{"schemaVersion":1,"slug":"inverse-time-characteristic-qualifier","name":"Inverse-time characteristic qualifier","kind":"symbol","abbreviation":"IDMT","aliases":["inverse time qualifier","IDMT marking","dependent-time qualifier"],"category":"measurement-protection","drawingTypes":["Schematic","SLD"],"referenceFamilies":["IEC"],"summary":"Qualifier showing that a measuring relay's operating time falls as the measured quantity rises above its setting.","description":["The inverse-time qualifier marks a measuring element whose operating time depends on how far the measured quantity exceeds the setting: the larger the overcurrent, the faster the trip. On drawings it appears with overcurrent and earth-fault elements that are graded against fuses and downstream relays.","The qualifier does not name the curve. Standard inverse, very inverse and extremely inverse curves grade very differently, and the curve type, time multiplier and pick-up current must be taken from the protection settings."],"readingNotes":["Identify the curve type from the settings record.","Read with the grading study for the feeder."],"drawingChecklist":["State curve type","State time multiplier","Reference grading study"],"distinction":"An inverse-time qualifier marks a current-dependent delay; a definite-time qualifier marks a fixed delay.","facts":[{"label":"Meaning","value":"Operating time falls as the quantity rises"},{"label":"Typical use","value":"Overcurrent and earth-fault grading"},{"label":"Preview","value":"Independent educational concept; consult the applicable standard and project legend for the exact graphic"}],"references":[{"sourceId":"iec-diagrams","relevance":"IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced."},{"sourceId":"breaker-characteristics","relevance":"Public explanation of time-current characteristics used in protective devices."}],"related":["definite-time-delay-measuring-qualifier","51-time-overcurrent","maximum-current-monitoring-relay"],"artwork":"s28-inverse-time-characteristic-qualifier","publishedOn":"2026-10-28","updatedOn":"2026-10-28","provenance":"independent-public-research","plainEnglish":{"title":"What does Inverse-time characteristic qualifier mean?","introduction":"The inverse-time additional drawing mark marks a measuring element whose operating time depends on how far the measured quantity exceeds the setting: the larger the overcurrent, the faster the trip. On drawings it appears with overcurrent and earth-fault elements that are graded against fuses and downstream relays.","example":"The additional drawing mark does not name the curve. Standard inverse, very inverse and extremely inverse curves grade very differently, and the curve type, time multiplier and pick-up current must be taken from the protection settings.","drawings":"Identify the curve type from the settings record. Read with the grading study for the feeder. An inverse-time additional drawing mark marks a current-dependent delay; a definite-time additional drawing mark marks a fixed delay.","terms":[{"name":"Coil","meaning":"A wound conductor that can produce a magnetic field when energised."},{"name":"Rating","meaning":"A value stating the conditions for which equipment is specified."},{"name":"Schematic","meaning":"A drawing showing function and connections rather than physical appearance."}],"diagram":"flow","steps":["Identify the device and form","Read the connections and state","Check the equipment reference"],"caption":"A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.","sources":[{"label":"IEC graphical symbols for diagrams","url":"https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/"},{"label":"Fundamental characteristics of a circuit-breaker","url":"https://www.electrical-installation.org/enwiki/Fundamental_characteristics_of_a_circuit-breaker"}],"detail":"additional drawing mark showing that a measuring relay's operating time falls as the measured quantity rises above its setting. An inverse-time additional drawing mark marks a current-dependent delay; a definite-time additional drawing mark marks a fixed delay.","profileId":"reviewed-symbol-20261028"},"url":"https://eloragrid.ai/symbols/inverse-time-characteristic-qualifier","illustration":{"url":"https://eloragrid.ai/tools/electrical-library/illustrations/inverse-time-characteristic-qualifier.svg","scope":"Original educational concept; not an official standard graphic or product footprint","license":"https://creativecommons.org/licenses/by/4.0/"},"sources":[{"id":"iec-diagrams","title":"IEC graphical symbols for diagrams","publisher":"IEC","url":"https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/","type":"standard","scope":"IEC 60617 and related symbol-design publications. Full database access requires a subscription.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced."},{"id":"breaker-characteristics","title":"Fundamental characteristics of a circuit-breaker","publisher":"Schneider Electric — Electrical Installation Guide","url":"https://www.electrical-installation.org/enwiki/Fundamental_characteristics_of_a_circuit-breaker","type":"manufacturer","scope":"Voltage, current, tripping and breaking-capacity concepts; not a product selection or settings prescription.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public explanation of time-current characteristics used in protective devices."}]}