It is a common misconception that UPS does not need surge protection.

The purpose of this article is to offer a basic understanding of how a standard, double-conversion UPS works and why a surge protector is an important element for reliable operation of that UPS.

The diagram to the left shows the three power paths in a standard on-line, UPS system. The primary power path includes:

1. A rectifier, which converts AC to DC.

2. An inverter that converts DC back to AC.

3. Batteries (DC) that supply power to the inverter in the event of a power failure. 

In addition to the normal rectifier/inverter path, there are two bypass switches that place the critical load on raw utility power.  The automatic “static” bypass is a make-before-break overlap transfer switch.  This switch will activate in the event of a failure in the normal power path.  The manual bypass, also known as the maintenance bypass, manually places the load on raw utility power for corrective or preventive maintenance.

A transient can gate the automatic (static) bypass switch closed, transferring the surge directly to the critical load. While most UPS systems specifications indicate that they do provide some transient protection, there is no status indication. A UPS is not a surge protector. The user will not know if the surge protection fails.  Transients have been known to damage the input harmonic filters of UPS systems causing an entire system meltdown. 

Some variations of the standard UPS shown in the diagram include dual input from two utility sources into the same UPS and external maintenance bypass, also using dual power supplies for the connected load.  Obviously, both of these power paths must be protected.

If a UPS system is in place, we can safely assume that the connected load is of critical importance.  A surge protector, placed on the panel (or panels) that feed the UPS, is an inexpensive solution to a potential disaster.

If you have any questions or would like to learn more about these systems and how to apply surge protection, feel free to call 1-800-851-1508 or email Paul Moraff at