A power transfer switch is a device designed to power a load from multiple voltage source. In case of home generator applications,it is used to connect building's electrical wiring either to standby generator or to utility power.

When the Alternate Current(AC) line is normal,TS allows powering the load from utility,when AC fails TS connects the load to the generator while isolating it from mains. A transfer switch connects to the circuit in your electrical panel that you need most during a power outage, such as furnace,garage door opener,water heater etc. It also protects the utility workers from getting hurt by dangerous back feeding from unsafe generator installations.

Generator transfer switches can be manual,automatic or combination of both manual and automatic.
An Automatic Transfer switch includes a control circuit that senses main voltage. When utility power is interrupted,it starts up the generator,disconnects the load from the utility and connects it to the generator. When the generator is disconnected then it normally goes into a cool down process and then automatically shuts down.