Automatic Transfer Switch (ATS)

An automatic transfer switch (ATS) is a device that automatically switches a data center's electrical load to a backup source. It acts when it senses loss or degradation of the primary utility feed, typically switching to a diesel generator plant without operator intervention. Conventional open-transition ATS units complete the switch in roughly 3-6 seconds, well inside the 10-second threshold set by NFPA 110 for emergency power systems; UPS batteries bridge the gap during that window and for the additional time generators need to start and stabilize, typically another 10-30 seconds. Closed-transition ATS designs used on some data center campuses can complete a make-before-break transfer in under 100 milliseconds, avoiding any interruption to the load. ATS placement and redundancy (single, dual, or paired with static transfer switches downstream) is a core design decision in 2N and distributed-redundant electrical topologies and a frequent focus of Uptime Tier commissioning tests.