Principle of work:
　 In the chart N is the commonly used power source, R is the emergency power supply, when the commonly used supply voltage is normal, the switch work when automatically is keeping off, regardless of Qr is at "gathers" or "the minute" the condition, all first completes Qr minute Qn to gather the procedure, guaranteed the commonly used power source puts through the load. When the commonly used power source appears the breakdown, in the time delay scheduled time the load from the commonly used power source cut to the emergency power supply, and restores normally when the commonly used power source, the load returns to the commonly used power source.