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

[Oxbow16]

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

[Roys]

The RCD will trip if your neutral and earth touch even with the mcb switched off.

[Oxbow16]

That's 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

[Nickfromwales]

You're fine, other than the obvious lack of hands on electrical skills....

Touching neutral or live to earth with the RCD on and the MCB off will inevitably ( nuisance ) trip the RCD. That's normal / expected, so nothing to worry about there.

Yes, isolate the live, neutral and earth wires from the rose to the pull-cord and that's you done in terms of electrical isolation / making the pull-cord and fan safe for removal.

FYI you should not be testing continuity with cables connected, as that can yield misleading results.   

[ProDave]

Is there a fan isolator switch (there usually is)  If so turn that off, it isolates neutral as well.

[Oxbow16]

12 hours ago, Nickfromwales said:

You're fine, other than the obvious lack of hands on electrical skills....

 

 

Thanks for that!  I'm not sure what it's based on though!  

 

12 hours ago, Nickfromwales said:

FYI you should not be testing continuity with cables connected, as that can yield misleading results.   

 

Yes, all cables were disconnected when testing for continuity.  

 

12 hours ago, Nickfromwales said:

Touching neutral or live to earth with the RCD on and the MCB off will inevitably ( nuisance ) trip the RCD. That's normal / expected, so nothing to worry about there.

Yes, isolate the live, neutral and earth wires from the rose to the pull-cord and that's you done in terms of electrical isolation / making the pull-cord and fan safe for removal.

 

Great, thanks for the explanation.  

 

12 hours ago, ProDave said:

Is there a fan isolator switch (there usually is)  If so turn that off, it isolates neutral as well.

 

No, just a pull cord to turn it on/off.  

 

In any case, it's all done now - fan removed and I replaced the worn ceiling rose while I was at it.

 

Appreciate all the help that got me there  

 

Many thanks and have a great weekend.