Relationship of X/R, IP, and IRMS´ to Asymmetry in Resistance/Reactance Circuits

The short-circuit current interrupting and/or withstand ratings of electric power distribution equipment are determined by the transient asymmetrical current available during the equipment testing procedure. The degree of asymmetry is a function of the X/R ratio and the point on the voltage wave at which the short-circuit is initiated. Power distribution equipment short-circuit current ratings, with rare exceptions, are given in symmetrical amperes. It is important to know what effect the stresses imposed by the transient asymmetrical current, which cannot be measured by standard ammeters, will have on the equipment during the first few cycles of current flow. Presented is an accurate evaluation of instantaneous asymmetrical current and equivalent asymmetrical rms current characteristics, during the transient period, by direct calculation. The data thus obtained should prove valuable to power distribution equipment designers and test engineers as well as equipment application engineers.