The Protective Role of the UPS

Most people are aware that a UPS is a system that provides battery backup in the event of a mains failure. However UPSs protect sensitive onsite equipment from many other externally-generated electrical disturbance types as well. Authored by Matt Henley, technical services manager for Uninterruptible Power Supplies Limited, a Kohler company, this article looks at why today’s critical loads are vulnerable, the type of electrical disturbances that threaten them and how UPS systems can offer protection. Most people are aware that uninterruptible power supply (UPS) systems provide battery backup during a mains failure. However today’s critical loads require protection from mains disturbances as well as backup power during complete blackouts. The implications for UPS users can be revealed by looking at the nature of the critical loads, the type of electrical problems that can occur, and how UPS topology can provide the protection required. Today’s critical loads Loads are typically critical to their owning organisations’ viability when they involve processing, either of products or of data – and, in either case, processing involves computers. A data centre with its dense population of computer racks is an obvious example; however computers are also embedded within process control systems, medical equipment, communications hardware, ATM and EPOS terminals. As we shall see, no computer, whether it’s a chip buried within an EPOS terminal or a large standalone mainframe, can tolerate mains-borne disturbances. These can cause immediate damage to hardware, or perhaps more seriously, loss or corruption of data. This in turn can mean production loss, possible production equipment damage, inappropriate control system operation, lost business due to EPOS failure, and time lost during repair and reset of affected equipment. Mains power problems The most obvious failure mode is a complete blackout lasting several seconds, minutes or hours. Such situations clearly demand battery and possibly generator backup to maintain any continued activity. However even blackouts of just a few milliseconds can have devastating effects on both hardware and software, as the computers are denied the opportunity for an orderly shutdown. A sudden power loss can for example cause hard drive heads to crash onto and possibly scratch the platters, damage themselves and destroy data. Blackouts can arise from supply line faults, accidents, thunderstorms and other factors. Spikes are short duration rapid voltage transitions superimposed onto the mains supply. Although short-lived, they can have large magnitude and therefore high energy. They can inflict both positive and negative voltage excursions, damaging and destroying electrical and electronic components. Software damage is also possible, which could extend as periodically read files may be corrupted and routine system processing compounds the errors. Spikes are most frequently and dramatically caused by locally grounded lightning strikes, particularly when induced into telecommunications cables. Other causes include thermostats or other equipment switching high electric currents, or load switching by the power companies. Common mode electrical noise arises from electrical disturbances between the supply lines and earth. Normal mode noise comes from line-to-line and line-to-neutral disturbances and can be caused by lightning strikes, load switching, cable faults, nearby radio frequency equipment and other factors. High frequency noise energy entering the earth line can affect sensitive circuits that use the supply earth as reference for internal control logic. This type of interference can be mains-borne or induced through communications cables or other external connections. It can be minimised by fitting surge suppression filters to offending equipment and providing adequate cable screening and earthing.