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  • Electrical safety for unearthed power supplies (IT systems)
  • Insulation monitoring equipment, insulation fault location systems
  • Power supplies for medical applications
  • Measuring and monitoring relays
  • Electrical safety for earthed power supplies (TN systems)
  • Using renewable energies efficiently: wind power, photovoltaics
  • Test systems for electrical equipment and medical technology


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 PE conductor in AC systems
  • Measuring circuit providing a high resistance against extraneous voltages and indication of extraneous voltages
  • Adjustable start-up delay, response delay and delay on release
  • Adjustable switching hysteresis
  • 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 with 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


  • Loop monitoring of motors
  • Loop monitoring of PE conductor connections for wire interruptions in electrical installations
  • Monitoring of earthing systems

Downloads for LINETRAXX® GM420

Manuals GM420 1.7 MB DE13.05.2019 10:31:41D00112
Manuals GM420 1.7 MB EN13.05.2019 10:31:49D00112
Datasheets GM420 319.1 KB DE13.05.2019 10:31:34D00112
Datasheets GM420 273.6 KB EN13.05.2019 10:31:35D00112
Datasheets GM420 275.5 KB ES13.05.2019 10:31:37D00112
Datasheets GM420 509.9 KB ZH13.05.2019 10:31:33D00112
Technical information Changed variants of relays from goldplated to silver-plated contacts 166.3 KB EN27.06.2022 09:03:08
Technical information Geänderte Relaisvarianten von vergoldeten Kontakten auf versilberte Kontakte 166.3 KB DE27.06.2022 09:03:08

Optional accessories for LINETRAXX® GM420

TypeArt. No.
Mounting clip for screw fixing (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… = push-wire terminal / B9… = screw-type terminal)