An uninterruptible power supply is a complex electronic device with a number of components that deteriorate with age and use. It is therefore in the client's interest to have regular maintenance carried out on the UPS to ensure that it is working correctly and that the components that are known to deteriorate with age and use are monitored.

On analogue designed UPS systems it is necessary to regularly monitor and adjust the operating parameters of the control circuitry to ensure that the UPS is operating in its most efficient mode. With the digital designed UPS systems there are fewer adjustments to be made as; unlike their analogue counterpart the circuits do not alter with the aging 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 with a reputable company specializing in power protection is strongly advised especially when you may require 24-7 callout 365 days of the year.

All the components mentioned here have a recognised design life dependent upon their operating conditions. Normal operating conditions would assume that the UPS and its battery is installed in a clean, dry environment with a constant ambient temperature not exceeding 20 ? C. Also it is generally accepted that it is more prudent to operate the UPS at a load not exceeding 80% of its kW rating.

Batteries

Battery life, it should be noted that UPS systems generally use the sealed valve regulated lead acid battery, is determined by the manufacturer and is generally quoted with the expectation that the battery will be operated in an ambient temperature of 20C and not exceeding 25C. The design life of the battery by the manufacturer will be quoted in terms of 3-5 year life or 7-10 year life, in exceptional cases it can be as much as 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.

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 bloc on float (normal operation) and after a set discharge time. This can identify any blocs 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 bloc 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 bloc can support falls, if the battery is continually used beyond its design life the risk of fire can become very real. This is due to the fact that the battery can no longer support the current demand generated by its internal chemical reaction and will overheat; possibly entering a thermal runaway situation where the chemical reaction cannot be stopped with the result 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 disputation to your mission critical load.

generally these have an operating life of 3 – 5 years dependent upon the manufacture. These fans are used for cooling the heat sinks of the power components as well as extracting excessive heat from the wound components such as transformers from the enclosure. Any failure will cause a rapid increase in temperature resulting in the inverter or rectifier/charger switching off 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 will allow this work to be carried out under controlled conditions rather than 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, also they provide a reservoir of instantaneous power for the inverter switching devices (thyristors, transistors or IGBT's). With time they become less effective and two things can occur; one is that the battery ripple current increases, the other is that the inverter can fail to operate under load as it cannot derive sufficient current from the rectifier/charger 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 some cases just leaking, in others 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 .

AC Capacitors; the ac capacitors act as a filter to ensure that the UPS output waveform is sinusoidal within a close tolerance, also they 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. They should be checked on a regular basis similar to the dc capacitors, this time for balanced currents in the case of a three phase output UPS system, case temperature and ‘purity' of the ac output waveform. In the event of failure the result is more dramatic, not only do the fire brigade arrive on site but the load is also lost.

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