The RPZ-1 relay is meant for use in control and protection circuits. Their task is to automatically switch the powered circuit for backup power supply in case of loss of main power supply. After reapplication of the main supply voltage a return switch is made. The RPZ-1 relay has two supply inputs (main and backup) and one output. Additionally an auxiliary change-over relay, informing about actual supplying source is built into the relay with terminals lead out to the plug.