Схемы, платы, компоненты

Загляни в параметр 14-50.. 

The circuit, as with other RFI filter

circuitry, can be sensitive to unbalanced phase-to-ground

voltages in the three-phase AC input line. This can

occasionally result in nuisance overvoltage alarms. For this

reason, the high frequency filter card contains a set of

relay contacts in the ground connection of the filter

capacitors. The relay is tied into the RFI/HF switch, which

can be switched on or off in 14-50 RFI 1. This disconnects

the ground references to all filters in case unbalanced

phase-to-ground voltages create nuisance overvoltage

conditions.