Automatic transfer switch [/ˌɔtəˈmætɪk ˈtrænsfər ˈswɪʧ/] n - An automatic transfer switch (ATS) keeps continuous watch upon its primary source (the utility, by way of the Where the Grid Comes In) and, when that source fails, transfers the load of itself to a secondary source (the The Standby Engine), transferring back when the primary returns.

It is not to be confounded with a Static transfer switch:

• An ATS switches between a source of generation and is electro-mechanical, with a transfer time of seconds. It is sized for the whole building and works hand-in-glove with the Uninterruptible power supply that spans the transfer gap.
• An STS switches between two already-live UPS feeds in milliseconds, to ward a single dual-corded load.

Modern ATS units can perform a closed-transition (“make-before-break”) transfer when both sources chance to be live, as when returning from generator to utility, paralleling the two briefly so the load never feels an interruption.

The ATS is a classic single point of failure, and so Redundancy topologies oft place independent ATS/generator pairs upon each power path, a requirement of the higher levels of the Uptime Institute tier classification.

Part of The Engines of the Hall.