A transfer switch is a mechanism that switches a load between two sources like a backup generator and the electric utility. Some transfer switches are manual – you have to physically flip a switch to initiate the transfer. Others are automatic and switch when they sense one of the sources has lost or gained power.

As well as transferring the load to the backup generator, an automatic transfer switch (ATS) may also command the backup generator to start, based on the voltage monitored on the primary supply. The transfer switch ensures that power from the generator is isolated while it’s running.