Main fuse trips

[10monty]

Not sure how to explain this really, but I'll give it a go anyway!

I recently re fitted my bathroom, which included fitting a new light and a switch outside to do away with the pull cord.
I obviously swithed the u/s lighting circuit off at the fuse board first, but when I cut the light cable in the loft it blew the main fuse, thus switching everything off.
If I cut the wires individually, ie live, then neutral seperately it doesn't do it.

The same thing happened the other day, but this time with the ring main for the sockets.

Any ideas people?

 

[ColJack]

define "main fuse"..

do you mean the RCCD?

does it have 100mA or 30mA printed on it somewhere?

if it does then it's supposed to do this..

it stops you getting a shock from the unswitched neutral.. the breaker only turns the live off..

 

[bongos]

Is it an MCB or RCD that trips?

It may be that when cutting the both the line & neutral cores together that they short on the pliers, thus causing the MCB to trip. If you cut each core individually then they won't. If this is the case then it suggests that the correct circuit is not isolated. Yous hould check that the circuit that you are working on is definately dead by using a multimeter or similar (NOT a neon screwdriver).

 

[ban-all-sheds]

Bongos - firstly Monty describes how the whole house goes off, which means it's not an MCB tripping.

Secondly he describes how he's seen this on two different circuits - how many wrongly wired or labelled circuits do you think he has?

Thirdly, even with an MCB to protect the circuit I think he'd have noticed a bit of a bang and part of his cutters going missing.

I guarantee you it's the RCD tripping because of a L-E fault.

 

[TicklyT]

..........I guarantee you it's the RCD tripping because of a L-E fault.

Don't you mean a N/E fault?
(Single pole isolation at MCB, so live disconnected, but a transient N/E fault as cutters bridge N-E when cutting cable will still trip RCD)

 

[ban-all-sheds]

Oh poo.

Yes, of course I mean a N-E fault

 

[rebuke]

10monty - I assume you notified this work to your LABC, since electrical work in a bathroom is notifiable under Part P of the building regulations. If you didn't, you may have trouble when you come to sell the house...

 

[10monty]

Bongos - firstly Monty describes how the whole house goes off, which means it's not an MCB tripping.

Secondly he describes how he's seen this on two different circuits - how many wrongly wired or labelled circuits do you think he has?

Thirdly, even with an MCB to protect the circuit I think he'd have noticed a bit of a bang and part of his cutters going missing.

I guarantee you it's the RCD tripping because of a L-E fault.

It is the 100ma rcd that trips, thus switching off the whole house, which is bloody annoying.

what is this neutral/earth fault that you mention?

 

[10monty]

10monty - I assume you notified this work to your LABC, since electrical work in a bathroom is notifiable under Part P of the building regulations. If you didn't, you may have trouble when you come to sell the house...

Er no, I wasn't aware of that.
Ignorance on my part i guess!

 

[ban-all-sheds]

It is the 100ma rcd that trips, thus switching off the whole house, which is bloody annoying.

Stop chopping through cables in one go then.

what is this neutral/earth fault that you mention?

The one you create when your cutters bridge neutral and earth....

Er no, I wasn't aware of that.
Ignorance on my part i guess!

Oh dear.

//www.diynot.com/wiki/electrics:part-p