RELETEK RS-MV31M Three Phase Voltage Monitoring Relay

The RELETEK RS-MV31M Three Phase Voltage Monitoring Relay is a compact protection device designed to monitor three-phase electrical supplies and protect motors and other three-phase equipment against phase failure and incorrect phase sequence. By continuously checking the presence and sequence of the three supply phases, the relay can disconnect the controlled circuit when a phase is lost or when the phase rotation is incorrect.

The RS-MV31M is designed for three-phase three-wire systems and monitors the three incoming phases through terminals L1, L2, and L3. It is intended for applications where maintaining the correct three-phase supply is essential for the safe operation of motors, pumps, compressors, fans, and other rotating equipment. RELETEK identifies the RS-MV31M specifically as a phase failure/phase reversal relay within its RS-MV31M–37M three-phase monitoring range.

The relay uses True RMS measurement and a microcontroller-based control system. True RMS measurement allows the monitoring circuit to evaluate the effective value of the AC supply rather than relying on a simple average-based measurement method. The unit continuously monitors the three-phase supply and reacts when the required phase conditions are no longer satisfied.

One of the main functions of the RS-MV31M is phase-failure protection. If one of the three phases is interrupted, the relay detects the abnormal condition and changes its output state. It also checks phase sequence, helping prevent a three-phase motor from operating with an incorrect phase rotation. The manufacturer's documentation specifies a trip time of less than 0.5 seconds for incorrect phase sequence and phase failure.

The RS-MV31M is designed around a nominal three-phase supply of 380V, 400V, or 415V, with an operating voltage range of approximately 266V to 540V. Its output consists of 1 normally open and 1 normally closed contact (1NO + 1NC) with a rated current of 8A at 250V AC-1. The relay output can be used as a control signal for a contactor, allowing the monitoring relay to protect higher-power motors and loads without switching the motor current directly.

For installation, the RS-MV31M uses a compact one-module DIN-rail housing and is designed for TH-35 rail mounting according to EN 60715. LED indicators provide visual information about the supply and relay/output condition, making the unit easier to monitor during commissioning and troubleshooting.

The RS-MV31M is therefore suited to motor control panels and three-phase electrical systems where protection against missing phases and reversed phase sequence is required. It should be distinguished from other members of the same family, such as models that additionally provide overvoltage, undervoltage, or phase-asymmetry monitoring.

Features

• Three-phase voltage monitoring

• Phase failure protection

• Phase sequence / reversal protection

• True RMS measurement

• Microcontroller-based operation

• Designed for 3-phase 3-wire systems

• 1NO + 1NC relay output

• 8A / 250V AC-1 contact rating

• LED indication for supply and output status

• Compact 1-module DIN-rail design

• TH-35 / EN 60715 mounting

Specifications

Specifications are based primarily on RELETEK's RS-MV31M–37M technical documentation.

Applications

• Three-phase motor protection

• Pump control panels

• Industrial automation systems

• Compressor protection

• Fan and ventilation systems

• Motor starter panels

• Three-phase machinery

• Electrical distribution and control panels

• Phase sequence monitoring

• Protection against phase-loss conditions