then the variable resistor S is varied till the galvanometer shows zero deflection (i.e. bridge is balanced). Lets say, the bridge is balanced for the value of S equal to S

_{1}Then,

Now, the switch K is thrown to the position 2 and the bridge is balanced by varying the resistor S. Say, the bridge is balanced at the value of resistor S is equal to S

_{2}. Then,

Now, putting the result of eq.(ii) in eq.(i),

Since the values of R

_{1}, R

_{2}, S

_{1}and S

_{2}are known, R

_{x}can be calculated. When R

_{x}is known, the distance from the test end to the fault point L

_{x}can be calculated as,

**L**

_{x}= R_{x}/rWhere, r = resistance of the cable per meter.