# Closed-transition transfer switch

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

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.

### Terms

- 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.

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

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.

## Important distinction

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

## Facts

- Transition: Make before break
- Overlap: Typically under 100 ms
- Required check: Synchronism
- 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 including transition types.
- [ATS controllers](https://www.deif.com/products/controllers/ats-controller/) — DEIF. Public transfer controller page describing overlap switching.

## 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.