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What is Automatic Transfer Switch, What Does It Do?

An automatic transfer switch
An automatic transfer switch
Resource: https://www.youtube.com/watch?v=Jp6HprY68qk

The automatic transfer switch is an integral part of many electrical systems today. It essentially helps to switch between two or more power sources, ensuring a continuous supply in an outage or other failure. ATSs find use in a variety of applications, from solar to emergency power systems that use generators and UPSs. Read this post for more about these switches.

What is Automatic Transfer Switch?

Let’s begin with the automatic transfer switch meaning. After that, we will see how the switch works, among other things, such as function and use.

An automatic transfer switch, ATS, is a smart switch that can sense when the primary power source it connects to has lost power. It then automatically switches to a secondary or emergency backup power source.

In most cases, the ATS connects to two power sources, such as a solar system and a backup utility grid. In some applications, the switch operates three power sources, such as when it connects to a utility source and two backup generators.

Automatic Transfer Switch Design

The automatic transfer switch design varies depending on the needs of the application. The two most common types are single-phase ATS and three-phase ATS. These will also come with different numbers of poles.

Automatic transfer switch dimensions also vary depending on the application. Large transfer switches are typically used in industrial settings and often include enclosures or even draw-out mechanisms to allow maintenance safety.

On the other hand, smaller automatic transfer switch sizes are commonly used in residential applications. This type is often easy to install and mainly classified as a low-voltage automatic transfer switch (below 1KV).

Automatic Transfer Switch Components

Although designs vary, the components of automatic transfer switch assemblies will usually comprise the following main parts:

  • Logic controller: this is the “brain” of the ATS. It monitors power sources and controls the switching action.
  •  Switching mechanism: the device that switches power from one source to another. It may be either a mechanical switch or it may use solid-state technology.

Other automatic transfer switch parts include contacts and control.

Automatic transfer switch function in a solar system
Automatic transfer switch function in a solar system
Resource: https://www.youtube.com/watch?v=KALcufvhLX8

What Does an Automatic Transfer Do?

After the automatic transfer switch definition, let’s move on and look at the device’s function. This will give you a better view of the device’s crucial role in electrical systems and help you understand why a home power system or business would need one.

Automatic Transfer Switch Function

The primary function of automatic transfer switch equipment is to ensure that an electrical system continues to operate even when the primary power source has failed. This happens automatically —without human intervention unless manual control provisions have been made.

For example, a solar ATS in a grid-connected PV system will switch to the solar battery when the grid goes down and vice versa. This ensures you do not experience any downtime due to a power outage or other failure, which could cause losses in a business and other inconveniences.

Automatic Transfer Switch Advantages

The ATS is an intelligent switch. It automatically transfers the load from one power source to another. There are several benefits to this. The advantages of an automatic transfer switch include:

  • Seamless power transition: the ATS enables a seamless transition from one power source to another. This is important in applications where an uninterrupted power supply is crucial, such as hospitals and data centers.
  •  Increased safety: An automatic transfer switch for a generator or other power source prevents back feeding, which can occur when a generator is connected to a grid without an ATS.

Back feeding can be dangerous as it can energize power lines that are being worked on by line crews.

  • Reduced downtime: Downtime can be costly, and an ATS helps reduce it. This is because the switch prevents disruptions caused by power outages and ensures critical systems are up and running even after an outage.
Automatic transfer switch with provision for non-automatic control
Automatic transfer switch with provision for non-automatic control
Resource: https://powerforum.co.za

How Do Automatic Transfer Switches Work?

Let’s now see how an automatic transfer switch works.

Essentially, ATSs operate by constantly monitoring the power of the primary power source. When the primary power source fails or drops below a certain level, the switch a

utomatically switches to the secondary or alternate power source.

Here is a sequence of the events that characterize automatic transfer switch operation in a grid-connected solar or generator backup system.

  • Loss of power from the utility grid (frequency or voltage dips on the utility grid)
  •  ATS detects the change and quickly connects to solar power or instructs the generator to start. (As soon as the generator is up to speed and stable, the switch transfers the load to it)
  •  The grid voltage and frequency return to normal. The ATS logic controller senses the return of power and transfers the load back to the utility grid.

The switch will also instruct the generator is shut down (in a generator backup system), but usually, after a delay to ensure grid stability as well as allow the generator to cool down)

Although the automatic transfer switch working is meant to be autonomous, many switches provide manual or semi-automatic control through remote switching. This allows users to override the auto function whenever or wherever necessary.

Automatic transfer switch for sola power system
Automatic transfer switch for sola power system
Resource: https://www.youtube.com/watch?v=mapU_ak2yI8

Manual vs. Automatic Transfer Switch

Transfer switch equipment is either manual or automatic. A non-automatic transfer switch requires an operator to physically throw the switch, while an automatic control does not. Auto transfer switches offer several advantages over manual types.

Apart from relieving the operator of the need to be present during a power outage or switchover, automatic switches are also faster in operation.

This is because they are equipped with sensors and logic controllers that constantly monitor power sources and quickly switch when necessary.

Furthermore, automatic transfer switches can be equipped with features that make them more flexible and user-friendly, such as remote control, generator exercise clock, and load shedding.

Automatic Transfer Switch Uses

Automatic transfer switches have a wide range of applications in residential, commercial, and industrial settings. Below are some examples:

  • Solar systems: as we discussed in the intro, an automatic transfer switch is a key component of any grid-connected solar system. Without one, you would need to manually switch to power sources whenever there is a grid failure.
  •  UPS systems: a UPS ATS is used in data centers and other critical applications where power should not be interrupted. The switch ensures that backup power is immediately available when needed.
  •  Generator backup systems: as we also discussed, an automatic transfer switch for generator backup systems to ensure the generator is only used when needed. This saves fuel and maintenance costs.
  •  Building management systems: an ATS in a building management system helps ensure that essential equipment such as elevators, lights, and security systems continue to operate even when there is a power outage.
  •  Industrial settings: ATSs are used in a variety of industrial settings, such as factories, where they help ensure that production can continue even when there is a power outage. The switch may be a generator, wind power, or solar automatic transfer switch.


Automatic transfer switches are critical parts of electrical systems that include emergency power sources. From conveniently switching between power sources to safely connecting and disconnecting loads during an outage, automatic transfer switches provide a variety of benefits. What’s more, these switches operate automatically, which means that they don’t require any input from the user.

Related Resource

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