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

[DIYNotIan]

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

 

[RandomGrinch]

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

Yes, it is - I've done it myself!
Most 30mA RCD's will trip with a fault current of between 18 and 23mA.
With the MCB off, you can still create a neutral to earth fault greater than this.

 

[Harry Bloomfield]

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

Yes! The neutral remains connected and the small voltage difference between that and the earth, passing through the RCD, produces enough current flow to cause a trip - as you found.

 

[DIYNotIan]

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

 

[sparkwright]

In layman's terms - you touched earth and neutral together, which causes the RCD/RCCB to trip.

 

[333rocky333]

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?

Believe it or not, you would be contravening the Regulations to do it intentionally

 

[bernardgreen]

Why an RCD can be tripped by a Neutral to Earth fault / short even though the MCB for the circuit is switched OFF

 

[winston1]

Believe it or not, you would be contravening the Regulations to do it intentionally

But as the regs are NOT statutory it would not matter. Go ahead and do it for your own satisfaction.

 

[DIYNotIan]

I'll leave the job until tomorrow, but couldn't resist testing it out - for my satisfaction - 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

 

[RandomGrinch]

I'll leave the job until tomorrow, but couldn't resist testing it out - for my satisfaction - 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

It may depend on what else you had switched on in the house at the time!

 

[DIYNotIan]

It may depend on what else you had switched on in the house at the time!

Bingo. It was because the cooker was on when it tripped, but wasn't when I tested. Cooker back on and it trips again. So all as it should be.

Many thanks

 

[DIYNotIan]

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

 

[333rocky333]

But as the regs are NOT statutory it would not matter. Go ahead and do it for your own satisfaction.

Sound advice, may as well bin the Regs book then, are you saying no regs matter or just ones that you desire.
The regs there for a reason, I assume to prevent stressing and possibly damaging the Rcd.

 

[winston1]

One should use ones common sense. How would a neutral earth connection stress a RCD?

 

[Risteard]

Believe it or not, you would be contravening the Regulations to do it intentionally

You would in fact be committing a criminal offence under ESQCR.