Surge Protection for ATS

When applying surge protection, there may be other power-quality devices installed in the electrical system that should be considered. These devices may include, but are not limited to, uninterruptible power supplies (UPS) and generators with Automatic Transfer Switches (ATS).  The purpose of this series of articles is to provide a basic understanding of how these systems work.


An ATS is a device that interfaces with a generator and the building's electrical system. It monitors the utility power and signals the generator to start if the utility power goes out of spec or drops out entirely (blackout). Backup power is now fed to the main utility panel or an emergency panel via the ATS.

If the ATS suffers damage resulting from a transient, the user may be left with no power to his emergency load from either utility or generator.

An ATS, typically, has two inputs and one output.  The inputs are a standby generator and utility power.  The ATS will automatically turn on the generator in the event of a power failure or it can be manually turned on due to an approaching storm or for UPS maintenance.  Most critical facilities consider their generators to be a more reliable and cleaner source of power than the utility.


An ATS generates switching transients much the same way a utility does. These transients can damage downstream equipment. A surge protector is always recommended downstream of an ATS.

Historically, the front end of an ATS did not need protection due to sufficient withstand ratings. However, modern ATS employs denser microcircuitry than ever before. Critical control boards suffer damage from transients on the input. If the generator is located inside the facility, the emergency input probably does not need protection. Generators located outside facility come into contact with nearby lightning strikes that can bring a surge directly into the building. A surge protector on the emergency feed is an important consideration.

The diagram above shows a simple make-before-break ATS.  There are other styles of transfer switches that include bypass-isolation, expensive and not very common, and static transfer switches, which are even more expensive and much less common, but the same basic principles apply.

If you have any questions or wish to learn more, feel free to call 1-800-851-1508 or email Paul Moraff at

Check out our full line of AC surge protectors that can protect an ATS application