Keba KeTop T100, User Manual, Page: 146 / 163

continuation of the device’s shielded housing. Pig tail connections are not suit-
able.
It may be necessary to earth cable shields and shielded housings for safety
reasons but this is not an effective EMC measure.
14.3.2.2 Interference suppressors, filter components
Filtering is always necessary when unshielded signal and power supply lines
are brought into shielded areas. Alongside the wanted signals, these lines of-
ten also carry interference signals that must not enter into shielded areas.
Filters should therefore guarantee the interference immunity of the device but
should also prevent the emission of interference from the device via unshielded
lines.
Unshielded lines are usually used when the wanted signals that are carried are
of a very low frequency. The normally high frequency interference signals are
separated from the wanted signals by means of frequency selective filtering
with the aid of low-pass filtering.
Low-pass filters must be sized in such a way that the lower frequency wanted
signals can pass through and the higher frequency interference signals are
filtered.
Multi-stage filters are often necessary for filtering. Nearly all filter combinations
contain Y-capacitors, i.e. capacitors that are connected to the filter housing for
the dissipation of interference currents. In order for these filters to function
correctly, the housing must be connected to a stable reference potential.
Since the interference signals must not enter into the shielded area, the po-
tential of the shielded housing must also be the reference potential for the filter
circuits.
Filter circuits or filter components must therefore be placed precisely where
the lines enter the shielded area. If the filters are placed inside or outside the
shield wall, this can lead to a field coupling between the filtered and unfiltered
lines.
The connection to the reference potential must be as low impedance as pos-
sible and therefore it is necessary for there to be contact over a large surface
area between the filter housing, which is usually metallic, and the shield wall
of the device. Pig tails may not be used to make such a contact.
Line filters
Typical line filters have a metal housing that has to be connected to the earth
wire (PE) for safety reasons and consist of a current-compensated choke (see
illustration) consisting of Y-capacitors, which are connected to the metal hous-
ing on one side, and X-capacitors (connected between the phases or between
phase and neutral).
CE Conformity, Directives and Standards KeTop T50VGA
© KEBA 2009
User's Manual V1.50 146