Murray loop bridge

[1][2] It has been used for more than 100 years[3] but is being replaced by the more precise time-domain reflectometer.

One end of the faulted cable is connected through a pair of resistors to the voltage source.

The bridge is brought to balance by changing the values of RB1 and RB2, which is achieved when:

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.

If the fault resistance is high, the sensitivity of the Murray bridge is reduced and the Varley loop may be more suitable.