Crest factor is the ratio of the peak signal 
amplitude to the RMS value of the signal. It 
determines the accuracy of AC measurement. 
If the crest factor is less than 3.0, voltage 
measurement will not result in error due to 
dynamic range limitations at full scale. 
If the crest factor is more than 3.0, it usually 
indicates an abnormal waveform as seen from 
the below table.