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Uninterruptible Power Supplies need regular inspection and service

Monday, August 20, 2018

Having no regular check of the functionality of the UPS is equivalent to hoping an insurance policy will continue to provide cover without premium payments. Contact us for a no obligation inspection of your system and quotation for a service contract 

The importance of regular inspection of UPS equipment for mission critical applications cannot be overstated. Because of the unpredictability of mains power failure incidents, and the distinct possibility of periods as long as a year or so with no mains interruption, you might not be aware that the UPS could be unable to ‘kick in’ when needed.

Layout of UPS systems

The diagram in Fig 1 is helpful in understanding the basics of operation and illustrates an UPS typical of larger installations, technically described as an ‘in line’ system. 

Normal mains electricity supply is alternating current (AC) as shown graphically at the input to the UPS, under mains availability conditions. The sine wave nature of voltage and current (AC as supplied by the electricity network) has to be mimicked by the UPS when mains power fails.

The power for your facility, in the event of an interruption in mains power comes from a battery bank, is shown in the diagram. The charge for the battery bank is supplied during times when power is available from the network. Batteries store direct current (DC) energy and require DC energy to charge them. The diagram shows a ‘converter’ block, basically changing the AC supplied by the network to DC for battery charge replenishment.

The basic architecture of an ‘in-line’ UPS

The last section of importance is the block labelled ‘inverter’, where DC from the battery bank is changed (inverted) back to AC. Something has emerged that you may not have been aware of: the in-line system illustrated is operating as an UPS even when mains power is available! In other words, the batteries are being continually charged (trickle-charged) and the inverter is always supplying AC to your critical loads.

You could be unaware that you have lost your critical power backup. In fig 2, the diagram as shown in the first figure is copied PLUS ‘an automatic bypass circuit’ shown in red. This bypass is automatically activated if there is a failure in the UPS, switching the critical load directly on to the network mains.

The function of the automatic bypass is to provide power in the event of a malfunction of the UPS

Batteries are an obvious area of concern, and further down, there is a brief description of typical tests conducted as to their state of health (SOH). There are other critically important areas as well.

The result of not regularly testing batteries regularly can result in catastrophic failures


In both figures 1 and 2, the capacitor bank, ahead of the battery bank is shown. Capacitors are essentially very short-term electric charge holding components, and very similar to batteries, for example, spilling electrolyte when they get hot, and therefore causing much damage. The function of capacitors is to ensure smooth trickle charging of the batteries during normal operation of the UPS. In fig 4, breakdown of capacitors is shown. Electrolyte has spilled, providing a fire hazard.

The middle capacitor has spilled electrolyte and presents a fire hazard

Power electronics

The converter (converting AC to DC) and the inverter (converting DC to AC) are examples of power electronics. Both use up power, can get very hot, and fail as a result. This is particularly the case for the inverter which is basically a group of very fast operating switches, typically 4000 or more on-off operations PER SECOND! These electronic switches use significant mounts of power and consequently require cooling. Inspection of the electronic switches is very necessary. Fans blow air over the metal ‘heat sink’ sections so that the electronic switches operate in a thermally stable environment and making sure this cooling is functioning properly is vital.

The left hand and centre transistors show signs of serious overheating


Battery banks in UPS systems comprise of paralleled strings (a string is simply a number batteries daisy chained in series to provide the required voltage). The strings are bundled together (paralleled) so that the battery bank can deliver not only the required voltage but also the power.

The VRLA batteries require frequent testing by the time 80% of capacity has been used and the testing frequency should be at yearly intervals. Part of a battery maintenance practice can include amp-hour data and in any event, the measurement of open circuit voltage and battery impedance. This latter parameter measures the resistance to current flow through the battery and this resistance builds up over time. A build-up in the value of the impedance parameter is an excellent predictor of battery failure. The combination of the open circuit voltage test and impedance provides the decision level for battery replacement. Record keeping is therefore essential and an integral part of a maintenance program.

Bypass circuit

In the event of a failure in the UPS, no matter what its cause might be, the automatic bypass circuit has to supply power to the critical load. In effect the UPS is taken off line, but in the process automatically switches the bypass circuit to ON, thus maintaining power for the critical loads. This particular test is conducted in consultation with the client because it involves a short disruption of supply.


For further information, please contact :


Power Parameters Pty Ltd,
Unit 24 & 25, 1 International Drive
Weastmeadows, VIC 3049
Freecall: 1 800 623 350 
Freefax: 1 800 067 263

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