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How can an office be assured that its computer systems are protected from any loss of utility power? Businesses have found that an uninterruptible power supply (UPS) is the technological insurance policy against this potential catastrophe.

Uninterruptible Power Supply (UPS) systems provide an interruption-free source of electricity. First developed over 25 years ago specifically for mainframe computer systems, UPS systems have been continually upgraded and made more compatible for today's smaller personal computer systems (PCs) as computer technology has evolved.

The ever-expanding use of computer hardware over the past two decades has increased the demand for these sophisticated systems. Financial and insurance companies, hospitals and firms that are computer dependent know the value of central UPS systems. Smaller firms that depend upon computers and PCs for their payroll, billing and other key functions are installing individual UPS units locally.

Today, scientists in the laboratory can save essential experiments from destruction or prevent distorted results by installing a UPS. Similarly, the CADD systems found in many architectural and engineering companies are backed up with a UPS.

Information Protectors
UPS equipment protects sensitive electronic hardware from voltage spikes and surges, sags, brownouts, blackouts and other power supply disturbances that can destroy data and damage equipment. The systems range in physical size and price according to the amount of electrical load served. A UPS system to back up a mainframe computer system can be large and require an entire room (for example 20 x 30 feet). For personal size computers, there are UPS modules manufactured small enough to sit on or beside a desk. Costs range from a few hundred dollars to hundreds of thousands of dollars. Large redundant UPS systems can cost as much as $1000/per kilovolt ampere. However, the investment can offset the financial loss a company could incur if its database is wiped out during a power system disruption.

A UPS assures that electrical problems in the power supply system do not reach the computer hardware. The goal is to minimize the risk of loss of memory, damage or malfunction due to loss of power, voltage variation, transients and harmonics (see glossary). There are two types of UPS systems currently available. One is a "static system" containing only solid state components and having no moving parts. The second is a rotary system powered by a motor driven generator.

Static UPS. A typical static UPS system consists of a rectifier (to convert utility AC to DC), storage batteries (the uninterruptible backup) and an inverter (to reconvert the DC back to AC for supply of loads). A static (solid state) switch is provided to transfer loads from the UPS source to an alternate utility source in the event of UPS system failure or maintenance.

Typically a static UPS system is "on-line". This means that during normal operation the incoming utility power is rectified (AC to DC), inverted (DC to AC) and supplied directly to the load. The internal conversion to DC provides the opportunity to interconnect a battery backup into the power supply chain. This is the feature that makes that system uninterruptible. In the event of power supply loss or disturbance, the batteries instantaneously supply loads.

The batteries in small systems can be integral to the rectifier/inverter or may be cabinet mounted. In large systems, the batteries are generally rack-mounted in remote rooms.

Rotary UPS. A rotary UPS system consists of an AC motor generator (M-G) set with a flywheel, as well as a rectifier, storage batteries, inverter, static switch and solid state circuitry. Both the motor generator set and the rectifier/battery/inverter combination are supplied by the incoming utility service. They represent parallel supply paths and either path is capable of supplying the load. A static switch selects the path to be utilized to supply the load. During normal operation, the M-G set powers the computer loads, while the off-line static section is on "stand-by" and charges the system batteries.

Upon loss of the utility feed, the control circuitry will disconnect the M-G set from the utility by opening the static switch and closing the inverter-output circuit breakers, allowing the system batteries to power the M-G set through the inverter. Mechanical energy "stored" in the flywheel allows the M-G set to continue to deliver its full-rated output for (generally) a minimum of 200 milliseconds. This provides sufficient time for the control system to sense a loss of utility supply and to transfer to the battery/inverter combination for supply of loads.

Which System is the Right System?
The difference between static and rotary UPS systems goes beyond their physical makeup. In fact, the performance and cost of both types is directly related to the technology employed. A comparison of the two systems should be performed prior to the commencement of design to allow the user to determine which system best meets their needs and falls within their budget constraints.

When designing a UPS system, it is necessary to know how much "reaction time" is required by the owner to achieve "soft shutdown". The element of time will determine the battery size required for the UPS. Batteries can provide an indefinite time period &151; the more (time) backup required, the higher the cost. Fifteen to twenty minutes is a typical backup time.

Heavy Equipment Requires Designer's Attention
AKF is presently involved in the design of UPS systems for several facilities, each of which contains a large data processing center. For these larger installations, the UPS equipment is heavy. Working closely with the structural engineer and architect enables the design of appropriate support systems.

Large UPS systems used for data centers contain system batteries that can weigh from 200 to 400 pounds per square foot &151; a very concentrated load. In some instances, steel plates need to be added under the battery racks in order to spread the load to the building floor system. Additional steel may be required to support the floor. Oftentimes, a company does not realize the value of the UPS system until a loss of data has occurred. The insurance provided by a UPS system should be considered by firms that could suffer extensive loss of time, data and/or revenue from a power failure. Although the term "power failure" is normally associated with blackouts and brownouts, there are occasional instances when the power shuts down or surges for milliseconds. These surges can create havoc for computer systems that are not backed-up with a constantvoltage UPS system.

Firms investing in a UPS system are advised to find qualified professionals to design and install the system properly. Additionally, potential customers should be certain that applicable code requirements are met by the product and the installer.

An office survey should be conducted by designated personnel to determine the computer usage within the firm and the method for maintaining alternative records for important data.

Many corporations would be surprised to learn the extent of their dependence upon the continuous operation of their computers. These firms would benefit significantly from a UPS system.

Oftentimes, a company does not realize the value of the UPS system until a loss of data has occurred.

Blackouts
A complete loss of power. The easiest power problem to detect, except for those with very short durations, these can have the most detrimental effects on equipment, data files, manufacturing processes and related areas.

Brownouts
These planned voltage reductions are relatively easy to detect and are often announced by the power companies in anticipation of excessive demands (summer for example). Generally, these fall within the plus and minus 10% working tolerances of most equipment. However, an overall voltage reduction can result in greater susceptibility to other voltage or noise problems.

Voltage Fluctuations (Surges)
When severe, these can be detected by flickering electric lights. Surges can result in damage to equipment. Periods of low voltage can cause a loss of computer-based data, improper functioning of realtime manufacturing systems with possible waste of materials, disturbance of continuity in security systems and erroneous readings in monitoring systems.

Transients
A sudden change in the electrical continuity can cause a transient voltage wave to be generated from the energy stored in circuit inductance and capacitance. The sudden surge in voltage appears as a spike in the system voltage waveform. The condition can occur when switching major power system components, such as capacitor banks. A lower level transient is caused when turning appliances on or off. The most severe transient occurs when systems fault. Lightning causes the same result.

Harmonics
Voltage waves that are a multiple (2x, 3x, 4x ...) of the supply frequency are harmonics. For example, 120hz and 180hz frequencies are the 2nd and 3rd harmonics of the base 60hz supply frequency. These harmonics are caused primarily by non-linear electrical loads (such as solid state computer equipment).