I hadn't really thought about this before, but with your common neutral/ground busbar, I guess to find a short on a branch circuit you would have to test before both are connected to the bar.
In the U.K., we always have separate neutral and ground busbars, even in places where PME is used and the ground is bonded to the incoming neutral.
It's standard practice on a new installation to do two final megger tests:
1. All switches closed, bulbs removed, appliances unplugged etc. Test between hot & neutral with all fuses in/breakers closed.
2. Temporarily strap hot to neutral and megger between them and the ground busbar. It's also acceptable to do two separate tests: hot to ground & neutral to ground.
We don't even need to disconnect any main cables for these tests on domestic work, because our main switches are D.P. and open the neutral as well.
