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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..
Interesting to note that unlike BS7671, IS10101 does not require the PEN to be earthed at two or more points. That standard merely describes TN-C-S as a “system in which the neutral and protective conductor functions are combined in a single conductor in part of the system...”. The example figures relating to the system show two electrodes, one at source and one in the distribution but they are bracketed with the words “earthing of system through one or more earth electrodes”. So the source electrode is one such electrode. Thus what Andy has drawn would be TN-C-S if the exposed parts in the installation are connected to a separate protective conductor. Any PNB system would also be described as TN-C-S in Ireland. I am still not sure what is going on in the OP but if N and E are combined in a single conductor then it is TN-C-S.
The point about the length of said conductor that OMS made is very reasonable but obviously not catered for in either standard.
Interesting to note that unlike BS7671, IS10101 does not require the PEN to be earthed at two or more points. That standard merely describes TN-C-S as a “system in which the neutral and protective conductor functions are combined in a single conductor in part of the system...”. The example figures relating to the system show two electrodes, one at source and one in the distribution but they are bracketed with the words “earthing of system through one or more earth electrodes”. So the source electrode is one such electrode. Thus what Andy has drawn would be TN-C-S if the exposed parts in the installation are connected to a separate protective conductor. Any PNB system would also be described as TN-C-S in Ireland. I am still not sure what is going on in the OP but if N and E are combined in a single conductor then it is TN-C-S.
The point about the length of said conductor that OMS made is very reasonable but obviously not catered for in either standard.
