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 · STS
Static transfer switch
A thyristor-based switch that transfers a load between two synchronised sources within a quarter cycle.
In plain English
What does Static transfer switch mean?
A static transfer switch uses back-to-back thyristor pairs on each of two sources to move a critical load from one supply to the other in a few milliseconds. It is used in data centres and process plants, typically downstream of two UPS systems, so that a failure in one UPS or its distribution does not interrupt the load.
A simple example
The single-line diagram shows two incoming feeders, each passing through an anti-parallel thyristor symbol or a solid-state switch box, joining at a common output. Mechanical bypass switches for maintenance are drawn in parallel with the static paths.
What the drawings help you do
Fast transfer only works when the two sources are close in phase. The symbol does not show the synchronisation window, the transfer inhibit logic on downstream faults or how transformer inrush is managed when the load includes transformers.
This particular reference
A thyristor-based switch that transfers a load between two synchronised sources within a quarter cycle. A static transfer switch uses thyristors and transfers in milliseconds; an automatic transfer switch uses mechanical contacts and takes much longer.
- Winding
- A coil of conductor inside an electrical machine or transformer.
- Phase
- One of the related AC waveforms in a power system.
- Signal
- Electrical information passed between devices, rather than necessarily the main power supply.
Read the source: Transfer switch fundamentals · AutoCAD Electrical: Semiconductor components
SLD / Schematic · STS
What it represents
A static transfer switch uses back-to-back thyristor pairs on each of two sources to move a critical load from one supply to the other in a few milliseconds. It is used in data centres and process plants, typically downstream of two UPS systems, so that a failure in one UPS or its distribution does not interrupt the load.
The single-line diagram shows two incoming feeders, each passing through an anti-parallel thyristor symbol or a solid-state switch box, joining at a common output. Mechanical bypass switches for maintenance are drawn in parallel with the static paths.
Fast transfer only works when the two sources are close in phase. The symbol does not show the synchronisation window, the transfer inhibit logic on downstream faults or how transformer inrush is managed when the load includes transformers.
How to read it
- Look for bypass switches around each thyristor path.
- Transfer is normally blocked for downstream faults; check the logic.
Information to show or reference
- State current rating and transfer time.
- Show both sources and maintenance bypasses.
- Reference synchronisation and inhibit logic.
A distinction that matters
A static transfer switch uses thyristors and transfers in milliseconds; an automatic transfer switch uses mechanical contacts and takes much longer.
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 context.
AutoCAD Electrical: Semiconductor components
Public semiconductor symbol family for the thyristor elements.
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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