Short-Circuit Calculations Khi đường dẫn dòng chảy hiện tại được định hướng một cách chính xác, áp lực của điện áp nguồn lực lượng cường độ bình thường dòng điện chạy qua các trở kháng tải. Trong thời gian này, các vật liệu cách nhiệt xung quanh các dây dẫn năng lượng ngăn ngừa hiện nay chảy qua bất cứ con đường nào khác hơn là thông qua trở kháng tải. Trong trường hợp này, trở kháng tải là đủ lớn để hạn chế dòng chảy hiện tại để "bình thường" giá trị thấp theo quy định của pháp. | Chapter 5 Short-Circuit Calculations When the current flow path is directed correctly the pressure of the source voltage forces normal current magnitudes to flow through the load impedances. During this time the insulation surrounding the energized conductors prevents current from flowing through any path other than through the load impedance. In this situation the load impedance is large enough to limit the current flow to normal low values in accordance with Ohm s law E I X Z Problems arise however when the conductor insulation fails permitting a shortened path for electron flow than through the load impedance. If the shortened path or short circuit also known as a fault permits contact between a phase conductor and an equipment grounding conductor this is known as a ground fault or a phase-to-ground fault. If however the shortened path instead permits contact between two or three phase conductors then it is known as a phase-to-phase fault. If a solid connection is made between the faulted phase conductor and the other phase wire or the equipment grounding conductor then the short circuit is identified as a bolted fault. 179 Copyright 2001 by The McGraw-Hill Companies Inc. Click here for Terms of Use. 180 Chapter Five In bolted faults little or no arcing exists the voltage drop across the very low impedance of the almost-nonexistent arc is very low and the fault current is of high magnitude. If an arcing connection is made between the faulted phase conductor and the other phase wire or the equipment grounding conductor then the short circuit is identified as an arcing fault with its associated lowered fault current flow. In certain systems this arcing fault current can be so low that it is not recognizable as a problem to the upstream overcurrent device. During such events excessive heat buildup around the arc can occur causing further damage to the otherwise sound electrical system or starting fires in nearby structures or processes. Even though the fault may be
