What a bypass does
A bypass is an alternative power path that feeds the load directly from the mains, bypassing the UPS inverter. It exists so that the load can keep running while the UPS is serviced, or so that the UPS can offload a fault or overload it cannot handle itself. Without a bypass, maintaining a UPS means switching off the equipment it protects.
Static (automatic) bypass
Built into online UPS, the static bypass uses thyristors to switch the load to bypass in milliseconds if the inverter faults, overloads beyond its capacity, or overheats. The transfer is automatic and seamless, so the load stays up even if the UPS itself has a problem. It is a key reliability feature of every online UPS we supply.
Maintenance (manual) bypass
A maintenance bypass is an external switch — often a make-before-break rotary switch — that lets an engineer isolate the UPS completely for service while the load runs on raw mains. For single UPS installations it is usually a wall-mounted external bypass panel; for parallel systems it is built into the switchgear. We always recommend a maintenance bypass on any critical installation.
Designing bypass in
When specifying a UPS, plan the bypass at the same time: a maintenance bypass for every critical system, and proper coordination so the bypass and UPS ratings match. For parallel N+1 systems, a centralised bypass cabinet simplifies the switchgear and allows modules to be added or serviced without dropping the load.
Frequently asked questions
What is the difference between static and maintenance bypass?
Static bypass is automatic and internal, switching the load to mains in milliseconds on a fault. Maintenance bypass is a manual external switch used to isolate the UPS for servicing while the load stays up.
Does every UPS have a bypass?
Online UPS have an internal static bypass. An external maintenance bypass should be specified separately for any critical installation so the UPS can be serviced safely.
Is bypass power clean?
No. On bypass the load runs on raw mains, so power quality is lower. That is why bypass is only used briefly for faults or maintenance, not for normal operation.
What is a make-before-break bypass?
A bypass switch that connects the bypass path before disconnecting the UPS path, so the load never loses power during transfer.