A Slow But Certain Threat That Can Compromise Your UPS


Among the many components of a UPS, any can eventually fail and cause catastrophic loss in absence of regular maintenance. But the most dangerous are the parts for which the failure isn’t immediately noticeable.

One such component is the capacitor. A capacitor is among the most ignored and least understood components of a UPS.

When capacitor failure happens, you may not notice right away. But it’s not safe to assume that the UPS is still functioning properly and that your performance will not be negatively affected.

More often than not, capacitor failure can push your UPS system into bypass mode, making the critical loads extremely vulnerable.

What do capacitors do?

Capacitor failure isn’t that simple

An AC input filter capacitor filters harmonic however at the output, the AC filter capacitors will contribute to provide a high-quality sinewave for the critical load. Each UPS usually has multiple capacitors working in combination to condition the power that gets supplied to the UPS.

Now what’s tricky with capacitors is that if one fails, another will step up and take the burden off the failed capacitor. So, you may not notice any significant effects instantly.

You may think that there is no reason to worry in such a case because the UPS is still working and there are more capacitors to account for the failure of one, but this isn’t necessarily true.

As proven by this white paper by Eaton the extent of damage that a capacitor failure can cause also depends on the condition it was in when it failed.

  • When a capacitor gets compromised in an “open” condition, it’ll simply stop working.
  • But when it fails in a “short” condition, it can cause a leak of the dielectric medium.
  • And if the venting puts too much pressure, this can lead to an explosive “pop”.

Although most capacitors are designed in such a manner that the electrolyte releases gently in case of a leak, but there are no guarantees that things will work as designed in the real world. In the worst case, the corrosive substance can reach and impact other components.

What’ll happen to the UPS?

If you’re still wondering how all this would affect the UPS, this again doesn’t have a simple answer. It depends on the current health of capacitors, whether they are functioning in series or in parallel, and the section where they are located.

If a capacitor has failed in an “open” condition, you’ll not even witness any leaks or signs of problem. Though the problem is there, and it can’t be ignored.

The performance of your UPS will be affected and its filtering ability obviously will not be the same as before. As the Eaton paper puts it, a typical three-phase UPS shifts to bypass mode when a capacitor in the power train gets compromised.


So what’s the solution?

Since it’s difficult to detect a capacitor failure, the best approach is to nip the problem in the bud before it arises. The steps below can ensure that your operations are safe from power failure:

#1. Maintenance

Your UPS capacitors should be subjected to a thorough assessment, typically when it reaches 5 years of life.  This inspection includes visual and thermal scanning; measurements to identify and resolve critical risks.

DC Capacitors : Every 5 years (Like batteries)

AC Capacitors Input/Output : 5 to 7 years

#2. Proactive capacitor replacement

Paper by Emerson a very useful resource to keep a check on life cycle of both AC and DC capacitors. It’s important that either you or your service provider knows everything that should be included in UPS lifecycle maintenance.


Your UPS capacitors should be subjected to a thorough assessment, typically after every 6 years. This inspection includes thermal scanning; visual scanning and more to identify and resolve critical risks.

#3. Consistent UPS maintenance

Regular maintenance from a trusted provider will make your UPS and operations more efficient and safe. This will include verification of your AC and DC capacitors filters as well as testing of fans, cleaning of filters, removal of dust, calibration of your unit and more.

As a result of quality maintenance and service, your UPS will continue to operate at a cooler temperature and cap life will significantly improve.

That’s why preventative maintenance is the only route one should adhere to. Sitting idle and doing nothing until a capacitor failure occurs can incur huge replacement costs down the line, along with intense pressure and frustration.

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