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Article 708: Critical Operations Power Systems

Nov 1, 2007 12:00 PM, By Michael A. Anthony, P.E., University of Michigan; Robert G. Arno, EYP Mission Critical Facilities

Sidebar: What Matters Gets Measured

Nestled in the U.S. Army's Ft. Belvoir complex outside of Washington D.C., the U.S. Army Corps of Engineers (USACE), Special Missions Office (SMO), a small team of engineers have labored on mission-critical facility design, operation, and maintenance for many years. In the early '80s, Secretary of Defense Caspar W. Weinberger signed a memorandum directing the Secretary of the Army, in coordination with the secretaries of the other military departments, to initiate investigative efforts, develop design criteria and standards, and propose solutions and implementation plans for the modernization of electrical power and systems at selected command, control, communications, computer, intelligence, surveillance, and reconnaissance (C4ISR) facilities. The Secretary of the Army, in turn, assigned this project to the U.S. Army Corps of Engineers, Power Reliability Enhancement Program (PREP).

Initially, PREP used survey data and criteria found in the 1991 edition of IEEE 493 because it was the best resource available at the time. Availability data on the pertinent factors (e.g., cause and type of failures, maintenance procedures, repair method, etc.) is necessary to characterize the performance of electrical equipment in service. Produced by volunteer IEEE committees, statistical samples were few and not suitable for military missions. PREP funded a data collection effort similar to the effort undertaken by the aerospace defense industry.

Concurrently, PREP was conducting analyses for various critical-mission customers throughout the Department of Defense. When PREP started using the data, it became obvious that additional data would be needed to reflect new technologies, the difference between calendar and operational time, etc., in many more facilities. It was also observed that new equipment was exhibiting significant increases in availability, with corresponding decreases in required maintenance and the occurrence of failures. Information was obtained on a variety of commercial and industrial facility types (including office buildings, hospitals, water treatment facilities, prisons, utilities, factories, universities, and bank computer centers) with varying degrees of maintenance quality. To provide the best reliability numeric, PREP workgroups recognized that technology was improving reliability as the data collection window was moving.

The PREP data collection effort rolled out into the latest editions of NFPA 70B, “Recommended Practice for Electrical Equipment Maintenance,” and the IEEE “Gold Book” is the culmination of a 24,000 man-hour effort to collect reliability, availability, and maintenance (RAM) data on 239 power generation, power distribution, and HVAC items, including gas turbine generators.

Additional technical manuals covering power security are available at the U.S. Army Corps of Engineer's Web site at http://www.army.mil/usapa/eng/index.html.

Sample risk assessment from the emergency management division of Washtenaw County, Mich. Note that earthquakes in southeastern Michigan are ranked relatively low and that infrastructure hazards are relatively high.

Sidebar: Reliability vs. Availability

Annex F contains a description of reliability and availability. For the purpose of this article, we refine and add to the use of these terms as follows:

reliability (lowercase “r”) — a number, typically expressed as a percentage that reflects the probability and frequency of failures and is expressed as a probability over a given duration of time cycles.

Reliability (uppercase “R”) — A term used in common language that reflects the overall state of a system (the Fine Print Note of Sec. 700.12, for example.

Availability — Always measured in terms of percentage of uptime vs. downtime; the closer to 100% the better.

Sidebar: Availability Comparison

As Annex F in the 2008 NEC indicates, the availability of a critical operations power system (COPS) is measured by the percentage of time that the system is in service. Given a specified level of availability, the reliability and maintainability requirements are then derived based on that availability requirement. Using these equations, you can compare two hypothetical systems. (MTBF = mean time before failure; MTTR = mean time to repair)

Although both systems availability metrics are the same, the systems are not equal.

Alpha: MTBF/(MTBF+MTTR) = 500/(500+0.5) = 0.999001

Bravo: MTBF/(MTBF+MTTR) = 20000/(20000+20) = 0.999001

An outage for one-half hour in system Alpha could be satisfied by a UPS, chilled water storage system, or other means. An outage in system Bravo for 24 hours, depending upon mission requirements, may be unacceptable regardless of the frequency.

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