I have to install a 100kva generator at an office block. He incoming mains goes through a 150A 3PH isolator. I have 2 150A 3PHcontactors that I need to use as the ATS, which will be installed next to the DB Board and isolator. There are also 3 surge arrestors in the DB board.Where do I interuppt the incoming power before or afer the isolator

I will assume an "isolator" is nothing more than a disconnect switch of some sort.

The ATS needs to be downstream of the disconnect.