The Danger of Using Ground as a Reference

In HVAC units, 240v circuits don’t have a neutral circuit conductor. However, they do have ground, which is a safety circuit that doesn’t carry power. If you use ground as a reference, you would read 120v at any point on the circuit, even if there are open switches. That’s because the electricity backfeeds through L2.

If you put one lead on the L2 side with the motor, you would read 240v on the other side of the open switch but 0v on the motor side. That’s because the open switch doesn’t allow the motor to do any work, so there is no electrical potential between the two leads.

Ground typically works as a reference on 24v or 120v circuits that are properly bonded between common, ground, and neutral. However, it isn’t an effective reference on improperly bonded 24v or 120v circuits, and it can lead to misdiagnosis if technicians attempt to use it as a reference on 240v circuits.