6 Earthing and shielding
| Earthing of shielded cables
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2484950000/00/03.2016
Manual UR67-PROIFBUS
6 Earthing and shielding
6.1 Earthing of shielded cables
Electrical and electronic systems must be designed such that
they are largely safeguarded against electrical interference,
thus enabling them to operate securely even in the case of
transient interference voltages.
Electrical interference can be introduced into electric circuits
in a variety of ways. The most frequent causes are due to
inductive interference. In addition, galvanic and capacitive
coupling as well as electrical fields and other processes are
causes for interference voltages. Here, high-frequency voltage
fluctuations – known as transients – are the cause of interfer
-
ence with a high level of effectiveness.
Shielded cables increase interference resistance
The sources of interference voltages can rarely be eliminated
and even then not always completely. Thus, it is necessary to
take measures to combat their effect. In general, the more ef-
fectively interference voltages can be kept away from circuit
elements or can be discharged, the less electrical circuits are
affected. This can be accomplished in a variety of ways with
varying levels of effectiveness. A very effective measure, in
particular for safeguarding against inductive effects, i.e. en-
suring “electromagnetic compatibility” (EMC), is the shield-
ing of electrically functional components to earth potential.
In doing so, for instance, components are installed in metal-
lic, earthed housings and the connecting lines are equipped
with shielding.
In general, it can be said that interference from cables can
be combated by routing cables as far away as possible from
each other, keeping the common return as short as possible
and using twisted-pair wire. Far better protection, however,
is provided by completely shielding of all cables. This is the
most effective measure that can be taken against the cou-
pling of interference signals.
The best type of shielding consists of a braided mesh sleeve
that uses individual wires made of non-magnetic materials
(copper, aluminium). The braided mesh should be sufficiently
large and also be as thick as possible. For cables that are
equipped with foil shields, it is necessary to be aware of the
low mechanical strength and the low current-carrying capac-
ity of the shielding.
Proper use of shielded cables
The shielding of cables will only result in the desired effect if
this is implemented properly. Incorrect earthing or the use of
improper components that perform their task inadequately
reduces or even totally eliminates the effect. Placing the
shielding at any spot on the earth potential will not suffice,
as this earth connection may have no effect on high frequen-
cies. In addition, ground loops must also be taken into con-
sideration. Furthermore, the shielding should be earthed over
a large surface area. Beyond that, the quality of the shield
conductor and earthing accessories is also important.
Weidmüller clamping bracket KLBUE 10-20 SC (Order No. 1712321001)
In practice, the shield is still often twisted and connected
to a terminal point. There is very high attenuation (voltage
drop) on these connections, especially for high-frequency
interference. Therefore, this type of shielding should not be
used, even for short cable lengths. The shielding of the cable
is practically negated and can, at best, be helpful for low-
frequency interference. We recommend that there is a large
amount of surface contact with the braided shield of the
cable.
There are generally four distinct types of coupling:
–
Galvanic coupling
–
Capacitive coupling
–
Inductive coupling
–
Radiation coupling
