Originally Posted by Jenn
. . .and there is no loose connection.
He can check connection integrity while current is flowing.
For relay contacts while rated current if flowing, less than 30 mV voltage drop due to contact impedance is good and more than 100 mV is bad.
For a Wirenut carrying 10A, 50 mV is way too much and means it was improperly installed. A good install shows much less drop.
I'd think these numbers would be smaller for a connection that uses tempered spring steel like in your HVAC stuff. The makers of these fasteners may give you pass/fail factory data by their QC people.