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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..
PNB can be seen as being either TNS like or TNC-S like.
Assuming only one NE bond and one point of earthing for the LV side, it can be treated as TNS, just with the NE link outside the transformer tank, all the failure modes and fault currents are the same,
If multiple links multiple transformers multiple earthing then treat more like PME.
If not you must assume HV and LV earthing is shared and never interrupt the common CPC while the HV side is connected, even if the LV is off, or the transformer core and all the metalwork attached to it, will be driven to some nasty high voltage by the capacitance between the HV windings and the core and tank.
PNB can be seen as being either TNS like or TNC-S like.
Assuming only one NE bond and one point of earthing for the LV side, it can be treated as TNS, just with the NE link outside the transformer tank, all the failure modes and fault currents are the same,
If multiple links multiple transformers multiple earthing then treat more like PME.
If not you must assume HV and LV earthing is shared and never interrupt the common CPC while the HV side is connected, even if the LV is off, or the transformer core and all the metalwork attached to it, will be driven to some nasty high voltage by the capacitance between the HV windings and the core and tank.
