GE GFK-2958E, Руководство пользователя

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GFK-2958E RSTi-EP User Manual November 2017 327
Earthing and Shielding
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
2
)of the connected cross-section. To make
sure a shielding concept is able to 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.