How Does an Air Conditioner Cabinet Become Energized?

The last two weeks I have been talking about electrical safety. Specifically, discussing the electrical hazard of an energized system cabinet. This week I would like to discuss ways that the cabinet of a system can become energized. First, for a cabinet to have a voltage other than ground, the cabinet is either not grounded, or the ground has failed. Failure to properly ground a metal cabinet is the first condition that sets up the electrical hazard. But another mistake or failed component is required to actually energize the cabinet. Some are obvious – such as a loose energized wire touching the cabinet. Others are less obvious, such as a grounded electric motor. If a motor is grounded (not shorted or open) and the equipment cabinet in which the motor is mounted is not grounded, when the motor is energized the cabinet will also become energized. Motors can sometimes be grounded and still operate. So you touch an operating unit and get shocked. Another failure that can energize a cabinet is a broken or missing insulator on heat strips. Most heat strips have one side that is wired hot all the time. If the ceramic insulators break and allow the strips to contact the frame holding them, the cabinet can become energized.

A sneaky way for a cabinet to be energized is to wire a 110 volt device into a 230 volt unit using one leg and ground. Basically, current will be going through ground anytime the 110 volt device operates. This alone won’t cause an energized case. But if the ground between the unit and the panel breaks or just gets a bad connection, now there will be a voltage between the case and the actual ground – and the case will be energized. To avoid this, don’t wire 110 volt devices this way. You either need a separate neutral AND a cabinet ground (4 wires), or you need a 230 – 110 transformer. This is why newer electric dryers have 4 prong plugs: two for the 230 volt hot legs, one for neutral for the motor, and one for a cabinet ground.

So here are a few simple rules to help you avoid creating an electrical hazard:

1. All metal cabinets and pipes should be grounded
2. Ground wires should never be used as part of an operating circuit.
3. When equipment has both 230 volt and 110 volt loads, the equipment either requires a neutral wire AND a separate ground wire, or a 230 volt to 110 volt transformer.