# Voltage directional relay

Canonical URL: https://eloragrid.ai/protection-functions/91-voltage-directional-relay
Publisher: Elora Grid
Updated: 2026-10-13
Lookup label: 91
Drawing use: SLD, Schematic
Reference families: IEEE / ANSI

Uses a voltage relationship with a defined direction criterion in a specialist protection or control scheme.

## What it represents

Uses a voltage relationship with a defined direction criterion in a specialist protection or control scheme.
In a drafting package, connect this reference to the compared voltage references and polarity convention. The associated drawing should trace the directional-voltage operating circuit; the number identifies the role and does not specify the terminal arrangement, ratings or operating settings.
This designation is useful when reading machine and legacy control drawings. Retain the documented function on an existing drawing and consult the original equipment manual before assigning it to a new design.

## How to read it

- Identify the compared voltage references and polarity convention.
- Trace the directional-voltage operating circuit.

## Information to show or reference

- Label the compared voltage references and polarity convention.
- Cross-reference the directional-voltage operating circuit.
- Identify the device tag, manual revision and logic drawing that define this implementation.

## Important distinction

Directional voltage 91 is not equivalent to directional power 32 or ordinary overvoltage 59.

## Facts

- ANSI / IEEE function number: 91
- Drawing context: The compared voltage references and polarity convention
- Application scope: Specialist or legacy machine-control use; interpret with the equipment legend.

## 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.