This issues Tech Brief covers two topics. Both relate to the interrupting rating of switchgear, X/R ratio and asymmetrical short

What is X/R ("X over R") ratio, and why is it important? First, X/R ratio is simply the ratio of the system reactance to the system resistance, looking back towards the power source from any point in a power circuit, assuming that a short circuit is applied to the system at that point. It is another way of stating the power factor of the source system. Mathematically, if power factor is equal to cosine theta, then X/R is equal to tangent theta. Note that this is the power factor of the system up to that point. It has absolutely no relationship to the power factor of any load on the system.
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