Hello -

I recently had to replace a light fitting, so I switched of the lighting
circuit at the MCB in the consumer unit, then carefully checked that the
light wasn't working any more. After dismantling the fitting, I also checked
that there was no measurable voltage in the circuit with a multimeter.

The thing that worries me is that while fiddling around with the bare wires,
the RCCB in the consumer unit tripped. Is this because the MCD only switches
the live wire and I inadvertently shorted the neutral wire to earth? Do I
have to de-energize the entire house just to work on one circuit? Is this
normal, or is there perhaps a fault in my house's wiring?

Thanks

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

Hello -

I recently had to replace a light fitting, so I switched of the lighting
circuit at the MCB in the consumer unit, then carefully checked that the
light wasn't working any more. After dismantling the fitting, I also checked
that there was no measurable voltage in the circuit with a multimeter.

The thing that worries me is that while fiddling around with the bare wires,
the RCCB in the consumer unit tripped. Is this because the MCD only switches
the live wire and I inadvertently shorted the neutral wire to earth? Do I
have to de-energize the entire house just to work on one circuit? Is this
normal, or is there perhaps a fault in my house's wiring?

Thanks



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.

Simon wrote:
Hello -

I recently had to replace a light fitting, so I switched of the lighting
circuit at the MCB in the consumer unit, then carefully checked that the
light wasn't working any more. After dismantling the fitting, I also checked
that there was no measurable voltage in the circuit with a multimeter.

The thing that worries me is that while fiddling around with the bare wires,
the RCCB in the consumer unit tripped. Is this because the MCD only switches
the live wire and I inadvertently shorted the neutral wire to earth?

Yes.

On 16 Jul 2006 01:59:45 -0700 someone who may be "
wrote this:-

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).

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.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

I recently had to replace a light fitting, so I switched of the lighting
circuit at the MCB in the consumer unit, then carefully checked that the
light wasn't working any more.

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.