Light circuit off at consumer board - but RCD / RCCB still tripped

[Max Rasa]

Hi

I'm removing an extractor fan from our WC, as it's just a toilet and sink and we never use hot water in there anyway...

The extractor is on the light circuit, but has it's own pull cord. I double checked by turning the fan and light on, then switching off the MCB. All good. I triple checked with a multi meter and all was 0. So after disconnecting I started checking for continuity so I knew which cable to pull on. Once sussed, I started wiggling it and pulling, at which point the RCD/RCCB tripped. I'm not great with all the consumer unit parts and what they're called, but it's a split unit and the left side tripped - which the same side as where I'd already turned off the MCB.

Think it'll probably be wise to get an electrician in to see what's what. Most the house lights are on a separate circuit so there's no rush.

But I'm really curious in the meantime....

- Is it possible to trip the RCD/RCCB by doing something on a circuit, even when the MCB for that circuit is off?

- If not, must I assume that something from another circuit is going on in the ceiling void?

Hope I'm making sense! Happy to add a couple of photos if it would help.

Cheers

 

[James]

It is possible to trip an rcd if the circuit has its individual breaker turned off.
a short from N to earth will do it nearly every time.

 

[littlespark]

It is possible to trip an rcd if the circuit has its individual breaker turned off.
a short from N to earth will do it nearly every time.

A short from N to E should do it every time, not nearly every time

 

[Max Rasa]

That's all great to hear and along the lines(ish) of what I hoped was the case. But didn't want to risk it without being certain.

In terms of being certain, is it safe to replicate what happened (MCB off, touch earth and neutral and see if it trips again)? Or is there any potential risk?

As for finishing the job, I thought it would be as simple as:

• removing the cable that connects the pull cord to the fan itself
• and then removing the cable that connects the pull cord to the ceiling rose.

But please do correct me if I'm wrong and if I could cause problems, open cans of worms, etc. I'm confident with the practical side of doing it, but like to make doubly sure on the theory.

Thanks a million for the help

 

[Lucien Nunes]

Linking N and E with the MCB off should be safe but it's not a very helpful test to do, because whether it will actually cause a trip depends on what is happening elsewhere in the system and might not always behave the same way. An L-E short always causes a trip becase there is 230V there to drive current through the fault. But N and E are approximately at the same potential; if they happen to be at the same potential at a given moment there is nothing to drive current through the fault and trip the RCD.

The small N-E potential that usually exists will depend on the load elsewhere causing voltage drop in the neutral conductors. With a TN-C-S supply, N and E are solidly linked at the head and the only thing to create VD with the lighting circuit off is the load of other circuits on the meter tails and the resistance of the main switch. With a TN-S supply there can be VD due to other installations on the supply mains, plus your own load on the service cable. So you could have a situation where you join N & E and nothing trips until your neighbour turns on their cooker or shower, or pulls a trigger on a big power tool that causes an spike on the N voltage due to the inrush.

In a nutshell, I am not surprised that handling the cables wth just the MCB off caused a trip, and would not bother to investigate further.

 

[Max Rasa]

That's a great and very helpful response @Lucien Nunes - and very much appreciated. Excellent timing too. No doubt as you were typing, I was in the WC with head torch on, touching neutrals to earths and wondering why it wasn't tripping. Meanwhile unbeknown to me you were explaining it.

Great stuff, many thanks.

 

[timhoward]

As for finishing the job, I thought it would be as simple as:

• removing the cable that connects the pull cord to the fan itself
• and then removing the cable that connects the pull cord to the ceiling rose.

If you are confident the ceiling rose feeds the pull cord and confident you have identified the correct cable, then I'd just turn off the MCB again, confirm dead again, and remove the ceiling rose connection first.
There is then also less chance that you'll trip the RCD when dismantling the pull cord or removing the fan cable.

 

[Max Rasa]

I should have added to the above.... When the initial tripping happened, the other half was cooking and so the oven was on. When I tested it wasn't. So that explains it.

@timhoward - great advice, thank you.

 

[Max Rasa]

Quick update to say I'm all done with the extractor fan removal and replaced the worn ceiling rose while I was at it. Appreciate all the help that got me there Have a great weekend everyone

 

[Zerax]

Congratulations on the successful removal of your extractor fan !

So now may not be the best time to mention the building regulations that require a fan if there is no opening window ?

 

[Max Rasa]

Hahaha! Two opening windows I'm pleased to report. Out of interest though, is that even the case for a small WC/cloakroom?

 

[telectrix]

Hahaha! Two opening windows I'm pleased to report. Out of interest though, is that even the case for a small WC/cloakroom?

all depends on the strength of your farts from lastnight's vindaloo.

 

[Max Rasa]

all depends on the strength of your farts from lastnight's vindaloo.

I'd best see if one of these would fit in that case

Mains ring or lighting circuit?