Protective relaying methods for reducing arc flash energy

Many papers presented on the subject of arc flash in the last few years addressed calculation methods, mitigation methods, and the continuing evolution of arc flash standards and requirements. The most recent change, National Electric Safety Code (NESC) 410.A3, went into effect January 1, 2009, requiring that all power generating utilities conduct arc flash assessments and provide a personal protective clothing system for any risk of an electric arc thermal energy greater than 2 cal/cm². This paper reviews the recent progress in the arc flash standards and requirements, and reviews protective relaying techniques used to reduce arc flash energy, primarily through the use of ¿high speed detection and operation¿. In some cases, the high speed zone is permanent and in others it is switched in during specific events such as maintenance or repair. The need for high speed ¿backup¿ arc flash protection will be discussed. Using a single high speed zone of protection to reduce arc flash energy, thus establishing the level of ¿personal protective clothing, begs the question ¿what if the single high speed zone fails¿? Several arc flash clearing time examples will be covered to demonstrate this concern. As arc flash standards continue to evolve, we need to be mindful of how the application of protective relaying can be used to minimize the effects of these events. As new engineers enter the market or move into positions that include responsibility for fault current and arc flash studies, regular review of this subject matter through the technical paper process is required.