What is the voltage between L. and N.?

Typically 240 Volts in the U.K.

Z.

The regulation 531.2.2 says that in TN-S system, the neutral need not be disconnected if the supply condition are such that the neutral conductor can be considered  to be reliably at earth potential.

Yes I know. But is it still good practice to fully isolate the outside air-con unit to protect repair technicians when repairing or servicing the unit?

461.2 might be more relevant.

Z.

Yes good practice, because it's:

(a) at height; and

(b) possibly will be maintained by someone who may be accessing from outside the building (i.e. outside the influence of the main bonding) - and the risk of a potential difference between N and Earth (rather than PE / MET) is greater. The risk is

Now, it's not a firm requirement of BS 7671. However, CDM Regs require the designer to consider the risks involved with installation, maintenance and decommissioning (even in non-notifiable projects, and in works carried out for a domestic Client). Note that all that's required to cause serious or fatal injury (from 'secondary effects' of electricity) when someone is working at height is a perceptive shock, which could be as little as 1 mA. This particular hazard is not covered for in BS 7671, but that doesn't mean you ignore it.

Equally, you could attest that there is another way to carry out the maintenance safely (isolate elsewhere with a 2-pole device upstream of the equipment isolator, e.g. main switch) - in which case that's what you document on your CDM risk assessment and no problem.

Thanks Graham.

Z.

(b) possibly will be maintained by someone who may be accessing from outside the building (i.e. outside the influence of the main bonding) - and the risk of a potential difference between N and Earth (rather than PE / MET) is greater. The risk is

So on TN, especially PME, systems, if we're isolating the N, should we be isolating the PE conductor too?

   - Andy.

plug and socket then if you want that, nothing like a 3ft contact gap.

Mike

I see little point in isolating PE as the outdoor unit is almost certainly linked to the indoor unit by copper pipes, that are bonded or earthed inside.

The air-con units that I saw were pretty much self contained and were not bonded inside the building. The pipes just ran at high level from outside to the inside of the room and were boxed in. The units would of course be earthed.

I was thinking of a maintenance person being outside on a metal ladder and working on the internals of the outside unit. Of course he could also be contacting between the earthed metal work and ground.

Z.