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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..
I agree that the cable connecting the system neutral bar to the case of the transformer sounds really suspect, however
I think that I am beginning to get the hang of this.
There are 5 conductors between the transformer and the panel: 3 lines, one neutral, one CPC (the armour). There is no description of a PEN conductor. So it must be TN-S.
The star point of the transformer must be earthed somewhere, but it could be PNB. From the SP Energy Networks document "In a PNB earthing system the LV neutral conductor is connected to an earth electrode at a point remote from the transformer at or near the customer's supply terminals."
So where is the LV earth electrode? It may very well be connected to the neutral terminal at the customer's supply terminals, but I am still confused as to why there is a green and yellow going back to the tank of the transformer. The SP Energy Networks document also says: "The transformer tank and associated HV metalwork shall be connected to the HV earth electrode." If there is a combined HV/LV earth electrode, that might explain things, but is it allowed?
I agree that the cable connecting the system neutral bar to the case of the transformer sounds really suspect, however
I think that I am beginning to get the hang of this.
There are 5 conductors between the transformer and the panel: 3 lines, one neutral, one CPC (the armour). There is no description of a PEN conductor. So it must be TN-S.
The star point of the transformer must be earthed somewhere, but it could be PNB. From the SP Energy Networks document "In a PNB earthing system the LV neutral conductor is connected to an earth electrode at a point remote from the transformer at or near the customer's supply terminals."
So where is the LV earth electrode? It may very well be connected to the neutral terminal at the customer's supply terminals, but I am still confused as to why there is a green and yellow going back to the tank of the transformer. The SP Energy Networks document also says: "The transformer tank and associated HV metalwork shall be connected to the HV earth electrode." If there is a combined HV/LV earth electrode, that might explain things, but is it allowed?
