What's going on there !!! everything tripped...

[Bolton_Bob]

Doing some redecorating and as part of that I decided to move a ceiling light about 1ft from where it was.

I turned the light on, went to the circuit breaker, flipped the switch and the light went out, so I figured the light must be on that circuit, so far so good (I thought}.

I took the light fitting off and fed the cable through the ceiling to its new position, by that time it was 1ft too long, so I decided to cut right through the cable. As soon as I did that every breaker in the house tripped, that's all that happened, no flash, no bang and I hadn't lit up or anything, but every breaker on the board tipped out.

What's going on there ?

Bob.

 

[spary]

There is a correct isolation procedure that you should follow which includes test equipment, do you have the correct test gear?
Did the RCD operate? if so could be just neutral fault when you cut the cable

 

[Darkwood]

HI Bob welcome to the forum.
It sounds like you have an rcd monitoring multiple circuits which still monitors your cables regardless if the correct mcb has been switch off or not, your method of functional isolation is very risky and relies on circuits been wired correctly too, for your safety I would recommend you isolate the full board in future an invest in some test equipment, a common issue and danger to even us knowledged Electricians is a borrowed/crossed Neutral that can see you getting a full 230v zap of an isolated circuit by simply taking the cables out of the joint, please avoid risking your life for convenience and ensure you are safe using the correct test equipment and knowledge of the risks of such like the afore mentioned borrowed Neutral.

 

[telectrix]

my money's on the RCD letting go when OP shorted N - E , even though L was isolated. and echo above posts. safe isolation is paramount. always test with an approved voltage tester before and after isolating circuit/installation.

 

[Bolton_Bob]

Thanks for the input Spary, as for the correct test gear that's a "no I don't" I'm just a retired joiner. My logic was that if I switch the light on, then throw the circuit breaker and the light goes off that I should be safe ? add to that when I cut through the cable there wasn't any flash or bang and thankfully no harm to me thanks to the RCCB, it was that {not all the breakers} that tripped, but how could that happen when I'd isolated the circuit that the light was on ?

 

[telectrix]

and lock off breaker/RCD whilst working. 'er indoors is bound to switch it back on when she finds that her hair straighteners don't work.

 

[Bolton_Bob]

HI Bob welcome to the forum.
Thanks, appreciated.

It sounds like you have an rcd monitoring multiple circuits which still monitors your cables regardless if the correct mcb has been switch off or not,
What's the difference between an RCB and an RCCB ? on the board it's listed as an RCCB/

your method of functional isolation is very risky and relies on circuits been wired correctly too,
Understood and appreciated, just can't figure out why the light would go out when I flipped the circuit breaker and yet the wires still be live ?

for your safety I would recommend you isolate the full board in future an invest in some test equipment,
It's a lesson learned for me now, if ever I go anywhere near any wires I'll throw the switch on everything, the main breaker that is...

a common issue and danger to even us knowledged Electricians is a borrowed/crossed Neutral that can see you getting a full 230v zap of an isolated circuit by simply taking the cables out of the joint, please avoid risking your life for convenience and ensure you are safe using the correct test equipment and knowledge of the risks of such like the afore mentioned borrowed Neutral.
Is a borrowed neutral legal ?

THANKS again (to all of you) for the feedback, you could well have saved an owd chaps life !

 

[Bolton_Bob]

Aye, well, if it wernt for 'er indoors (bangin on about doin' 'er ironin') I'd have thrown the main switch, so I know what you mean (thanks).

Add to that, the electrician that I used to work with YEARS ago dint just pull the fuse when he was working on something he'd put all the "Man at Work, machine Isolated" signs up, then he'd put the fuse in his pocket. That always stuck in my mind...

 

[Bolton_Bob]

So I guess it was a borrowed neutral that caused the problem ? believe me I won't fall for that ever again, and I do very much appreciate you lads taking the time to put me straight on that, it's decent, very decent of you.

 

[spary]

Electrical work isn't as easy as it might seem, it takes years to become qualified and another few years to fully understand

 

[Lucien Nunes]

A normal single-pole MCB does not completely isolate the circuit, it breaks the line conductor but not the neutral. This usually makes the circuit safe to touch but not if it has been incorrectly wired, hence cannot be assumed to be sufficient without checking. The RCD may still be able to detect a short-circuit between neutral and earth conductors even with the line conductor switched off at the MCB, which would have happened when you cut the cable.

A borrowed neutral is something different, where there are two circuits (and hence two MCBs) relying on one neutral conductor. Both MCBs must be switched off, even the one that does not feed the point you want to work on, before it is safe to touch. There is no evidence that you have a borrowed neutral.

 

[Bolton_Bob]

A normal single-pole MCB does not completely isolate the circuit, it breaks the line conductor but not the neutral. This usually makes the circuit safe to touch but not if it has been incorrectly wired, hence cannot be assumed to be sufficient without checking. The RCD may still be able to detect a short-circuit between neutral and earth conductors even with the line conductor switched off at the MCB, which would have happened when you cut the cable.

A borrowed neutral is something different, where there are two circuits (and hence two MCBs) relying on one neutral conductor. Both MCBs must be switched off, even the one that does not feed the point you want to work on, before it is safe to touch. There is no evidence that you have a borrowed neutral.

THANKS for the above explanation Lucien, much appreciated, I understand what happened now, thanks to your post and the input from the other guys, I'm indebted to you all for taking the time.

Bob.

 

[Statick]

Thanks for the input Spary, as for the correct test gear that's a "no I don't" I'm just a retired joiner. My logic was that if I switch the light on, then throw the circuit breaker and the light goes off that I should be safe ? add to that when I cut through the cable there wasn't any flash or bang and thankfully no harm to me thanks to the RCCB, it was that {not all the breakers} that tripped, but how could that happen when I'd isolated the circuit that the light was on ?

Sorry, ignore. Don't know how to delete post. I saw TS already sorted this out.

This is one of the possibility as mentioned by several posters.
I'm not sure if it's 3 wires or 2 wires you are cutting at the same time.
Assuming it's 3 wires and the live was isolated as per what you mentioned.
During the cutting of the wires, the neutral, live and earth wires all come into contact momentarily.
This means neutral is shorted to the earth and live. Since live is isolated, there is no effect.
But when the earth and neutral are shorted, there is a path for current to flow from neutral to earth.
If your light circuit has the neutral tapped from another circuit, then current can flow from that circuit via your light circuit and then to earth. This will cause the RCD to trip.
The same goes if you are cutting 2 wires with a pair of un-insulated scissors and you are also not insulated. The cutting of the borrowed neutral will allow current to flow through your body even though live is isolated and this is where the danger is.

 

[Lucien Nunes]

If your light circuit has the neutral tapped from another circuit, then current can flow from that circuit via your light circuit and then to earth

This is misleading. All neutrals are connected together in the consumer unit, so the current flow to earth will happen regardless of whether the neutral is 'borrowed' or not, whenever a potential exists between N & E at the CU due to normal voltage drop in the supply cables.