Suggestion for Isolation Equipment Selection

I would approach the new switchgear for this situation in this way:

Incoming just needs the bus riser - to isolate the whole feeder, open at the upstream main substation breaker. This is often where metering is installed as well.

Outgoing can be manually-operated, fused disconnect switches, with the fuses chosen for protection of the feeder cables to your two (new and existing) buildings, especially if underground. It may take some evaluation to coordinate with the upstream PS451 and your downstream protection. Unless you are going to have significant motor loads at the buildings (you did say they are residential), then a single cable fault won't cause too much heartache from single-phasing if one fuse blows.

Adjust the existing 7SJ602 if needed to (try to) coordinate with the fuse, but its main purpose is to protect the transformer. Someone should do a protective device coordination study on the whole scheme to see how it all fits.

What do you plan for main protection at the new building? Will it be similar to the existing? You will need to try to coordinate it with the upstream fuse, but again, it should protect its transformer first.

Thanks alot for the reply.

Yes, the building is residential but it might have some HVAC motorized loads.

The main protection will also be the same as for the existing building.

You suggested that fuse protection will be good enough for the cable feeder of existing building. But if some maintenance activity to be planned out only for the existing, how we isolate the exiting building feeder without any interruption to the new building feeder.

That is why each of the two outgoing units is a fused (load-break) disconnect switch. You open the switch for the feeder you want to isolate, and it is de-energized. The fuse provides the protection for the cable, the switch gives isolation / disconnection capability. Simple, reliable, and durable.

Thanks alot for your valuable suggestion