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Types of Automatic Transfer Switches:Working and Use

Automatic transfer switch with manual option
Automatic transfer switch with manual option
Resource: https://diysolarforum.com

Different types of automatic transfer switches are better suited for different applications. Anyone considering using an ATS should, therefore, carefully consider their needs before selecting a particular model. This article will discuss the four most common automatic transfer switch types.

About Automatic Transfer Switches

An automatic transfer switch, or ATS, is a device that is used to automatically switch between two different power sources. These devices find useful application where there is a need for a backup power source.

Because switching needs vary across different applications, there are a number of different types of automatic transfer switch equipment available on the market. These mostly vary in terms of the number of poles and the amperage or voltage rating.

Most of the time though, ATSs are classified based on their connection or power transition methods. So the types of automatic transfer switches in the market will be one of the following:

  • Make before break type (open transition)
  • Break before make type (closed transition)
  • Delayed transition types

Others include switches that allow a soft (gradual connection) and those that can be bypassed for maintenance purposes. Here, the automatic transfer switch types explained in more detail.

Automatic transfer switch for solar system
Automatic transfer switch for solar system
Resource: https://forum.arduino.cc

Types of automatic transfer switches

For anyone who is considering using an automatic transfer switch, it’s important to first understand the different types of these devices that are available. As mentioned, most ATSs are classified according to their transition method. Here is a detailed look at each automatic transfer switch type.

1. Open Transition Transfer Switch

An open transition transfer switch is named so for its break-before-make method of transition. During the power transfer sequence, the switch will first break its connection with one source before connecting to the other source.

An open transition ATS will, therefore, usually cause a momentary power outage. This is usually a few seconds, which for most applications, is acceptable. Nevertheless, for mission-critical systems, this type of automatic transfer switch might not be ideal.

As such, open transition ATS equipment is typically used in small applications, such as residential backup power systems. It’s also common in situations where a short period of outage would not cause concern, such as systems that power the lights in a building.

2. Closed Transition Transfer Switch

A closed transition transfer switch is just the opposite of an open transition ATS – it’s a make-before-break device. That means during the power transfer sequence, the switch will first make its connection with the alternate source before breaking its connection with the existing source.

The main advantage of using a closed transition ATS is that the momentary power loss during transfer is barely noticeable, only lasting for approximately 100 milliseconds. Because of its closed transition method, the switch provides a seamless transfer of power back to the primary source.

These types of automatic transfer switch methods are, therefore, often used in mission-critical applications where even a short power outage could cause problems. These applications include in the UPS systems of healthcare and security or communication centers.

3. Delayed Transition Transfer Switch

A delayed transition transfer switch is similar to a closed transition ATS. The main difference is in the timing of the power transfer or transition process, which is usually delayed for a few seconds (up to 10 seconds).

These types of automatic transfer switches are useful when powering an inductive load or when the load is a large electric motor (20HP and higher. When these loads are disconnected from live power, they usually produce short lived voltages that could cause damage or trip breakers unnecessarily.

The delay allows these voltages to decay to safe levels and prevent an inrush current. A delayed transition ATS, therefore, finds application in industrial settings where they ensure safe transitions from the primary to secondary power sources, and vice versa.

4. Soft Loading Transfer Switch

The soft loading transfer switch is a variation of the closed transition ATS. With this type of switch, rather than connecting to the new power source immediately, it will slowly ramp up the connection. This is done to avoid any sudden changes in voltage or amperage that could damage equipment.

These types of automatic transfer switch connections are used in applications where a sudden change in load could cause problems. For example, soft loading ATSs are often used in data centers to avoid disrupting sensitive electronic equipment.

5. Bypass Isolation Transfer Switch

The bypass transfer switch is technically two transfer switches in the same assembly, connected in parallel. The primary ATS serves as the main switch, transferring the load to the emergency source and vice versa. The bypass switch, on the other hand, comes into use during routine ATS maintenance.

The bypass ATS is usually a manual type, although some Switches come with both switches automatic. To allow for removal without disconnecting the load during, the primary ATS is usually installed in a draw out carriage.

Bypass isolation transfer switch is commonly used in critical installation where the system must remain powered throughout the process to test or inspect the ATS. It allows technicians to bypass or isolate the main switch for maintenance operations.

Installing automatic transfer switch
Installing automatic transfer switch
Resource: https://www.youtube.com/watch?v=XiQPWpfx3nY

Open vs. Closed Transition Transfer Switch

The main difference between an open and closed transition transfer switch is how the power is transferred from one source to another. An open transition switch will briefly disconnect the power before transferring it to the other source, while a closed transition switch will seamlessly transfer the power without noticeable interruption.

Depending on your needs, either type of ATS can be a good choice. If you need to avoid any power disruption, a closed transition switch is the better option.

However, if your electrical system can stand a few seconds of outage, an open transition ATS switch might be a better choice since it will briefly disconnect the power before reconnecting it to the other source.

Ultimately, the best type of automatic transfer switch for you will depend on your specific needs and application, so be sure to consider every aspect of the electrical system.


There are different types of automatic transfer switches and models in the market today. These mostly differ in terms of their operation, features, and applications. Selecting the most appropriate unit will, therefore, require a careful evaluation of the electrical system but, most importantly, a clear understanding of the types of these switches.

We hope this article has helped you understand the basics of automatic transfer switches and the different types that you can use in your home or business power system.

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