# Differential protection

Canonical URL: https://eloragrid.ai/protection-functions/87-differential
Publisher: Elora Grid
Updated: 2026-10-13
Lookup label: 87
Drawing use: SLD, Schematic
Reference families: IEEE / ANSI

A function comparing documented electrical quantities across a protection zone.

## What it represents

Differential protection compares quantities associated with the boundaries of a protection zone according to the selected implementation. Equipment and application suffixes can distinguish transformer, busbar, line or other arrangements. The base number alone does not define the zone or measurements.
On a power drawing, keep this function reference connected to the selected equipment and its complete designation. A multifunction relay can implement several numbered functions. This entry explains the reference; it supplies no settings or commissioning procedure.

## How to read it

- Identify the protected zone and each measurement boundary.
- Read CT associations, polarity and any device-specific compensation from the protection documentation.

## Information to show or reference

- Identify the protected equipment or zone.
- Reference measurement boundaries.
- Link the relay application and trip destinations.

## Important distinction

Transformer differential, busbar differential and other differential applications are not interchangeable merely because they share the base number 87.

## Facts

- ANSI / IEEE function number: 87
- Equipment implementation: Consult the selected device manual
- Suffix interpretation: Application and manufacturer documentation

## Public references

- [Public reference table of ANSI device-function numbers](https://www.se.com/es/es/faqs/FA34639/) — Schneider Electric. Public manufacturer table used to check the base function-number name.
- [IEEE C37.2-2022: power-system device function numbers](https://ieeexplore.ieee.org/document/9984140) — IEEE. Normative publication reference for the device-function convention.
- [Relion 620: functions, codes and symbols](https://techdoc.relays.protection-control.abb/r/620-series-IEC-60870-5-103-Point-List-Manual/2.0-FP1-IEC/en-US/Functions-codes-and-symbols) — ABB. Public examples of manufacturer-specific function names and suffixes.

## 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.