There are circumstances under which even the best of UPSs are unable to provide full on-line protection. Even though modern modular UPSs can achieve as much as 99.9999% availability, the chance of a UPS failure does still exist.
This is where static switches are invaluable. A static switch is used to transfer the load between the UPS inverter output and a bypass supply. This article by UPS Ltd explains in more detail how static switches contribute to power continuity.
The static switch – typically using semiconductor silicon controlled rectifiers (SCRs) rather than mechanical switching elements – is an intelligent device that decides whether to use the UPS inverter output or the raw mains to supply the load. This decision is made by logic within the switch and/or the UPS’s control system, which continuously monitors the bypass and inverter voltages.
Thus, static switches are essential for ensuring no-break transfers between inverter and bypass, and for ensuring personnel safety during on-line UPS maintenance and repairs.