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Uninterruptible power supplies (UPS) have become commonplace products in most businesses today. Their primary purpose, of course, is to protect against utility power outages. They are intended to supply power to electrical equipment whenever utility power is lost. However, UPS equipment is routinely marketed as being a cure-all for every AC power anomaly. UPS products are sold not only as backup power sources, but as noise filters, regulators, and surge suppressors. In practical applications, though, only the very expensive top end UPS devices can truly support all those claims. While lower end UPS equipment can be designed to filter and to regulate the AC power that is supplied to its equipment loads, they cannot provide adequate protection against transient power surges.
All UPS equipment is not the same. These devices come in models that differ from each other dramatically in design, configuration, and capability. They are available in Standby, Line Interactive, and Double Conversion design configurations. While generally speaking, they do share the same electronic building blocks in three basic operating stages, those stages are distinguished from the different UPS types by their design complexity and by their intended function.
Any UPS will normally employs a rectifier in its first operational stage to convert the incoming AC to DC. Batteries are usually found in the second stage. The DC output from the rectifier charges the batteries as it supplies power to the inverter, the third basic UPS stage. The inverter changes the DC power at its input back into AC at its output. That output, of course, dispenses power to its loads. Most UPS devices employ bypass circuits that allow their equipment loads to circumvent the UPS and draw AC power directly from the utility. Higher end products only make use of their bypass lines during maintenance procedures or when the UPS is inoperative. However, low end and midrange products exploit them during their normal operating parameters as their inverter stages are only intended to power their equipment loads during an actual utility power outage. While these UPS products will often utilize small metal oxide varistors (MOV) on their utility power inputs to support their surge suppression claims, the realized levels of real-world surge protection are minimal. An inexpensive surge strip that can be purchased at any hardware store will do as much. And even that practice may be changing as the UL1449 safety requirements pertaining to surge suppression components have become much more stringent as of February 2007. Most of the TVSS products that met the applicable UL requirements before that date are not able to do so now. To be deemed safe by UL without exceptions, TVSS components must be subjected to far more intensive testing parameters than before. The cost involved with incorporating true TVSS protection within low and mid range UPS design parameters renders that effort impractical. Low and midrange UPS users are far better served protecting their critical electronic equipment loads, including their UPS equipment, with quality TVSS products that are designed for that specific purpose.
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