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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 a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.
No it doesn’t! The star point is within the transformer tank and is never brought out apart from split phase transformers or some specialist traction transformers. The star point will be brought out to a neutral terminal in the LV connection box. There will be no connection to earth inside the tank. Whether the single connection to earth is in the connection box or 100m away, it is still TNS. In order to be TNC-S, you would need a cable core that provides the neutral and earth function, in this application there isn’t one.
if you take a good transformer and measure between the LV winding and the tank, it will be open circuit. Between the HV winding and the tank, it will be open circuit. Between the HV winding and the LV winding it will be open circuit.
. . . In a TN-S system, the PE must be connected to the star point itself. If there is any connexion between N and PE anywhere else, it becomes TN-C-S because there are then two conductors wired in parallel.
No it doesn’t! The star point is within the transformer tank and is never brought out apart from split phase transformers or some specialist traction transformers. The star point will be brought out to a neutral terminal in the LV connection box. There will be no connection to earth inside the tank. Whether the single connection to earth is in the connection box or 100m away, it is still TNS. In order to be TNC-S, you would need a cable core that provides the neutral and earth function, in this application there isn’t one.
if you take a good transformer and measure between the LV winding and the tank, it will be open circuit. Between the HV winding and the tank, it will be open circuit. Between the HV winding and the LV winding it will be open circuit.
