{"schemaVersion":1,"slug":"81-frequency","name":"Frequency protection","kind":"function","abbreviation":"81","aliases":["underfrequency","overfrequency","ROCOF","81U","81O","81R"],"category":"measurement-protection","drawingTypes":["SLD","Schematic"],"referenceFamilies":["IEEE / ANSI"],"summary":"A function responding to a documented frequency condition.","description":["Frequency protection responds to frequency conditions according to the selected implementation. Underfrequency, overfrequency and rate-of-change functions can be distinguished by additional suffixes and device documentation. A generic 81 reference does not determine which variant is applied.","On a power drawing, keep this function reference connected to the selected equipment and its complete designation. A multifunction relay can implement several numbered functions. This entry explains the reference; it supplies no settings or commissioning procedure."],"readingNotes":["Read the complete designation, not only the base number.","Identify the voltage or other input from which the selected device derives its frequency measurement."],"drawingChecklist":["State the applied frequency function.","Reference the measurement and relay.","Link any trip, alarm or load-control action."],"distinction":"Underfrequency, overfrequency and rate-of-change are different functions even when their designations share the number 81.","facts":[{"label":"ANSI / IEEE function number","value":"81"},{"label":"Equipment implementation","value":"Consult the selected device manual"},{"label":"Suffix interpretation","value":"Application and manufacturer documentation"}],"references":[{"sourceId":"device-numbers","relevance":"Public manufacturer table used to check the base function-number name."},{"sourceId":"ieee-functions","relevance":"Normative publication reference for the device-function convention."},{"sourceId":"abb-functions","relevance":"Public examples of manufacturer-specific function names and suffixes."}],"related":["27-undervoltage","generator","power-converter"],"artwork":"function","publishedOn":"2026-10-13","updatedOn":"2026-10-13","provenance":"independent-public-research","url":"https://eloragrid.ai/protection-functions/81-frequency","illustration":{"url":"https://eloragrid.ai/library/illustrations/81-frequency.svg","scope":"Original educational concept; not an official standard graphic or product footprint","license":"https://creativecommons.org/licenses/by/4.0/"},"sources":[{"id":"device-numbers","title":"Public reference table of ANSI device-function numbers","publisher":"Schneider Electric","url":"https://www.se.com/es/es/faqs/FA34639/","type":"manufacturer","scope":"Spanish-language public reference table. Used to check device-number names; no claim that every function is implemented in every device.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public manufacturer table used to check the base function-number name."},{"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":"Normative publication reference for the device-function convention."},{"id":"abb-functions","title":"Relion 620: functions, codes and symbols","publisher":"ABB","url":"https://techdoc.relays.protection-control.abb/r/620-series-IEC-60870-5-103-Point-List-Manual/2.0-FP1-IEC/en-US/Functions-codes-and-symbols","type":"manufacturer","scope":"Published product mappings between function names, IEC 61850 identifiers, IEC mnemonics and ANSI designations. These are product examples, not universal equivalences.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public examples of manufacturer-specific function names and suffixes."}]}