CSE Pure Power - Summer 2008 - (Page 14)

❯❯ PURE POWER // SUMMER 2008 14 cover story operation. Suppose the state emergency management agency issues a directive that all city ﬁre departments must have backup power available to at least “three nines” (99.9%), less than the 72 hours required in 708.22-C. Three nines would not be an unfamiliar metric: the power provided by most U.S. utilities is available out to three and four nines. The agency is asking for utility-like availability at the DCOA using the power sources listed in Part III of 708. Identifying an acceptable risk is one of the most difﬁcult decisions for higher governmental authorities to make. While the cost of additional nines is a well-known parameter in the business continuity industry guided by NFPA 1600, “Standard on Disaster/ Emergency Management and Business Continuity Programs,” these agencies would have to look at the economics of additional nines to public service organizations with different missions. Getting availability out to three nines could be costly if fuel storage or emissions restrictions apply to the site. As the availability table in Annex F shows, three nines of availability translates into a potential 8.76 hours of downtime in a year. If getting to three nines means periodic testing that triggers a local cap on emissions, designers may have to investigate other feasible sites. They could develop vehicle-mounted generators covered in 708.20(F)(6) as an option, along with fuel-handling logistics. Administrative options should not be overlooked in the hunt for savings. Limiting the use of parts of the overall facility provides a workaround, sometimes as simple as scaling the size of the DCOA to the number of people or the level of training of the people who will use it. To achieve 708 objectives with a single engine-generator set suggests a considered design in which administrative procedures and power chain hardware is balanced carefully. One characteristic of the reliability engineer’s art is how administrative options are translated into the system www.purepowermagazine.com Figure 2 ITEM Utility Dist swbd Circuit breaker – draw out type Generator Circuit breaker – draw out type Auto transfer switch Dist swbd Circuit breaker – molded case Circuit breaker – molded case Rectiﬁer Static switch Inverter Battery/cap Battery LABEL UTL SB1 CB4 GEN CB4 ATS1 SB1 CB4 CB4 RECT SS4 INVR1 BAT CH BAT Sample parts list from which a statistical framework can be built and the numbers run. Each component has a failure rate measured in mean time to failure and a mean time to repair, both measured in hours. model. Translation of system needs or requirements to reliability numeric is critical in establishing a facility meeting the 708 requirements. Caution should be exercised here not to fall into a “cookie cutter” approach. STEP 3: BASE CASE: ESTABLISH AVAILABILITY POTENTIAL Several different tools exist to model the availability potential of the COPS design. In general, reliability engineers use modeling tools with the capability of statistical simulation to incorporate variations in component capacities associated with consumables such as water and fuel. Failure rate and repair time data play an important role in the analysis of a system to determine whether it meets requirements. Figure 2 is a tabulation of COPS components along the power chain of our simple study system. Each component in the tabulation has an availability derived from an extensive data collection effort and is expressed (usually as a decimal) in terms of potential availability per year. IEEE reliability experts began collecting data on failure rate and repair more than 20 years ago, published in the Gold Book7. This effort ran parallel to an international trend in total quality management (TQM) that employed statistical and probabilistic methods to improve component and system quality. A reference to TQM also shows up in Annex F. Realizing the importance of gathering failure rate and repair cost data to its own mission for civilian infrastructure security, the USACE funded the most comprehensive reliability data collection effort to date. The failure data used in doing any DCOA/COPS study should be included in the appendix of any risk assessment. Unlike the cable reactance data in the wiring tables of Chapter 9 of the NEC—some of which is the better part of 100 years old now—power chain component reliability data are far more dynamic, reﬂecting improvements to “commodity” components of a COPS such as engines, generators, transfer switches, and UPS systems. None is a commodity in a strict economic sense. They are components, lumped as parameters within a system, and manufacturers are always improving them. Because they tend to be factoryassembled and can be shipped to the system site, they can be treated as interchangeable commodities in a COPS. The core value of a COPS lies in the proper application of those commodity components to produce an initial and continually veriﬁable nameplate availability. Skilled reliability engineers will apply their judgment in reconciling the competing requirements for redundancy and simplicity. They will hit the availability target with the least number of components, concentrating dollars on the components that will yield the largest value. Construction and maintenance activities are reciprocal partners in that effort, so the reliability engineer will balance attention to both the speciﬁcs and the interconnectedness of the ﬁrst-cost/longterm O&M cost conundrum. With commercial software, one can enter the failure and repair data from http://www.purepowermagazine.com

Table of Contents Feed for the Digital Edition of CSE Pure Power - Summer 2008

CSE Pure Power - Summer 2008 Contents In the News Industry Roundup Risk Assessments for COPS Grounding Requires More Power Systems to Protect Healthcare Important Changes Coming in NFPA 70E A Look at Arc-Resistant Switchgear Agencies and Associations New Products Ad Index

CSE Pure Power - Summer 2008

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