Roger and William, as I say, I'm not an EE, but I have looked into these issues. I'm just passing along wisdom from folks who are and know these things.
Putting a do-it-yourself DPDT switch in the 220v input line is an easy and tempting way to kill all power to the lathe (or other equipment) in an emergency. There are 220v DPDT magnetic switches, even "paddle" or "knee" switches, available online for under $15. I know of turners who have done this. When a VFD is involved, though, we should be aware of the other considerations.
If one is used, I suggest thinking of it only as a substitute for "pulling the plug", and making sure anyone who uses the lathe knows why. If under the circumstances you would pull the plug, use the switch. So, if the lathe is actually operating, that means only in an emergency, never merely as a convenient way to shut down the lathe. But even then, be sure to wait two or three minutes before flipping the switch back on. Absent an emergency, I would use the switch only after the lathe has been shut down normally and you want to disconnect from the line power to avoid damage from power surges. In the latter case, though, a DPDT switch only disconnects the two hot legs, not the ground leg. So a lightning ground strike might conceivably reach the lathe anyway. And even with respect to the hot legs, the disconnect is only the gap between the switch contacts. Pulling the plug from the receptacle would still seem to be the best protection.