Protective relay engineers have long been concerned with protecting power systems and all of the equipment associated with those systems. We routinely apply relays to limit damage to apparatus (e.g., transmission and distribution lines, power transformers, buses, generators, motors, etc.) and protect against, or reduce, the impact of electrical disturbances on the larger power system (e.g., shedding load for frequency or voltage variations). Safety for personnel has always been a concern, but in the past several years there is a heightened awareness of the importance of safety around electrical apparatus, as reflected in recent regulations and standards [1] [2]. In particular, industry and utilities alike recognize that arc-flash events can cause dangerous and potentially fatal levels of heat, ultraviolet radiation, blast pressure, flying shrapnel, and deafening sound waves. The existing standards mainly deal with the heat energy from the arc flash. The energy produced by an arc-flash event is proportional to voltage, current, and the duration of the event (V·I·t). Design engineers have a few options to reduce system voltage or fault currents (e.g., grounding practices and application of current-limiting fuses), but the best and most direct ways to reduce arc-flash hazards are to reduce fault-clearing times and use wireless communications to reduce the need for technicians to be in harms way. In most cases, clearing times are reduced via more complete use of microprocessor relays features and other technologies already available. Similarly, digital relay communications and secure wireless communications devices allow engineers and technicians to converse with relays from a safe distance. In this paper, we include some important industry definitions of arc flash and ways of measuring arc-flash hazards. We then examine the use of existing technologies, including digital relays and communications capabilities, to implement reduced trip times using instantaneous overcurrent relays, a fast bus trip scheme, differential schemes, and light detection. We use a typical industrial switchgear lineup as an example of how to implement these schemes. Finally, we quantify the levels to which we can reduce arc-flash energy and its impact on safety.
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