If an external high level air-con unit has a rotary isolator positioned next to it, is it good practice to wire the isolator so that it disconnects both L. and N. of the supply?
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.
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