UPS Bypasses

There are two primary types of bypass, known as Static and Maintenance Bypass:
Static Bypass

A static bypass is fitted to almost all On-line UPS; it forms a second line of defence. Should the inverter fail, the static bypass ensures the load drops automatically onto the mains input feed.
Maintenance bypass

With an On-line UPS, if an inverter fails, it will, in all probability, occur whilst mains power is present.

A static bypass is in almost all cases part of the UPS internal circuitry and may be invoked manually using an external switch. In larger UPS, it will probably synchronise the UPS output with the mains cycle before switching, so may take a few seconds to engage.
Maintenance Bypass

Known more descriptively as a wraparound bypass, this is fitted externally to the UPS and enables the UPS to be isolated for maintenance or repair without interrupting power to the load. On larger UPS the internal static bypass should be engaged before throwing the maintenance bypass. Some bypasses have interlocks to ensure this correct operation.

After setting the UPS into bypass mode (to synchronise with the mains supply), Sw1 is operated to provide raw mains to the load. Sw2 may be opened to allow safe working on the UPS or may be left closed if the engineer wishes to make tests with the power on.