TPN+E

Yes it can be a TN-S system. If a 4 pole breaker is used the neutral should be late break. All systems I have worked on used 3 pole breakers with removable neutral links.

In a TN-S System, the Source Neutral is earthed solidly at the source. And, the equipment earthing is thro this earthed neutral of the source. But, there are seperate conductors emanating from the earthed source neutral for distributing the neutral and for the protective conductor. So, in a three phase system, you will have L1, L2, L3, N & PE - totally five conductors or wires running thro the distribution system. The neutral may or may not be broken; in single source systems, it is not broken to save cost on a fourthe pole of the switchgear. But, in multiple source systems, it is prudent practice to go for neutral breaking - with a 4 Pole Switchgear.

But, note that in a TN-C system, you MUST NOT break the neutral.

Sir

We have a low voltage switchgear with a 4P main for normal and 4P main for emergency and tie breaker of 4P the scheme is a double ended substation so the neutral of secondary of xformer will go to the 4th pole and at the same time it is solidly grounded. My question is can we apply REF (Restricted Earth Fault Relay) or ordinary ground fault protection on the 4P breaker?

I can understand the Scheme as it is more common. The choice of either REF or BEF or both is not influenced by the arrangement in your case. YOu can have either or both. REF is mandatory if the TRF. is more than 1 MVA capacity.

electricalexperts65

thank u sir for the prompt reply. More power and godbless.