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.

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.