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  • Murray loop bridge is a bridge circuit used for locating faults in underground or underwater cables. It has been used for more than 100 years. One end of the faulted cable is connected through a pair of resistors to the voltage source. Also a null detector is connected. The other end of the cable is shorted. The bridge is brought to balance by changing the values of RB1 and RB2, which is achieved when: which is equivalent to: The value of resistance Rₓ is proportional the length Lₓ, thus the location of the fault can be calculated: where L is the total length of the cable under test - a value proportional to Rg. The method assumes a single fault exists, of low resistance compared with the undamaged cable insulation resistance, and that the cable conductors have uniform resistance per unit length.