1. Capacitors are designed to smooth out voltage fluctuations. Fairly simple devices that range in size from a thumbnail to a soda can, a capacitor’s main function is to store and release electrical energy. Generally encased in aluminum or chromium-plated cylinders, capacitors can be either AC or DC in nature. DC capacitors primarily smooth out voltage fluctuations by eliminating peaks and filling in valleys. While AC capacitors — found primarily in the UPS’s input and output filters— have many of the same characteristics as DC versions, they also reduce harmonic distortion on the utility input to the backup power UPS and connect directly to the critical load output to help control the waveform shape of the output voltage.

2. The number of capacitors in a UPS varies. A typical UPS contains a dozen or more different types of capacitors, from those that even out the power supplied to the UPS processor to others that regulate power flowing to protected equipment. The inventory of capacitors inside a UPS varies tremendously depending on the kVA rating of the unit. To put it in perspective, a typical personal computer contains about 50 capacitors, while a 750 kVA three-phase UPS likely has hundreds. Even the smallest UPSs rely on dozens of capacitors.

3. Capacitors degrade over time. Just like UPS batteries , capacitors have a definitive lifespan and will break down after a certain amount of time. While a typical capacitor might be rated by the manufacturer for seven years of round-the-clock use, under favorable operating conditions it could potentially deliver up to 10 years. Wet capacitors, when properly designed and fabricated, show very gradual changes in essential characteristics when operated at normal rated voltages. However, the paper, aluminum foil and electrolyte inside capacitors are subject to normal breakdown. As those materials age and start to degrade physically and chemically, they lose capacitance and the ability to properly perform their job.

4. Multiple factors influence a capacitor’s lifespan. The operational lifetime of a capacitor depends on a number of dynamics, including the function of the rated voltage, applied voltage, the amount of over-current, ambient temperature and thermal resistance. Failures increase in likelihood the older the capacitors get. When a capacitor fails, others have to take over the workload, which will also shorten UPS capacitor shelf life.

5. Keep them cool and clean. As mentioned, adverse operating conditions can hasten the demise of capacitors. The hotter the operating temperature, the shorter its lifespan will be. Keep in mind that heat is derived not only from external environmental temperatures but from internal sources such as a clogged or dirty air filter.

6. You may not be aware of a UPS capacitor failure. When a capacitor fails in an “open” condition, it simply stops working as intended. This kind of failure is quiet and can pass unnoticed. On the other hand, if it fails in a “short” condition where it develops a dielectric leak, the capacitor will often vent with a loud pop that sounds like a firecracker. Because electrolyte is conductive, a spill can make unintended connections that can disrupt UPS performance or interact with other components inside the equipment. For these reasons, many electrolytic capacitors have a scoring mark at one end, which is designed to rupture and leak gently, rather than allowing the container to vent suddenly and spread electrolyte across a wide area.

7. Capacitor failure can impact UPS functionality. Depending on where the failed capacitor is located, the number of capacitors working in series or parallel, and the overall health of other components, it’s possible that a failure won’t dramatically impact the UPS. For instance, if capacitors are operating well below their voltage rating, one or two might readily be able to take over for a failed capacitor without significantly affecting UPS operation. And while a failed capacitor in the power train can diminish the unit’s overall filtering ability, it won’t render the unit nonfunctional. However, some capacitor issues can spiral out of control quickly. When Spiceworks asked members to share their worst IT disaster, one follower summed up his UPS capacitor failure in this way: “Basically one of the capacitors got dried out, Positive met Negative and they loved each other. Boom. They had a baby and named it Fire.” Be sure to read the full account, because the pictures alone are worth a thousand words.

There are other times when the end result is not so dramatic. For instance, when a capacitor in the power train fails, most three-phase UPSs will transfer to bypass mode. During this time, the UPS — while operational — won’t actually provide protection to downstream equipment. In rare cases, a failed capacitor can disrupt power to the logic processor, but leading UPSs are designed to execute a safe and orderly shutdown in that unlikely event. However, it is never wise to have components shorting out inside your electronic equipment, regardless of whether the piece is mission-critical to the overall architecture.

8. UPS capacitor replacement is essential. Capacitor replacement is part of regular UPS preventive maintenance. Based on the manufacturer’s rated service life, Eaton recommends swapping out capacitors used in the primary power-train portion of the UPS after seven years. However, newer UPSs typically have electrolytic DC capacitors that are rated for the full life of the UPS.

9. Preventive maintenance is the best approach to maximize capacitor life. When an individual capacitor fails, it’s often a sign that others are not optimally performing. Left unchecked, this could trigger the failure of additional capacitors and ultimately diminish UPS performance. By the time degradations show up on monitoring reports, the damaging process has perpetuated itself and the replacement cost will be higher than if end-of-life capacitors had been identified and replaced earlier.

While complete power capacitor replacements are recommended to proactively safeguard UPSs against catastrophic failure and reduce the risk of unplanned downtime, maintaining the components on a regular basis is equally imperative. Ensuring the recommended ambient temperature, humidity and cleanliness of capacitors will go a long way toward maximizing their lifespan and reliability. Predictive monitoring is one of the best tools to safeguard your UPS against potential issues such as capacitor failure. As the first cloud-based analytics service for data center infrastructure to predict the failure of power components, Eaton PredictPulse Insight adds predictive analytics to Eaton’s next generation PredictPulse remote monitoring service, shifting power monitoring from a reactive to a proactive model.  Collecting and analyzing data from connected power infrastructure devices, the service provides Eaton with the insight needed to make recommendations and take action on your behalf.

10. Engage the original manufacturer. Only a certified technician should replace capacitors, preferably the original UPS manufacturer and not a third-party service organization. The company that designed your equipment will know it better than anyone else, and will provide technicians that are trained, certified and have access to factory-authorized parts, proprietary diagnostic software and engineering updates.

Now that you understand what capacitors are, what their role is and why they’re essential to the performance of your UPS, it is clear that proactive maintenance for these components is essential. To maximize the performance and reliability of your UPS, treat capacitors as the perishable commodity that they are.