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  • Undervoltage, overvoltage and frequency monitoring in 3(N)AC systems 0…500 V
  • Integrated energy backup
  • Asymmetry, phase failure and phase sequence monitoring
  • Adjustable start-up delay, response delay and delay on release
  • Various monitoring functions selectable < U, > U, < f, > f

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. Due to adjustable response times, installation-specific characteristics, such as device-specific start-up procedures, short-time voltage fluctuations, etc. can be considered. The devices are supplied by the system being monitored and utilize an internal energy backup.


  • 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, > U, < f, > 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


  • 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

Downloads for LINETRAXX® VMD421H

CE Declarations of Conformity VMx42x 93.7 KB 2022/11/04 09:31:2004.11.2022 09:31:20
Manuals VMD421H 1.9 MB EN2021/06/21 05:34:2121.06.2021 05:34:21D00138
Datasheets VMD421H 363.3 KB EN2019/05/13 10:31:3613.05.2019 10:31:36D00138
Quickstarts VMD421H 940.6 KB EN2021/06/16 06:59:1916.06.2021 06:59:19D00138

Optional Accessories for LINETRAXX® VMD421H

TypeArt No.
Mounting clip for screw fixing (1 piece per device)B98060008


TypeNominal system voltage Un*Response valueArt. No.**
VMD421H-D-33(N)AC 70…500 V, 15…460 HzAC 70…500 VB73010007

* absolute values
**(B7… = push-wire terminal / B9… = screw-type terminal)