Transfer Switch

Transfer switches allow switching from a primary power source to a secondary or tertiary power source and are employed in some electrical power distribution systems. Most often transfer switches can be seen where emergency power generators are used to back up power from the utility source. The transfer switch allows safely switching from utility power to emergency generator power while maintaining isolation of each source from the other. The switch may be either a manual switch, an automatic switch, or a combination of manual and automatic. In a home during a power outage, for example, the transfer switch allows isolation of the owner's critical circuits (e.g. cooling, refrigerator, lighting) from the utility service, allowing for operation of the generator without backfeeding to the utility, which can damage utility equipment and hurt (or kill) utility workers. 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.