{"schemaVersion":1,"slug":"thermistor-motor-protection-relay","name":"Thermistor motor protection relay","kind":"symbol","abbreviation":"PTC","aliases":["PTC thermistor relay","motor thermistor relay","thermistor protection unit"],"category":"measurement-protection","drawingTypes":["Schematic"],"referenceFamilies":["IEC","Manufacturer"],"summary":"Relay that monitors PTC thermistors embedded in motor windings and trips when their resistance rises at the rated temperature.","description":["A thermistor protection relay is connected to a chain of PTC thermistors built into a motor's windings. When the windings reach the thermistors' rated response temperature, their resistance rises sharply and the relay opens its contact in the motor control circuit, protecting against overheating that a current-based overload cannot detect.","Typical causes it catches are blocked ventilation, high ambient temperature and frequent starting. The symbol does not show whether the relay detects a short-circuited sensor loop, whether it latches, or how it is reset, which affect safety where automatic restart is possible."],"readingNotes":["Follow the sensor loop to the motor terminal box.","Check reset type and short-circuit detection."],"drawingChecklist":["Show sensor loop terminals","State reset type","Reference motor control trip contact"],"distinction":"A thermistor relay measures winding temperature directly; a thermal overload relay models heating from current.","facts":[{"label":"Sensor","value":"PTC thermistors in motor windings"},{"label":"Output","value":"Trip contact in motor control circuit"},{"label":"Preview","value":"Independent educational concept; consult the applicable standard and project legend for the exact graphic"}],"references":[{"sourceId":"pub-function-eaton-motor","relevance":"Public motor protection function comparison including thermal protection."},{"sourceId":"iec-diagrams","relevance":"IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced."}],"related":["motor-overload-trip-nc-contact","49-thermal-protection","resistance-temperature-detector"],"artwork":"s28-thermistor-motor-protection-relay","publishedOn":"2026-10-28","updatedOn":"2026-10-28","provenance":"independent-public-research","plainEnglish":{"title":"What does Thermistor motor protection relay mean?","introduction":"A thermistor protection relay is connected to a chain of PTC thermistors built into a motor's windings. When the windings reach the thermistors' rated response temperature, their resistance rises sharply and the relay opens its contact in the motor control circuit, protecting against overheating that a current-based overload cannot detect.","example":"Typical causes it catches are blocked ventilation, high ambient temperature and frequent starting. The symbol does not show whether the relay detects a short-circuited sensor loop, whether it latches, or how it is reset, which affect safety where automatic restart is possible.","drawings":"Follow the sensor loop to the motor terminal box. Check reset type and short-circuit detection. A thermistor relay measures winding temperature directly; a thermal overload relay models heating from current.","terms":[{"name":"Winding","meaning":"A coil of conductor inside an electrical machine or transformer."},{"name":"Normally open / normally closed","meaning":"The contact state in the normal condition defined for the drawing."},{"name":"Coil","meaning":"A wound conductor that can produce a magnetic field when energised."}],"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":"EMR motor protection: public function comparison","url":"https://www.eaton.com/us/en-us/catalog/electrical-circuit-protection/motor-relays.html"},{"label":"IEC graphical symbols for diagrams","url":"https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/"}],"detail":"Relay that monitors PTC thermistors embedded in motor windings and trips when their resistance rises at the rated temperature. A thermistor relay measures winding temperature directly; a thermal overload relay models heating from current.","profileId":"reviewed-symbol-20261028"},"url":"https://eloragrid.ai/symbols/thermistor-motor-protection-relay","illustration":{"url":"https://eloragrid.ai/tools/electrical-library/illustrations/thermistor-motor-protection-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-eaton-motor","title":"EMR motor protection: public function comparison","publisher":"Eaton","url":"https://www.eaton.com/us/en-us/catalog/electrical-circuit-protection/motor-relays.html","type":"manufacturer","scope":"Public motor-relay application mappings including jam/stall protection, current input variants, breaker failure and motor differential. Suffix interpretation applies to the cited family.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public motor protection function comparison including thermal protection."},{"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."}]}