{"schemaVersion":1,"slug":"81u-underfrequency-protection","name":"Underfrequency protection","kind":"function","abbreviation":"81U","aliases":["low frequency","under frequency","UF protection","ANSI 81U","IEEE 81U"],"category":"measurement-protection","drawingTypes":["SLD","Schematic"],"referenceFamilies":["IEEE / ANSI","Manufacturer"],"summary":"Supervises frequency below the defined operating criterion.","description":["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."],"readingNotes":["Identify the voltage-derived frequency source and minimum-voltage supervision.","Trace the underfrequency trip or staged load-shedding output."],"drawingChecklist":["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."],"distinction":"Underfrequency measures frequency magnitude; rate-of-change function 81R evaluates its change over time.","facts":[{"label":"Function designation","value":"81U"},{"label":"Base function","value":"81 — Frequency protection"},{"label":"Published mapping","value":"Eaton E-Series function mapping"},{"label":"Suffix scope","value":"Application or manufacturer specific; check the complete drawing legend and selected device manual."}],"references":[{"sourceId":"pub-function-eaton-brochure","relevance":"Public primary-source function or application mapping used to identify this designation; wording and drafting notes are original."},{"sourceId":"ieee-functions","relevance":"Device-number and suffix convention. This page is not a reproduction of the full licensed standard."}],"related":["81-frequency","voltage-transformer"],"artwork":"pub-function-box","publishedOn":"2026-10-13","updatedOn":"2026-10-13","provenance":"independent-public-research","url":"https://eloragrid.ai/protection-functions/81u-underfrequency-protection","illustration":{"url":"https://eloragrid.ai/library/illustrations/81u-underfrequency-protection.svg","scope":"Original educational concept; not an official standard graphic or product footprint","license":"https://creativecommons.org/licenses/by/4.0/"},"sources":[{"id":"pub-function-eaton-brochure","title":"E-Series protective relays: public family comparison brochure","publisher":"Eaton","url":"https://www.eaton.com/content/dam/eaton/products/electrical-circuit-protection/protective-relays-and-predictive-devices/e-series-protective-relays-br026001en.pdf","type":"manufacturer","scope":"Public function-name and suffix mappings across feeder, motor, transformer and generator relays. Linked as documentation only; no illustrations or product drawings are copied.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public primary-source function or application mapping used to identify this designation; wording and drafting notes are original."},{"id":"ieee-functions","title":"IEEE C37.2-2022: power-system device function numbers","publisher":"IEEE","url":"https://ieeexplore.ieee.org/document/9984140","type":"standard","scope":"Device function numbers, acronyms and contact designations. The full publication may require access through IEEE.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Device-number and suffix convention. This page is not a reproduction of the full licensed standard."}]}