# Minimum-impedance measuring relay

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

Measuring relay that compares voltage with current and operates when the apparent impedance falls below a set value.

## In plain English

### What does Minimum-impedance measuring relay mean?

A minimum-impedance relay divides measured voltage by measured current and operates when the result falls below its setting, indicating a fault close enough to fall inside its reach. It is used as generator and transformer backup protection and is the basic idea behind distance protection.

### A simple example

The relay needs both CT and VT inputs, so the symbol should always be read with both connections. It does not show the characteristic shape on the impedance plane, any directional element, or the loss-of-voltage blocking needed to avoid operation when the VT supply fails.

### What the drawings help you do

Confirm both voltage and current inputs are shown. Check for VT fuse-failure blocking. A minimum-impedance relay uses voltage and current together; an overcurrent relay uses current alone.

### This particular reference

Measuring relay that compares voltage with current and operates when the apparent impedance falls below a set value. A minimum-impedance relay uses voltage and current together; an overcurrent relay uses current alone.

### Terms

- Winding: A coil of conductor inside an electrical machine or transformer.
- 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

A minimum-impedance relay divides measured voltage by measured current and operates when the result falls below its setting, indicating a fault close enough to fall inside its reach. It is used as generator and transformer backup protection and is the basic idea behind distance protection.
The relay needs both CT and VT inputs, so the symbol should always be read with both connections. It does not show the characteristic shape on the impedance plane, any directional element, or the loss-of-voltage blocking needed to avoid operation when the VT supply fails.

## How to read it

- Confirm both voltage and current inputs are shown.
- Check for VT fuse-failure blocking.

## Information to show or reference

- Show CT and VT inputs
- State reach and delay
- Reference blocking logic

## Important distinction

A minimum-impedance relay uses voltage and current together; an overcurrent relay uses current alone.

## Facts

- Measured quantity: Apparent impedance from voltage and current
- Publisher reference token: HCR1F41 (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 Minimum Impedance 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.