A perfect sinusoidal shaped signal, be it acoustic, voltage or
current signal is per definition a "clean", pure,
one-frequency signal. When another frequency is overlaying the
original signal, the signal becomes distorted and no longer
perfectly sinusoidal. Especially when the overlaying or interfering
signal has a constant mathematical relationship to the original
sinusoidal signal, for instance twice the frequency, or half the
wavelength of the original signal.
is the interfering frequencies with a constant mathematical
relationship to the original signal. Hence the term
"Harmonics".When dealing with harmonic distortion on the
mains supply, the interfering frequencies will then derive from that
basic frequency, 50Hz or 60Hz.
The problems with harmonics distortion arise when electronic devices
generate harmonics on the mains, and thereby "pollute" the
source for other electronic devices. In sensitive environments, such
as hospitals, airports, banks and other buildings or institutions
with sensitive electronic equipment, it is crucial that the harmonic
distortion is kept at a minimum. Several authorities also demands
compliance to IEC or other Standard Organization regulations.