Mains circuit tripping

[srobarts]

I know this is electrical rather than computer but I was not sure which other part of Pprune to post in to pick the brains of the Ppruners.
Last night in the middle of dinner the circuit breaker for the sockets in the main part of the house (we have an extension with its own distribution board). I switched off and unplugged everything on the ground floor and reset the trip. The switch tripped again. I turned off the ground floor trip. And reset the main trip and all was OK. I switched back on the ground floor sockets trip and the main trip switch tripped again. I am now looking at a series of extension cables running from the extension and upstairs to power the boiler, fridge and freezer and of course the broadband router and wondering where to look next before I have to call an electrician in.
Can anyone help. Why is the main trip tripping rather than just the one for the groundfloor. The main trip is 80A RCD the ground floor is a 32A. Two thoughts have occurred to me.
1) the ground floor trip is faulty
2) it is the RCD that is tripping and some water has got into a socket or cable - heaven alone knows how I find that one.
Any suggestions would be gratefully received. At least the beer is still cold and the easter weekend decorating has been put on hold.

[Guest 112233]

This is definatly one for a qualified electrician. - Sorry - I'm speaking from experence of a CB contact fused togather by a short circuit on an oven. -Not obvious There can be lots of causes.

CAT III

[srobarts]

CAT III
Thanks for taking the time to read and reply.
Certainly not obvious - I had a hunch to check the sockets in the conservatory which lost a roof panel in high winds a few weeks ago and although promptly replaced some rain got into the conservatory. The first socket I opened had a rather shocked slug sitting across the earth and live connections with loads of slug slime around him to make a good contact. I have no idea how he got in there.
Conservatory circuit disconnected and all is well. I will get qualified electrician in after weekend to check circuit and replace slimed socket.
SWiMBO told me I'm not off decorating duties!

srobarts

[Avtrician]

Just use soap n water to wash the unit.....:E:E

Wot could possibly go wrong.

Good fault finding by the way.

[taxydual]

A nice little earner for the Sparky. A kamikaze slug!! Whatever next?

Agreed, excellent fault finding! Well done, that man!

[west lakes]

The reason for the 32A MCB (miniature circuit breaker) not tripping and the 80A RCD (residual current device) tripping is due to their differing designs and duties.

The 32A MCB is a pure overcurrent device and will not trip until the current through it exceeds 32A e.g. live to earth, live to neutral or too many appliances switched on, many are designed with an overload factor often to allow for the higher current flow as motors (fridges, hoovers) start as well, so it could be a figure slightly higher than 32A.It will only be connected into the live conductor.

The 80A RCD will also have another setting (usually 100mA (milliamp i.e. .1 amp) for a main unit) These devices are connected to both the live and the neutral conductors.
Again it is designed to protect from overloads above 80A, subject to the comment above, additionally it also measures (simple explanation) the difference between the current flow in the live and the neutral. If these values differ by more than the 100mA owing to a fault between the live and earth or between the neutral and earth the device will operate.
So your slug in the socket though not causing a live to earth fault in excess of 32A was causing one above 100mA.

Of course if you use mains power tools outside you should connect them via a 30mA RCD socket or adaptor.

[al446]

It sounds to me like whoever designed your installation got the wrong configuration of board. The main breaker should be just, not an RCD and all final power circuits should be protected by their own individual RCDs, lighting circuits should be protected by MCBs, usually 6A. This is to prevent the house being plunged into darkness due to power fault and allows you to identify individual faulty circuits while maintaining supply to the rest of the installation. Discuss with spark who comes to (unnecessaryily) renew socket.

[srobarts]

Thanks for all the help.
I have looked again at the distribution box. It has two switches labelled main. One red 100A which switches everything off and one RCD which only covers the three CBs for the sockets(ground, first floor and garage) There are four CBs which are not affected by the RCD - Cooker, some kitchen sockets both 32A, ground and first floor lights both 6A.
The lights did indeed stay on when the trip went. Since the sparks who I will get is the one who installed it all when the extension was built I am guessing he will say he designed it correctly!

srobarts

[west lakes]

The design is correct, the main problem being in the cost and space in the board taken up with 32A RCD's coupled with the need for a different design of internal busbars to cope with RCD's in the outgoing circuits

[srobarts]

Thanks a lot west lakes. I am guessing this is why we ended up with two distribution boxes when the extension was built as there was not enough space in one for the extra circuits.

[al446]

Try another spark. I was one for many years until I changed career for one less damaging on the body, knees get shot. Sounds like a legal but imperfect design to me, especially with a kitchen socket circuit not protected by RCD, lots of water knocking round kitchens.

[Keef]

That sounds scary! The RCD should protect ALL the circuits - unless (like me) you have several RCDs on different bits of the system. As west lakes said, the MCB won't trip unless the circuit is pulling more than 30 amps (or whatever the rating is).

I'm a coward. I have 30mA fast-acting RCDs. Even 30mA can give you a nasty surprise.

If the slugged socket isn't in a kitchen or a bathroom, you don't need a sparks to replace it for you. If you want one to do so, fair enough.

[west lakes]

You don't need a sparkie for like for like replacement anywhere, only for modifications.

A problem that does occur is nuisance tripping caused by appliances, they are not as such faulty, but the earth leakage can in some cases exceed 30mA. Common issues are kettle/cooker/washer elements, starter capacitors in motors and flourescent fittings.

The greatest risk is, as I commented earlier, the use of equipment outside the house in fact with some earthing systems RCD's in this case are mandatory for all suppies (sheds, garages) outside the house.

[al446]

Indeed KEEF, the ideal is to have individual RCDs but to use them on lighting circuits is overkill. And I agree with the non-requirement for a spark to change socket.