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What Does A Bypass Switch In A UPS System Do?

A static bypass switch automatically and instantaneously transfers the load to the mains electricity supply when there’s an internal fault or failure with the UPS system.  

It is a common characteristic of an Online UPS. In effect, running on bypass the circumvents the UPS (i.e. rectifier, batteries, and inverter) and ensures power continuity while the UPS is fixed or swapped out.

This is known as a ‘safe failure to mains’ and occurs for various reasons, such as a fault within the UPS or an overload (short-circuit). Once the fault is cleared, the bypass automatically transfers the load back to the UPS.

However, operating on bypass does not filter or condition the input or output supply as is the case with typical double conversion online UPS systems, so should be used sparingly.  

Internal static bypass switches should not be confused with an external maintenance bypass switch, which is an option that enables the UPS to be powered down manually for maintenance while the critical load is powered directly from the mains. 

External maintenance bypass switches tend to be mechanical in the form of a wraparound or rotary switch or set of circuit breakers.  

Line Interactive and Offline UPS systems do not typically incorporate an internal automatic bypass. An alternative solution is to install an external maintenance bypass with an automatic transfer feature that automatically transfers the load to the mains if there’s a failure with the UPS.


Examples Of Static And Maintenance Bypass In A UPS System



Learn more about bypass switches in UPS systems with this ‘Tech Talk’ with our Technical Services Manager Jason Yates: 



Further reading: