RCD puzzle

[ivixor]

Hi,

Having some problems reconnecting the swa cable from my garage/workshop. Here's what I've established:

RCD does not trip when main switch in garage is off, or when all fuses are removed.

RCD trips only after power is used - ie it does not trip until you turn something on.

All circuits in the garage are affected in the same way.

I've double and triple checked that the neutral in the CU is connected to the correct neutral block. I've also checked the SWA circuit using a multimeter for any shorts to earth either on phase or neutral.

Any ideas before I give up and call an electrician?

Cheers

 

[westie101]

Neutral to earth fault on one of the circuits in the garage

 

[ericmark]

I would concur with westie101 likely neutral earth fault. When no power is being used neutral and earth will be the same voltage so no current flows to earth. As power is used there will be a volt drop on the neutral so neutral and earth no longer the same voltage so current flows and trip will open.

 

[JohnW2]

I would concur with westie101 likely neutral earth fault.

Same here, but ...

When no power is being used neutral and earth will be the same voltage so no current flows to earth. As power is used there will be a volt drop on the neutral so neutral and earth no longer the same voltage so current flows and trip will open.

IMO, that really isn't a very good (or correct) explanation. Apart from anything else, if there is a N-E fault (i.e. E and N connected) in the garage, then neutral and earth obviously would be at the same potential. The simpler, standard, explanation is that if any load is connected to a circuit protected by the RCD is question, some of what should be neutral current will go to earth instead of along the neutral conductor, thereby creating an L-N imbalance (N current less than L current) which will cause an RCD to operate.

Kind Regards, John

 

[bernardgreen]

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

 

[LearningSpark]

ivixor, where abouts are you in Cambridgeshire? If you are near Peterborough I may be able to help.

 

[ivixor]

Thanks all,

I suspected it must be a N-E fault somehow. I did disconnect one suspect circuit entirely (old and unused storage heater). Looks like I need to disconnect all circuits and reconnect one at a time, or perhaps better, get the multimeter out again.

LearningSpark - south of Cambridge near the Essex border i'm afraid. Looks like I have a few more tests to do though.

The garage circuits were fine before I disconnected the SWA. So I spent a long time trying to work out if it was a fault in the reconfigured cable. Still not convinced there isn't.

I suppose it's *possible* that something in there somehow developed a fault during the two weeks or so it was disconnected, seems unlucky though.

 

[flyingsparks]

You need a insulation resistance meter or megger really, a multi meter won't necessarily show up the short.

 

[bernardgreen]

A friend is looking into the matter of earth leakage measurements. He and I both feel that a DC measurement at 500 volts of earth leakage will give a different result from an AC test at 230 volts.

The chemical effects at DC are different to the effects at AC. With DC across a damp junction electrolysis can create an insulating oxide layer that would be destroyed by AC, ( electrolytic capacitors depend on that oxide layer ).

 

[pjcomp]

The garage circuits were fine before I disconnected the SWA.

Assuming you didn't make any changes to the circuits in the garage, then logic suggests the fault must lie in the connection of the SWA.

pj