Undervoltage monitor in three-phase mains (each phase against neutral) with fixed threshold value, fixed hysteresis and integrated testing key. It has been developed especially for emergency lighting to DIN VDE 0108. The device can also be used for monitoring an individual phase. All unoccupied inputs have to be connected to the connected phase. If there is an inverse voltage due to the consumer, which exceeds the adjusted threshold value, there is not any fault message. OK message: Relay is activated (contacts 11-14 and 21-24 closed), LED is off.Fault message: Relay is deactivated (contacts 11-14 and 21-24 open), LED is on.Key pressed: Relay is being deactivated (contacts 11-14 and 21-24 open), LED lights up.