# Percentage-biased differential current relay

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

Differential relay form whose operating threshold rises in proportion to the through current, set as a relative value.

## In plain English

### What does Percentage-biased differential current relay mean?

This relay form represents differential protection where the operate quantity is the difference between currents entering and leaving a zone, and the threshold is a percentage of the through current. The bias keeps the relay stable when CT errors grow during heavy external faults or transformer tap changes.

### A simple example

On a drawing the relay box is fed from CT sets at each boundary of the protected zone. The symbol does not show the slope settings, the harmonic restraint used for transformer inrush, or the vector-group and ratio compensation, all of which must be taken from the relay settings record.

### What the drawings help you do

Locate every CT set that bounds the protected zone. Check CT polarity so through current cancels in the relay. A biased differential relay raises its threshold with through current; a plain differential current relay uses a fixed threshold.

### This particular reference

Differential relay form whose operating threshold rises in proportion to the through current, set as a relative value. A biased differential relay raises its threshold with through current; a plain differential current relay uses a fixed threshold.

### Terms

- Winding: A coil of conductor inside an electrical machine or transformer.
- Coil: A wound conductor that can produce a magnetic field when energised.
- Rating: A value stating the conditions for which equipment is specified.

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: Current monitoring relays](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-E0DDA638-4664-4F1E-B191-29A4C40C4A0A.htm)
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/)


## What it represents

This relay form represents differential protection where the operate quantity is the difference between currents entering and leaving a zone, and the threshold is a percentage of the through current. The bias keeps the relay stable when CT errors grow during heavy external faults or transformer tap changes.
On a drawing the relay box is fed from CT sets at each boundary of the protected zone. The symbol does not show the slope settings, the harmonic restraint used for transformer inrush, or the vector-group and ratio compensation, all of which must be taken from the relay settings record.

## How to read it

- Locate every CT set that bounds the protected zone.
- Check CT polarity so through current cancels in the relay.

## Information to show or reference

- Show all zone CTs
- Mark CT polarity
- Reference slope and restraint settings

## Important distinction

A biased differential relay raises its threshold with through current; a plain differential current relay uses a fixed threshold.

## Facts

- Measured quantity: Difference current relative to through current
- Publisher reference token: HCR1F30 (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: Current monitoring relays](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-E0DDA638-4664-4F1E-B191-29A4C40C4A0A.htm) — Autodesk. Public AutoCAD Electrical relay listing that names the Differential Current Protection - Relative Value 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.