Abstract: A microprocessor controlled automatic transfer switch is provided with an improved voltage sensing scheme which utilizes an open-delta transformer and a control routine for determining voltage level or phase loss. The invention requires only two transformers for sensing three-phase power. An actuator protection feature is also provided to limit the duration of power applied to and to protect the actuator solenoids on the automatic transfer switch. Also provided are improved diagnostic annunciation features which provide specific information regarding switch position and sensing element and/or actuator malfunction.

Abstract: A simplified mechanical and electromechanical interlock system connecting automatic transfer and bypass/isolation switches in a bypass/isolation automatic transfer switch is provided, whereby the risk of hazard resulting from the misconnection of power sources through automatic transfer and bypass/isolation switches is reduced by prevention of closure of the two switches into opposite or dead sources is prevented. Related retractable table structure is also provided for moving the automatic transfer switch from a frame in which the switches are housed.