Consulting-Specifying Engineer - October 2007 - (Page 52)

SCCR and interrupting ratings It should be noted that there are different ratings involved in overcurrent protection. Short circuit current rating (SCCR) is the same as withstand rating. The rating represents how much short circuit current a device can withstand without self-destructing. “A withstand rating is the maximum RMS symmetrical short-circuit current at which the equipment has been tested under specified conditions,” explained Farrell and Valvoda. “At the end of the test the equipment must be in ‘substantially’ the same condition as prior to the test.” SCCR is applicable to non-interrupting equipment including switches; busway or bus duct; switchgear and switchboards; motor starters; types of faults is that the energy in a bolted fault condition is dissipated in the faulted equipment while an arcing fault releases energy out into the surrounding environment.” Coordination issues Selective coordination minimizes downtime caused by nuisance tripping. Joe Schomaker, senior product manager at Cooper Bussmann, said that selective coordination involves “isolating an overloaded or faulted circuit from the remainder of the electrical system by having only the nearest upstream overcurrent protective device open. Without selective coordination, a single faulted circuit can shut down an entire facility.” Prior to NFPA 70E and the work done to develop IEEE 1584, most facilities were designed to protect the electrical system and its loads from damage while avoiding nuisance interruptions. However, protecting workers from arcing faults is now a necessary part of the equation. Safety should never be an afterthought. On the surface it appears that selective coordination and safety from arc flash hazards are opposites. However, some believe that coordination and safety can be achieved in the same system. “Protecting people, while protecting the system and providing continuity of service are not mutually exclusive goals,” said Joe Weigel, product manager, Square D Services, Schneider Electric, in Nashville, Tenn. “But the people-protection aspect is causing electrical designers to reconsider some of the design practices that they traditionally employed—including their choices of overcurrent protective devices.” Weigel said the primary strategy to reduce the incident energy released during an arcing fault is to detect and clear the fault as quickly as possible. Because of this, there is often a fine line between optimal arc flash energy reduction and system coordination to provide continuity of service. “For example,” Weigel said, “one way to significantly reduce the arcing fault incident energy release is to lower the ‘instantaneous’ setting on the circuit breaker trip unit (if it has that function). However the instantaneous setting cannot be randomly set at its minimum setting or nuisance tripping is likely to occur as the loads attempt to start. So a time-current coordina- Facilities are engaged in intense investment efforts to comply with NFPA 70E covering arc flash. contactors; MCCs; and similar equipment. If a short circuit occurs, every component in the system through which the fault current flows must safely withstand the effects of the current, which include heating and magnetic stresses. The protective device must break the current path reliably and safely. Article 100 of the NEC defines interrupting rating as “the highest current at rated voltage that a device is intended to interrupt under standard test conditions.” The Fine Print Note (FPN) to Article 100 states “Equipment intended to break current at other than fault levels may have its interrupting rating implied in other ratings, such as horsepower or locked rotor current.” Section 110-9 of the NEC states: “Equipment intended to break current at fault levels shall have an interrupting rating sufficient for the system voltage and the current which is available at the line terminals of the equipment … Equipment intended to break current at other than fault levels shall have an interrupting rating at system voltage sufficient for the current that must be interrupted.” Arc flash considerations Existing facilities are investing intense efforts in complying with the NEC and NFPA 70E. It’s more difficult to change the status quo than to engineer in compliance at the beginning. However, opportunities for consulting engineers exist for both existing and new projects. New facilities can be designed correctly before construction begins. Existing facilities must be analyzed, and modified if necessary. Regardless of whether an electrical system is being designed for a new facility or an existing one, an arc flash hazard analysis must be done to ensure workers are protected from this potentially lethal threat. It is necessary to know the bolted-fault-current value when doing arc flash analysis calculations. It is also necessary to know the available fault current, which should be available from the utility. Bolted fault current is not the same as arcing current. “Arcing faults differ in the fact that the current actually flows through ionized air causing an arc,” Lottman said. “The major difference between these two There is often a ‘fine line’ between optimal arc flash energy reduction and system coordination to provide continuity of service. tion study is required, and the coordination study is a critical element of the arc flash hazard analysis for that reason. Once the instantaneous has been properly set based on the coordination study and arc flash analysis, it should never be reset by anyone; unless changes in the arc flash incident energy potential release is considered.” “Circuit breakers can be used in selectively coordinated electrical systems,” said Kenneth Cybart, senior technical sales engineer at Littelfuse, Des Plaines, Ill. “But specifiers must overlay the time-current 52 Consulting-Specifying Engineer • OCTOBER, 2007

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Consulting-Specifying Engineer - October 2007 Contents On the Web In the News M/E Roundtable How To Beat the Energy Codes Professional Practices Codes & Standards A Capital Job on Engineered Buiding Systems Going Green in Data Centers Overcurrent Protection: Fuses or Breakers? Third-Party Liability Management Report New Products Product Spotlight Jobs/Classifieds Specifier's Notebook

Consulting-Specifying Engineer - October 2007

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