Patent ID: 6941233

Claim:
A control arrangement for a source-transfer switching system of the type that includes first and second solid-state switches that are controlled to selectively supply a load circuit connection with an alternating-current waveform via one of the two first and second solid-state switches, the control arrangement comprising: transfer control means for controlling the first and second solid-state switches to selectively supply the load circuit connection from one of the first and second solid-state switches, said transfer control means further comprising first means for inhibiting a change in the supply of the load circuit connection between the first and second solid-state switches when a fault is detected that is downstream of the control arrangement, said first means further comprising second means for comparing sensed current and voltage at various points along the alternating-current waveform to determine if the sensed current establishes a downstream fault current, said first means further comprising third means operative when said second means does not establish a downstream fault current for delaying determination for a predetermined time interval if the point along the waveform is within a predetermined range of a voltage zero point, and fourth means for determining that a downstream fault current is not present if the second means does not establish a downstream fault current by the end of the time interval.