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8 EMC DIRECTIVES
(3) Cables
The cables extracted from the control panel contain a high frequency noise
component. On the outside of the control panel, therefore, they serve as
antennas to emit noise. To prevent noise emission, use shielded cables for the
cables which are connected to the I/O modules and intelligent function modules
and may be extracted to the outside of the control panel.
The use of a shielded cable also increases noise resistance.
The signal lines (including common line) of the programmable controller, which
are connected to I/O modules, intelligent function modules and/or extension
cables, have noise durability in the condition of grounding their shields by using
the shielded cables. If a shielded cable is not used or not grounded correctly, the
noise resistance will not meet the specified requirements.
(a) Grounding of shield section of shield cable
1) Ground the exposed shield section of the shielded cable close to the
module. When the grounded cables and the not yet grounded cables
are bundled, the cables might be induced to electromagnetic.
2) Ground the exposed shield section to spacious area on the control
panel. A clamp can be used as shown in Figure 8.2.
In this case, mask the inner wall surface when coating the control panel,
and contact the exposed shield section with the clamp at the exposed
bare metal surface.
Shield section
Screw
Clamp fitting
Shield cable
Exposed bare
metal surface
Figure 8.1 Part to be exposed Figure 8.2 Shield grounding (Correct example)
Note) The method of grounding with a vinyl-coated wire soldered onto
the shielded section of the shielded cable as in shown Figure 8.3
is not recommended. Doing so will raise the high-frequency
impedance, resulting in loss of the shielding effect.
Shield cable
Wire
Solderless terminal, crimp contact
Figure 8.3 Shield grounding (Incorrect example)