RCD tripping on ring main after new CU Install? - Electrician Talk - Professional Electrical Contractors Forum

Dave Marchesi · 04-19-2009, 11:15 AM

I have just fitted a new 17th edition Consumer unit as a replacement for an old bs3036 rewireable fuseboard and i have had some problems with the ring main circuit.

The problem is that the RCD (63 Amp - 30ma) keeps tripping, i have found a broken neutral in one of the walls and replaced the faulty wire and can now get continuity on both the neutral and live conductors but as soon as i connect the circuit into the RCD it keeps tripping out. The RCD is working ok with two other circuits so its not faulty.

The load on the ring main is neglible with only the central heating controls drawing any current.

Does anyone please have any idea what might be causing this tripping?

**MDShunk** · 04-19-2009, 11:22 AM

So... what have you checked yet? Seems like a megger would sort that out pretty quickly. The RCD is obviously seeing a ground fault in excess of 30ma. Perhaps disconnect some or all of the loads at first to just check the ring main itself and see if the RCD holds? That's what I'd do, as a quick test of whether you're looking for an equipment fault or a wiring fault.

Dave Marchesi · 04-19-2009, 11:38 AM

Ive not tested the circuits insulation resistance yet

OK i'll test the earth conductor continuity and run an insulation resistance test on the circuit and see what that tells me.

what if the circuit passes the megger test?

thanks

LawnGuyLandSparky · 04-19-2009, 11:54 AM

Can you break the ring up and test to the 1st point, then add to the 2nd point, etc, or are your rings non-splicable?

**MDShunk** · 04-19-2009, 11:56 AM

Quote:

Originally Posted by Dave Marchesi

what if the circuit passes the megger test?

thanks

It won't.

cornishsparks · 04-19-2009, 04:21 PM

I know this is probably bleedin obvious but have you got a borrowed neutral from some where, or got one end of the ring neutral connected to one neutral bar and the other neutral on the bar controlled from the second RCD, its easil done on replacements it would give you an instant trip when you tried to put any kind of load on.
Theese 17th edition units have three or four flying blue leads kicking around inside i have had them connected wrong from the factory instalation.

Dave Marchesi · 04-20-2009, 08:55 AM

Cornishsparks, i have checked and the RCD's are both installed correctly with no shared neutrals, i have found one drilled wire in a wall and replaced it but the RCD still trips. but thanks for the thought anyway :-)

i have done a megger test today on the circuit and i get:

L - N 14.22 Meg Ohms
L - E 14.17 Meg Ohms
N - E 0.00 Meg Ohms

so it appears i have a Neutral - Earth short in the circuit somewhere.

i suppose the next step is to test each section of the circuit until i find the shorted section?

Dave Marchesi · 04-20-2009, 08:59 AM

You are right MDShunk it failed the megger test on the N-E conductors

looks like i better hunt down where the short is by testing each section of the circuit.

Do the fancy 200 quid plus cable fault finders pick up this kind of short circuit and are they worth having?

Thanks DM

flip73 · 04-20-2009, 05:29 PM

as mentioned by lawn guy just split the ring in the middle making 2 radials test to find the shorted side then keep doing the same until you arrive at the leg with the fault

LawnGuyLandSparky · 04-21-2009, 05:23 PM

Quote:

Originally Posted by Dave Marchesi

You are right MDShunk it failed the megger test on the N-E conductors

looks like i better hunt down where the short is by testing each section of the circuit.

Do the fancy 200 quid plus cable fault finders pick up this kind of short circuit and are they worth having?

Thanks DM

Maybe if I knew what a quid was...

Trimix-leccy · 04-22-2009, 05:33 PM

1 quid = 1 english pound and at todays exchange rate about $1 as well

**MDShunk** · 04-22-2009, 11:34 PM

Quote:

Originally Posted by Trimix-leccy

1 quid = 1 english pound and at todays exchange rate about $1 as well

Don't they make calamari from a quid?

Trimix-leccy · 04-23-2009, 04:25 PM

Quote:

Originally Posted by MDShunk

Don't they make calamari from a quid?

in my attempt to be multi-cultural I made a stab at learning Greek for my holidays.

Every morning I would greet the waiters, pool man , reception with 'calimares'. towards the end of my second week i was told that I ahd been actually saying 'squid'! I should have said 'calis mare'.....bu88er!

I am now learning Arabic