Murray and Varley Loop Test
For determining the location of an earth fault or short-circuit fault in an underground cable, we perform loop test. Murray and Varley loop test are the two types of the tests we can perform to locate an earth fault or short circuit fault in an underground cable. For performing loop test, we should ensure that a sound cable runs along with the grounded cable or short-circuited cable.
Unless the resistance of the fault point is very high, the value of the fault resistance does not affect the result of the test.
Murray and Varley loop test is based on Wheatstone bridge principle.
Murray Loop Test
The figure below shows the test method for Murray loop test.
As these test is based on the Wheatstone bridge principle, P and Q are the two ratio arms which consists of step resistors or slide wire, G being the galvanometer, B is the battery, KB is the battery key and KG is a galvanometer key.
A low resistance link is used to connect the faulty cable with the sound cable. By adjusting the resistance of the ratio arms P and Q the bridge is balanced. When the bridge is balance the galvanometer will indicate zero deflection.
For a balanced condition of the bridge,
Here, r is the resistance of one of the cables when it is free from fault.
If l is the length of the cable in meters and r/l is the resistance per meter length of the cable. Then
The circuit connection of short-circuit fault is similar to that of the earth fault test. Here, instead of using the earth as a return path, one of the short-circuiting cable is used as a return path.
The circuit connection is shown below:
The bridge is balanced as done in earth fault test.
For balanced bridge condition,
The quantity R+X is the known quantity and it is the total resistance of the loop.
Since all the quantity on the right side of the above expression is known, we can then calculate the X which is the distance of the fault from lower end ratio arm Q.
Varley loop Test
In this test we will be requiring a sound cable in addition to the existing cable. The figure below shows the circuit diagram to perform the Varley loop tests for earth fault and short-circuit fault.
Here we will measure the total loop resistance.
In this test we will fix the ratio arms P and Q and we obtain the balancing of the bridge by adjusting or varying the known resistance S. The battery has a key ‘K. We should obtain the balancing condition of the bridge for both the positions of the key i.e. for the key on stud 1 and stud 2 respectively.
Under balance condition with the key on stud 1 then for earth fault or short-circuit fault,
S1 is the value of setting resistance S when the key is in position 1.
Under balance condition with the key on stud 2 then for earth fault or short-circuit fault,
S2 is the value of setting resistance S when the key is in position 1.
From equation (i) and (ii) we have
Thus, if we know the value of P, Q, S1 and S2, we can therefore determine the value of X.
Difference Between Murray and Varley Loop Test
The main difference between these two tests is that in Murray test we obtain the total loop resistance by the relation R = ρ l/a. But in the case of Varley test, we have the provision of measuring the total loop resistance instead of using the relation of the resistance.