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What earthing arrangement is this?
Former Community Member
The supply is from a private transformer in a four core cable 3ph + n. The cable armour is earthed and connected to the MET. However there is also a green and yellow cable connected to the neutral terminal at the main isolator going back to a the transformer casing. The transformer is only 5 or 6 metres away. I think this must have been intended to make it a tncs supply but seems to me to just create parallel neutral conductors. Or is it tn-s-c-s? I have only been able to go off visual inspection because I could not disconnect the supply..
In my copy of BS 7671 it seems to be worded slightly differently - mentioning only "The earthed point of the source, or an artificial neutral". Thus the neutral (star point) of the source could be distinct from the earthed point of the source.
Good point. System diagrams in BS7430 suggest 'source' to be distinct from the distribution cable and the consumer terminals (where its PNB electrode is shown). But BS7671's diagrams aren't so clear as that. Anyway: seeing it as you suggest helps avoid the strangely broad definition of PEN.
it doesn't matter how long the conductor between the star point ("0") and the N-PE link is - it's still just a neutral conductor.
Personally I prefer to think of protective conductors in term of the path of the consumer's connection to Earth - rather than the Earth Fault Loop - as the latter includes the conductors back to the star point - which seems to cause more than its fair share of confusion.
That certainly appeals as reasonable. If I had to give a defining feature of TNCS systems and PEN conductors - in the way I still see them in spite of my surprise at BS7430 - it would be along the lines of whether a single conductor-break with only load current present (no earth fault, negligible leakage) can raise the potential of PE in the installation relative to 'remote earth'. (However, when trying in an earlier posting to see how the claim of "PNB implies TNCS" might be more reasonable than it initially seemed, I did note a couple of ways in which the PNB 'neutral' is a bit special. Its continuity does have a relevance to shock protection that's more than for just a plain load-neutral conductor, although this could be the same for some connections inside a transformer in a non-PNB system.)
In my copy of BS 7671 it seems to be worded slightly differently - mentioning only "The earthed point of the source, or an artificial neutral". Thus the neutral (star point) of the source could be distinct from the earthed point of the source.
Good point. System diagrams in BS7430 suggest 'source' to be distinct from the distribution cable and the consumer terminals (where its PNB electrode is shown). But BS7671's diagrams aren't so clear as that. Anyway: seeing it as you suggest helps avoid the strangely broad definition of PEN.
it doesn't matter how long the conductor between the star point ("0") and the N-PE link is - it's still just a neutral conductor.
Personally I prefer to think of protective conductors in term of the path of the consumer's connection to Earth - rather than the Earth Fault Loop - as the latter includes the conductors back to the star point - which seems to cause more than its fair share of confusion.
That certainly appeals as reasonable. If I had to give a defining feature of TNCS systems and PEN conductors - in the way I still see them in spite of my surprise at BS7430 - it would be along the lines of whether a single conductor-break with only load current present (no earth fault, negligible leakage) can raise the potential of PE in the installation relative to 'remote earth'. (However, when trying in an earlier posting to see how the claim of "PNB implies TNCS" might be more reasonable than it initially seemed, I did note a couple of ways in which the PNB 'neutral' is a bit special. Its continuity does have a relevance to shock protection that's more than for just a plain load-neutral conductor, although this could be the same for some connections inside a transformer in a non-PNB system.)
