Can you tell me when troubleshooting hvac tech will check voltage 240 v on outside condensing unit . Then he took 1 probe and touch a ground screw and put the other probe on one side of contactor and read 120v
He said he does this v to ground to insure there is voltage I am not sure what this means can you explain this to me
The reason he checked voltage to ground.

Also what is control voltage anyway when you check with a voltmeter?