Mission Critical - Winter 2008 - (Page 38)

Beyond the Nines continued MUT = Mean Uptime MDT = Mean Downtime System Reliability Another aspect of component failure rates is the definition of “failure.” In establishing failure rates for components, the manufacturer and the end-user must agree on this point, otherwise component failure rates have no common basis of meaning. Per IEEE Std. 493-2007, IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems. The definition of failure is the termination of the ability of a component or system to perform a required function. Predictably, disagreements can arise regarding the precise meaning of this definition. Maintenance Time MUT MDT Availability = MUT MUT + MDT A hallmark of a well-designed data center power system is maintainability. The necessity of maintainability is unavoidable: In order to keep component MTBFs to their normal, nearly constant values, maintenance must be performed. Otherwise, the wearout line of the bathtub curve may be crossed, with a resulting increase in MTBF. Maintenance can also help pinpoint abnormal sources of component deterioration, such as overloaded circuits, improperly set protective devices, and changing voltage conditions, etc. As illustrated in figure 2, the reliability of the system is not constant but rather decreases over time. Ideally, when reliability reaches the lowest acceptable level, maintenance brings the system back to an acceptable level, and the process repeats. In reality, however, the need for maintenance is rarely quantified with this degree of detail. For a maintainable system, the reliability of the system during the repair time does not go to zero as shown in figure 2, but only to a reduced level. The better the maintainability of the system the higher the reliability level during maintenance. Parallel power paths and proper switching devices to allow component isolation, etc. help achieve such maintainability. The need for benchmarking is another driver of component maintenance is. During maintenance, tests such as the high-potential tests and thermal scans are performed. The results of such tests are most meaningful when they are tracked over time. An abrupt change in a test result usually signals a problem, and the best way to notice such a change is by comparing with test results from previous maintenance periods. Computerized storage of such records facilitates this process. Figure 2. Illustration of system reliability changes with time nent life is known as wearout. Wearout makes it necessary to follow the manufacturer’s recommended maintenance schedule in order to avoid lowering the reliability of the system. Between the infant mortality and wearout points the failure rate is approximately constant. It is this period of time, and this period of time only, for which the results of a reliability analysis that utilizes constant component MTBFs is valid. It is problematic to use constant MTBFs because it is simply either very difficult or impossible to give such a number without survey data for most types of components. Most component types have many failure modes, and these failure modes are not necessarily independent of one another. Attempts to supply such data based upon any method other than field history data (which can take years to build up) or a rigorous reliability study of the component in question can result in overly optimistic results. Failure Rate (Failures/ Year) Infant Mortality Wearout Commissioning Commissioning is not just the simple startup of components, which only tests the component in question and is designed to bring it up to the point where it can be energized. At the highest level, commissioning tests whole systems and across systems to make sure all components work together properly. Commissioning takes a system-level approach, with the goal being to ensure that the facility is functioning according to its intended purpose. It should test Constant MTBF assumption valid only during this period Time Figure 3. Illustration of time-varying nature of component failure rate 38 | 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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