In this article, Keith Moran, Operations Manager at KUP explains the UPS’s vital role during mains presence as well as in blackouts and shows the importance of selecting on-line systems.
Most people in IT are aware that UPSs use batteries to seamlessly take over the critical load if the incoming mains supply fails. While they do indeed fulfil this role, UPSs also perform an equally critical protection function throughout normal operation, with mains available. This is because any mains supply is liable to many types of disturbance apart from total blackout – and these have the potential to damage or destroy any unprotected sensitive load.
Data centre loads are typically described as critical, both in the sense of their availability to the application relying on them, and of their own dependence on high-quality power at all times.
A power supply transgression’s immediate effect could be to cause an equipment failure arising from component damage. However, even if the supply problem only halts the load rather than damaging it, the consequences can still be serious. An unexpected hardware stop will cause a software crash, leading to data loss or corruption. Business transactions will be interrupted and lost, exposing the enterprise to wider financial implications and loss of reputation.
In a manufacturing environment, the results could be equally if not more serious; control systems could be driven into inappropriate operation, for example. Both production equipment and product can be damaged, with time needed for cleaning up as well as repairs.
So, how serious is the threat to your particular installation, and what can you do to mitigate it? The answer depends on three factors; the type of power disturbances that could appear on your mains supply, the type of equipment to be protected and its susceptibility to these problems, and the steps you take to provide protection.
Spikes are short duration rapid voltage transitions superimposed on the mains waveform. They can inflict both positive and negative voltage excursions, damage or destroy electrical and electronic components, and corrupt software. Software problems may be particularly difficult to track down and rectify, as they may not show until some time after the damage occurred.
Spikes are typically caused by equipment switching high electrical currents, or load switching by power companies.
Surges are voltage increases above normal mains levels that exceed one cycle. They typically appear after a large load is switched off or following load switching at substations.
Sags are drops in the mains supply that can last for several cycles. They are generated similarly to negative spikes but have a much longer duration. Sags are very common occurrences that are usually the result of switching on large loads like air conditioning equipment, or starting rotating machinery. Sags can cause a computer re-boot if the mains voltage falls so low that the computer believes it has been switched off.
Brownouts are identical to sags but are of much longer duration and generally more serious. They arise when the mains supply cannot cope with the present load demand, so the generating company drops the overall network voltage. Brownouts can last for several hours in extreme circumstances.
Blackouts are complete power losses, where the mains supply fails totally. Caused by supply line faults, accidents, thunderstorms and a range of other conditions, they have an obvious, sometimes devastating effect.
Computers typically have specified upper and lower limits for steady state slow averaged rms line voltage variations of between ±5% to ±10%, depending on the manufacturer, but will tolerate short duration line voltage excursions outside those limits. The shorter the duration of the excursion, the greater the excursion which can be tolerated. Computer equipment resilience to mains disturbances is very limited; protection from the mains supply at all times is essential.
The most important measure is to select a UPS with on-line double conversion topology which will protect against frequency variations and voltage issues. This provides the highest level of power protection because it positions the rectifier and inverter as barriers between the supply and the load; these remove mains-born noise and transient voltages. Importantly, the inverter maintains its level of supply regulation even when it is operating from the UPS battery during a power failure.
As well as filtering out events like spikes, the UPS also protects against excursions beyond a preset voltage range caused by surges or sags. It does so by switching to battery power, as it would for a complete blackout.
Above, we have seen how a critical load can be threatened as much by live utility mains supply problems as by blackouts. It also becomes clear that to provide true UPS protection against all potential supply contingencies, selecting a system with dual on-line topology is essential. Users then enjoy both protection from mains aberrations, and battery autonomy during blackouts.