{"schemaVersion":1,"slug":"insulation-monitoring-relay","name":"Insulation monitoring relay","kind":"symbol","abbreviation":"IMD","aliases":["insulating relay","insulation monitoring device","HCR1F44"],"category":"measurement-protection","drawingTypes":["Schematic","SLD"],"referenceFamilies":["IEC","Manufacturer"],"summary":"Relay that continuously checks insulation resistance of an unearthed (IT) system to earth and alarms on a first fault.","description":["An insulation monitoring relay is connected between the conductors of an unearthed (IT) system and earth, and injects a measuring signal to determine insulation resistance. It warns when insulation falls below its setting so a first fault can be located before a second fault causes a trip.","The symbol shows the device and its connections but not the measuring method, which matters on systems with drives or large DC components. Only one insulation monitor should normally be active on a galvanically connected system, so the drawing should show any disconnection or interlock between parallel systems."],"readingNotes":["Check that only one monitor is active on each connected IT system.","Identify alarm outputs and test button wiring."],"drawingChecklist":["Show system and earth connections","State alarm thresholds","Reference interlock between parallel monitors"],"distinction":"An insulation monitoring relay measures insulation resistance of an unearthed system; a residual current monitor measures leakage current in an earthed system.","facts":[{"label":"Measured quantity","value":"Insulation resistance to earth"},{"label":"Publisher reference token","value":"HCR1F44 (Autodesk identifier; not an IEC database ID)"},{"label":"Preview","value":"Independent educational concept; consult the publisher and applicable standard for the exact graphic"}],"references":[{"sourceId":"pub-symbol-monitoring-relays","relevance":"Public AutoCAD Electrical relay listing that names the Insulating Relay block; the illustration is independently drawn."},{"sourceId":"iec-diagrams","relevance":"IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced."}],"related":["iec-insulation-failure-indication","residual-current-monitor","earth-fault-voltage-relay"],"artwork":"s28-insulation-monitoring-relay","publishedOn":"2026-10-28","updatedOn":"2026-10-28","provenance":"independent-public-research","plainEnglish":{"title":"What does Insulation monitoring relay mean?","introduction":"An insulation monitoring relay is connected between the conductors of an unearthed (IT) system and earth, and injects a measuring signal to determine insulation resistance. It warns when insulation falls below its setting so a first fault can be located before a second fault causes a trip.","example":"The symbol shows the device and its connections but not the measuring method, which matters on systems with drives or large DC components. Only one insulation monitor should normally be active on a galvanically connected system, so the drawing should show any disconnection or interlock between parallel systems.","drawings":"Check that only one monitor is active on each connected IT system. Identify alarm outputs and test button wiring. An insulation monitoring relay measures insulation resistance of an unearthed system; a residual current monitor measures leakage current in an earthed system.","terms":[{"name":"DC","meaning":"Direct current: electricity flowing in one direction."},{"name":"Coil","meaning":"A wound conductor that can produce a magnetic field when energised."},{"name":"Terminal","meaning":"An identified point where a conductor connects."}],"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":"AutoCAD Electrical: Miscellaneous monitoring relays","url":"https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-31F4CF79-F8E0-4E5A-8C51-4D71640ACDB8.htm"},{"label":"IEC graphical symbols for diagrams","url":"https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/"}],"detail":"Relay that continuously checks insulation resistance of an unearthed (IT) system to earth and alarms on a first fault. An insulation monitoring relay measures insulation resistance of an unearthed system; a residual current monitor measures leakage current in an earthed system.","profileId":"reviewed-symbol-20261028"},"url":"https://eloragrid.ai/symbols/insulation-monitoring-relay","illustration":{"url":"https://eloragrid.ai/tools/electrical-library/illustrations/insulation-monitoring-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-symbol-monitoring-relays","title":"AutoCAD Electrical: Miscellaneous monitoring relays","publisher":"Autodesk","url":"https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-31F4CF79-F8E0-4E5A-8C51-4D71640ACDB8.htm","type":"cad-reference","scope":"Public vendor catalogue naming and functional variants. Vendor block names are not IEC identifiers. Linked for reference; no graphics or CAD files are redistributed.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public AutoCAD Electrical relay listing that names the Insulating Relay block; the illustration is independently drawn."},{"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."}]}