Abstract
A power quality monitor doesn’t measure power quality. Instead, it measures voltage and current, thousands of times per second, and every other number it ever shows you—RMS, kVAR, THD, Pst, a dot on an ITIC curve, etc.—are the result of arithmetic performed on those two streams of samples. Two kinds of wire go into a PQ monitor, and they work on completely different physical principles. One touches the circuit. One deliberately does not. This paper is all about understanding why.