Introduction to Short Circuit Analysis （介绍短路分析）.pdf

www.PDH PDH Course E204 www.P Introduction to Short Circuit Analysis Course Content A short circuit is a low-resistance connection established by accident or intention between two points in an electric circuit. This excessive electric current potentially causes circuit damage, overheating, magnetic stress, arcing, fire or explosion. The amount of current that is available in a short circuit is determined by the capacity of the system voltage sources and the impedances of the system, including the fault. In circuit analysis, the term short circuit is used by analogy to designate a zero-impedance connection between two nodes. This forces the two nodes to be at the same voltage. In an ideal short circuit, this means there is no resistance and no voltage drop across the short. The electrical opposite of a short circuit is an open circuit, which is infinite resistance between two nodes. What causes a short circuit? Several scenarios can lead to a short circuit; #1: When two bare conductors touch, a short circuit occurs. In figure below, a short is caused due to broken insulation. Another type of short circuit occurs when some conductive object such as tool or an animal, accidentally gets into an overhead power line. If the object touches both the lines at the same time, the electricity has a short circuit path available to return to the source before it goes to the customers electric service. If the object is connected to the ground, the earth can act as a short circuit path. #2: In the figure below, a short is caused by improper wiring. Note the effect on current flow. Since the resistor has in effect been replaced with a piece of wire, practically all the current flows through the short and very little current flows through the resistor. Electrons flow through the short (a path of almost zero resistance) and the remainder of t