When we have a transformer or a stand-by generator feeding to a bus, it is mandatory that at least either of the Incomers or the bus coupler must be TPN or 4-Pole Breaker please refers IS 3043.
In multi incomer power feeding systems, we cannot mix up the neutrals of incoming powers to other Power Source so we can use TPN or 4P breakers or MCB instead of TP MCB to isolate the Neutral of other power sources from the Neutral of incomer power in use. We can use 4 Pole ACB instead of TP for safety reasons. If there is power failure and DG sets are in running condition to feed the loads, if there is some unbalance in loads (which is practically unavoidable in L.V. distribution system), depending of quantum of unbalance, there will be flow of current through Neutral. During this time, if Power Supply Utility Technicians are working, and if they touch the neutral conductors (which is earthed at their point) they will likely to get electric shock depending on the potential rise in common neutral due flow of current through Neutral conductor as stated above. Even fatal accident may occur due the above reason. As such, it is a mandatory practice to isolate the two Neutrals.
We can use 4-pole breakers or TPN Breakers when the system has two alternative sources and, in the event of power failure from the mains, change-over to the standby generator is done. In such a case, it is a good practice to isolate the neutral also.
4 pole circuit breakers have advantages in the case when one of the poles of the device will get damage, and it also provides isolation from neutral voltage.
Normally, Neutral is not allowed to break in any conditions, (except special applications) for human & equipment safety. So for single incomer power fed systems, 3P breaker is used, where only phases are isolated during breaking operations.
Where We have dual Power like in DG & other electricity supply sources, it is required to isolate neutral, where neutral needs to be isolated in internal network TPN MCB or 4P MCB can be used.