A static transfer switch is used for switching an output load from a first power source to a second power source. A first switching unit has a first pair of silicon controlled rectifiers (SCR). A second switching unit has a second pair of SCRs arranged in an anti-parallel configuration. The control circuit determines whether at least one of the SCRs is activated in accordance with the voltage between the gate and the cathode of the at least one SCR. If the voltage between the gate in the cathode is at least 150 mV, the control circuit indicates that the at least one of the SCR is activated.

A static transfer switch is used for switching an output load from a first power source to a second power source. A first switching unit has a first pair of silicon controlled rectifiers (SCR). A second switching unit has a second pair of SCRs arranged in an anti-parallel configuration. The control circuit determines whether at least one of the SCRs is activated in accordance with the voltage between the gate and the cathode of the at least one SCR. If the voltage between the gate in the cathode is at least 150 mV, the control circuit indicates that the at least one of the SCR is activated.