Why does my RCD keep tripping?

I mentioned in another thread that I had a rewire done last year and my electrician had done a few things that in hindsight appear a bit odd. I've got two other electrical issues too, and just wanted to run these past the people on here. I apologise in advance if the terminology I'm using here is incorrect, but hopefully you can understand it.

1) We are renovating the entire house. This involves having to remove sockets and switches quite a lot, and then to re-wire them when the plasterer has finished. I do this myself. Obviously, I switch off the MCB relevant to the circuit I'm working on when I do this. However, I notice that I will still occasionally trip the RCD that controls the bank of MCBs that the circuit I am working on is on. E.g. I've got on RCD that controls upstairs lights, downstairs sockets and some other stuff; and another that controls downstairs lights and upstairs sockets. There is then the Big Switch that controls the two RCDs. I had to put the sockets back in the lounge the other day after the plasterer finished and the walls were painted. I turned off the MCB for the downstairs ring circuit, tested it was dead and went to work, but I still tripped the RCD. The upstairs lights went off, the downstairs lights stayed on. It doesn't always happen, but I've noticed that it almost always happens when I have to cut the cables, even if it is just a few mm.

I assume that this shouldn't be happening? Is there any reason why it is happening?

2) The light the dining room occasionally blinks, and often dims a bit all on its own. It's irritating. I assumed that it was because there was some loose connection in the ceiling rose, which was left hanging from the ceiling (rather than fixed to it) for about 6 months as we renovated, but it's all fixed in place now and I had a check of the wires and they are all tightly fixed.

Again - I assume this should not be happening? Is there any reason why it is happening?

Thanks for your help with this, I really appreciate it. I did try to google it, but couldn't find anything UK-specific.

The MCB only disconnects the Live, The Neutral remains connected,

Shorting the Neutral to Earth while working will cause the RCD to trip if there is a load on another circuit protected by the RCD.

DavidA1982 said:

2) The light the dining room occasionally blinks, and often dims a bit all on its own. It's irritating. I assumed that it was because there was some loose connection in the ceiling rose, which was left hanging from the ceiling (rather than fixed to it) for about 6 months as we renovated, but it's all fixed in place now and I had a check of the wires and they are all tightly fixed.
.

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If your light system is of the "loop in" type, then the power feed will come from another light. If there is a bad connection, it could be at that point.
I guess that none of the other lights in the house flicker?

I think it's loop-in i.e. the power and switch cables go directly to the ceiling roses rather than junction boxes in the ceiling. It is just that light that flickers.

DavidA1982 said:

I think it's loop-in i.e. the power and switch cables go directly to the ceiling roses rather than junction boxes in the ceiling. It is just that light that flickers.

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If the flickering light is the last one in the circuit (hence no others downstream to flicker), it's still possible that the problem is a loose connection in the loop-in rose which is next upstream of the flickering one. Since you probably won't know in which order the lights are wired, you may have to just check the connections in all the roses on the circuit (logically starting with those physically closest to the flickering one).

Kind Regards, John

DavidA1982 said:

1) We are renovating the entire house. This involves having to remove sockets and switches quite a lot, and then to re-wire them when the plasterer has finished. I do this myself. Obviously, I switch off the MCB relevant to the circuit I'm working on when I do this. However, I notice that I will still occasionally trip the RCD that controls the bank of MCBs that the circuit I am working on is on. E.g. I've got on RCD that controls upstairs lights, downstairs sockets and some other stuff; and another that controls downstairs lights and upstairs sockets. There is then the Big Switch that controls the two RCDs. I had to put the sockets back in the lounge the other day after the plasterer finished and the walls were painted. I turned off the MCB for the downstairs ring circuit, tested it was dead and went to work, but I still tripped the RCD. The upstairs lights went off, the downstairs lights stayed on. It doesn't always happen, but I've noticed that it almost always happens when I have to cut the cables, even if it is just a few mm.

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It will do this, as has already been mentioned in previous posts, when the CPC/earth touches the neutral conductor and not ideal if you require other electrical equipment to be powered via this RCD, whilst working on this circuit.
The solution is to temporarily remove the neutrals of this circuit from the busbar.

DavidA1982 said:

2) The light the dining room occasionally blinks, and often dims a bit all on its own. It's irritating. I assumed that it was because there was some loose connection in the ceiling rose, which was left hanging from the ceiling (rather than fixed to it) for about 6 months as we renovated, but it's all fixed in place now and I had a check of the wires and they are all tightly fixed.

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This problem could be resolved elsewhere, could have loose fittings any previous connection to this light, they could be at another light fitting or junction, but I would start at the switch.

bernardgreen said:

The MCB only disconnects the Live, The Neutral remains connected,

Shorting the Neutral to Earth while working will cause the RCD to trip if there is a load on another circuit protected by the RCD.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

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Thanks for that, very interesting. I guess I'm a bit concerned now that this did NOT happen in my last house when I was doing the same sort of thing!

Anyway, I'll just put one of those orange lever-arm wago clips on the neutral cables while I'm working for now, that should stop it happening most of the time. It's not a massive problem when it does happen, it just means I have to re-programme a few clocks.

DavidA1982 said:

Thanks for that, very interesting. I guess I'm a bit concerned now that this did NOT happen in my last house when I was doing the same sort of thing!

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Either you last house didn't have RCDs or else you were particularly careful (or very lucky!).

DavidA1982 said:

Anyway, I'll just put one of those orange lever-arm wago clips on the neutral cables while I'm working for now, that should stop it happening most of the time. It's not a massive problem when it does happen, it just means I have to re-programme a few clocks.

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That sounds like a sensible plan. It is surprisingly easy to let N and E come into contact, and it only has to be for a few milliseconds! As you say, if it happens it's usually no big deal, but is definitely a nuisance!

Kind Regards, John

DavidA1982 said:

Thanks for that, very interesting. I guess I'm a bit concerned now that this did NOT happen in my last house when I was doing the same sort of thing!

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In my experience, the incidence of tripping when a neutral is in contact with an earth connection also depends on the supply type.

I have observed that the problem appears to be less prevalent in TN-S supplies than TNC-S.

Especially if it takes out lighting which you need.

Or it's the first time it's ever happened to you

ban-all-sheds said:

... Or it's the first time it's ever happened to you

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Very much so. I suspect that, like many before me, I experienced a very unpleasant feeling in my stomach, assuming that (despite all the precautions) I must have somehow been working on a live circuit!

Kind Regards, John

Taylortwocities said:

In my experience, the incidence of tripping when a neutral is in contact with an earth connection also depends on the supply type. ... I have observed that the problem appears to be less prevalent in TN-S supplies than TNC-S.

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That's interesting. I need to think about that, to see if I can find some way of rationalising it. On very brief, initial thinking, what you say sounds 'the wrong way around'. With TN-S there is scope for there to be an appreciable pd between N and E at the origin of the installation - so, even in the absence of any loads at all within the installation, there is presumably potential for a N-E short downstream of the installation to result in current through just the N side of the RCD. On the other hand, with TN-C-S there cannot be any N-E pd at the origin of the installation.

It sure happens with TT, too

Kind Regards, John

I have no idea why, but the most modern hager RCDs do not seem to trip with a N>E short on an isolated circuit.

RF Lighting said:

I have no idea why, but the most modern hager RCDs do not seem to trip with a N>E short on an isolated circuit.

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As I'm sure you understand, that really makes no sense. In the presence of any significant loads on any of the circuits (or sometimes even in the absence of loads) an N-E short will inevitably result in a >30mA imbalance in the RCD, no matter what make it is - which, equally inevitably, will cause the RCD to trip (assuming that modern Hager ones actually work ).

Having said that, knowing your approach to such matters, have you undertaken an experiment and confirmed that (in the presence of loads) a deliberate N-E short does not cause a 'modern Hager RCD' to operate? If so ... well, I don't know!!!

Kind Regards, John

RF Lighting said:

I have no idea why, but the most modern hager RCDs do not seem to trip with a N>E short on an isolated circuit.

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Well they should if there is more than a 60 milli Amp load on a circuit on that RCD. The load required depends on the ratio of impedances of the two routes from neutral bus bar to Neutral - CPC joint in the cut out. ( the pecked grays in this diagram ).