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Earthing for shielded cables
Earthing for shielded cables
Electrical and electronic systems should be designed and
installed so that they are essentially protected against
electrical interference, so that they also function reliably in
the event of transient interference voltages.
Electrical interference is introduced into circuits in many
different ways. The most frequent cause is inductive
interference effects. However, conductive or capacitive
coupling plus electrical elds and other phenomena can also
cause interference voltages. In these cases high-frequency
voltage oscillations – the so-called transients – are very likely
the cause of the interference.
Shielded lines enhance interference immunity
The sources of interference voltages can never be eliminated
completely. Therefore, we have to take measures to deal
with their effects. Generally, it is true to say that the more
effectively we can keep interference voltages away or
discharge them from circuit elements, the smaller are their
disturbing effects. This can be done in many ways – with
differing degrees of effectiveness. One really effective
measure for protecting against inductive inuences, i.e.
guaranteeing the electromagnetic compatibility, is to shield
the electric functional components at earth potential. One
way of doing this is to install components in metal, earthed
housings and to shield the connecting lines.
Generally, it is true to say that counteracting the interference
effects of lines is feasible by laying the lines as far apart as
possible, keeping the common return as short as practical,
or using twisted lines. A far better method of protection,
however, is to provide a continuous shield for all lines. This
is the most effective measure that can be taken against the
coupling together of interferences.
The best form of shielding consists of a braided hose of
individual wires made from a non-magnetic material (copper,
aluminium). The braiding should be sufciently robust and as
solid as possible. Care must be taken with lines protected by
a foil shield because of the foil’s low mechanical strength
and low current-carrying capacity.
Correct use of shielded lines
Adding shielding to lines achieves the desired effect only
when they are properly designed and installed. Incorrect
earthing or the use of components that do not function
satisfactorily reduce the effect or even nullify it altogether. It
is not sufcient to connect the shielding to earth potential
at just any point because it could be the case that this earth
connection is inadequate for high frequencies. In addition,
we must also watch out for earth loops, the shielding must
be earthed over a large area, and the quality of the shield
bonding lines and earthing accessories is also important.
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B.1212028840000
Lightning and surge protection for control and instrumentation signals