I know that there are relays that monitor voltage, especially for 3 phase. These could be wired to a contactor for the equipment or maybe to a relay placed in the control circuit. Or, if he's got some serious bucks, install shunt trip breakers & wire them into the low voltage sensor.
TimeMark, ASCO & many others make this type of relay, check with your local electrical supply house.
[This message has been edited by Tom (edited 03-12-2002).]
Few things are harder to put up with than the annoyance of a good example.