TN-S System

Not necessary to use 4-Pole Breakers at the Incomer unless this source is going to be connected to a bus wherein there is another standby source is also connected. If you have only one source at your plant you can go for all three pole breakers (both incomers as well as outgoings). Or if you have good money to invest, it is a good practice to go for all four pole breakers (both the incomer as well as the outgoings).

Alternately, if your plant has got another source too, wg=hich could be a stnad by, the at least one of the incomers or the bus coupler MUST be a four pole breaker.

Hi electrical expert

You are indeed expert thank you for the prompt reply, we have in our plant 2 xformer, connected on the low voltage switchboard there is tie on the center and we have a dynamic UPS combination of motor and generator, the main CB is 4P for 2 xformer, including the tie but the feeder breakers are 3P is this correct scheme of TN-S?