CSE Pure Power - Summer 2008 - (Page 15)

15 ❮❮ PURE POWER // SUMMER 2008 user-deﬁned libraries into branch and node input data screens similar to the manner in which short circuit or load ﬂow studies take in system data. The output is a reliability block diagram as shown in Figure 3. STEP 4: HITTING THE TARGET To focus on the areas needing the most attention in the system improvement process another set of useful tools are Failure Modes and Effects Critically Analysis (FMECA) and Fault Tree Analysis (FTA). Each method is described in Annex F, Part I, and each employs a different probability distribution equation, but both accomplish the identiﬁcation of weak members of in a system that contribute to prospective unavailability. These procedures focus efforts to critical branches and nodes of the system needing the most improvement. These quantitative methods are new to the NEC, but are not new to other NFPA documents. For example, NFPA 1600 mentions FTA in its Annex A. Annex A of the 2006 “Vehicular Fuel Systems Code – NFPA 52” also mentions FMECA as a hazard analysis method. FMECA and FTA methods are standard operating procedure in many industries that continually seek to improve designs for products and processes. They are required to comply with safety and quality requirements, such as ISO 9001, QS 9000, ISO/TS 16949. Substantial how-to information speciﬁc to mission critical power systems appears in another technical manual.8 Figure 4 is a summary tabulation of the input and output from a commercial reliability program. The mean time before failure (MTBF) and mean time to repair (MTTR) are calculated from all of the upstream components of the sample system. One can verify the availability by using the equation: Availability = MTBF/ (MTBF+MTTR) = 115,900.10/(115900.10+ 1.55) = 0.999987 Note that four-nines availability exceeded the initial target of three nines. If four nines is too expensive, the reliability engineer might try to “value engineer” the COPS with less expensive components at initial construction to be compensated with a more robust testing plan in the long run. Design engineers also can run cases to demonstrate to the inspector how COPS availability erodes over time unless maintenance tasks are performed, or how reliability might grow as commodity components burn-in. Other cases could be run to show how toofrequent use, coupled with over-testing, would trigger a longer repair time in a high-speed generator due to manufacturer recommended engine overhaul. STEP 5: Start 1::1 Ref Des: UTL PN: UTL Ref Des: Dist Swbd PN: SB1 Ref Des: CB NC PN: CB4 Ref Des: GEN PN: GEN3 Ref Des: CB NC PN: CB4 1::2 Sb Ref Des: Dist Swbd PN: SB1 1::1 Ref Des: ATS PN: ATS1 Ref Des: BAT CH PN: BAT 15 Failure: Uniform LB: 0.245 Ref Des: BAT PN: BAT Ref Des: LV CB PN: CB2 Ref Des: RECT PN: RECT 1::2 Sb Ref Des: INVR PN: INVR1 Ref Des: LV CB PN: CB2 Ref Des: SBS PN: SBS4 1::2 Sb Ref Des: Dis Swbd PN: SB1 End Figure 3 Commercial software can create a reliability block diagram like this one. FIGURE 4 MTBF (hrs) Utility and generator 115,900.10 to ATS feeding a switchboard, UPS module with battery and static bypass switch. MTTR (hrs) 1.55 AVAILABILITY 0.9999867 PERFORMANCE TESTS The development and implementation of functional The results of the sample COPS hitting the prescribed performance tests (FPT) availability target. described in Part II of 708 applies an operational should be a multilevel simulated failure perspective to system exercise to ensure system readiness. design. Functional tests are developed Ensuring its nameplate availability during design and performed after conthroughout the lifecycle of the COPS struction to demonstrate that the COPS can be accomplished by applying NFPA system will function according to the 70B, Recommended Practice for Maintedesired nameplate availability. Since nance of Electrical Equipment. FPTs are based on the actual installaBusiness continuity companies are tion, if the system changes, the tests seeking business models wherein the may need to be modiﬁed. commodity portion of sophisticated Although baseline test results are systems (UPS, on-site generators, required in 708.8, the FPT described in transfer switches) can be released Annex F is optional material, “included turnkey to a supplier who can build to for informational purposes only.” The availability speciﬁcation and manFPT would be not just commissioning age the supply chain to keep it there. of components or subsystems, but a full Emerging speciﬁcations for connectsystem-wide performance test exercisability and maintainability allow ing as many functions as possible. It www.purepowermagazine.com 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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