When the normal power is abnormal,the spare power work at once,which therefore ensures the continuirty,reliability,and of power supply. When the normal power supply is normal,it will automatically switches back from the backups power to the normal power supply. Specially designs for household Din Rails installation.Convenient in instabllation and maintenance. With two station designs (commonly used for A and standbys for B),suitable for systems with 50-60hz and rate current 63A. The Switches is mainly used for testing whether normal or spare power is normal or not.It can Switches power supply in 20ms to build uninterruptible power supply.