Cooling the Arc Flash Hot Spot

Abstract

Maximum arc flash energy is seen at the secondary of transformers before the first protective device. Reducing transformer primary protection trip times to cool this “hot spot” starts with quicker tripping on the secondary. Zone selective interlocking (ZSI) is a proven means of avoiding the delays of traditional coordination time intervals but interlocking of all devices within a system can raise concerns about construction and testing complications. How much of an arc flash benefit would it be to interlock only main and tie breakers where interlock capable communication exists for automated transfer scheme functions. To determine benefits, arc flash hazards are first reduced to the extent possible with traditional measures. Communication elements are analyzed to determine those having the speed and reliability required for protection quality interlocking. ZSI is then applied between main and tie breakers to demonstrate how the hazards of the arc flash hot spot can be brought under control.