Side-by-side reference

Understand the difference.

Compare the meaning, drawing context and source references of two entries. A similar graphic or shared abbreviation does not establish equivalence.

First referenceCompare with

CTTS

Symbols

Closed-transition transfer switch

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.

Drawing use: SLD
Application: Microgrid systems
Reference family: IEC / IEEE / ANSI

A distinction that matters

A closed-transition switch overlaps the two sources; an open-transition switch always breaks before it makes.

Full reference

ATS

Symbols

Automatic transfer switch

An automatic transfer switch monitors the normal supply and, when voltage or frequency goes out of limits, signals the generator to start and transfers the load to the alternate source once it is ready. When the normal supply returns and is stable, it transfers back after a time delay. Most units use an open-transition break-before-make sequence.

Drawing use: SLD / Schematic
Application: Microgrid systems
Reference family: IEC / IEEE / ANSI

A distinction that matters

An automatic transfer switch operates without an operator; a manual transfer switch needs someone to move it.

Full reference

Original educational previews. Follow the source references on each full entry for the appropriate standard or selected product.