This is still confusing to me. I am not sure about furnaces, we don't have them. High efficiency A/C units are inverter deals and don't even know what the frequency or voltage is. The first thing they do is rectify this into DC at 1.4x the line voltage and chop it into 20kz or so. Then they make whatever voltage they need.
The only thing that makes sense is they are using the ground for something like the igniter or that some control system like the thermostat is not being powered.
