What is an Automatic Transfer Switch ATS panel? How does it work? What does it do?

What is a Automatic Transfer Switch ATS panel? 

An Automatic transfer switch panel, or ATS panel, is a type of transfer panel used with a diesel generator to automatically switch between the mains and generator in the event of a power failure. The generator will start / stop automatically depending on the mains supply.

Why is an automatic transfer switch ATS panel important? 

A transfer switch (either manual or automatic) is required in every country when installing a generator at premises with a mains supply. It is required by law for good reason. A transfer switch avoids:

• The mains power coming into contact with the generator, which would almost certainly burn out if this happened.
• It stops the generator from back feeding the mains when its failed, endangering the lives of electricity utility workers.

In terms of importance, both the manual and automatic switch perform the same function, however the automatic transfer switch ATS panel does the process automatically, saving time and reducing the power outage length. 

 This is the internals of a small ATS panel that uses a motorized switch to change over - other panels may also use contactors, MCCB's and ACB's, depending on their size and customer requirements.

How does and automatic transfer switch ATS panel work?

There are two main types of automatic transfer switch panel, ones with built in mains detection and panels without mains detection. Both work differently. This article describes the basic, normal operating function. Different panels can have slightly different functions and more advanced control systems.

Automatic Transfer Switch ATS panels with Mains Detection

Automatic transfer switch ATS panels with mains detection built in monitor the mains supply. When they detect a mains failure, normally using a mains failure relay, they will disconnect from the mains and send a signal to the generator to start. Once the generator has started, it will send an "available" signal back. When the ATS receives this, it will switch to the generator supply. 

Automatic Transfer Switch ATS panels without Mains Detection

Automatic transfer switch ATS panels without mains detection need the mains detection built into the generator or elsewhere. When the generator detects a mains failure, it will send a signal to the ATS to disconnect from the mains and start automatically. Once the generator has started, it will send a signal to the generator to start. When it receives this, the ATS will switch to the generator supply. 

What are the advantages/disadvantages of an automatic transfer panel?

Here is a summary of the advantages and disadvantages of a manual transfer panel over an automatic transfer switch.

Advantages of a automatic transfer panel

• You need to switch it every time the mains fails and you want to run the generator.
• You need to start the generator manually each time.
• If you aren't nearby, you cannot start the backup power.
• It takes longer - the automatic panel can react as soon as the mains fails.

Disadvantages of a manual transfer panel

• Cost - the Manual transfer panel is much cheaper than an automatic panel.
• Control - the automatic switch will cause the generator to start even if you aren't home - unless you remember to turn off the generator.
• Wiring - there are no control cables needed, just the electricity supply cables.

How does a contactor based ATS panel work?

There are various designs of ATS panel. Typically, a mains failure relay is fitted in an electrical enclosure, with two contactors. The contactors are mechanically and electrically interlocked. This means there is a mechanical device, called an interlock, that ensures the two contactors cannot be closed at the same time. The contactors need electrical power to close (you can get both DC and AC types).

The electrical interlock is designed in the circuitry, to prevent the contactors from energizing at the same time. While the mechanical interlock should stop both of them closing even if this were to happen, this secondary circuit is an added protection. Usually, a closed contact is fitted to each contactor. These contacts, normally (i.e. when the contactor is not energized) are closed. When the contactor is energized, they open. If you have two contactors, A and B, the circuit to close contactor A is wired through the auxiliary of B. Thereby when B is closed, it is not possible to energize A. The same is true for B, this is wired through the auxiliary of A. This provides the electrical interlock.

When the mains fails, the ATS sends a remote start (normally volts free) to the generator. The generator runs up and when at the required power and voltage, the ATS will open the mains contactor and close the generator contactor. When the mains returns, the generator contactor is opened and the mains contactor is closed.

Because both contactors are never closed together, even when the mains returns, there will be a short break in electrical supply as the contactors change over.