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Surviving unexpected interruptions.

System risks increase with multiple ground connections.

The use of uninterruptible power supply (UPS) to protect a company's vital electronic equipment, such as local area networks, communications systems, and computer workstations and/or mainframes, has expanded as recovery costs of system failure have increased. By providing continuous, conditioned power to all critical loads, UPS systems ensure that equipment will operate normally during power disturbances such as outages, brownouts, transients, or surges. Typically, a UPS system is specified based on an assortment of risk. Key factors in selecting a UPS system include: the type of equipment to be protected; UPS system kW rating; degree of redundancy required; provisions for on-line maintenance; transfer speeds; and the mode of system operation.

One critical consideration related to UPS systems that is frequently overlooked is the method of distributing and grounding the UPS output power particularly in applications requiring multiple UPS systems.

To maintain reliable communications between separate electronic systems, the instantaneous voltage differential between the components should be as close to zero as possible. Otherwise, the resultant circulating currents can corrupt data, especially as data transmission rates escalate. Higher data transmission rates and faster processing speeds (now approaching 1,000 MHz) have contributed to the sensitivity of DP equipment to power supply disturbances. As data transmission speeds increase, the scale of the problem continues to grow, and thus the proliferation of UPS systems in an attempt to mitigate the problem.

Multiple UPS installations increase the risk of unexplained computer glitches. These problems may be the result of high momentary potential differences between interfacing components. These differences are triggered by internal or external electrical events such as lightning strikes to utility lines or building steel, power distribution system faults, substation circuit breaker reclosures, harmonics current distortion, and other forms of transients. Significant differences between the ground potential of separate UPS systems can cause circulating currents, surges, spikes, and transients in communication lines. These momentary differentials can reach levels high enough to cause component failure and/or data corruption, even though each system component is served by a UPS that is presumed to protect against such conditions.

A single point-of-connection of all UPS neutrals to the building system ground can avoid many of these problems. By minimizing the possibility of potential differences between interconnected components, corporations can significantly reduce the associated risks of system failure or transmission problems. The cost to link a company's UPS systems to a single point-of-connection to earth ground varies. Factors such as configuration of existing wiring, relative location of the UPS systems and DP components, and the magnitude of loads impact the cost.

By performing a cost-benefit analysis, a corporation can determine the most appropriate plan of action. The costs to install UPS systems with a single common ground point in new facilities can be nominal. Older facilities may require extensive, costly rewiring if the UPS system ground cannot be easily segregated, An alternative that many companies implement to maintain reliable data transmission is fiber-optic communication lines. By replacing copper wiring with fiber optics, the communication systems essentially become immune to disturbances in the power source.

While investment in the appropriate systems is critical to maintaining proper operations, equally important is how the system is configured. A single point-of-connection for UPS grounding significantly reduces the risk of electronic system problems. The investment to properly ground all UPS systems is as important as procurement of the right equipment. The proper system solution is usually unique to each company's needs, and rests with the user's risk assessment to provide an appropriate level and quality of reliable, conditioned power.

UPS Success

The following are a few items to consider when choosing to install a UPS system:

* To maintain reliable communications between separate electronic systems, the instantaneous voltage differential between the components should be as close to zero as possible.

* Corporations can significantly reduce the associated risks of system failure or transmission problems by minimizing the possibility of potential differences between interconnected components.

* A company can determine the most appropriate plan of action by performing a cost-benefit analysis.

* The investment to properly ground all UPS systems is as important as procurement of the right equipment.

Robert H. Craemer, PE, is a senior electrical project engineer with Bala Consulting Engineers in Wynnewood, PA.
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Title Annotation:system failures and uninterruptible power supply units
Author:Craemer, Robert H.
Publication:Buildings
Date:Jul 1, 1998
Words:707
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