Why does an external maintenance bypass need to be interlocked with the UPS?

An external wrap-around UPS maintenance bypass lets you isolate a UPS system completely for maintenance or replacement while the load stays powered from the connected mains power supply. The external bypass panel usually has three switches: UPS input, UPS output and bypass. Interlocking makes sure they can only be operated in the correct order. If the bypass closes while the UPS inverter is still supplying the load, two unsynchronised sources connect, which can damage the UPS and drop the load. Interlocking is done with mechanical key interlocks (such as Castell keys), or with auxiliary contacts wired back to the UPS so it transfers to its internal static bypass before the external bypass closes. The safe sequence is: (1) UPS to internal static bypass, (2) Close the external bypass, (3) Open the UPS output and finally (4) Open the UPS input. To return to UPS operation, reverse these steps. Rotary bypass switches with ‘UPS / Bypass / Test’ positions build this interlocking into the switch itself.