An Uninterruptible Power Supply UPS, system comprising: an input to receive input AC power; an output to provide AC output power to a load; a neutral connection; an automatic voltage regulation AVR, transformer having a primary winding and a secondary winding; a plurality of relays (104, 106, 108, 118) coupled to the input, the output, and the primary winding, the plurality of relays configured to selectively couple the primary winding to the input and the output; a bypass relay configured to selectively couple the primary winding of the AVR transformer to the neutral connection; a DC/AC inverter circuit coupled to the secondary winding and configured to be coupled to a backup power source and to receive backup DC power from the backup power source; and a controller configured to operate the plurality of relays, the bypass relay, and the DC/AC inverter circuit to provide the output AC power to the load derived from at least one of the input AC power and the backup DC power, wherein in a backup power mode of operation, the controller is further configured to operate the plurality of relays and the bypass relay to decouple the AVR transformer from the input and couple the primary winding to the output, to operate the DC/AC inverter circuit to convert the backup DC power into converted AC power provided to the secondary winding, to monitor the AC output power, the controller being characterized in that it is configured to identify, based on the monitored AC output power, that the bypass relay has decoupled the primary winding from the neutral connection in a welded relay condition, wherein in monitoring the AC output power the controller is further configured to monitor an output voltage level and an output current level at the output, wherein the controller is further configured to compare the monitored output voltage level to an output voltage threshold, wherein in response to determining that the output voltage level is less than the output voltage threshold, the controller is further configured to compare the output current level to an output current threshold, and wherein in response to determining that the output current level is less than the output current threshold, the controller is further configured to identify that the bypass relay has decoupled the primary winding from the neutral connection in the welded relay condition. an input to receive input AC power; an output to provide AC output power to a load; a neutral connection; an automatic voltage regulation AVR, transformer having a primary winding and a secondary winding; a plurality of relays (104, 106, 108, 118) coupled to the input, the output, and the primary winding, the plurality of relays configured to selectively couple the primary winding to the input and the output; a bypass relay configured to selectively couple the primary winding of the AVR transformer to the neutral connection; a DC/AC inverter circuit coupled to the secondary winding and configured to be coupled to a backup power source and to receive backup DC power from the backup power source; and a controller configured to operate the plurality of relays, the bypass relay, and the DC/AC inverter circuit to provide the output AC power to the load derived from at least one of the input AC power and the backup DC power, wherein in a backup power mode of operation, the controller is further configured to operate the plurality of relays and the bypass relay to decouple the AVR transformer from the input and couple the primary winding to the output, to operate the DC/AC inverter circuit to convert the backup DC power into converted AC power provided to the secondary winding, to monitor the AC output power, the controller being characterized in that it is configured to identify, based on the monitored AC output power, that the bypass relay has decoupled the primary winding from the neutral connection in a welded relay condition, wherein in monitoring the AC output power the controller is further configured to monitor an output voltage level and an output current level at the output, wherein the controller is further configured to compare the monitored output voltage level to an output voltage threshold, wherein in response to determining that the output voltage level is less than the output voltage threshold, the controller is further configured to compare the output current level to an output current threshold, and wherein in response to determining that the output current level is less than the output current threshold, the controller is further configured to identify that the bypass relay has decoupled the primary winding from the neutral connection in the welded relay condition.