MU150 and MU250 are voltage relays that combine various protections and starting delay. MU150 is used for single-phase monitoring while MU250 is used for 3-phase monitoring. Microprocessor based numerical relay Undervoltage Overvoltage Delay-on 2 voltage-free output contacts Voltage and frequency display ANSI Code : 27, 47, 59, 60 3-Phase Unbalance Phase loss Phase sequence With or without neutral connection Phase to phase or phase to neutral monitoring