We use floating control circuits on our emergency fans. You want the fans to run to destruction in the event of a fire. One wire grounding won't take us out of service but you lose your safety margin if you don't detect it. I install AC ground detectors which drive a Ground OK "GOK" status relay. I also install a Power OK "POK" status relay. These are both N.O.H.C. relays. The "POK" relay is driven via control power through the local "On,Hand,Remote" switch and 2 phase loss relays from the 480V side. This gives me the ability to use POK & GOK together as a very informative dual status point where:
0,0 = Power Fail
0,1 = Hand switch off of remote or phase missing.
1,0 = Ground Fault
1,1 = Normal
It's all gravy when your ground detectors help you get other info that you needed anyway.
The down side if any, is the infrequent question, "Why am I only reading 60 VAC to ground?" I tell them that the ground detector is establishing a high impedance, half point reference, and suggest a better location for a meter probe.
Joe
