The function
of a line filter is to permit
the flow of supply currents in
the range 0 to 400Hz and to prevent
the flow of undesired frequencies
which in general extend from 10KHz
to 1GHz.
This is achieved by adopting basic
low pass filter configurations
using shunt capacitors and series
inductors to attenuate high frequencies.
The low pass filter is effective
in either direction and thus can
be used to prevent excessive emissions
from the equipment to the mains
line and limit the flow of spurious
signals into the equipment from
the mains line.
Due
to unknown parameters such as
the source impedance of the unwanted
signals, it may be difficult to
predict the precise behaviour
of a filter in any given equipment,
hence direct measurements under
standard conditions are made using
prescribed procedures and apparatus
to confirm the performance of
an iec line filter. Resistors
are incorporated into the filter
for damping resonances as well
as providing safety discharge
paths and non-linear devices such
as metal oxide varistors and zener
diode combinations are used to
deal with voltage transients.
Suppression components may be
required in the earth conductor
as well as the current carrying
conductors to achieve satisfactory
performance.
All components used
must be rated for the application
in accordance with international standards
and care must be taken to avoid generating
spurious magnetic fields which can
cause mechanical vibration, noise
and local heating.
