I saw an advertisement for the new Wylex "domestic" three phase consumer unit which has a three pole main switch with the neutral wired directly into the neutral bars.

I thought all domestic installations have to have a switched neutral, I checked BS7671 and found I was wrong. 

The risk of the neutral being not connected when the phases are - which requires an offset contact in the 4 way switch block, is considered worse than the risk of leaving it connected.

And to be fair to the DNOs in most cases the neutral really is at or very near earth potential of course.

I have also been shown pictures of an expensive mistake with wring to a 3p and n main switch in  large installation where the neutral was not routed through the offset contact, but one of the phases was. The switch was just fine, but sometimes when it was operated for maintenance  some unrelated random failure would occur. Apparently It took ages to work out what was happening.

Mike.

Well my reading of it is that 4-pole switching is recommended (537.2.6) and since no one seems to know what the hell 461.2 means, it would seem reasonable to drop the reference to it in 537.2.6 and run with the recommendation in the first bit of that regulation.  

Is this conclusion correct? You can use single pole devices to isolate circuits or equipment if the neutral is earthed and does not need to be disconnected, as regulation 537.2.6 says. Regulation 461.2 says that you do not need to disconnect the neutral or PEN conductor if the supply conditions make sure that the neutral conductor is reliably at earth potential. This means that the neutral is connected to the earth at the supply source, and it cannot become live because of a fault or other reason. What conditions are required to decide that there is no possibility of the neutral becoming live ?

if the supply conditions make sure that the neutral conductor is reliably at earth potential.

Ah, but which "earth potential"? Say you had a TT installation - is your local "Earth" potential the same as Earth at the source? The traditional answer was no - not just because of possibility of the actual soil being at different potentials in different places, but because of certain faults on the network (not just broken PENs but also accidental earthing of line conductors) can mean that the supply's electrode can be dragged very significantly away from 0V. In many ways it seem that what they're trying to say is whether N can be at a significantly different potential to not Earth (as in the potential of the general mass of the earth) but to the installation's earthing system (main earth terminal, c.p.c., bonding conductors etc.). In that context the dependency on being in an installation equipped with main bonding makes a lot more sense.

I would agree that the new regulation wording doesn't convey intelligible clarity (at least to me).

There is a bit of an elephant in the room if we carry this thinking to its logical conclusion though - if we take a TN system outside of the main equipotential zone, we might decide that N my at times be at a significantly different potential to its surroundings, and so warrant isolation of N - but then the c.p.c. will be at a similar potential to the N - so shouldn't the c.p.c. be isolated as well?

    - Andy.

but then the c.p.c. will be at a similar potential to the N - so shouldn't the c.p.c. be isolated as well?

Enter the argument that leads to the use of double insulated hedge trimmers, lawnmowers etc.

Arguably if we had not spent most of the 1960s taking them out, you could have provided special 2 pin sockets for double insulated outdoor equipment. Perhaps not the ones with split pins though.,,,

Mike

but then the c.p.c. will be at a similar potential to the N - so shouldn't the c.p.c. be isolated as well?

Enter the argument that leads to the use of double insulated hedge trimmers, lawnmowers etc.

Arguably if we had not spent most of the 1960s taking them out, you could have provided special 2 pin sockets for double insulated outdoor equipment. Perhaps not the ones with split pins though.,,,

Mike