What Does An Automatic Transfer Switch (ATS) Do In A UPS System?

Automatic Transfer Switches (ATS) are typically used to provide resilience for smaller uninterruptible power supplies (below 10 kVA) that can’t operate in parallel.  

The ATS has two AC input power sources (‘A’ and ‘B’) and if/when one fails the loads are automatically and instantaneously transferred to the other. 

Automatic Transfer Switches like Riello UPS's Multi Switch ATS can also be used instead of Power Distribution Units (PDUs) for plug and play loads. In addition, ATSs protect against short-circuits and offer the ability to switch output power connections on and off remotely over a network. 



The ‘A’ and ‘B’ power sources feeding an Automatic Transfer Switch can be configured in a variety of combinations: 

  • ‘A’ supplied from the output of the UPS and ‘B’ from the mains supply 
  • Both ‘A’ and ‘B’ supplied from two separate UPS outputs 
  • Both ‘A’ and ‘B’ supplied from two separate mains supplies. 

The above example highlights an ATS with two input supplies and up to eight outputs. 


Further Reading: