# Overfrequency protection

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

Supervises frequency above the defined operating criterion.

## What it represents

Supervises frequency above the defined operating criterion.
The drafting record should connect this function to the frequency measurement source and enabled overfrequency stage, then trace the generator or interconnection overfrequency output. Keep the complete designation and equipment tag together so that the SLD, relay logic and terminal schedule describe the same implementation.
The cited mapping is a public manufacturer example (Eaton E-Series function mapping). Suffixes are not universal equivalences across products, and this reference supplies neither a protection setting nor certification of the selected scheme.

## How to read it

- Identify the frequency measurement source and enabled overfrequency stage.
- Trace the generator or interconnection overfrequency output.

## Information to show or reference

- Label the frequency measurement source and enabled overfrequency stage.
- Cross-reference the generator or interconnection overfrequency output.
- Retain the complete suffix, selected device tag and manufacturer manual revision.

## Important distinction

The suffix O is the letter O for overfrequency; use the complete label to avoid confusion with a zero in search or drawing text.

## Facts

- Function designation: 81O
- Base function: 81 — Frequency protection
- Published mapping: Eaton E-Series function mapping
- Suffix scope: Application or manufacturer specific; check the complete drawing legend and selected device manual.

## Public references

- [E-Series protective relays: public family comparison brochure](https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/protective-relays-and-predictive-devices/e-series-protective-relays-br026001en.pdf) — Eaton. Public primary-source function or application mapping used to identify this designation; wording and drafting notes are original.
- [IEEE C37.2-2022: power-system device function numbers](https://ieeexplore.ieee.org/document/9984140) — IEEE. Device-number and suffix convention. This page is not a reproduction of the full licensed standard.

## 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.