Question

Marvo · Dec 14, 2012

Okay, I have a question for the trainees.

If you don't have an IR (megger) tester at your disposal and you had a CU with an RCD that keeps tripping, how would you go about finding the low IR fault.

Mark1985 · Dec 14, 2012

Good question..my guess would be turn off all the mcb's,reset the rcd then turn on each circuit 1 at a time to find out what circuit is causing the rcd to trip..then you've narrowed it down to 1 circuit...then if possible inspect that circuit to see if theres anything obvious,ie damaged cable etc..

Am i on the right track???

Marvo · Dec 15, 2012

Mark1985 said:
..then you've narrowed it down to 1 circuit...

Hi Mark, good reply.

Assume you find that the tripping problem appears to be caused by a combination of 3 or 4 circuits being powered up at the same time. You've unplugged all appliances to rule them out and there's no crossed or borrowed neutrals on the circuits.

You do have access to a standard tester but it's not picking up the insulation fault on the ohms scale.

Which way now?

Floody · Dec 16, 2012

Need to visually inspect every load to make sure a live/neutral isn't touching anything e.g back box, and nothing is going down to earth

Mark1985 · Dec 16, 2012

could you use a clamp meter,to either clamp the live and then the neutral to check the current is equal,and none is flowing to earth as this would cause the rcd to operate,or could you clamp the cpc to see if any current is flowing...although im not sure if this would be possible as it would of tripped..

Floody · Dec 16, 2012

Fingers crossed

x

Paul.M · Dec 16, 2012

Mark1985 said:
,or could you clamp the cpc to see if any current is flowing...although im not sure if this would be possible as it would of tripped..

Your on the right line mate.

Marvo · Dec 16, 2012

Floody said:
Need to visually inspect every load to make sure a live/neutral isn't touching anything e.g back box, and nothing is going down to earth

All the loads (appliances) were unplugged and if the tripping fault occurs when a combination of circuits are powered up I'm afraid it's unlikely to be a N-E short. You're spot on with the visual inspection technique but it's always nice to focus your search to the largest leakage faults first and not to waste time chasing the very small faults that normally wouldn't be a problem.

Mark1985 said:
could you use a clamp meter,to either clamp the live and then the neutral to check the current is equal,and none is flowing to earth as this would cause the rcd to operate,or could you clamp the cpc to see if any current is flowing...although im not sure if this would be possible as it would of tripped..

I like this. You might not get accurate leakage readings by clamping the CPC because the current might be finding a parallel earth path through moisture for example.

You could however get an accurate leakage reading by clamping the main supply L+N at the same time which is great because then you can then assess the leakage on each individual circuit by resetting them one at a time. Obviously you can then decide which circuits require remedial action, you define exactly what the circuit supplies and do what Floody suggested and start visually checking.