Fault Locating With The VM Series Products

Locating the route of a cable is sometimes just the start of the process. The operator very often needs to find the location of a fault to undertake repair work on the cable. There can be many different types of faults, but where there is damage to the cable, the conductor is in contact with the ground, the optional addition to the VM-560 locator kit, the VM-510FFL+ Standalone A-frame fault locator can be used to identify the exact position of the fault.

The process of detecting a fault to ground can be a complicated and laborious task. The VM-510FFL+ simplifies the whole process and provides a remarkably accurate pinpoint of the fault. (Within a few cm/inches).

The process is a simple. First remove all ground bonds from the cable. Fig 5 The idea is to make the fault the only contact the cable has with a ground. Having isolated the cable, the transmitter can be connected to the faulty cable and the other connection cable connected to an independent earth. (A small ground rod is supplied for this purpose. The transmitter is switched on and set to the 8kFF mode. This is a complex signal but suffice to know that the main frequency is 8Hz which is used to detect the fault.

The low-frequency component of the fault find signal (8Hz) travels along the cable and leaks into the earth at the fault location. This sets up ground voltages around the defect in a similar way a garden watering hose would squirt water in all directions from a hole in the pipe.
The fault currents that cause the pooling of voltage at a fault return through the ground and complete the circuit back at the earth stake.

Fig 5

The VM-510FFL+ is designed to locate and measure the fault currents in the ground. To do this the A-frame is switched on and the spike of the A-frame is inserted in the ground over the cable. A dB reading indicates the level of fault current/voltage at that point. The spikes are placed in the ground at regular intervals along the cable. As the fault is approached, the signal level will increase as the ground currents increase as the fault is neared. The signal reaches a maximum just before and after the fault.

Fig 6

Phase-sensitive measurements are made on the fault currents, which are interpreted as simple forward/back arrows so the user is steered to the fault. A left/right indicator is also shown ensuring the operator keeps to the centerline of the cable. The VM-560FF kit, with the addition of the VM-510FFL+ stand-alone A-frame, is the ideal short drop and fault finder combination.