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Why Use An External Maintenance Bypass Switch With A UPS?

An external maintenance bypass switch enables a UPS system to be electrically isolated – taken out of the critical power circuit – for safe UPS maintenance or unit replacement.

It enables service work to be undertaken without disrupting the critical load. Without an external bypass switch, such as Riello UPS's Multi Pass, to allow for preventive maintenance opportunities, the only safe alternative is to power down the entire critical load, not just the UPS system.  

External maintenance bypasses tend to be circuit breaker or rotary switch-based. It will use a manual switching method to transfer the load between UPS output and the mains power supply. This can comprise of a single transfer switch (three or four pole) or an array of isolators.

An external bypass can often be wall or rack-mounted – known as a Wrap-Around Bypass, these are generally installed in a separate cabinet or panel next to the UPS system. 

If there are concerns over unauthorised use, external bypasses can include a key switch. It can also incorporate mechanical interlocks, known as Castell, to make sure the bypass operates in the correct sequence and doesn’t damage the UPS or the load.

External maintenance bypass shouldn’t be confused with the internal maintenance bypass built into some UPS, which only allows for safe maintenance work in isolated parts of the UPS – there may still be other “live” parts such as the busbar connections.  

In addition, external maintenance bypass shouldn’t be mistaken for the static bypass switch in online UPS, which is a safety function in case of fault or failure with the uninterruptible power supply itself. Note that some external maintenance bypasses do incorporate an automatic transfer function which acts in the same way as a static bypass within a UPS.  

External maintenance bypasses tend to be circuit breaker or rotary switch-based.  



Benefits Of Using An External Maintenance Bypass 

  • Capability for total isolation that allows for UPS maintenance with no disturbance to the load 
  • Safe and user-friendly with a simple switching sequence that doesn’t risk back-feeding the UPS 
  • Electrical interlocking ensures no-break power transfer without the need for complex and expensive key interlock arrangements 
  • Eliminates the need for additional maintenance switches or key exchange boxes, which can reduce costs 
  • Is a standard system using readily available switchgear 
  • Versatile as can be built to include full MCB (Miniature Circuit Breaker) or MCCB (Moulded Case Circuit Breaker) protection or just isolation, depending on site requirements 
  • Easily customisable to incorporate larger terminals for oversized cables 
  • Option to incorporate shut trip facilities for Emergency Power Off (EPO) on input and bypass switches 
  • Can be installed outside normal hours, which enables the UPS to be installed during normal hours without the need for further shutdown. 


Learn the differences between centralised and decentralised UPS bypass systems in this ‘Tech Talk’ video guide: 



Further reading: