{"schemaVersion":1,"slug":"21-distance-protection","name":"Distance protection","kind":"function","abbreviation":"21","aliases":["distance relay","impedance relay","ANSI 21","IEEE device 21"],"category":"measurement-protection","drawingTypes":["SLD","Schematic"],"referenceFamilies":["IEEE / ANSI"],"summary":"Uses measured voltage and current to assess an electrical distance or impedance characteristic.","description":["Uses measured voltage and current to assess an electrical distance or impedance characteristic.","In a drafting package, connect this reference to the protected line CT and VT circuits. The associated drawing should trace the distance zones and associated breaker trip paths; the number identifies the role and does not specify the terminal arrangement, ratings or operating settings."],"readingNotes":["Identify the protected line CT and VT circuits.","Trace the distance zones and associated breaker trip paths."],"drawingChecklist":["Label the protected line CT and VT circuits.","Cross-reference the distance zones and associated breaker trip paths.","Identify the device tag, manual revision and logic drawing that define this implementation."],"distinction":"Distance protection uses impedance-related criteria; directional overcurrent 67 uses current and directional supervision.","facts":[{"label":"ANSI / IEEE function number","value":"21"},{"label":"Drawing context","value":"The protected line CT and VT circuits"},{"label":"Application scope","value":"Protection or apparatus-control role; exact implementation is defined by the equipment documentation."}],"references":[{"sourceId":"pub-function-sel-numbers","relevance":"Public manufacturer number/name association. Drafting explanations are independently written."},{"sourceId":"ieee-functions","relevance":"Normative device-number publication; exact edition wording and compliance require the licensed standard."}],"related":["current-transformer","52-ac-circuit-breaker"],"artwork":"pub-function-box","publishedOn":"2026-10-13","updatedOn":"2026-10-13","provenance":"independent-public-research","url":"https://eloragrid.ai/protection-functions/21-distance-protection","illustration":{"url":"https://eloragrid.ai/library/illustrations/21-distance-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-sel-numbers","title":"ANSI device-number and suffix reference table","publisher":"Schweitzer Engineering Laboratories","url":"https://selinc.com/pt/products/tables/ansi/","type":"manufacturer","scope":"Public Portuguese-language base-number and application-suffix table. Used to cross-check public number associations. Application-specific and historical labels need the selected drawing legend; this table is not a substitute for the licensed IEEE standard.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public manufacturer number/name association. Drafting explanations are independently written."},{"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 device-number publication; exact edition wording and compliance require the licensed standard."}]}