Automatic transfer switches work by switching the load between the power grid and the generator. When these switches sense the available power drop from the power grid, they switch the load to the generator and signal the generator to start. When the power returns to the grid, they switch the circuits back to the grid, and after the generator cools during the no load cycle, they automatically shut down its engine.
If the owner has the backup generator hardwired into the home, building codes require a transfer switch to isolate workers trying to restore the lines from electricity from the generator. Codes also require switches to bear safety approvals, although there are some reported problems with counterfeit switches.
The automatic transfer switch monitors both power sources at all times. It uses preset values to determine when to start and stop the generator to provide a transfer that is as seamless as possible. The time delay feature lasts 1 second, although transfer switches are available with delays ranging from 0 to 6 seconds. The purpose of the delay is to provide time for the system in meeting electrical codes.
Automatic transfer switches require regular testing to ensure they work properly in case of a power failure. Automatic systems have a manual control switch that simulates a power failure to ensure the generator is ready for use.