RCD keeps tripping all MCBs are off

dil1976

grumbler said:

Section62 said:

grumbler said:

RCD can be tripped by a neutral wire touching earth.

But if all MCBs are off it seems to be no current to trip the RCD.

Current flowing on the neutral side to 'earth' would create an inbalance across the RCD and is enough to make it trip (in the right circumstances).

Yes, this is obvious.

The question was in the last sentence of my post that you excluded from the quote - what can cause this current when all live wires are disconnected by MCBs.

If all live conductors are disconnected then its not possible for a RCD to trip. Do know that both the LINE and NEUTRAL are both live conductors 

grumbler

I do know about LINE and NEUTRAL, but don't like this terminology. By "all" I meant all live wires connected to the appliances/sockets MCB.

I edited my above post - added some suggestion.

dil1976

grumbler said:

Section62 said:

grumbler said:

RCD can be tripped by a neutral wire touching earth.

But if all MCBs are off it seems to be no current to trip the RCD.

Current flowing on the neutral side to 'earth' would create an inbalance across the RCD and is enough to make it trip (in the right circumstances).

Yes, this is obvious.

The question was in the last sentence of my post that you excluded from the quote - what can cause this current when all live wires are disconnected by MCBs.

ra9jd, switch all MCBs off, including lighting, and I bet the RCD will stop tripping. In this case the fault is a neutral wire touching earth somewhere in the sockets circuits. If you feel confident enough you can find out which circuit exactly by disconnecting one by one neutral wires in the CU .

Switching off the MCBs will only stop the tripping if they are double pole and the neutral is fed through them

grumbler

dil1976 said:

grumbler said:

Section62 said:

grumbler said:

RCD can be tripped by a neutral wire touching earth.

But if all MCBs are off it seems to be no current to trip the RCD.

Current flowing on the neutral side to 'earth' would create an inbalance across the RCD and is enough to make it trip (in the right circumstances).

Yes, this is obvious.

The question was in the last sentence of my post that you excluded from the quote - what can cause this current when all live wires are disconnected by MCBs.

ra9jd, switch all MCBs off, including lighting, and I bet the RCD will stop tripping. In this case the fault is a neutral wire touching earth somewhere in the sockets circuits. If you feel confident enough you can find out which circuit exactly by disconnecting one by one neutral wires in the CU .

Switching off the MCBs will only stop the tripping if they are double pole and the neutral is fed through them

No, you seem to be missing the point.

In the CU only earth wires are split in two sets; the neutral wires aren't and are all connected together.

My guess was that even when all sockets MCBs were off, part of the current flowing through the lighting circuits was leaking to the sockets neutrals and then to sockets earth thus tripping the sockets RCD.

If you switch all other MCBs as well, even single pole, there can be no current in the lighting circuits and nothing to leak. If the RCD is still tripping, then it's faulty.

Chickereeeee

A faulty RCD  is a possibility. They can refuse to be reset, or trip after a short delay, as described.

Risteard

grumbler said:

I do know about LINE and NEUTRAL, but don't like this terminology. By "all" I meant all live wires connected to the appliances/sockets MCB.

I edited my above post - added some suggestion.

It's the correct terminology. Neutral is a live conductor as it is intended to carry current in normal operation. Line (or phase) and neutral are live conductors. By convention, a PEN conductor is not classed as a live conductor however.

grumbler

Risteard said:

grumbler said:

I do know about LINE and NEUTRAL, but don't like this terminology. By "all" I meant all live wires connected to the appliances/sockets MCB.

I edited my above post - added some suggestion.

It's the correct terminology.

Did I say "incorrect"?

OK, I don't like the 'correct' terminology. And when I say 'live' and 'neutral' instead, everybody understands what I mean and there is no place for any ambiguity.

Section62

grumbler said:

Section62 said:

grumbler said:

RCD can be tripped by a neutral wire touching earth.

But if all MCBs are off it seems to be no current to trip the RCD.

Current flowing on the neutral side to 'earth' would create an inbalance across the RCD and is enough to make it trip (in the right circumstances).

Yes, this is obvious.

The question was in the last sentence of my post that you excluded from the quote - what can cause this current when all live wires are disconnected by MCBs.

My post was an answer to that question - there will be a voltage on the neutral side of the RCD, even if all the 'live' (brown) side MCBs are switched off.  If a final circuit connected to the RCD has a neutral-'earth' fault then that voltage will result in current flowing through the neutral side of the RCD (to 'earth').

This is an example of where the terminology relating to the word 'live' is really important.  The 'neutral' conductor (whilst connected to the supply) is a 'live' conductor - and capable of delivering voltage and current in exactly the same way the (brown) 'live' can.  Thinking that blue=neutral=zero volts is an error... and a cause of confusion in situations like this one.

I left out your last sentence in what I quoted because talking about "sockets neutral" and "lighting neutral" only adds to the confusion.  On the supply side the neutrals will be commoned together (at some point, usually in the CU) -  so for practical purposes (on the supply side) they should be seen as one and the same. The voltage on the supply-side 'neutral' terminals on both RCD's in a split-load board will be (practically) the same. Appreciating that point should help in understanding why an RCD can trip even when all the 'live' (brown) MCB's are off.

grumbler said:

If you switch all other MCBs as well, even single pole, there can be no current in the lighting circuits and nothing to leak. If the RCD is still tripping, then it's faulty.

This is incorrect.  The RCD might be faulty, but this test isn't going to give you that answer.  The neutral conductor is 'live' (see above), hence a current can flow through the RCD and cause it to (quite correctly) trip.

47Revisited

Well I'm just going by what the OP says.

The Lighting RCD is tripping on what will almost certainly be a radial circuit. So lights, switches, extractor fan maybe. Whatever!  That circuit is the problem.

Unless there is a borrowed neutral or the RCd is wired up wrong..wrong neutral bar etc

Or the OP has sockets appliance's attached to his lighting circuit??

Checking connections at all accessories and the consumer unit is the first and cheapest way to approach the problem.

If he feels he is not capable of checking that (Specifically the consumer unit) then he needs to employ the services of a decent Spark.

47Revisited

@dil1976 ,^^