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

udaman · Post by **udaman** » Fri Dec 17, 2021 9:15 pm

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

Someone-Else · Post by **Someone-Else** » Fri Dec 17, 2021 9:23 pm

No pictures required, your description was fab.

When you turn off an MCB (The little one that says 6A) you are disconnecting the Live only. When you were pulling the cable the Neutral and earth touched, this will trip your RCD.

All you have to do now is carry on and get the cable out (Probably connected to the light) then switch the RCD back on, then the MCB you switched off first.

udaman · Post by **udaman** » Fri Dec 17, 2021 9:58 pm

Your response was fab too :)

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.

Cheers

Someone-Else · Post by **Someone-Else** » Fri Dec 17, 2021 10:25 pm

Go for it (Removing the cables) but do it in the morning so you can see.

udaman · Post by **udaman** » Fri Dec 17, 2021 11:21 pm

Yep, will leave the job until tomorrow, but couldn't resist testing it out and so had a go at touching earth to neutral and nothing happened. Did it with each of the three cables separately and no trip... Any ideas?

Ta

udaman · Post by **udaman** » Fri Dec 17, 2021 11:32 pm

Scrap that last post. I've since learnt that it depends what else is happening in the house - electricity wise. The cooker was on when it tripped. WHen I tested just now it wasn't, and that's why it didn't.

Cheers

Neelix · Post by **Neelix** » Sat Dec 18, 2021 9:02 am

This is precisely why dual RCD fuseboard need to be banned.

A neutral to earth fault on any of the circuits will trip one of the RCD's - RCBO boards are far better

udaman · Post by **udaman** » Sat Dec 18, 2021 10:40 am

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 :)

Neelix wrote: ↑Sat Dec 18, 2021 9:02 am This is precisely why dual RCD fuseboard need to be banned.

A neutral to earth fault on any of the circuits will trip one of the RCD's - RCBO boards are far better

That's not good news :( We had a partial rewire and replaced consumer units in the house not ling after moving in in 2019. There were three CUs, there's now two. The electrician said they were good units and I was happy to trust him on that. Should I not have been?

Cheers

Someone-Else · Post by **Someone-Else** » Sat Dec 18, 2021 12:18 pm

Worry not, it is only the opinion of Neelix. I too have a duel RCD board.

Neelix, it is nothing personal, everyone has an opinion.

Neelix · Post by **Neelix** » Sat Dec 18, 2021 9:16 pm

Someone-Else wrote: ↑Sat Dec 18, 2021 12:18 pm Worry not, it is only the opinion of Neelix. I too have a duel RCD board.

Neelix, it is nothing personal, everyone has an opinion.

And would argue that duel RCD boards don’t comply with the regs.

Someone-Else · Post by **Someone-Else** » Sat Dec 18, 2021 10:50 pm

Neelix wrote: ↑Sat Dec 18, 2021 9:16 pmAnd would argue that duel RCD boards don’t comply with the regs

I thought you would.

Neelix · Post by **Neelix** » Sat Dec 18, 2021 11:22 pm

Someone-Else wrote: ↑Sat Dec 18, 2021 10:50 pm
Neelix wrote: ↑Sat Dec 18, 2021 9:16 pmAnd would argue that duel RCD boards don’t comply with the regs
I thought you would.

That’s because I know and understand them

Neelix · Post by **Neelix** » Sun Dec 19, 2021 4:24 pm

And watch out for the next revision of the regs!

The manufacturers have influenced the changes so much that mandatory installation of AFDDs is likely to happen .

This means dual rcd boards would go and hugely expensive AFDD boards enter the frame

Them the cow boy sparks inspect earlier editions of the regs, hand out C2’s and hey presto persuade owners to part with more money

This is a huge scandle in the making

Hugh's Candle · Post by **Hugh's Candle** » Sun Dec 19, 2021 6:55 pm

That's a great username, I fully concur with Neelixs' sentiment, dead right.