Undervoltage measuring relays detect when the supply voltage falls below the limit and pass this on to higher-level controls or signaling devices. They are used to switch off electrical consumers when the voltage is too low. The devices in this series report undervoltages with a fixed hysteresis. An LED on the front signals the current status. The changeover contact allows flexible use. The RUR 1 has a fixed switching threshold. Properties: Switching threshold RUR 1: 0.85 x Un, Switching threshold RUR 3: adjustable from 160 V to 240 V, no external supply voltage required, potential-free changeover contact, module width only 1 pitch unit, protection class IP 20, mounting type: quick attachment to mounting rail, installation position as desired, areas of application: automatic control of emergency power generators and emergency lighting in all types of buildings and industrial plants. Notes: Single-phase networks can be monitored by connecting all measuring inputs to an outer conductor. If electrical consumers generate reverse voltages that are greater than the threshold value Us. the detection of a phase conductor failure is no longer possible.