Earthing neutral

To ensure it is only earthed at one point - if it were bare (and buried) then it  would effectively be multiple earthed

Agreed - even if not buried a bare conductor might be subject to accidental or incidental earthing.


Remember that this conductor is the N of the system (as well as providing the reference to Earth) - and multiply earthing neutrals can be a bad idea. Generally we attempt to keep things safe - at least under single fault conditions (either insulation failure or conductor continuity failure) - and a multiply earthed neutral can be a problem in that respect. Think of a N conductor earthed to separate electrodes at two different points, and then a break in the N conductor somewhere between the two - any current still flowing through the N will cause a significant voltage gradient around each electrode - so not only is the system itself no longer entirely referenced to earth, but you have a likely lethal voltage between the two electrodes and whatever is connected to each of them - not good for a single simple fault. That's one reason why we don't normally have PEN conductors in consumer's installations and in DNO systems where they are used they go to extraordinary lengths (well beyond normal BS 7671 requirements) to keep PEN conductors intact. With N fully insulated and Earthed at just one point any single fault shouldn't be dangerous - bits of the system might well stop working and (insulated) bit of the N conductor may well be a long way from 0V - but it should still be safe.


The problem is more obvious where the N is distributed and so expected to carry large currents from the loads - but even in this case where it isn't the N could still be carrying some circulating currents between the sources (imagine one source trying to produce 229.99V L-N and the other 230.01 volts) - so you can't rule out the possibility of N carrying a few amps between the sources - so the same approach applies.


That's not to say you couldn't use a multiply-earthed (TN-C-S) system if you really wanted - but then it wouldn't be a TN-S system and that particular diagram wouldn't apply.


   - Andy.

To ensure it is only earthed at one point - if it were bare (and buried) then it  would effectively be multiple earthed

Agreed - even if not buried a bare conductor might be subject to accidental or incidental earthing.


Remember that this conductor is the N of the system (as well as providing the reference to Earth) - and multiply earthing neutrals can be a bad idea. Generally we attempt to keep things safe - at least under single fault conditions (either insulation failure or conductor continuity failure) - and a multiply earthed neutral can be a problem in that respect. Think of a N conductor earthed to separate electrodes at two different points, and then a break in the N conductor somewhere between the two - any current still flowing through the N will cause a significant voltage gradient around each electrode - so not only is the system itself no longer entirely referenced to earth, but you have a likely lethal voltage between the two electrodes and whatever is connected to each of them - not good for a single simple fault. That's one reason why we don't normally have PEN conductors in consumer's installations and in DNO systems where they are used they go to extraordinary lengths (well beyond normal BS 7671 requirements) to keep PEN conductors intact. With N fully insulated and Earthed at just one point any single fault shouldn't be dangerous - bits of the system might well stop working and (insulated) bit of the N conductor may well be a long way from 0V - but it should still be safe.


The problem is more obvious where the N is distributed and so expected to carry large currents from the loads - but even in this case where it isn't the N could still be carrying some circulating currents between the sources (imagine one source trying to produce 229.99V L-N and the other 230.01 volts) - so you can't rule out the possibility of N carrying a few amps between the sources - so the same approach applies.


That's not to say you couldn't use a multiply-earthed (TN-C-S) system if you really wanted - but then it wouldn't be a TN-S system and that particular diagram wouldn't apply.


   - Andy.