# 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 .
- For a non-sinusoidal signal, the crest factor can be either greater than or less than .

The crest factor for the current drawn by non-linear loads is commonly much higher than . 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**= 0.6A

_{M}**THD**= 145%

_{i }**Crest factor**= 3.75

**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**= 670V

_{M}**THD**= 6.2%

_{u}**Crest factor**= 1.34