UPS Components Can Fail – Why?

UPS components can and do fail

Why? How does that happen?

An uninterruptible power supply (UPS) is a complex electronic device.
It has a number of components that deteriorate with age and use. Therefore, it’s in the client’s interest to have regular maintenance carried out on the UPS.
This is important to ensure that it is working correctly. Monitor components do deteriorate with age and use.

Analogue UPS System

On analogue designed UPS systems it is necessary to regularly monitor and adjust the operating parameters of the control circuitry. This will ensure that the UPS is operating in its most efficient mode. With the digital designed UPS systems, fewer adjustments need to be made.

Unlike their analogue counterpart, the circuits do not alter with the ageing of their components.
However, in addition to the batteries and fans, there are a number of components in the power circuits that need to be monitored and replaced on a regular basis. A maintenance contract is strongly advised. Especially when you may require 24-7 callout 365 days of the year. Aim to choose a reputable company specialising in power protection.

All the components mentioned here have a recognised design life dependent upon their operating conditions. Normal operating conditions for the UPS and its battery, are a clean and dry environment with a constant ambient temperature- not exceeding 20 degrees Celsius. Also, generally it is more prudent to operate the UPS at a load not exceeding 80% of its kW rating.


Battery life is determined by the manufacturer. The ambient temperature for batteries is 20C. It is noteworthy, that UPS systems generally use sealed valve regulated lead acid battery.
The design life of the battery by the manufacturer will be as following:
3-5 year life
7-10 year life
10-12 years

The lower figure quoted normally takes into account both the AC ripple on the DC charging current of larger UPS systems and also the fact that on smaller systems the battery has less airflow round it.

Battery Maintenance

It is possible to monitor the life of the battery by carrying out a regular (every six to twelve months) controlled battery discharge, checking each block on float (normal operation) and after a set discharge time. This can identify any blocks that may be prematurely failing, equally, as a matter of ‘good practice’, one should not patch a 3-5 year life battery string on year 3 as the new block will tend to cause further problems.

An alternative method of monitoring batteries is to carry out impedance testing and monitor the figures over a period of time against the recommended values set by the battery manufacturer.

During the course of its life, the valve regulated sealed lead acid battery will give up hydrogen due to the chemical activity experienced by discharges (controlled and mains failures) and the ripple current supplied by many UPS systems on the DC bus. This hydrogen is irreplaceable, and as a result, over a period of time the AH that the block can support falls. A high of fire is continually using batteries beyond their design life. As a result, the battery can no longer support the current demand generated by its internal chemical reaction and will overheat. Likely to then enter a thermal runaway situation where the chemical reaction cannot be stopped, resulting in of uncontrolled fire.

When the time arrives for replacement of batteries, use a company that will both remove the batteries and replace them at the same time, to avoid any disruption to your mission critical load.


Generally these have an operating life of 3 – 5 years dependent upon the manufacturer. These fans are used for cooling. The heat sinks off the power components as well as extracting excessive heat from the wound components. Any failure will cause a rapid increase in temperature, likewise resulting in the inverter or rectifier/charger switching off. This would be due to the over temperature alarm and the load transferring to the utility mains (reserve supply) via the static switch. The load will now be unprotected from any utility mains fluctuations.

Some UPS manufacturers will build fan redundancy into the design of their uninterruptible power supplies, providing indication of the fan failure without compromising the load due to inverter shut down. A regular replacement routine during maintenance visits is essential. It will allow this work to be carried out under controlled conditions. Instead of waiting for failure and the resulting loss in the protection of the load.

DC Capacitors

As mentioned above, they have a smoothing effect on the battery charging current and provide a reservoir of instantaneous power for the inverter switching devices. These devices are thyristors, transistors or IGBT’s.
With time they become less effective and two things can occur;

  • possibly the battery ripple current increases
  • the inverter can fail to operate under load.

This is due to it not deriving sufficient current from the rectifier/charger. Especially to supply the switched mode waveform that makes up the sinusoidal output waveform.

In extreme cases, the chemical reaction can become so fierce that the safety valves will open. In other cases, just leaking or even causing a fire. Interestingly enough the inverter will often transfer the load to reserve without interruption.

Of course, there is the cost of the fire brigade’s attendance and the refill costs of the extinguishant. The normal design life for DC capacitors is similar to the battery, being approximately 5 years. This can be monitored by checking the AC ripple current during regular maintenance of your Uninterruptible Power Supplies.

capsAC Capacitors

The AC capacitors act as a filter to ensure that the UPS output waveform is sinusoidal within a close tolerance, and also act as a reservoir for out of phase loads.
Again, the capacitors are similar in some respects to the DC capacitors as to what damage they can cause if neglected. Regularly check your DC capacitors. Check for balanced currents in the case of a three phase output UPS system. Also, temperature and ‘purity’ of the ac output waveform. In the event of failure the result is more dramatic. The fire brigade arrive on site and the load is also lost.

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