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Loop test in cable cannot locate the
Open circuit fault
Short circuit fault
Earth fault
None of above
Open circuit fault
Loop tests, such as the Murray or Varley loop tests, operate on the principle of a Wheatstone bridge circuit. These tests require a continuous metallic path (a loop) from the bridge to the fault and back to accurately measure the fault resistance/location, which is impossible in the case of an open circuit fault where the conductor continuity is broken.
Loop tests, such as the Murray or Varley loop tests, operate on the principle of a Wheatstone bridge circuit. These tests require a continuous metallic path (a loop) from the bridge to the fault and back to accurately measure the fault resistance/location, which is impossible in the case of an open circuit fault where the conductor continuity is broken.
RxтАЛ=RтЛЕQPтАЛ тАФ Standard Wheatstone bridge balance condition for short/earth faults
IloopтАЛ=0 тАФ In open circuit fault, the loop current is zero, rendering the test non-functional
In a loop test, the faulty cable is looped with a healthy cable of similar characteristics at the far end. A bridge circuit is formed using the resistance of the conductors. Because an open circuit represents an infinite resistance path, the bridge cannot be balanced, and the loop current required to calculate the distance to the fault cannot be established.
Murray loop test is suitable only for ground faults and short-circuit faults.
Open circuit faults are typically located using capacitance measurement tests (e.g., Capacitance bridge).
Loop tests rely on forming a closed electrical circuit to establish a bridge balance.
The accuracy of the loop test depends on the homogeneity of the cable and the availability of a healthy pilot wire.
High accuracy for low resistance faults
Independent of fault resistance in certain configurations
Requires a healthy cable of equal cross-section
Cannot detect open circuit faults
Underground power cable fault location
Communication cable testing for short-circuit faults
Option B (Short circuit) and Option C (Earth fault) are both successfully detectable via loop tests because they permit the flow of bridge currents.
Open circuit faults in cables change the line capacitance, which is why capacitive bridge tests are used instead.
A is correct тАФ Loop tests require a continuous metallic path to balance a Wheatstone bridge, which is absent in an open circuit fault.
For open circuit faults, always remember the 'Capacitance Test'. If the question asks for fault location and it involves 'continuity', think Loop Test (Murray/Varley).