3 floors but one floor keeps blowing a fuse- Notes from the tech today

Ok. Done here for now. Blown fuse again, replaced and system came on. Checked outdoor unit wiring inside, looks fine. Replaced contactor as it did have some high resistance. Fingers crossed that takes care of it. If not we may have to change out the other contactor or defrost board. I'm trying to keep cost down and start with the least expensive part that could cause it. Please keep me posted. You can arm the alarm again. Enjoy your weekend!

Any ideas? Where do we look for what keeps causing IG to trip? Things we have recently done- changed the water meter (required for insurance) and just the weather is changing.Thank you!