# Frequency protection

Canonical URL: https://eloragrid.ai/protection-functions/81-frequency
Publisher: Elora Grid
Updated: 2026-10-13
Lookup label: 81
Drawing use: SLD, Schematic
Reference families: IEEE / ANSI

A function responding to a documented frequency condition.

## What it represents

Frequency protection responds to frequency conditions according to the selected implementation. Underfrequency, overfrequency and rate-of-change functions can be distinguished by additional suffixes and device documentation. A generic 81 reference does not determine which variant is applied.
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

- Read the complete designation, not only the base number.
- Identify the voltage or other input from which the selected device derives its frequency measurement.

## Information to show or reference

- State the applied frequency function.
- Reference the measurement and relay.
- Link any trip, alarm or load-control action.

## Important distinction

Underfrequency, overfrequency and rate-of-change are different functions even when their designations share the number 81.

## Facts

- ANSI / IEEE function number: 81
- 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.