What is an ATS (automatic transfer switch)

An ATS (automatic transfer switch) is a system that automatically transfers the load from the main power source to a backup source when voltage is lost. Thanks to the ATS, critical equipment keeps running even during a fault in the grid. In this article the engineers at Atlas Scada explain how it works.

Why an ATS is needed

Many facilities cannot be left without electricity for even a minute: fire-fighting pump stations, boiler houses, medical and server rooms, and continuous-process plants. An ATS provides them with uninterrupted power from a second (backup) feed or a generator.

How an ATS works

The system continuously monitors the voltage on the main feed. When it is lost or goes outside the permissible limits, the ATS transfers the load to the backup feed after a set delay. When the main feed is restored, two scenarios are possible

• With automatic return — the load returns to the main feed.
• Without return — the load stays on the backup feed until switched manually.

The choice of scenario depends on the facility's requirements and the type of load.

What an ATS panel consists of

• Incoming devices (contactors or motorized circuit breakers) for the main and backup feeds.
• A controller or transfer control logic.
• Voltage and phase monitoring relays.
• Interlocks that prevent both feeds from being switched on at the same time.

Where it is used

• Water-supply and fire-fighting pump stations.
• Boiler houses and heat substations.
• Commercial and public facilities with critical loads.
• Server rooms, medical institutions, and production sites.

What to pay attention to

• Transfer time — for critical loads a minimal interruption is important; sometimes a UPS or generator is added.
• Reliable mutual interlocks — a "cross" connection of the feeds must never be allowed.
• Monitoring not only the presence but also the quality of the voltage (phase imbalance, sags).

How an ATS differs from a UPS and a generator

An ATS switches between two grid feeds, but there is a brief interruption at the moment of transfer. An uninterruptible power supply (UPS) holds the load with no interruption thanks to batteries, but only for a short time. A diesel generator provides autonomy for hours, but does not start instantly. At critical facilities they are combined: the UPS covers the interruption, the ATS switches the feed, and the generator provides long-term autonomy.

One-way and two-way redundancy

In a simple scheme the backup feed only backs up the main one. In a two-way scheme (mutual redundancy) either of the two feeds can be the working or the backup one — this increases flexibility and reliability for critical facilities.

Reliability matters most

The key thing in an ATS is correct interlocks and logic that prevent both feeds from being switched on at once and rule out false transfers. That is why an ATS panel must be designed and assembled by specialists, not improvised.

The key points in brief

An ATS is "insurance" for the power supply: when the main grid fails, the equipment automatically transfers to the backup and keeps running. For fire-fighting pump stations an ATS is mandatory. Atlas Scada designs and assembles ATS panels and pump stations with power redundancy — reliably and to standards.