The test points first.
There are five of them. the one on it's own is the 0v common.
then, moving away from the 0v, in this order and measured relative to 0v:
You should have on your incoming supply (relative to ground) 120v per phase or 220v between all the phases. If oyu don't, you may have a fuse gone on the incoming supply.
Turn the isolator off to measure the incoming supply as you have a transformer that will create the impression that you have a low phase if one is open.