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Why Install A Bypass Isolation Transformer In A UPS System?

Isolation transformers transfer electricity from a source of alternating current (AC) to equipment or devices while isolating these devices from the power source, typically for safety reasons.  

This galvanic isolation protects against electric shock, suppresses electrical noise to sensitive devices, and enables power transfer between two circuits that must not be connected. 

There are several reasons why isolation transformers are used: 

  • Keep the load from generating harmful harmonics 
  • Keep harmonics on the distribution bus from continuing downstream to sensitive loads 
  • The output of the UPS may be at a different voltage to the load, so the transformer acts as a step-up/down voltage converter 


If the isolation transformer is located between the distribution bus and the UPS system, anything generated by the UPS or load is isolated from the bus, while anything from the bus is isolated from the load, even in bypass mode. 

For example, when a generator is installed, it is common to use four pole changeover switchgear or contactors when transferring from mains to the generator. This results in the traditional neutral-earth reference being lost during transition, which can cause phase voltages to rise and damage sensitive single-phase loads. 

Adding a bypass isolation transformer allows an electrical contractor to earth the UPS output neutral, eliminating this problem.  

Single-phase bypass transformers should be installed on smaller systems where the UPS output neutral needs to be earthed.  

For a transformerless UPS, two isolation transformers are required on the input supplies to provide complete neutral separation, including the bypass supply.


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