Harmonic distortion indicators - Crest factor

The crest factor is the ratio between the value of the peak current or voltage(I_M or U_M) and its r.m.s. value.

• For a sinusoidal signal, the crest factor is therefore equal to [math]\displaystyle{ \sqrt 2 }[/math].
• For a non-sinusoidal signal, the crest factor can be either greater than or less than [math]\displaystyle{ \sqrt 2 }[/math].

The crest factor for the current drawn by non-linear loads is commonly much higher than [math]\displaystyle{ \sqrt 2 }[/math]. It is generally between 1.5 and 2 and can even reach 5 in critical cases.

A high crest factor signals high current peaks which, when detected by protection devices, can cause nuisance tripping.

Examples:

Figure M7 represents the current absorbed by a compact fluorescent lamp.

I_r.m.s. = 0.16A
I_M = 0.6A
THD_i = 145%
Crest factor = 3.75

Fig. M7 – Typical current waveform of a compact fluorescent lamp

Figure M8 represents the voltage supplying non-linear loads through a high impedance line, with a typical "flat top" distorted waveform.

V_r.m.s. = 500V
V_M = 670V
THD_u = 6.2%
Crest factor = 1.34

Fig. M8 – Typical voltage waveform in case of high impedance line supplying non-linear loads