Isolating mains wire for plasterboard replacement

FreeBear

Section62 said: Yes (if you only turn off the circuit breaker/fuse), because the neutral (and 'earth') wire(s) will still be live.

If the earth is live, you have some serious issues that would need urgent attention.

I had been doing some work on a ceiling and had left the cable for a light fitting dangling in free air. Had turned off the MCB (fuse) for that circuit.. Head accidentally made contact with the bare wires, and the RCD immediately tripped. I should have put some insulated terminals & tape over the bare wires.

Section62

FreeBear said:

Section62 said: Yes (if you only turn off the circuit breaker/fuse), because the neutral (and 'earth') wire(s) will still be live.

If the earth is live, you have some serious issues that would need urgent attention.

...

With a PME supply the earthing arrangement is bonded to the supply neutral at the service cutout, so really should be considered to be as 'live' as the installation neutrals.  AIUI this is one of the reasons why there are special requirements with PME supplies - including bonding arrangements - because there are fault conditions where the 'earth' within the installation can reach a voltage where it poses a danger to someone in contact with an 'earthed' part and true earth. E.g. touching a metalic outside tap whilst standing on damp ground.  The idea of bonding (AIUI) is to make sure the potentially dangerous conductive metalwork in the house remains at the same potential (hence 'equipotential') as the installation's 'earthing' system.  That doesn't help with the outside tap scenario, but would if someone was holding a faulty appliance and touched a central heating radiator.

A big risk with a PME supply is a broken PEN conductor on the supply side.  That could get nasty.  There's also risks involved with diverted neutral currents.  One of these types of fault won't necessarily be obvious to someone working on the installation until they start disconnecting things (or correctly test for voltage/current first).

But overall it is why the 'earth' conductor needs to be treated with the same respect as the brown and blue wires because it is 'live' in the sense of being connected to an electrical supply system and possibly at a voltage that would hurt, or worse.

You're absolutely right though, in this scenario it is a serious issue which needs urgent attention from a professional.

Grandad2b

Section62 said:

hessodreamy said:

FreeBear said:

Isolating the circuit in question by flipping the circuit breaker (fuse) will leave the neutral connected - If you have an RCD, as soon as that neutral is touched, the RCD will trip. If there is no RCD, there is still a risk of an electric shock.

Hang on - switching this circuit off at the fuse box still gives you a risk of electrocution? 

Yes (if you only turn off the circuit breaker/fuse), because the neutral (and 'earth') wire(s) will still be live.

The 'Main Switch' or whole-house RCD should be double-pole and turning them off should isolate both the brown and blue (red/black) conductors.

Unless there is a fault you can't get electrocuted by touching the neutral. If the neutral touches earth then you will probably trip out the RCD which is less than ideal. Turning off the main switch doesn't disconnect the earth so that will be just as live as if you'd done nothing at all.

Section62

Grandad2b said:

Section62 said:

hessodreamy said:

FreeBear said:

Isolating the circuit in question by flipping the circuit breaker (fuse) will leave the neutral connected - If you have an RCD, as soon as that neutral is touched, the RCD will trip. If there is no RCD, there is still a risk of an electric shock.

Hang on - switching this circuit off at the fuse box still gives you a risk of electrocution? 

Yes (if you only turn off the circuit breaker/fuse), because the neutral (and 'earth') wire(s) will still be live.

The 'Main Switch' or whole-house RCD should be double-pole and turning them off should isolate both the brown and blue (red/black) conductors.

Unless there is a fault you can't get electrocuted by touching the neutral. If the neutral touches earth then you will probably trip out the RCD which is less than ideal. Turning off the main switch doesn't disconnect the earth so that will be just as live as if you'd done nothing at all.

The bit in bold involves making a whole load of assumptions - which goes to the heart of the comments FreeBear and I made.  Turning off the power on a MCB doesn't isolate the whole circuit - it remains partially live, and so needs to be treated with respect.

Grandad2b

A whole load of assumptions? One or two maybe. Perhaps you could make a list so I can be better informed in future.

grumpy_codger

FreeBear said:

hessodreamy said:

I've got a plasterer coming to put in some new plasterboard on a wall with a mains socket. I've already ripped off the old plasterboard so the socket is loose. When he fits the new board he'll want to pull the cable through a small hole so I need to remove the wires from the socket while he works. What do I need to do to make it safe? Wago blocks? 

I could just turn off that fuse and run an extension from downstairs for the power he'll need, but if there's a more elegant solution I'll use it.

I've used a length of 30A terminal strip to temporally terminate a ring main. Wrap a bit of insulation tape round the block for a little bit more protection and then tuck the whole lot in to the back box. But I'm guessing you will be using drywall boxes - Ideally, they need to be fixed in to the plasterboard before the skim coat is applied. But the cutouts are unlikely to be big enough to thread the cables AND terminal strip through...

- that's what I usually do . Possibly with adding some PVC tape in extreme cases.

I know, I know... Do this at your own risk.

Risteard

Section62 said:

FreeBear said:

Section62 said: Yes (if you only turn off the circuit breaker/fuse), because the neutral (and 'earth') wire(s) will still be live.

If the earth is live, you have some serious issues that would need urgent attention.

...

With a PME supply the earthing arrangement is bonded to the supply neutral at the service cutout, so really should be considered to be as 'live' as the installation neutrals.  AIUI this is one of the reasons why there are special requirements with PME supplies - including bonding arrangements - because there are fault conditions where the 'earth' within the installation can reach a voltage where it poses a danger to someone in contact with an 'earthed' part and true earth. E.g. touching a metalic outside tap whilst standing on damp ground.  The idea of bonding (AIUI) is to make sure the potentially dangerous conductive metalwork in the house remains at the same potential (hence 'equipotential') as the installation's 'earthing' system.  That doesn't help with the outside tap scenario, but would if someone was holding a faulty appliance and touched a central heating radiator.

A big risk with a PME supply is a broken PEN conductor on the supply side.  That could get nasty.  There's also risks involved with diverted neutral currents.  One of these types of fault won't necessarily be obvious to someone working on the installation until they start disconnecting things (or correctly test for voltage/current first).

But overall it is why the 'earth' conductor needs to be treated with the same respect as the brown and blue wires because it is 'live' in the sense of being connected to an electrical supply system and possibly at a voltage that would hurt, or worse.

You're absolutely right though, in this scenario it is a serious issue which needs urgent attention from a professional.

A PEN conductor, by convention, is not considered to be a live conductor. Although obviously a neutral conductor is a live conductor.