You only have a choice in a TN- system and that is subject to 461.2. My question would be why would you not want neutral isolation?

As a broken N on 3-phase systems can cause havoc (feeding 400+V to things expecting 230V) the traditional policy is to keep N as reliable as possible - and as moving parts are generally less reliable than solid joints, the usual approach is to use 3P incomers except when it's necessary to open N for isolation (typically in TT systems). Fashions differ though and there can be other consideration - e.g. if there are RCDs upstream.

   - Andy.

What about the idea of back fed neutral currents?

So 4 pole isolator incomers should not be used on dist boards fed by 3 phase PME supplies? I wondered about this when I looked at an installation with multiple dist boards - should I have bonded them together with some 16mm earth?

So 4 pole isolator incomers should not be used on dist boards fed by 3 phase PME supplies? I wondered about this when I looked at an installation with multiple dist boards - should I have bonded them together with some 16mm earth?

I wouldn't go as far as "should not" - as with many things it's really down to the designer to weigh up the pros and cons for any particular situation -  and different people will naturally give different weights to different aspects. Some may anticipate having RCDs upstream (e.g. perhaps of the possibility of local generation) or may have other valid reasons the prefer N isolation (circuits going outdoors perhaps). The design of some modern isolators (with reliable showing of contact position and N closing first/opening last) may well mean that the chances of closing L if N is jammed open is much reduced compared with old experience. Often it boils down to something between 5 and 7 of one vs half a dozen of the other.

I'm not sure that an additional PE bond would help, given that N and PE would be sully separated at that point - but I might well have missed something in your thinking there.

    - Andy.

What about the idea of back fed neutral currents?

What sort of situation do you have in mind?

   - Andy.