Tracing an electrical fault?

[A.Lee]

A friend asked me to have a look at his kitchen light on Sunday. He turned it off, there was a pop, and the circuit breaker tripped.

He tried to reset the CB, but it wouldnt reset, just immediately tripped.

So, I go, thinking blown bulb or somesuch thing. The circuit covered most of downstairs, with all of those lights off obviously.

After checking numerous switches, disconnecting all the lights etc. I was no closer to finding a fault The living room had 3 switches with three lights connected to the 3 switches - all 3 way - lights on if one switch is flicked, off if another is flicked. The back of the switches were like a spaghetti farm.

I removed the switches in turn, to see if one was faulty. By chance, the first one was removed, and the CB could be turned on, with all lights working apart from the kitchen and living room. The switch was checked, and no leakage to earth, and switched fine on the meter. So now it seemed there was a fault somewhere in the wiring. I came to the conclusion that I couldnt fix it.

He then got a sparky in on Monday. He was there 4 hours. Apparently, one set of wires ran inside a metal sheath cover (the type you can use a nail to hold on). When originally fitted, one cable was too close to the nail, and had been compressed, so much so that the live wire had finally been pushed out of its insulation, 10 years after being fitted, and had touched earth, causing the CB trip.

He also could not understand why there were so many wires around, and had to call in his Son to help him find the fault.

So, the query is, if this ever happens again, how can you trace such a fault, without stripping out all the wires?

Ta Alan.

[Harry Bloomfield]

A.Lee expressed precisely :

With a great deal of difficulty, to be honest.

There are ways that can help, but often it comes down to isolating each cable until you find the cable which has gone faulty, then guessing at the route the cable takes then using guess work and intuition to find the problem. You can buy equipment which puts a signal down the wire to enable its route to be traced. You can also buy equipment which pulses a signal down the cable and displays how far down the cable a short is in metres. Problem is, that both items are expensive.

[John Rumm]

Well the first steps would be to disconnect the circuit at the CU and do resistance checks between each of the conductors to work out what sort of fault you are dealing with - i.e. L to N or a L to E short. Problems like that you describe above are more likely to be down L to E faults.

If you can't see a short with a multimeter, then you need to try again with an insulation tester.

Once you have established the nature of the fault, you can see if it goes away with the bulbs removed and each switch position tried.

If that is inconclusive, then start to break down the circuit to isolate the fault. For example at a ceiling rose you can disconnect the downstream section of a circuit, and see which "half" the fault is in. You can then usually reconnect, and move the break to a different half until you have isolated which section of cable is causing the problem.

Sometimes you can jump forward by analysing what has changed - i.e. has someone just fixed something to a wall etc, or changed a light fitting.

[NT]

Divide & conquer is the best way