Neutral Isolation. A Safety Necessity or a Practical Nightmare?

Note that in countries that use BS7671 but do not have an equivalent to that rule in the ESQCR, your decision may not be the same, and in countries like Ireland , where the underpinning regulations are the same as those that drive '7671, but the national variant is not, then you may well be permitted to permanently connect neutrals to earth consumer side anyway so long as it is safe to do so.

Mike.

Hi Andy, 

You are right about the generator connection to the system you shall use a 4-pole change-over switch. 

My comment was mostly above the question about the confusion of neutral isolation. With three poles or single pole switch.

Many thanks 

Arya

The disconnection of a fixed neutral conductor on TN C S and TN C system is unnecessary

Perhaps not always true - say you had local generation as a switched alternative to the grid - the change-over switch would need to switch (and isolate) N to comply with 444.4.7. (Wouldn't be needed in a pure TN-C system of course but the ESQCR prohibits PEN conductors in consumer's installations.)

That then begs the question, as far as the downstream installation is concerned, how reliable is the N connection with Earth? Moving contacts are generally less reliable than solid joints, and not all disconnection devices guarantee not to close the L contacts if the N has failed to close, or prevent the N from opening if the L contacts have welded closed. If you consider that the N downstream of the change-over switch isn't reliably held at close to Earth potential, can you justify working on such a conductor without isolation?

   - Andy.

• The disconnection of a fixed neutral conductor on TN C S and TN C system is unnecessary and could introduce more significant levels of risk and harm.

• Disconnection and temporary opening of a Fixed neutral link can result in a situation where the link is not replaced, which can lead to serious con. sequences.

Always prove dead before commencing working 

Phase to phases

Phase to neutral 

Neutral to Earth 

• In situations where neutrals are shared the disconnection of the link/conductor can lead to significant voltage instability on dependant connected circuits. in TN-C-S and TN-C. 

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

That NZ article is remarkable, and I take away two things,

firstly that a system of registered electricians is no guarantee of intelligent thought,

and secondly  that

While it makes a song and dance about inaccurate test results, clearly no test at all was done really as even a £5 plug in socket checker  would have made it obvious there was a problem.

Part of me also wonders if there was no RCD in circuit, or if there was but it failed to operate.

Mercifully both here and there, such events are rare enough to make the news, which I suppose is something.

Mike.

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.

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 ?

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.  

I have experienced the reversed polarity scenario, but I was not expecting a double fault scenario.

There has been an extremely sad incident in New Zealand, an electrician has been found to be responsible for the death of a builder, from the news report’s I have been reading over the last few weeks it appears that over twenty years ago someone wired a light with two-way switching using the circuit protective conductor as a strapper with some red insulation tape wrapped around the end of the CPC in the switch box. The electrician connected a new cooker hood to the existing cable, unaware of how it was wired.

www.nzherald.co.nz/.../