Conventional automatic transfer-switch units rated 225 A and greater have separate arcing contacts. Transfer time is the same as for electrical operation. Control circuit automatically disconnects from electrical operator during manual operation. Load transfer without interruption, through momentary interconnection of both power sources not exceeding 100 ms. Controlled by in-phase monitor and sensors confirming both sources are present and acceptable.
Initiation occurs without active control of generator set.
Controls ensure closed-transition load transfer closure occurs only when the two sources are within plus or minus 5 electrical degrees, maximum and plus or minus 5 percent maximum voltage difference. Relay compares phase relationship and frequency difference between normal and emergency sources and initiates transfer when both sources are within 15 electrical degrees, and only if transfer can be completed within 60 electrical degrees. Transfer is initiated only if both sources are within 2 Hz of nominal frequency and 70 percent or more of nominal voltage. Pickup voltage is adjustable from 85 to 100 percent of nominal, and dropout voltage is adjustable from 75 to 98 percent of pickup value. Factory set for pickup at 90 percent and dropout at 85 percent.