Tips for locating a fault in wiring

13 April 2018

I have to locate a fault in wiring; what can I do?

Depending how often you need to deal with having to remedy a fault in wiring, will have a direct impact on how we answer your question.

The quick and easy answer to ‘how does one locate a fault in wiring’ is…take a course.  This holds true if you’re going to get involved in serious fault locating and don’t want to spend a lot of time digging empty holes.

The AVO Institute, a subsidiary of Megger, offers a class on how to deal with this situation. Fault locating in wiring is quite involved and includes numerous levels of test equipment and techniques. The AVO Institute goes over all of this in depth during its class.

If locating a fault in wiring is just an isolated incident that needs to be addressed, then we suggest you start with a handheld TDR (time domain reflectometer). 

Testing with a TDR

For telecom/datacom/twisted pair/coax and similar applications, the TDR should be all you need. Standardization keeps these cables well within the testing parameters.

Testing power cable is something different. You could be measuring anything from Romex to 4/0. A TDR may work, mainly for bolted faults and complete opens, but for more sophisticated ‘high resistance’ faults (above 100Ω), an arc reflection system (TDR and ‘thumper’ combination) may be required.

First, the TDR must have two parallel conductors. A pulse is sent down through the insulation between the conductors and a portion of the energy echoes back from any change in resistance, as by a fault. The tester times the pulse and tells the operator how far the fault is located.

At times single core wire is used. This presents a major problem. TDR technology is based on two parallel conductors being separated by a constant width of insulation. A TDR may work on two single-core wires pulled together in conduit, but will generally give a large error. Using the TDR in this instance is a ‘better-than-nothing test’, but you may end up digging several holes.

Modern quality testers have reduced, but not eliminated, operator involvement. Many of the variables (gain, distance, end of cable) can be manipulated if desired, but will also work automatically. The one parameter that the operator must still provide is the velocity of propagation (VoP) for the type of insulation. This should be part of the wire spec and indicates how fast the TDR impulse will travel. The operator enters this datum into the TDR. Don’t know it? OK, guess. Again, it will just introduce an error, and the more wrong you are, the bigger the error. But you can still get in the vicinity, dig a hole, and find the fault.