# Static transfer switch

Canonical URL: https://eloragrid.ai/symbols/static-transfer-switch
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: STS
Drawing use: SLD, Schematic
Reference families: IEC, IEEE / ANSI

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.

### Terms

- 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.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Transfer switch fundamentals](https://www.cummins.com/sites/default/files/2026-04/2023-sep-transfer-switch-fundamentals.pdf)
- [AutoCAD Electrical: Semiconductor components](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-AC954072-7314-4682-88AF-38160D61E9A2.htm)


## 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.

## Important distinction

A static transfer switch uses thyristors and transfers in milliseconds; an automatic transfer switch uses mechanical contacts and takes much longer.

## Facts

- Switching element: Anti-parallel thyristors
- Transfer time: Sub-cycle
- Typical location: Downstream of two UPS systems
- Preview: Independent educational illustration; consult the applicable standard and project legend for the exact graphic

## Public references

- [Transfer switch fundamentals](https://www.cummins.com/sites/default/files/2026-04/2023-sep-transfer-switch-fundamentals.pdf) — Cummins. Public transfer switch fundamentals context.
- [AutoCAD Electrical: Semiconductor components](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-AC954072-7314-4682-88AF-38160D61E9A2.htm) — Autodesk. Public semiconductor symbol family for the thyristor elements.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.