81
Frequency protection
A function responding to a documented frequency condition.
Protection functions
Protection functions · 81U
Underfrequency protection
Supervises frequency below the defined operating criterion.
SLD / Schematic · 81U
What it represents
Supervises frequency below the defined operating criterion.
The drafting record should connect this function to the voltage-derived frequency source and minimum-voltage supervision, then trace the underfrequency trip or staged load-shedding 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 voltage-derived frequency source and minimum-voltage supervision.
- Trace the underfrequency trip or staged load-shedding output.
Information to show or reference
- Label the voltage-derived frequency source and minimum-voltage supervision.
- Cross-reference the underfrequency trip or staged load-shedding output.
- Retain the complete suffix, selected device tag and manufacturer manual revision.
A distinction that matters
Underfrequency measures frequency magnitude; rate-of-change function 81R evaluates its change over time.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
E-Series protective relays: public family comparison brochure
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
Device-number and suffix convention. This page is not a reproduction of the full licensed standard.
About this graphic
This illustration was authored independently by Elora Grid to explain the concept. It is not reproduced from a customer drawing, an official standard graphic or a manufacturer CAD file. Its correspondence to an exact IEC or IEEE symbol identifier has not been certified.
SVG and DXF downloads contain this original artwork. DXF uses unscaled drawing units; a GA envelope supplies no product dimensions, clearances or fabrication information. Original artwork is available under CC BY 4.0. Third-party references retain their own terms.
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