Advances and enhancements are continually being made on static and electronic-
trip devices. For example, the older type static- and electronic-trip elements
measured peak and/or average currents and then scaled these currents
to rms values based on the properties of pure sine wave. Therefore, the older
static- and electronic-trip elements along with the electromechanical analogtype
trip elements are susceptible to tripping problems due to harmonic currents
generated by nonlinear loads. The nonlinear loads are variable speed
drives, switch-mode power supplies, electronic ballasts, and the like. The current
electronic units are truly microprocessor based and are programmed to
sample the current waveform at required intervals to calculate the effective
rms value of the load currents. Microprocessor trip elements with rms sensing
avoid false tripping problems due to harmonic current peaks and sense