# Automatic transfer switch

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

A mechanically switched changeover device that automatically moves a load from a failed normal source to an alternate source.

## In plain English

### What does Automatic transfer switch mean?

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.

### A simple example

Single-line diagrams show a changeover contact with two incoming poles labelled N and E or S1 and S2 and one outgoing load connection, or two interlocked switching devices. The controller and generator start signal are often noted beside it, and a neutral pole may be switched for separately derived systems.

### What the drawings help you do

The symbol does not show the timing, whether the neutral is switched, or whether the transition is open, closed or delayed. Those features determine earthing behaviour, motor reconnection and coordination with the network operator.

### This particular reference

A mechanically switched changeover device that automatically moves a load from a failed normal source to an alternate source. An automatic transfer switch operates without an operator; a manual transfer switch needs someone to move it.

### Terms

- Normally open / normally closed: The contact state in the normal condition defined for the drawing.
- Terminal: An identified point where a conductor connects.
- 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)
- [ATS controllers](https://www.deif.com/products/controllers/ats-controller/)


## What it represents

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.
Single-line diagrams show a changeover contact with two incoming poles labelled N and E or S1 and S2 and one outgoing load connection, or two interlocked switching devices. The controller and generator start signal are often noted beside it, and a neutral pole may be switched for separately derived systems.
The symbol does not show the timing, whether the neutral is switched, or whether the transition is open, closed or delayed. Those features determine earthing behaviour, motor reconnection and coordination with the network operator.

## How to read it

- Check whether three or four poles are switched.
- Read the transition type from the notes.

## Information to show or reference

- State current rating, poles and transition type.
- Show both sources and the generator start signal.
- Reference controller timing settings.

## Important distinction

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

## Facts

- Switching: Mechanical, interlocked
- Typical transition: Open, break-before-make
- Inputs: Normal and alternate sources
- 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.
- [ATS controllers](https://www.deif.com/products/controllers/ats-controller/) — DEIF. Public automatic transfer controller page describing mains and generator transfer.

## 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.