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MELSEC-Q
8 EMC AND LOW VOLTAGE DIRECTIVE
8.1.3 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 cable 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
connected to the PLC I/O modules and intelligent function modules use shielded
cables to assure noise resistance under the conditions where the shield is earthed. If a
shielded cable is not used or not earthed correctly, the noise resistance will not meet
the specified requirements.
When the shield of a shielded cable is earthed to the cabinet body, please ensure that
the shield contact with the body is over a large surface area. If the cabinet body is
painted it will be necessary to remove paint from the contact area. All fastenings must
be metallic and the shield and earthing contact must be made over the largest
available surface area. If the contact surfaces are too uneven for optimal contact to be
made either use washers to correct for surface inconsistencies or use an abrasive to
level the surfaces. The following diagrams show examples of how to provide good
surface contact of shield earthing by use of a cable clamp.
(1) Earthing of shielded of shield cable
(a) Earth the shield of the shielded cable as near the module as possible taking
care so that the earthed cables are not induced electromagnetically by the
cable to be earthed.
(b) Take an appropriate measures so that the shield section of the shielded
cable from which the outer cover was partly removed for exposure is earthed
to the control panel on an increased contact surface. A clamp may also be
used as shown in the figure below. In this case, however, apply a cover to
the painted inner wall surface of the control panel which comes in contact
with the clamp.
Shield section
Screw
Clamp fittin
g
Shielded cable
Paint mask
Note) The method of earthing by soldering a wire onto the shield section of the
shielded cable as shown below is not recommended. The high frequency
impedance will increase and the shield will be ineffective.
Shielded cable
Wire
Crimp terminal