Question about RCDs for the electricians here

cliveb

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I needed to do some work on a lighting circuit. After disabling the appropriate circuit breaker, I was surprised that cutting a cable caused the RCD on the CU to trip.
There can't have been any power on that circuit, so why did it happen?

I have a hypothesis: I think the neutrals and earths of all the circuits are tied together at the CU.
So does cutting the cable (which obviously shorts the earth to neutral) create a leakage path between neutral and earth for other circuits that are live at the time which causes the RCD to trip?
That sort of makes sense to me, but of course I could be way off.

If that is the case, it seems a little irritating that it's necessary to power down the whole house just to work on a lighting circuit.
Doing so means you can't plug a work light into a socket to see what you're doing. How do real electricians deal with this?
 

RichA

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I'm no electrician, but spent a day revising what I'd forgotten about domestic electrics to do a job a couple of weeks ago.
I generally avoid it, but I'd get a proper spark in, if I were you. What you've described shouldn't happen. It sounds like a fault somewhere. They'll track down in minutes what might take you a week of random trial and error.
 

PJ87

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I needed to do some work on a lighting circuit. After disabling the appropriate circuit breaker, I was surprised that cutting a cable caused the RCD on the CU to trip.
There can't have been any power on that circuit, so why did it happen?

I have a hypothesis: I think the neutrals and earths of all the circuits are tied together at the CU.
So does cutting the cable (which obviously shorts the earth to neutral) create a leakage path between neutral and earth for other circuits that are live at the time which causes the RCD to trip?
That sort of makes sense to me, but of course I could be way off.

If that is the case, it seems a little irritating that it's necessary to power down the whole house just to work on a lighting circuit.
Doing so means you can't plug a work light into a socket to see what you're doing. How do real electricians deal with this?

You should in no way have to power down the house to work on the lights

I will add im not a sparky

However out CU for example has power up, power down, lighting up, lighting down .. then other bits like the cooker, loft, car charger etc

I installed a ring spot light on an existing outside light (side night whoever wired up the extension out the back put the lights on the main ring... great.. only for the kitchen) so cut the RCD for the power down to install the light

my garage for example has sockets and light breakers.. my mate had to rewire the lights and was able to plug in a light to see watch he was doing

id say get a sparky in but also buy one of them live circuit pens .. never work on electrics without checking nothings flowing and it makes it so simple
 

Green Man

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If your installation is relatively new your hypothesis is correct. Under the latest regulations all circuits have to be protected via RCD. It used to be that lighting circuits didn’t need this protection.
 

cliveb

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OK, thanks to everyone that has replied with their input. Just to make things clear, there is absolutely no way that any current was flowing in the cable I cut. Not only did I switch off the circuit breaker, having cut the cable (and then reset the RCD) I double checked that nothing was live.

If your installation is relatively new your hypothesis is correct. Under the latest regulations all circuits have to be protected via RCD. It used to be that lighting circuits didn’t need this protection.
Thanks. Do I take it that you are an electrician?
Yes, the RCD on the CU does indeed protect all circuits. This is not a surprise to me. What did surprise me is that it tripped when I shorted out a circuit that was disabled.
When you say my "hypothesis is correct", do you mean that it is indeed because all the neutrals and earths are tied together? (Switching off the circuit breaker only disconnects the live, of course).
 

Green Man

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OK, thanks to everyone that has replied with their input. Just to make things clear, there is absolutely no way that any current was flowing in the cable I cut. Not only did I switch off the circuit breaker, having cut the cable (and then reset the RCD) I double checked that nothing was live.


Thanks. Do I take it that you are an electrician?
Yes, the RCD on the CU does indeed protect all circuits. This is not a surprise to me. What did surprise me is that it tripped when I shorted out a circuit that was disabled.
When you say my "hypothesis is correct", do you mean that it is indeed because all the neutrals and earths are tied together? (Switching off the circuit breaker only disconnects the live, of course).

When you switch off the MCB all it does it cut the live wire. The Neutral is still connected so shorting out the cable between neutral and earth would trip the RCD.
 

SaintHacker

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I'm a qualified electrician however i won't offer any advice as its so many years since I stopped working in the trade and things have changed so much I could well be giving you duff advice. What I would advise you is ignore any advice on here from people who are not sparks, do yourself a favour and get it looked at by someone who knows what they are talking about and has the qualifications to back it up.
 

Fade and Die

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I needed to do some work on a lighting circuit. After disabling the appropriate circuit breaker, I was surprised that cutting a cable caused the RCD on the CU to trip.
There can't have been any power on that circuit, so why did it happen?

I have a hypothesis: I think the neutrals and earths of all the circuits are tied together at the CU.
So does cutting the cable (which obviously shorts the earth to neutral) create a leakage path between neutral and earth for other circuits that are live at the time which causes the RCD to trip?
That sort of makes sense to me, but of course I could be way off.

If that is the case, it seems a little irritating that it's necessary to power down the whole house just to work on a lighting circuit.
Doing so means you can't plug a work light into a socket to see what you're doing. How do real electricians deal with this?

You are correct it is the shorting of the Neutral to Earth which trips the RCD, there is nothing wrong with the device it’s just doing it’s job but it can be a nuisance, to overcome this you can disconnect the relevant Neutral at the CCU or be careful when cutting the cable, avoiding cutting the N & E together. Depending on the age of your CCU you will normally have one or two RCDs. This style of fuse board will be gradually phased out by RCBO type boards which have individual RCD/MCB devices, saves all the hassle of nuisance tripping.
 

Beezerk

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You are correct it is the shorting of the Neutral to Earth which trips the RCD, there is nothing wrong with the device it’s just doing it’s job but it can be a nuisance, to overcome this you can disconnect the relevant Neutral at the CCU or be careful when cutting the cable, avoiding cutting the N & E together. Depending on the age of your CCU you will normally have one or two RCDs. This style of fuse board will be gradually phased out by RCBO type boards which have individual RCD/MCB devices, saves all the hassle of nuisance tripping.

Yes, it has detected a fault to earth which is why it tripped like explained here.
William Alex should be along shortly to explain it in layman’s terms for us thickos ?
 

cliveb

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You are correct it is the shorting of the Neutral to Earth which trips the RCD, there is nothing wrong with the device it’s just doing it’s job but it can be a nuisance, to overcome this you can disconnect the relevant Neutral at the CCU or be careful when cutting the cable, avoiding cutting the N & E together. Depending on the age of your CCU you will normally have one or two RCDs. This style of fuse board will be gradually phased out by RCBO type boards which have individual RCD/MCB devices, saves all the hassle of nuisance tripping.
Thanks, you've put my mind at rest. The house actually has two separate CCUs, each with one RCD.
It was a pain when the RCD tripped. My wife was in the shower at the time and was not best pleased. It also pulled the rug out from our media servers, one of which is a bit temperamental to reboot.
But the work is now done and all is well. The biggest task was cutting out and making good the chipboard floor to actually get at the circuit. At least it was 60cm boards and not great big 8x4 ft sheets. There's something to be said for good old-fashioned floorboards.
 

jim8flog

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But the work is now done and all is well. The biggest task was cutting out and making good the chipboard floor to actually get at the circuit. At least it was 60cm boards and not great big 8x4 ft sheets. There's something to be said for good old-fashioned floorboards.

Absolutely. Especially when the builders build brick walls on top of the chip board. A couple of my bedroom floors look like jigsaw puzzles.
 
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