automatic transfer switch (ATS)

What is an automatic transfer switch (ATS)?

An automatic transfer switch (ATS) is a device that automatically transfers a power supply from its primary source to a backup source when it senses a failure or outage in the primary source. Following the failure, the ATS automatically invokes the backup source to minimize power supply disruptions.

What is the purpose of an automatic transfer switch?

The main purpose of an ATS, as a critical component of a functional power system, is to connect the electrical equipment in a facility to backup power in case the facility's primary power source goes down or fails. Implementing an ATS in the power system ensures that all machines that require continuous or near-continuous uptime continue operating even if there is a power outage or failure.

An ATS is commonly implemented anywhere that machine downtime can have a catastrophic impact on organizational productivity, service delivery, or even human lives. Hospitals, data centers and factories are among the many facilities that can benefit by implementing an ATS since it ensures minimal disruptions or downtime to power availability.

ATS vendors include APC, Dell, Cummins Power Generation, General Electric, Western Telematic, and ABB.

Functions of an ATS

Detecting power failures is among the most important functions of any ATS. It does this by monitoring the voltage and frequency of the primary power source continuously. When these parameters go out of range, the ATS initiates load switching and transfer to the secondary power source.

Since the ATS is connected to both primary and backup power sources, it serves as an intermediary between equipment and the power supplies, acting as an electrical relay. Designed to carry current continuously, it keeps all electrical loads connected to either of the power sources (primary or secondary). To prevent downtime, the ATS remains connected to the power distribution circuits, even under circumstances involving short-circuit or fault currents.

An ATS can also act as a redundant, rack-mounted power supply for equipment connected to a power source with only one cord. Finally, the ATS is meant to both sense when the primary source is restored and retransfer the load to it (from the secondary source) post-restoration.

How does an ATS work?

By default, the ATS is connected to the primary utility power source. When a failure occurs in this system, the ATS invokes a standby power source, such as an uninterruptable power supply. An ATS can also start up more long-term backup power systems, such as local diesel generators, to run electric equipment until utility power from the primary source is fully restored.

The typical load-switching process includes these steps:

• The primary power source fails.
• The ATS checks that the power from the secondary source is stable and within acceptable voltage and frequency tolerance levels.
• The ATS automatically and near-instantaneously switches the load circuit to the secondary source.
• When the primary source is restored, the ATS returns the load from the secondary source to the primary source.

Depending on its design, an ATS can switch or transition the connection between primary and secondary power sources in one of three ways:

• Closed transition. Known as a make-before-break transfer, this is when the ATS connects equipment to the secondary power before closing the connection to the primary source, ensuring that machines will operate without interruptions and preventing downtime.
• Open transition. Known as a break-before-make transfer, this occurs when the ATS first breaks the equipment's connection to the primary source and only then connects it to the backup source.
• Delayed transition. In this method, the ATS breaks the connection to the primary source before connecting to the backup source by adding a delay between the connections, usually to allow inductive load residual voltages to dissipate before the transition.

ATS arrangements

Facilities can choose from many ATS arrangements, which include the following:

• Utility-utility. The ATS switches between two utility power sources that provide redundancy in the power system and enable quick restoration of service if the primary source goes down.
• Generator-generator. The ATS switches between two generators. This is a common arrangement in remote facilities.
• Utility-generator. The ATS switches between a utility service-powered primary power source and one or more standby generators.

ATS devices are also implemented in facilities with three power sources, either a utility-generator-generator or utility-utility-generator. In both cases, an additional backup source (generator) is included to ensure uninterrupted power even if both the primary and first secondary sources fail.

Internal control logic in an automatic transfer switch

The ATS is both intelligent and self-acting due to its internal control logic. This logic allows the ATS to monitor the frequency and voltage of the primary and alternate power sources, and to work automatically and autonomously to invoke the backup power system in the event of a power failure in the primary source.

The ATS' logic is part of an automatic controller, which is a microprocessor that continuously monitors the primary and secondary power sources. The controller also keeps an eye on these sources' voltage and frequency to:

• Identify failure in the primary power source.
• Automatically switch the load circuit to the secondary/backup source.
• Return the load to the primary source when utility power is restored.

Human operators can also use an ATS' manual or nonautomatic operational modes, usually by pulling a handle, rotating a switch, or pressing a button, to initiate the load transfer process from the primary to the secondary power source.

Importance of ATS in data centers

Automatic transfer switches are essential components of emergency power systems in any facility. Data centers in particular rely on these components to prevent downtime. In any data center, it is vitally important to have uninterrupted power. This is the only way to ensure that all connected systems, particularly critical systems, remain up and running at all times.

Uninterrupted power is also essential to prevent data loss, maintain data integrity, protect systems from the physical damage that might result from electrical disturbances like voltage spikes, brownouts, or blackouts, and most importantly, to ensure operational and business continuity. By supporting these goals, an ATS plays a vital role in any data center and is commonly deployed as part of a proactive approach to business continuity and disaster recovery (BCDR) planning

That said, to ensure uninterrupted power and prevent power outages, the systems in the power path must be properly maintained and tested.

Data center administrators need to monitor and maintain the ATS routinely to ensure reliable operations and switching of the load circuit (from primary to backup power source) in the event of a power outage. ATS units can channel considerable amounts of energy during the switching process, which can put heavy stress on the electrical contacts involved, so regular ATS maintenance is critical.

Generators, UPS systems and batteries work together to safeguard data centers against power outages, fluctuations and grid failures. Learn about data center power backup options admins work with to know what system is best for you.