For determining the location of an earth fault or short-circuit fault in an underground cable, we perform a loop test. Murray and Varley loop tests are the two types of tests we can perform to locate an earth fault or short circuit fault in an underground cable. For performing a Murray and Varley loop test, we should ensure that a sound cable runs along with the grounded cable or short-circuited cable.
In the Murray and Varley loop test, 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 the Wheatstone bridge principle.
Here we will be discussing both the loop test, Murray and Varley loop test in detail. Also, we will be looking at the key difference between Murray and Varley loop test.
Murray Loop Test
The figure below shows the test method for the Murray loop test.
Both of the Murray and Varley loop test is based on Wheatstone bridge principle. As this test is based on the Wheatstone bridge principle, we can consider P and Q to be the two ratio arms which consist 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 cables is used as a return path.
The circuit connection is shown below:
The bridge is balanced as done in the 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 the lower end ratio arm Q.
Varley loop Test
In the Varley loop test, we will be requiring a sound cable in addition to the existing cable as in the Murray loop test. 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 of 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 Murray and Varley loop test is that in the Murray test we obtain the total loop resistance by the relation R = ρ l/a. But in the case of the Varley test, we have the provision of measuring the total loop resistance instead of using the relation of the resistance.
Hence Murray and Varley loop test can be used in order to determine the cable faults in underground cable.