{"schemaVersion":1,"slug":"closed-transition-transfer-switch","name":"Closed-transition transfer switch","kind":"symbol","abbreviation":"CTTS","aliases":["make-before-break transfer switch","closed-transition ATS","bumpless transfer switch"],"category":"microgrid-systems","drawingTypes":["SLD"],"referenceFamilies":["IEC","IEEE / ANSI"],"summary":"A transfer switch that briefly parallels two synchronised sources so the load moves without interruption.","description":["A closed-transition transfer switch connects the load to the new source before disconnecting it from the old one, with both sources paralleled for a short time, typically less than 100 milliseconds. A synchronism check confirms that voltage, frequency and phase angle are within limits before the overlap is allowed.","Drawings show two switching devices or a transfer switch with both contacts able to close, plus a synchronism check function and often a note on maximum overlap time. Where the overlap is longer than the network operator allows, a soft-load transfer with a paralleling controller is used instead.","Even a momentary parallel connection can require approval from the network operator and protection against the generator feeding a utility fault. The symbol does not show the overlap timer, its backup trip or the interface protection."],"readingNotes":["A synchronism check function should be associated with the switch.","Read the maximum overlap time permitted."],"drawingChecklist":["State overlap time and synchronising limits.","Show synchronism check and backup trip.","Reference network operator approval."],"distinction":"A closed-transition switch overlaps the two sources; an open-transition switch always breaks before it makes.","facts":[{"label":"Transition","value":"Make before break"},{"label":"Overlap","value":"Typically under 100 ms"},{"label":"Required check","value":"Synchronism"},{"label":"Preview","value":"Independent educational illustration; consult the applicable standard and project legend for the exact graphic"}],"references":[{"sourceId":"transfer-switches","relevance":"Public transfer switch fundamentals including transition types."},{"sourceId":"s28-deif-ats-controller","relevance":"Public transfer controller page describing overlap switching."}],"related":["automatic-transfer-switch","25-synchronism-check","transfer-switch","static-transfer-switch"],"artwork":"s28-closed-transition-transfer-switch","publishedOn":"2026-10-28","updatedOn":"2026-10-28","provenance":"independent-public-research","plainEnglish":{"title":"What does Closed-transition transfer switch mean?","introduction":"A closed-transition transfer switch connects the load to the new source before disconnecting it from the old one, with both sources paralleled for a short time, typically less than 100 milliseconds. A synchronism check confirms that voltage, frequency and phase angle are within limits before the overlap is allowed.","example":"Drawings show two switching devices or a transfer switch with both contacts able to close, plus a synchronism check function and often a note on maximum overlap time. Where the overlap is longer than the network operator allows, a soft-load transfer with a paralleling controller is used instead.","drawings":"Even a momentary parallel connection can require approval from the network operator and protection against the generator feeding a utility fault. The symbol does not show the overlap timer, its backup trip or the interface protection.","terms":[{"name":"Normally open / normally closed","meaning":"The contact state in the normal condition defined for the drawing."},{"name":"Phase","meaning":"One of the related AC waveforms in a power system."},{"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":"Transfer switch fundamentals","url":"https://www.cummins.com/sites/default/files/2026-04/2023-sep-transfer-switch-fundamentals.pdf"},{"label":"ATS controllers","url":"https://www.deif.com/products/controllers/ats-controller/"}],"detail":"A transfer switch that briefly parallels two synchronised sources so the load moves without interruption. A closed-transition switch overlaps the two sources; an open-transition switch always breaks before it makes.","profileId":"reviewed-symbol-20261028"},"url":"https://eloragrid.ai/symbols/closed-transition-transfer-switch","illustration":{"url":"https://eloragrid.ai/tools/electrical-library/illustrations/closed-transition-transfer-switch.svg","scope":"Original educational concept; not an official standard graphic or product footprint","license":"https://creativecommons.org/licenses/by/4.0/"},"sources":[{"id":"transfer-switches","title":"Transfer switch fundamentals","publisher":"Cummins","url":"https://www.cummins.com/sites/default/files/2026-04/2023-sep-transfer-switch-fundamentals.pdf","type":"manufacturer","scope":"Public educational presentation about source transfer. Manufacturer graphics and complete circuits are not redistributed.","checkedOn":"2026-10-13","reuse":"link-only","relevance":"Public transfer switch fundamentals including transition types."},{"id":"s28-deif-ats-controller","title":"ATS controllers","publisher":"DEIF","url":"https://www.deif.com/products/controllers/ats-controller/","type":"manufacturer","scope":"Public automatic transfer controller description including generator-mains mode and overlap switching.","checkedOn":"2026-10-28","reuse":"link-only","relevance":"Public transfer controller page describing overlap switching."}]}