Can RCD 'detect' a light fitting short - even if switched off?

[Darrylle]

I have an RCD for the house, and have 3 spotlights to the rear of the house, which I'm in the process of replacing with LED spots.
One wall switch 'feeds' all of them in series. Whilst fitting one of the lights (with only the wall switch off (and taped down!)) - the RCD flipped. I tested this again, and it did the same. There was no current to any of the light fitting wires.
This confused me at first, but then thought: "Hmm can the RCD still sense a 'short' of any of the earth, live or neutral wires?".
Is this possible - even if all wiring is 'correct' and the main light switch is most definitely 'off'?

 

[telectrix]

it's common. you have only switched off the L. when doing the work you short N to E and the RCD will trip as it sees some small current from N to E but no current from L, so that is an imbalance, it trips.

 

[loz2754]

Yes. A short from earth to neutral can cause an RCD to trip in a circuit that is switched off.
Edited - thanks to @telectrix.

 

[telectrix]

Yes. A short from live to neutral can cause an RCD to trip in a circuit that is switched off.

another member still half asleep. N- E.

 

[loz2754]

another member still half asleep. N- E.

Haha caught me out! Indeed neutral to earth. Oops..zzzzzzzz

 

[pc1966]

As covered above, in a typical installation the N & E are nominally at the same voltage. In fact, it is that commonality that defines the neutral cable out of the current carrying set.

But cables have resistance & inductance, and currents are flowing elsewhere, so if you measure N - E you will often see a small voltage, usually less than 5V and often just a fraction of a volt. So if there is a N-E short then some of that current flowing elsewhere can find its way round the short and if that reaches the 15-30mA trip threshold for the RCD, it fires.

N-E shorts are often a problem as, for example, you find the RCD trips when someone uses the shower. You assume it is the shower's circuit that is at fault (which for a L-E leak it would be) but in reality the fault could be N-E on another circuit and it is only when enough current flows that it trips.

Many an electrician and DIY'er has wasted hours trying to pin down the fault if they don't know to start off insulating testing N as well as L to rule this out!

 

[Darrylle]

Brilliant! Thanks so much guys. I'm not a 'sparky' (obviously), and only attempt 'simple' jobs.
This was a 'niggle' that was winding me up.

Thanks again.

 

[littlespark]

It can catch out even the best of us.

Changing a socket or light switch to decorative.... turn off the mcb.... working away, then pop! Half the house off!

“sorry that was me”

“huh! You should have turned the power off.... call yourself an electrician!”

“f*** off!”

 

[Timbo]

One wall switch 'feeds' all of them in series.

I hope they aren't!