# Phase-angle or out-of-step protection

Canonical URL: https://eloragrid.ai/protection-functions/78-phase-angle-or-out-of-step-protection
Publisher: Elora Grid
Updated: 2026-10-13
Lookup label: 78
Drawing use: SLD, Schematic
Reference families: IEEE / ANSI

Supervises the angular relationship or loss of synchronism of an electrical system or machine.

## What it represents

Supervises the angular relationship or loss of synchronism of an electrical system or machine.
In a drafting package, connect this reference to the voltage/current measurement references and protected machine. The associated drawing should trace the out-of-step trip, separation or blocking outputs; the number identifies the role and does not specify the terminal arrangement, ratings or operating settings.

## How to read it

- Identify the voltage/current measurement references and protected machine.
- Trace the out-of-step trip, separation or blocking outputs.

## Information to show or reference

- Label the voltage/current measurement references and protected machine.
- Cross-reference the out-of-step trip, separation or blocking outputs.
- Identify the device tag, manual revision and logic drawing that define this implementation.

## Important distinction

Out-of-step protection during operation is distinct from a 25 closing-permission synchronism check.

## Facts

- ANSI / IEEE function number: 78
- Drawing context: The voltage/current measurement references and protected machine
- Application scope: Protection or apparatus-control role; exact implementation is defined by the equipment documentation.

## Public references

- [ANSI device-number and suffix reference table](https://selinc.com/pt/products/tables/ansi/) — Schweitzer Engineering Laboratories. Public manufacturer number/name association. Drafting explanations are independently written.
- [IEEE C37.2-2022: power-system device function numbers](https://ieeexplore.ieee.org/document/9984140) — IEEE. Normative device-number publication; exact edition wording and compliance require the 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.