# Time negative-sequence overcurrent

Canonical URL: https://eloragrid.ai/protection-functions/51q-time-negative-sequence-overcurrent
Publisher: Elora Grid
Updated: 2026-10-13
Lookup label: 51Q
Drawing use: SLD, Schematic
Reference families: IEEE / ANSI, Manufacturer

Supervises negative-sequence current using a time-dependent operating criterion.

## What it represents

Supervises negative-sequence current using a time-dependent operating criterion.
The drafting record should connect this function to the three phase CT measurements used to derive negative-sequence current, then trace the unbalance-sensitive timed trip and coordination reference. 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 ETR family). 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 three phase CT measurements used to derive negative-sequence current.
- Trace the unbalance-sensitive timed trip and coordination reference.

## Information to show or reference

- Label the three phase CT measurements used to derive negative-sequence current.
- Cross-reference the unbalance-sensitive timed trip and coordination reference.
- Retain the complete suffix, selected device tag and manufacturer manual revision.

## Important distinction

51Q is a negative-sequence time-overcurrent element; generator thermal unbalance protection may use a separate machine-damage model.

## Facts

- Function designation: 51Q
- Base function: 51 — Time overcurrent
- Published mapping: Eaton ETR family
- Suffix scope: Application or manufacturer specific; check the complete drawing legend and selected device manual.

## Public references

- [ETR-4000 and ETR-5000 transformer protection function tables](https://www.eaton.com/us/en-us/catalog/electrical-circuit-protection/transformer-relays.html) — 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.