When I first started working with VFDs at GE, we required a set of contacts in the load side disconnect switch that would open as soon as you started to open the switch, since a lot of the non-informed folks would see the local safety disconnect switch and use it like a light switch to turn the motor on and off.
The reason for the contacts was to send a signal to the VFD's control circuitry that its load was about to go to zero.
The VFD tended to freak out if it suddenly lost its load and would do some crazy stuff, ranging from fuse-blowing to electronics-smoking.
That signal gave the VFD the notice that it needed so that it would behave normally and would not damage itself.
