Weidmüller 1334950000, Manual

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7 Earthing and shielding
47 Manual u-control 2604080000/03/08.2020
7 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 opera-
tion 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 conduc -
tor 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 of the connected cross-section. To make sure
a shielding concept is able to work properly, the shield im-
pedance must be 10 times larger than the impedance of the
earth potential.
Resolver
To other
servo amplifiers
Mains
power
filter
+25 V
XGND
BRAKE
PE
U2
V2
W2
L1
L2
L3
+DC
–DC
X1
X6 X2
X5
PC RESOLVER
SW +
SW -
AGND
PSTOP
NSTOP
DGND
I/O
ENABLE
BTB
BTB
SHIELD
WPE
KLBÜ
KLBÜ
Shielding
Earthing
KLBÜ
KLBÜ
KLBÜ
Booster
Personal
computer
3
Control signals
digital analogue Positioning signals CNC – GND CNC – GND
CONTROL
Connection diagram of a frequency converter
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 con -
nected 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 safe-
guards for personnel and equipment. The following sections
describe the complexity and special characteristics in more
detail.