Integrity testing of isolation means in an uninterruptible power supply

The present invention provides an uninterruptible power supply (10) wherein the isolation means (14') is automatically tested during normal operation to ensure that it is capable of effectively isolating the input from an auxiliary power supply (16) during emergency operation. The uninterruptible power supply (10) comprises an input for coupling to a main power supply (1), an output for connection to an electrical load (2), an auxiliary power supply (16) and selective isolation means (14') configured to isolate the auxiliary power supply (16) from the input during emergency operation. First means (26,28) establish a reverse polarity across the isolation means (14') for a test period (”T 2 ) during normal operation, and second means (30,24) monitor a voltage proportional to an input voltage (U in ) to the isolation means (14) and output an error signal (DT Error ) if the monitored voltage (U in ) remains or rises above a first reference value (U ref2 ) during the test period (”T 2 ).