strange fusebox/light fitting problem

[newsquestions2003]

Hi

I was changing a light fitting and have run into an unexpected difficulty. I don't know very much about electricity, but as it seemed little more complicated than swapping the wires out of the ceiling from going into piece of plastic into another, I thought I'd be okay.

Before taking off the old fitting I flicked the switch at the fusebox for the downstairs lighting circuit, which seems to be turning off the current in the wires (I've touched them and am not dead!).

However while attempting to connect the new fitting the main fuse tripped, cutting all electricity to the house. I thought this was just chance so I flipped the fuse back on turning on the electricity (leaving the lighting circuit switched off). Again after a little bit of fiddling the main fuse flipped.

I wonder what could cause this - I thought if the fuse controlling the lighting circuit were switched off then there would be no electricity in these wires so they shouldn't cause another fuse to flip. I wonder if anyone has any idea what is going on. I have isolated the wires with insulating cable and am leaving it well alone until I know why this is happening!

Thanks!

[Owain]

the MCB (miniature circuit breaker) on the lighting circuit only isolates the live (phase) wire, leaving the neutral connected.

Your main "fuse" i.e. RCD (residual current circuit device) will trip on both a live-earth short and a neutral-earth short. It is likely you touched the earth and neutral together whilst "fiddling".

Owain

[ctl]

Don't mess if you don't know what you're doing! The common problem I used to see was people taking a fitting down and ending up with two or three mains cables poking out of the cieling. Then all they would do is connect all the red ones up and all the black ones before changing to the current colour coding. So each time the switch went ON it shorted out the mains supply and blew the fuse. They forgot one cable might be used for the switch wire and the other might have been a supply going to or coming from another fitting. It was funny as people would swear they never touched it or did a thing and it worked perfectly before! At least you have admitted it, so it makes it easier to fix.

[John Rumm]

Not what is happeniong here by the sounds of it though...

I am with Owain on the Neutral to Earth short tripping a RCD suggestion. Sometimes it is hard to avoid even whey you anticipate it and try not to!

[meow2222]

RCD tripping doesnt mean you'll get bit or anything's live. Just leave it switched off till you're done.

NT

[Christian McArdle]

Indeed. You shouldn't work on a non-isolated circuit. Turning off an MCB does NOT isolate a circuit. If you must work on a circuit with only the MCB off, then treat it as live working.

Christian.

[meow2222]

I take a neutral position on that one, being a down to earth kinda person.

NT