Installation In Control Box - Mitsubishi A-Melsec A1FXCPU User Manual

5. EMC DIRECTIVE, LOW VOLTAGE DIRECTIVE

5.1.2 Installation in control box

The PLC is open equipment (equipment incorporated into other apparatus) and must
be installed in a conductive control box. This not only ensures safety but also has a
great effect on the shielding of PLC-generated noise with the control box.
(1) Control box
(a) The control box should be conductive.
(b) When fixing the top and bottom plates, etc. of the control box with bolts, mask
the paint to permit surface contact.
(c) To reserve electrical contact with the control box, mask the paint on the bolts
used for mounting the inner plates to the control box so that conductivity may
be reserved on the widest possible surface.
(d) To reserve low impedance at high frequencies, connect the control box to the
ground with thick ground wires.
(e) The hole of the control box should be 10cm max. in diameter. If it is more than
10cm, electric waves may leak.
(2) Routing the power and ground wires
Route the ground and power wires of the PLC as described below:
(a) Provide a control box grounding point near the built-in power supply Section of
the A1FXCPU module and connect the LG and FG (LG: line ground, FG:
frame ground) terminals of the power supply module to the ground with the
thickest and shortest possible wires (about 30cm or less long) to that point.
Since the LG and FG terminals are designed to draw noise generated inside
the PLC to the ground, the lowest possible impedance must be reserved.
In addition, make sure to wire the ground cable short as the wires are used to
relieve the noise, the wire itself carries large noise content and thus short
wiring means that the wire is prevented from acting as an antenna.
(b) The ground wires drawn from the grounding point should be twisted with the
power wires. Doing so can release noise from the power wires to a larger
ground. However, when a noise filter is fitted to the power wires, this twisting
may not be needed.
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MELSEC-A