# Insulation monitoring relay

Canonical URL: https://eloragrid.ai/symbols/insulation-monitoring-relay
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: IMD
Drawing use: Schematic, SLD
Reference families: IEC, Manufacturer

Relay that continuously checks insulation resistance of an unearthed (IT) system to earth and alarms on a first fault.

## In plain English

### What does Insulation monitoring relay mean?

An insulation monitoring relay is connected between the conductors of an unearthed (IT) system and earth, and injects a measuring signal to determine insulation resistance. It warns when insulation falls below its setting so a first fault can be located before a second fault causes a trip.

### A simple example

The symbol shows the device and its connections but not the measuring method, which matters on systems with drives or large DC components. Only one insulation monitor should normally be active on a galvanically connected system, so the drawing should show any disconnection or interlock between parallel systems.

### What the drawings help you do

Check that only one monitor is active on each connected IT system. Identify alarm outputs and test button wiring. An insulation monitoring relay measures insulation resistance of an unearthed system; a residual current monitor measures leakage current in an earthed system.

### This particular reference

Relay that continuously checks insulation resistance of an unearthed (IT) system to earth and alarms on a first fault. An insulation monitoring relay measures insulation resistance of an unearthed system; a residual current monitor measures leakage current in an earthed system.

### Terms

- DC: Direct current: electricity flowing in one direction.
- Coil: A wound conductor that can produce a magnetic field when energised.
- Terminal: An identified point where a conductor connects.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [AutoCAD Electrical: Miscellaneous monitoring relays](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-31F4CF79-F8E0-4E5A-8C51-4D71640ACDB8.htm)
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/)


## What it represents

An insulation monitoring relay is connected between the conductors of an unearthed (IT) system and earth, and injects a measuring signal to determine insulation resistance. It warns when insulation falls below its setting so a first fault can be located before a second fault causes a trip.
The symbol shows the device and its connections but not the measuring method, which matters on systems with drives or large DC components. Only one insulation monitor should normally be active on a galvanically connected system, so the drawing should show any disconnection or interlock between parallel systems.

## How to read it

- Check that only one monitor is active on each connected IT system.
- Identify alarm outputs and test button wiring.

## Information to show or reference

- Show system and earth connections
- State alarm thresholds
- Reference interlock between parallel monitors

## Important distinction

An insulation monitoring relay measures insulation resistance of an unearthed system; a residual current monitor measures leakage current in an earthed system.

## Facts

- Measured quantity: Insulation resistance to earth
- Publisher reference token: HCR1F44 (Autodesk identifier; not an IEC database ID)
- Preview: Independent educational concept; consult the publisher and applicable standard for the exact graphic

## Public references

- [AutoCAD Electrical: Miscellaneous monitoring relays](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-31F4CF79-F8E0-4E5A-8C51-4D71640ACDB8.htm) — Autodesk. Public AutoCAD Electrical relay listing that names the Insulating Relay block; the illustration is independently drawn.
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/) — IEC. IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.