Mission Critical - Winter 2008 - (Page 34)

Beyond the Nines BY BILL BROWN, P.E. ability distribution, both shown in figure 1 (see next page), characterize such behavior. Assuming that time between failures of a given component behaves according to the exponential probability density and that the distribution leads to the assumption that there is a constant mean time between failures (MTBF is defined as the mean exposure time between consecutive failures of a component) and associated failure rate for that component (the mean [arithmetic average] number of failures of a component or system per unit exposure time) creates a fundamental issue. The use of constant MTBF and Failure Rate in reliability analysis is based upon the assumption that both the probability density and distribution functions for the time between component failures are exponential. Engineered systems can be considered to consist of multiple components, each of which has its own set of failure characteristics. System-level reliability analysis considers how these components connect to form the system and uses this information to calculate the various aspects that describe the reliability of the system. There are a number of methods available to do this. Whatever method is used, there is capability to calculate certain “reliability indices” for the system. The most common of these is availability, defined as the ability of an item — under combined aspects of its reliability, maintainability, and maintenance support — to perform its required A practical approach to data center power system reliability M odern data center power systems represent the ultimate in optimization for reliability. This is a necessity, since the computer and IT equipment that these power systems serve are very sensitive to even momentary loss of power. Tier IV systems (Uptime Institute classification system) rank at the highest level and boast a representative site availability of 99.99 percent, or four nines. But just what does availability really mean? Can a single number really communicate all the necessary information about the reliability of a power system? What about individual component statistics such as MTBF? Do other characteristics, such as maintainability, affect how often there will be problems with power to critical equipment? The Nines Logic dictates that the longer a given component is in service, the more likely failure becomes. The exponential probability density and its associated exponential prob- Bill Brown, P.E., is the lead engineer for Critical Power Consulting with Schneider Electric’s Engineering Services group. He provides electric power consulting engineering support for the Square D Critical Power Competency Center. 34 | Mission Critical Winter 2008

Table of Contents Feed for the Digital Edition of Mission Critical - Winter 2008

Mission Critical - Winter 2008 Table of Contents Editorial: What Have We Been Doing? Cronin’s Workshop: Imagining the Perfect Data Center Power And Cooling Solution Digital Power: The Next Edge in Business Resiliency Zinc Whiskers: Keeping Up With The Green Movement Mission Critical Care: Shipping, Staging, and Installation Legal Perspectives: “A Reliable Energy Supply Is Key to Data Center Success” Continued Payback, Instant Protection Beyond the Nines The Green Information Technology Revolution Health Care Network Offers Glimpse of the Future of Backup Power High-Density Data Centers – Discuss, Design, Deploy High Performance Data Centers Call for Innovative Cooling Techniques More Data in the Data Center Five Considerations for Powering Data Center Racks New Products Industry News Events Classifieds Letters Ad Index Datacenter Wiki: wiki-Mission Critical

Mission Critical - Winter 2008

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