I actually thought it was because I was running my spindle at low speed (12000rpm) with a 43mm bit taking a few mm off my spoil board. I thought, that the spindle had become hot inside and shorted out somewhere because I noticed a "spark" in the air. The outside of the spindle was just slightly warm.

But sparks are fast, circuit breakers not far behind. The HY VFD, actually caught fire inside. I watched the fire for a wee while (as us men do) before it went out.

Heres the thing. Immediately after the event (and the fire inside the VFD going out) I disconnected the shielded output lead off the VFD and put a meter on it to see where or if the spindle was shorted. It all measured about 2.5ohms between all phases. Open circuit to ground. Visual inspection of the plug, lead, I am anal I looked. Obviously not enough.

I bought a new HY VFD. Before I plugged it in, I megga checked the lead with 1000v. I wired the new VFD in, and have run the setup for about 3 hours while I make a new Z Axis (I decided to upgrade my spindle) to accept a larger motor. Yesterday, CRACK, same deal without the fire in the VFD. This time a 6mm bit not even working hard.

Again, meter on the lead. 2.5 ohms between phases. Pull the plug off, and now I see the issue. This lead, still measures open circuit between phases. Still megga checks fine. Hold it, twist it, keep that 1000v onto it, no problem. But occasionally and thats enough, it has an internal short inside. I guess this is "hey look what happened to me". I am not concerned today I am going to install a new plug to see if my setup still runs. The VFD powered up yesterday without the motor connected. Strange that the short is intermittent but thats life.