We use Murray Loop Test for identifying the location of earth fault and short circuit fault in an underground power cable. In this test, we simply apply the principle of Wheatstone bridge to find out the location of the fault.
Murray Loop Test for Ground Fault
Suppose there is a ground fault at X meter away from one end of the cable. For arranging Murray Loop Test we need one sound cable runs in parallel with the faulty cable. Now we have to provide a very low resistance (ideally shorted) connection between the far end of both cables. Then we connect the ratio arms A and B of a Wheatstone bridge with the other end of these cables as shown in the figure below.
After arranging this simple circuit connection, we need to adjust the resistance value of the ratio arms. We continue this adjustment until the reading of the galvanometer comes to zero. That means the circuit of Murray Loop Test has become balanced. Then we note down the adjusted resistance value of A and B.
After recalling the theory of the Wheatstone bridge circuit, we can write
Here, R + X is the total resistance of the sound and faulty cable, when they are in series. From, resistance ratings provided by the manufacturer of the cable, we can easily get the value of R + X.
Since we have already noted the value of resistance of A and B, and we have found out the value of R + X, we can easily find out the value of X.
As shown in the circuit diagram of Murray Loop Test, X denotes the resistance between the operating point and faulty point of the cable under test. From X, we can very easily calculate the distance of the fault location from the operating location. Because we can easily find the resistance per unit length of the cable conductor from the cable rating data sheet provided by the cable manufacturer.
Murray Loop Test for Short Circuit Cable Fault
In that case, we only have to connect the battery to the head of the second short-circuited cable instead of connecting it to the ground. Other arrangements of the test are exactly the same as the test for a ground fault. Here, in the same way, by adjusting resistances of the ratio arms, we have to bring the balanced condition of the circuit. Then we have to calculate the location of the fault with the help of calculated resistance value from the operating point to the faulty point of the cable under test.
We can draw a schematic diagram of the connection of Murray Loop Test for finding the short circuit fault location.
Here, also at the balanced condition,
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