Novel approach to ARC flash mitigation for low voltage equipment

Abstract

As industry pursues safer workplaces with arc flash mitigation programs, they face major challenges in effectively reducing incident energies on the secondary of medium voltage to low voltage (MV-LV) transformers. When an arc fault occurs on the transformer secondary, primary currents can be below the short circuit range of the primary protection. Typical E-rated fuses can take over 2 seconds to clear while high side device 51 relays will have a waiting period before initiating a breaker trip. Solutions that adjust the LV main breaker's (if applied) protective functions for faster response, do not address arc fault issues on the bus connecting this main breaker to the transformer secondary. This paper presents a method of applying a high speed grounding switch (HSGS) at a location between the MV E-rated fuse load terminal and the transformer's primary connection. Since the transformer's primary voltage is reduced to zero in milliseconds, the low voltage arc flash event collapses quickly to limit incident energies to less than 1.2 cal/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Applications can include MV-LV transformers up to 5000kVA.