Consumer unit RCCB trips when replacing light fitting why?

On Sun, 16 Jul 2006 05:06:59 +0100, Simon wrote (in article ):

This is normal behaviour and is because the neutral is not disconnected.

Normally on a split consumer unit, lighting circuit MCBs are located before the RCD, the idea being that a trip on a protected circuit does not put the lights out.

You could move the lighting MCB if there is a spare way before the RCD.

You can also fit RCBO devices (essentially combined MCB and RCD) on every circuit, but they are fairly expensive.

If it's annoying, you can identify the neutral for the circuit at the CU and temporarily disconnect while you are working on the circuit.

Depending on the earthing arangements in your house, the RCD may control all circuits or sockets only. Regardless of this, you should use the main switch, not just a MCB, before working on wiring.

As the previous poster said, the MCB only breaks live, neutral is still connected. Neutral is often a few volts different from earth, so shorting them does cause a current to flow. Really, safe working should be with the main switch off (you never know if someone might have switched live and neutral wiring on a lighting circuit).

Incidentally, I learned this the hard way many years ago as a somewhat inexperienced employee - I was disconnecting a hefty bit of 3-phase gear from the circuit breaker, had dropped the phase wires out, and as I dropped the neutral wire I touched a screwdriver between neutral and earth - big spark! I learnt with that nasty (but harmless) surprise that 3 phase breakers don't always break neutral, and that in large buildings quite a bit of current can flow between them.

On 16 Jul 2006 01:59:45 -0700 someone who may be " snipped-for-privacy@gglz.com" wrote this:-

I once nearly lost a man who only turned the main switch off in a building. He had opened the switch to check the outgoing cable. However, it had been wired up backwards and what appeared to be the outgoing cable was in fact the incommer. The unenergised outgoing cable was positioned behind the shields, the energised incommer was bare.

It took a great deal of willpower on my part not to have the thing ripped out on the spot, but eventually I settled for a warning notice until the thing was rewired a few weeks later.

You shouldn't work on a system unless the main switch is off. If you can't do this, you should regard it as live working. The MCB should just be regarded as a functional switch, not an isolator.

Christian.