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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..
If only 3 phases and neutral come from the transformer to the main panel and there is a link between the neutral in the panel and the installation earth then that is TN-C-S (PNB). PNB is one of the 2 types of TN-C-S.
That would be TN-S, John - all that's happened is that the physical star point has been expressed out to the Tier 1 switchboard in order to physically make the N-E bond and reduce the N-E offset for the installation.
If it's a DNO transformer serving a single consumer they would describe it as PNB - but would also tell you it's TN-S, despite telling you to bond it as if it were PME
If only 3 phases and neutral come from the transformer to the main panel and there is a link between the neutral in the panel and the installation earth then that is TN-C-S (PNB). PNB is one of the 2 types of TN-C-S.
That would be TN-S, John - all that's happened is that the physical star point has been expressed out to the Tier 1 switchboard in order to physically make the N-E bond and reduce the N-E offset for the installation.
If it's a DNO transformer serving a single consumer they would describe it as PNB - but would also tell you it's TN-S, despite telling you to bond it as if it were PME
