Earthing And Shielding - Emerson PACSystems RSTi-EP User Manual

User Manual
GFK-2958J
The terms earths and shields are classified according to their relation to human safety or
system safety. An earth is installed primarily to protect human life, and for this reason it is
referred to as the protective earth (PE) conductor. A shield, on the other hand, serves to
ensure the trouble-free operation of an electrotechnical system as well as electromagnetic
compatibility.
The main differences between the two terms are therefore the electrical design and
installation. A shield is not designed to transfer power, even though leakage currents can
flow on it – something which must be avoided. In contrast, a PE conductor must be capable,
at least in the short term, of discharging high residual currents (IEC 60947-7-2). The
corresponding short-term current resistance of the PE connection must be 120 A/mm²
(77419.2 A/in
work properly, the shield impedance must be 10 times larger than the impedance of the
earth potential.
The following figure shows how these two topics relate to each other in application. As
shown in the figure below, the cable's shielding is connected to the earth potential so that
the shield's current can be discharged. Depending on the sensitivity of the system, an
attempt is made to create separate potential areas for this. However, it is still typical to mix
the areas, i.e. the shielding has a common equipotential bonding (earth). This figure shows
how the number of shields and PE conductors that need to be connected can increase quite
rapidly (in this case only one component is used). The shielding and earthing systems must
be planned carefully to provide adequate safeguards for personnel and equipment. The
following sections describe the complexity and special characteristics in more detail.

Earthing and shielding

Earthing and Shielding
2 ) of the connected cross-section. To make sure a shielding concept is able to
Chapter 7
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