Crest factor is a measure of a waveform, such as alternating current, showing the ratio of peak values to the average value. In other words, crest factor indicates how extreme the peaks are in a waveform.

True RMS meter is specified by Crest Factor because it tells you how badly the waveform is distorted. Crest factor can be calculated by dividing the peak value of the current by the effective true RMS value (see figure).

For a perfect sine wave the Crest factor is 1.414, and the more the signal is distorted, the higher the factor gets because of the ever sharper peaks. This means that a true RMS meter with a maximum Crest factor of 1.5 will still give you incorrect readings on distorted waveforms, and is only capable of measuring close-to-perfect sine waves. Normally, a Crest factor of up to 3 is more than adequate for most power distribution measurements.

Figure: Crest Factor