The crest factor is the ratio of peak value to RMS value of waveform as shown in below figure. This ratio is also called to peak-to-RMS ratio. The crest factor is expressed as follows:
Crest Factor (peak-to-RMS ratio)= (peak value)/(RMS value)


For the power meter, crest factor has two meanings. One is a specification of the power meter itself. It expresses that the instrument has the capability of correct measurement how much distorted waveform. The other is the measuring of the crest factor of the input voltage or current of an input signal. It means to know quality of an input signal.

For a measuring instrument, the crest factor expresses the size of the dynamic range for an input signal. We define the crest factor as the size of dynamic range based on rated range value (RMS value).
For example, Crest factor of the WT2000 is three or six. It means possible to measure the input signal that peek value is three or six times larger than rated range value.
For example, using 100Vrms range and 1Arms range in the WT2000, available input voltage and current signals are as follows:

If input RMS value is lower than rated range value, the power meter can measure signal with more large crest factor. Below table shows between available input signal and the crest factor.

Input signal range which is defined accuracy, is within 10 to 110% of rated range. Therefore, in CF=3, the crest factor as the size of dynamic range for the WT series is up to 30 (at 10% of rated range value input) within the range of defined accuracy.
For your information, the WT series can display value when it puts the signal that is more than 0.5% of rated range value. If the input RMS value is 0.5% of rated range and input peek value is three times larger than rated range value, the crest factor becomes 600.
If an input exceeding the value shown in the specifications is measured, the peak area of the waveform is clipped to cause an error.

Below table shows RMS value, crest factor and form factor for a typical waveform. The form factor is defined as RMS value/MEAN value. The crest factor of sine wave is the square root of 2.
If it calculates crest factor of input signal and compares this crest factor with the square root of 2, it knows how much distortion ratio of input waveform.

Below figure shows thyristor waveform characteristic for firing angle versus crest factor.

Below figure shows pulse waveform characteristic of pulse duty cycle (=t/T) versus crest factor.