Automatic Transfer Switch Working Principle: All Details

The poor state of the power supply calls for alternative sources of power generation and automation of electrical power generation to back up the utility supply. Over time, automation of electrical power supply has become so vital as the rate of the power outage is predominantly high. Therefore, a changeover or automatic transfer switch was developed. An automatic transfer switch has been widely used in the electrical industry. Especially generator manufacturers and panel builders are familiar with this unique device. I will explain to you all you need to know about automatic transfer switch (ATS) and you will fully understand its operating principle.

What is an Automatic Transfer Switch?

The automatic transfer switch (ATS) is a device that is used to switch the power supply from the main supply to an emergency supply to reduce problems caused by faulty conditions in the public network to the minimum level. The automatic transfer switch is the safest way to connect a backup power supply to your home or facility.

The automatic transfer switch (ATS) can be located in the following systems.

1-Main Distribution Board

ATS for backing up the power supply for most of the facility loads with Main-Main (or Utility-Utility or Transformer), Main-Tie-Main, etc. power supply configurations.

2-Sub-Distribution Board

ATS for backing up the power supply for selected critical facility loads with Main-Main or Main-Gen power supply configurations.

3-Fire Fighting Panel (or Emergency Distribution Board)

ATS for backing up the power supply for loads belonging to the firefighting extinguishing system.

How Does an Automatic Transfer Switch Work?

A building cannot be connected to both the primary and backup power at the same time. The working principle of the transfer switch is based on a fast switch between the energy sources. Automatic transfer switches are used to switch electrical energy between primary and backup power sources quickly and safely. 

A typical transfer sequence includes these steps:

1. The primary power source fails.
2. When power from the generator or the backup utility feed is stable, the transfer switch shifts the electrical load to the backup power source. The transfer either occurs automatically or is executed manually.
3. When primary power is restored, the transfer switch shifts the load from the backup power source to the primary source.

Most transfer switches can support multiple operation modes through the addition of configurable options.

1-Manual mode

In manual mode, the transfer is performed manually, by pushing a button or moving a handle. The human operator has full control over the transfer process.

2-Non-automatic mode

In non-automatic mode, the operator manually initiates a transfer by pressing a button or rotating a switch that causes an internal electromechanical device to electrically operate the switching mechanism.

3-Automatic mode

In automatic mode, the transfer switch controller completely manages the transfer. The transfer is triggered when the automatic controller senses unavailability or loss of source power and operation is typically performed by an electric solenoid or motor.

Reference: ABB White Paper, How to select an Automatic Transfer Switch class, Socomec Catalogue, Eaton Catalogue