Okay guys I've heard enough hooey. The VFD shield is acting as an antennae because of the RF noise that VFDs generate. Forget earth grounds and stuff because if it seems to fix the problem you can be sure of one thing, you didn't actually find the problem or fix it. It will come back. Two issues here. Safety grounding which is intended to keep all grounded conductors from ever having an unsafe voltage difference even when 5,000 amps are flowing in a dead short. Safety grounding is all about creating as many ground loops as you possibly can. You can never know where the current will actaully flow so the idea of a clean ground is a myth. Control NOISE grounding is a completely different animal. The goal here is to never create a ground loop and only have one path straight to one ground for every device. No daisy chaining allowed. Manufacturers, and PCs in particular make it tough because they hide additional grounds in places you'd never expect like on ports, sensors etc. A simple shielded cable plugged in a PC often creates a ground loop because there is a ground pin on the connectors at both. Plug into the PC powered from the wall outlet and connect to a device powered from the wall outlet and you have a ground loop. So for everything you connect you need to use an ohm meter to determine if it is already connected to ground before you connect any more grounds.