Automatic Transfer Switch (ATS) is often installed where a backup generator is located, so that the generator may provide temporary electrical power if the utility source fails. An ATS not only safely switches-in the backup generator as a temporary source of electric power; it also commands the backup generator to start, based on certain conditions it continuously monitors on the electric utility feed; and it isolates the backup generator from the electric utility, when the generator is on and is providing temporary power. The control capability of a transfer switch may be manual only, or a combination of automatic and manual. The switch transition mode (see below) of a transfer switch may be Open Transition (OT) (the usual type), or Closed Transition (CT).
For example, in a home equipped with a backup generator and the usual type of ATS designed for use with a backup generator, if an electric utility outage occurs, the ATS will: a) command the backup generator to start; b) once the ATS sees that the generator is ready to provide electric power (many residential backup generator sets, powered by either natural gas or diesel fuel, are ready in less than 10 seconds), the ATS breaks the home's connection to the electric utility and connects the generator to the utility side of the home's main electrical panel. At this point, the generator supplies power to the home's electric load (lights, air conditioner, etc.), and the generator is not connected to the electric utility. It's important that the backup generator's output not connect to the electric utility (this condition is called backfeeding), because if the home's generator and transfer switch are not specifically designed for this mode of operation, the generator or the utility's equipment may be damaged; and utility personnel or the home's residents may be injured or even killed.
Automatic transfer switches continually monitor the incoming utility power. Any anomalies such as voltage sags, brownouts, spikes, or surges will cause the internal circuitry to command a generator to start and will then transfer to the generator when additional switch circuitry determines the generator has the proper voltage and frequency. When utility power returns or no anomalies have occurred for a set time, the transfer switch will then transfer back to utility power and command the generator to turn off, after another specified amount of "cool down" time with no load on the generator.
A transfer switch can be set up to provide power to only critical circuits or entire electrical (sub)panels. Some transfer switches allow for load shedding or prioritization of optional circuits, such as heating and cooling equipment.
