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Insulation monitoring device for unearthed
AC/DC control circuits (IT systems)


BENDER earth fault relay IR425


The ISOMETER®s of the IR425 series monitor the insulation resistance of unearthed AC/DC control circuits (IT systems) 0…300 V. DC components existing in AC/DC systems do not influence the operating characteristics. An external supply voltage allows de-energised
systems to be monitored too.


• AC/DC control circuits in the industrial sector, mechanical engineering, power plants, elevators, automation systems etc.

• AC/DC control and auxiliary circuits in accordance with DIN EN 60204-1 “Electrical equipment of machines”, IEC 60204-1, EN 60204-1

• AC/DC auxiliary circuits in accordance with DIN VDE 0100-725 (VDE 0100-725)

• Smaller AC/DC IT systems such as lighting systems


The currently measured insulation resistance is indicated on the LC display. In this way any changes, for example when circuits are connected to the system, can be recognised easily. When the value falls below the preset response values, the response delay “ton” starts. Once the response delay “ton” has elapsed, the alarm relays “K1/K2” switch and the alarm LEDs “AL1/AL2” light up. Two separately adjustable response values/alarm relays allow a distinction to be made between prewarning and alarm. If the insulation resistance exceeds the release value (response value plus hysteresis), the alarm relays return to their initial position. Insulation faults are distinguished according to AC and DC faults (indication ±). If the fault memory is enabled, the alarm relays remain in the alarm state until the reset button is pressed or until the supply voltage is switched off. The device function can be tested using the test button. The parameterisation of the device can be carried out via the LC display or the function keys integrated in the front plate.

Connection monitoring:

The connections to the system (L1/L2) and to earth (E/KE) are either automatically checked every 24 h, or by pressing the test button or when supply voltage has been connected. In case of interruption of a connecting lead, the alarm relay K2 switch, the LEDs ON/AL1/AL2 flash and the following message appears on the display: “E.02” signals a fault in the connecting leads to the system, “E.01” signals a fault in the connecting leads to PE. After eliminating the fault, the alarm relays return to their initial position either automatically or by pressing the reset button.

Preset function:

After connecting the device for the first time, the nominal system voltage is measured and the response values are set automatically.

Measurement method:

The ISOMETER® IR425 uses the AMP measuring principle.


The ISOMETER® of the IR425 complies with the requirements of the device standards: DIN EN 61557-8 (VDE 0413-8), IEC 61557-8, ASTM F 1669M-96 (2007).

Wiring diagram

BENDER earth fault relay IR425 wiring diagram


BENDER earth fault relay IR425 dimensions

Technical data

BENDER earth fault relay IR425 technical data


Ordering information & Accessories

BENDER earth fault relay IR425 ordering information