Automatic Transfer Switches
The purpose of an ATS is to automatically switch the power source feeding a building's electrical system from the main utility line to a backup generator when there is a power outage.
It monitors the incoming voltage from the utility line. If that voltage drops below a set threshold indicating a power loss, the ATS will start the generator and allow it to stabilize its voltage and frequency output.
Once the generator is providing steady power at the right voltage and frequency, the ATS will switch the building's circuits from being fed from the main utility line over to being fed from the generator. It transfers the loads safely and quickly in order to prevent any significant power disruption in the building.
The ATS will continue providing electricity from the generator as long as necessary during an outage. When the ATS senses the main utility line voltage has returned and stabilized at the normal level, it will transfer the electrical loads back to the main line and shut down the generator.
Sometimes these units are known as automated transfer switches or Automated Mains Failures (AMF)