When transferring between two live sources, such as when normal power has returned or during generator set test/exercise periods, a system designer can select different types of transfer switches to minimize or even eliminate the electrical disturbance to critical loads.
The obvious basis for selecting a transfer switch is reliability, but in some cases, the choice is simply based on convenience. In general, the transfer switch that minimizes the disturbance that can occur when switching between sources will be more expensive. Not only does it cost more to completely eliminate the switching disturbance, the added complexity can reduce the reliability of the transfer device and even the generator equipment.