Consumer Unit Trips When Cutting Cable

[benthecowman]

I have just installed some new ceiling mounted spotlights in my bedroom. Before starting work I flicked the switch on the Consumer Unit that cuts power for that circuit. I presumed that all would be safe to then work on that circuit. However, when I cut into the twin and earth cable (to strip it ready for connecting) the entire house lost power. It seems that cutting into the cable caused the main switch on the Consumer Unit to trip even though there was no power in the circuit I was working on.

Can someone please expain why this should happen....

Is there a major fault with the wiring in my house??

Is this normal???

Thanks

Ben

 

[streetlighter]

Quite normal if the whole house is protected by an RCD. When you turned off the circuit breaker for the lighting you disconnected only the Phase ( Live ) conductor the earth and neutral where still connected. Cutting through the cable caused an inbalance between neutral and earth which made the RCD trip and all the power go off.

 

[Spark123]

It is pretty normal if you have one RCD protecting the whole premises. When you cut the cable you will have bridged the earth and neutral, these will have been at slightly different potentials and a small amount of current would therefore flow. This imballances the RCD and trips it. The bad thing about the setup that is sounds like you have is that a single fault on a circuit can plunge your whole house into darkness, assuming of course you only have one RCD.

 

[Jim2287]

your 'fuse board' most probably has an RCD as the main switch.

RCDs will trip if there is a Neutral-Earth fault (you cutting cable)

Turning the lighting circuit off only disconnects the live (phase) conductor. not neutral

 

[RF Lighting]

Yep it's normal.

Turning off the MCB disconnects the phase only.

Cutting the cable causes a N>E short, an inbalance in the RCD which trips as a result.

 

[benthecowman]

Thanks for your rapid responses!

That has given me some peace of mind at least.

The house is pretty old and in a rural location - and was probably rewired some time ago I fear. I think that here is only one RCD and indeed if the power trips because of an appliance overload then the whole house does plunge into darkness.

I will consider forking out for an electician to have a look at updating the Unit/circuits.

Thanks again.

 

[Steve]

Yeah perfectly normal that. The old N-E short thing. This trips an RCD, as N and E are still connected - the MCB only disconnects the phase conductor.

 

[streetlighter]

Is the supply to the house via overhead power lines ? If so this may be a TT install an explain the choce of whole house RCD protection. Any chance of a picture of the meter and fusebox area ?

 

[benthecowman]

Can get you a picture of the fuseboard area tomorrow - am away tonight.

What is a "TT install" Streetlighter?

 

[Spark123]

A TT system is where for one reason or another the electricity supply company doesn't supply you with an earth (or for another reason you chose not to use it) therefore you have to provide your own earthing by installing an electrode in the ground. As the resistance of the ground between your electrode and the suppliers electrode will almost certainly higher than that of an electricity suppliers supplied earth there may not be enough current flowing in the event of a fault to trip an MCB etc hence you need to have RCDs protecting your circuits.