Consulting-Specifying Engineer - October 2008 - (Page 72)

Codes & Standards Series method of AIC ratings A series AIC rating is defined by Cutler-Hammer as “A short-circuit interrupting rating assigned to a combination of two or more over-current protection devices. The short-circuit rating of the upstream device (main) fully rated system, and series-connected system. A selectively coordinated system is a system in which “all breakers are fully rated, and upstream breakers must have adequate short-time withstand ratings and short time delay adjusting must comply with National Electric Code 240.86 requirements, specifically marking, motor contribution, and engineering supervision in existing installations. Testing requirements The tests required by UL for a single breaker are: • Calibration of the test breaker • “O” test in which the fault is initiated with the breaker in the closed position • “CO” test in which the same breaker is closed on the fault • Calibration test to ensure the breaker is still functional • Dielectric test. The testing required for a series rating is the same testing as noted above for the main and branch circuit breakers individually—the interrupting ability and the intermediate ability tests. Depending on the results of these tests, additional testing may be required. In addition, UL may require follow-up testing. Pay is principal and electrical department head of FEA Consulting Engineers. He is an expert in low- and medium-voltage distribution system design. REFERENCES Beatty, Bill. 100% Rated Molded Case Circuit Breakers. White paper, 1994. Beatty, Bill. Series Ratings and Molded Case Circuit Breakers. White paper, 1994. Bussmann Mfg. Division. Engineering Dependable Protection for an Electrical Distribution System – Part I A Simple Approach to Short Circuit Calculations. St. Louis: McGraw-Edison Co., 1968. National Electrical Manufacturers Assn. Series Ratings, Molded Case Circuit Breaker Section, National Electrical Manufacturers Assn., 1994. National Fire Protection Assn. (NFPA). National Electrical Code (NEC). Clifton Park, N.Y.: Delmar Cengage Learning, 2005. In a series-connected system, “low-level arcing faults are still cleared by the downstream breaker alone.” However, under high fault conditions, both the upstream and downstream breakers would open, eliminating service to the affected portion of the system. must be at least equal to the available system fault current. The high interrupting capacity rating of the downstream device can be less, providing factory tests have been conducted to verify the combination rating.” Typically, the fault current is calculated at every point within the system. The main switchboard will have an AIC rating calculated by the methods shown above. Then, all of the devices at the main switchboard will have an AIC rating that is the same based on the interrupting capacity determined in the calculations. For example, if the fault current calculation was 72,300 amps at the main switchboard, the next standard AIC rating would be selected. A 100,000 AIC rated switchboard usually is specified. All of the circuit breakers at the main switchboard, including the main and the distribution circuit breakers, have the same AIC rating of 100,000. In the series rating method, the main breaker would have an AIC rating of 100,000, but the distribution breakers on the main switchboard can have a rating less than 100,000, based on factory tests of the combination of the breakers. The combination of the two breakers is rated at 100,000. Cutler-Hammer defines three types of systems: selectively coordinated system, capabilities.” This type of system has the highest reliability, requires calculations for the entire system, and is the most expensive of the three noted by Cutler-Hammer. In addition, fuses are sometimes required to obtain selective coordination. A fully rated system is a system in which “all the over-current protective devices have a short circuit interrupting rating at least equal to the available short circuit current.” This type of system is less reliable than a selectively coordinated system because multiple breakers may open during a fault and it requires calculations for the entire system. A fully rated system is less expensive than a selectively coordinated system. In a series-connected system, “low-level arcing faults are still cleared by the downstream breaker alone.” However, under high fault conditions, both the upstream and downstream breakers would open, eliminating service to the affected portion of the system. This type of system operates similarly to the fully rated system where multiple breakers may open to clear the fault; however, it does not require calculations for the entire system because tables are used to determine the required combination. This system is the least expensive of the three types. If a series-connected system is used, it also 72 Consulting-Specifying Engineer • OCTOBER 2008

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Consulting-Specifying Engineer - October 2008 Contents Viewpoint Letters News M/E Roundtable Cities Under One Roof Eight Design Tips for Integration Is 'Green' Really New? Down the Drain Product of the Year Awards Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - October 2008

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