What do you mean by its 'isolated from the rest of the system'?
Your P.P. and USB port is at the same reference or common to one side of your AC to the compressor and any other AC motor or device.
This is due to the P.C. P.S. being at Earth ground as well as any AC device due to the neutral at Earth Ground also.
You need to bond or equi-potential bond all metal work on the machine to a central point, together with the service ground conductor.
Al.