The multi-functional VMD421 series voltage relay is designed to monitor the frequency, undervoltage and overvoltage and the voltage between two threshold values (window discriminator function) in 3(N)AC systems. The voltages are measured as r.m.s. values. The currently measured value is continuously shown on the LC display. The measured value required to trigger the alarm relay is stored. Thanks to the VMD’s adjustable response times, installation specific characteristics, such as device-specific start-up procedures, short-time voltage fluctuations, etc. can be fully configured for specific applications. The devices are supplied by the system being monitored and utilise an internal energy backup.


  • Monitoring of voltage-sensitive machines and electrical installations
  • Switching machinery and equipment on and off at a certain voltage level
  • Monitoring of stand-by and emergency supply systems
  • Supply voltage monitoring of portable loads
  • Protection of three-phase motors against phase failure and phase open-circuit
  • Transformer protection, asymmetrical load can be recognised

Product Features

  • Undervoltage, overvoltage and frequency monitoring in 3(N)AC systems 70…500/288 V
  • Without external supply voltage
  • Integrated energy backup
  • Asymmetry, phase failure and phase sequence monitoring
  • Various monitoring functions selectable U, f
  • Start-up delay, response delay, delay on release
  • Adjustable switching hysteresis
  • r.m.s. value measurement (AC+DC)
  • Digital measured value display via multi-functional LC display
  • Preset function (automatic setting of basic parameters)
  • LEDs: Power On, Alarm 1, Alarm 2
  • Measured value memory for operating value
  • Continuous self monitoring
  • Internal test/reset button
  • Two separate alarm relays (one changeover contact each)
  • N/C or N/O operation and fault memory behaviour selectable
  • Password protection for device setting
  • Sealable transparent cover
  • Two-module enclosure (36 mm)
  • Push-wire terminal (two terminals per connection)
  • RoHS compliant

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