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The GM420 series loop monitor is designed to monitor the resistance of PE conductor connections in AC systems and in de-energised systems. The extraneous voltage Uf between the terminals E and KE must not exceed AC 12 V. The ohmic resistance of the conductor loop and the AC extraneous voltage Uf, if existing, will be indicated on the display. The currently measured value is continuously indicated on the LC display. If the measured resistance value increases above the set response value, the alarm will be activated and stored. Adjustable time delays allow installation-specific requirements to be considered.


  • Loop monitoring of the protective conductor in AC systems
  • Measuring circuit with high resistance to external voltage and display of the external voltage
  • Adjustable start-up, response and off-delay
  • Adjustable switching hysteresis
  • Digital measured value display via multi-functional LCD display
  • Preset function (automatic basic parameterization)
  • Indicator LEDs for operation, Alarm 1, Alarm 2
  • Measured value memory for trigger value
  • Permanent self-monitoring
  • Internal test/reset button
  • 2 separate alarm relays 1 changeover contact each
  • Quiescent/working current and fault memory behavior selectable
  • Password protection for device setting
  • Seal-able transparent cover
  • 2-module housing (36 mm)
  • Spring terminal (two terminals per connection)
  • RoHS compliant


  • Loop monitoring for motors
  • Loop monitoring in electrical systems for PE conductor interruption
  • Monitoring of earthing systems

Downloads for LINETRAXX® GM420

CE Declarations of Conformity GM42x 87.0 KB 2022/08/12 06:17:2412.08.2022 06:17:24
Manuals GM420 1.6 MB 2022/11/23 07:28:2723.11.2022 07:28:27D00112
Datasheets GM420 273.6 KB EN2019/05/13 10:31:3513.05.2019 10:31:35D00112
Technical Information Changed variants of relays from goldplated to silver-plated contacts 166.3 KB EN2022/06/27 09:03:0827.06.2022 09:03:08

Optional Accessories for LINETRAXX® GM420

Mounting clip for screw mounting (1 piece per device, accessories)B98060008

LINETRAXX® GM420 Variants

TypeMeasuring range
loop resistance
measuring range
interference voltage
supply voltage USArt. no. *
GM420-D-10...100 ΩAC 0...50 VDC 9.6 V...94 V /
AC 16...72 V, 15...460 Hz
GM420-D-20...100 ΩAC 0...50 VDC 70...300 V /
AC 70...300 V, 15...460 Hz

*(B7... = spring terminal / B9... = screw terminal)