{"schemaVersion":1,"slug":"91-voltage-directional-relay","name":"Voltage directional relay","kind":"function","abbreviation":"91","aliases":["directional voltage","ANSI 91","IEEE device 91"],"category":"measurement-protection","drawingTypes":["SLD","Schematic"],"referenceFamilies":["IEEE / ANSI"],"summary":"Uses a voltage relationship with a defined direction criterion in a specialist protection or control scheme.","description":["Uses a voltage relationship with a defined direction criterion in a specialist protection or control scheme.","In a drafting package, connect this reference to the compared voltage references and polarity convention. The associated drawing should trace the directional-voltage operating circuit; the number identifies the role and does not specify the terminal arrangement, ratings or operating settings.","This designation is useful when reading machine and legacy control drawings. Retain the documented function on an existing drawing and consult the original equipment manual before assigning it to a new design."],"readingNotes":["Identify the compared voltage references and polarity convention.","Trace the directional-voltage operating circuit."],"drawingChecklist":["Label the compared voltage references and polarity convention.","Cross-reference the directional-voltage operating circuit.","Identify the device tag, manual revision and logic drawing that define this implementation."],"distinction":"Directional voltage 91 is not equivalent to directional power 32 or ordinary overvoltage 59.","facts":[{"label":"ANSI / IEEE function number","value":"91"},{"label":"Drawing context","value":"The compared voltage references and polarity convention"},{"label":"Application scope","value":"Specialist or legacy machine-control use; interpret with the equipment legend."}],"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":["voltage-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/91-voltage-directional-relay","illustration":{"url":"https://eloragrid.ai/library/illustrations/91-voltage-directional-relay.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."}]}