Capacitor Failure in UPS Systems

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Riello White Paper
Capacitor Failure in UPS Systems

Capacitors play a crucial role in a UPS system by helping to smooth, filter, and store energy. A typical UPS contains dozens of different types of capacitors in both the main power section and at the printed circuit board (PCB) level.

This White Paper from Riello UPS, focuses on the former – the capacitors in the main power section. These components range in size from a miniature drink can through to a tube of Pringles.

All capacitors in the power section of the UPS are subjected to potential high frequency switching and UPS loads, as well as stresses caused by the physical and electrical operational environment.

How long do capacitors last for?

Along with batteries, capacitors are the UPS components most prone to failure. Capacitors age over time, losing the ability to perform their job.

The electrolyte, paper, and aluminium foil inside the capacitor degrades physically and chemically. Several factors, such as excessive heat or current, can speed up this rate of deterioration.

Accepted industry best practice recommends replacing capacitors between years 4 to 8 to minimise the risk of a serious failure. In specific roles or locations, some capacitors may need to be swapped as early as year 4 to prevent failure.

Factors that can affect capacitor lifespan

Capacitor lifespan is shortened by any detrimental environmental extremes (i.e. hot or cold temperatures) and its workload.

  • Excessive current
  • Overuse
  • Excess heat
Signs a capacitor is about to fail

There are several tell-tale signs a service engineer might spot when carrying out routine preventive UPS maintenance, including:

  • Oil leakage
  • Deformation
  • Scorched wires connected to capacitor
  • Burnt valve cap protrusion
  • Increase in temperature
  • Capacitance
What happens when a capacitor fails?

There are two main types of capacitor failure. Firstly, they can fail in an “open” condition where they basically stop working. This type of breakdown can often pass undetected, as there’s little visible evidence something has gone wrong.

The second option is a “short” condition failure, where there’s an obvious leak of the dielectric medium. Sometimes there’s even a loud popping sound like a firework.

In some cases there is a third condition where the capacitor will fail gradually and fall out of tolerance.

How does capacitor failure affect a UPS?

This is dependent upon where the capacitor is located, the overall health of the capacitors, and whether or not they’re working in a series or parallel.

Ultimately however, capacitor failure will negatively impact the performance of a UPS. Its filtering ability will suffer, there will be more issues with harmonics and electrical noise, the energy storage volume will reduce, and battery strings can be damaged.

In the worst-case scenario, a serious capacitor failure will trigger the UPS to switch to bypass mode, which leaves the critical load unprotected.


There is no standard age at which a capacitor will fail. To ensure the ongoing performance and reliability of your UPS system, it is advisable to treat them as consumables and proactively plan their replacement before they reach the end of their rated service life.

The recommended time frame for this is between years 4 to 8, depending on location and role.

To find out more, read the full Riello white paper entitled: Capacitor Failure in UPS Systems. This is available via the link found further down this page.

White Paper from Riello UPS


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