Anyone who has ever learned a lesson the hard way never forgets the lesson.  One such truth is that all Uninterruptible Power Supplies are not created equal. 

This is increasingly important in the digital age - because unlike analog dials and knobs which stay put during a power fluctuation, digital signals can be lost during a power variance.  And big-time problems and expense can follow.   Here's a quick primer on some power quality terms that may shed some light on some problems you may be experiencing.

Offline (or line interactive) UPS  
This type of uninterruptible power supply can be best viewed as "batteries in a box" because it monitors incoming line voltage and supplements it with battery power when the voltage drops below a certain level.  The transfer time, which is inherent with line interactive technology, is typically 4-6 milliseconds.  This is noticeable to both human observers and more importantly sensitive electronic equipment.  The result can be lost or corrupted data.

Online (double conversion) UPS
This type of uninterruptible power supply takes incoming utility alternating current (AC) and converts it to direct current (DC) which charges the batteries.  The direct current (DC) is then converted back to alternating current (AC) which is supplied to the protected load as clean power at the desired voltage.  Dropping the incoming line voltage does not cause activation of the batteries since the input AC is not the primary power source. Therefore an input power failure does not cause a transfer - so there is no transfer time (and no interruption of power to the protected load).

The Bottom Line
Harmonics present with offline UPS:    Will be passed through to the protected load.  

Harmonics present with online UPS:    Will be mitigated because a clean sine wave is created by the double conversion technology.

A Real-World Example
A major bottling plant learned a difficult and expensive lesson on the differences in UPS topology when they were having problems with their bottling line.  Power quality problems were causing their "batteries in a box" UPS to kick on.  But the transfer time - 4-6 milliseconds - was of sufficient duration to cause their PLC's to drop the instructions from memory.  When power from their line interactive UPS was returned, the PLCs re-started at step 1 - and all of the product currently on the bottling line was scrapped.  Naturally this was frustrating - and expensive. 
 
Switching to Staco double conversion UPS solved the problem.  Since replacement of the multiple UPS units, the line has not had a single shutdown.  Scrap rates plummeted, efficiency increased, and life is good once again on this bottling line.

Spare Yourself the Cost & Aggravation!
While lessons learned the hard way are permanently etched in memory - they can be expensive.  Spare yourself the cost and aggravation.  Visit www.stacoenergy.com or http://www.stacoenergy.com/ups/on-line-uninterruptible-power-supply.htm to learn more about UPS topologies and which may be best for you. Better yet, contact your local Staco Energy representative and discuss your current situation.  They'll guide you through the UPS selection process!