What is the benefit of an external UPS maintenance bypass switch?

Some uninterruptible power supplies include a built-in UPS maintenance bypass for service purposes. This does not fully isolate the UPS but provides a safe working environment for the UPS engineer. An external UPS maintenance bypass switch allows the full isolation of the uninterruptible power supply which can be safely shutdown without interruption of power to the critical load which is then supplied by the mains power supply. The shutdown UPS system can then be removed and swapped out.