ATS
Automatic transfer switch
A mechanically switched changeover device that automatically moves a load from a failed normal source to an alternate source.
SLD / Schematic
Symbols · CTTS
Closed-transition transfer switch
A transfer switch that briefly parallels two synchronised sources so the load moves without interruption.
In plain English
What does Closed-transition transfer switch mean?
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.
A simple 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.
What the drawings help you do
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.
This particular reference
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.
- Normally open / normally closed
- The contact state in the normal condition defined for the drawing.
- Phase
- One of the related AC waveforms in a power system.
- Terminal
- An identified point where a conductor connects.
Read the source: Transfer switch fundamentals · ATS controllers
SLD · CTTS
What it represents
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.
How to read it
- A synchronism check function should be associated with the switch.
- Read the maximum overlap time permitted.
Information to show or reference
- State overlap time and synchronising limits.
- Show synchronism check and backup trip.
- Reference network operator approval.
A distinction that matters
A closed-transition switch overlaps the two sources; an open-transition switch always breaks before it makes.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
Transfer switch fundamentals
Public transfer switch fundamentals including transition types.
ATS controllers
Public transfer controller page describing overlap switching.
About this graphic
This illustration was authored independently by Elora Grid to explain the concept. It is not reproduced from a customer drawing, an official standard graphic or a manufacturer CAD file. Its correspondence to an exact IEC or IEEE symbol identifier has not been certified.
SVG and DXF downloads contain this original artwork. DXF uses unscaled drawing units; a GA envelope supplies no product dimensions, clearances or fabrication information. Original artwork is available under CC BY 4.0. Third-party references retain their own terms.
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Synchronism check
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SLD / Schematic
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