Method for forcing the commutating of a bypass scr of a ups system and ups system

In an uninterruptible power supply (UPS) system comprising an inverter to supply auxiliary AC power form a DC source, forcing the commutating or switching of a bypass silicon controlled rectifier (SCR) by: detecting an outage of a mains supply of the UPS system (S10); turning on the inverter of the UPS system with a normal output polarity (S12); switching off the bypass SCR (S14); turning off the inverter (S16), and; turning on the inverter with a reverse output polarity for a predetermined time interval so that a reverse current is drawn via the bypass SCR (S18). Preferably, the predetermined time interval is 2 milliseconds, after which the inverter is turned off for 0.3 milliseconds, then turned on with a normal polarity. The SCR may comprise two antiparallel connected thyristors with the gate of each thyristor controlled by a microcontroller which monitors the mains power to detect faults and controls the inverter. The microcontroller may also monitor an inverter current limit signal from the inverter to register that the fault is an upstream fault.