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MELSEC-Q
8 EMC AND LOW VOLTAGE DIRECTIVE
8.1.2 Installation instructions for EMC Directive
The PLC is open equipment and must be installed within a control cabinet for use. This
not only ensures safety but also ensures effective shielding of PLC-generated
electromagnetic noise.
(1) Control cabinet
(a) Use a conductive control cabinet.
(b) When attaching the control cabinet's top plate or base plate, mask painting
and weld so that good surface contact can be made between the cabinet
and plate.
(c) To ensure good electrical contact with the control cabinet, mask the paint on
the installation bolts of the inner plate in the control cabinet so that contact
between surfaces can be ensured over the widest possible area.
(d) Earth the control cabinet with a thick wire so that a low impedance
connection to ground can be ensured even at high frequencies.
(e) Holes made in the control cabinet must be 10 cm (3.94 inch) diameter or
less. If the holes are 10 cm (3.94 inch) or larger, radio frequency noise may
be emitted.
In addition, because radio waves leak through a clearance between the
control panel door and the main unit, reduce the clearance as much as
practicable.
The leakage of radio waves can be suppressed by the direct application of
an EMI gasket on the paint surface.
Our tests have been carried out on a panel having the damping
characteristics of 37 dB max. and 30 dB mean (measured by 3 m method
with 30 to 300 MHz).
(2) Connection of power and earth wires
Earthing and power supply wires for the PLC system must be connected as
described below.
(a) Provide an earthing point near the power supply module. Earth the power
supply's LG and FG terminals (LG : Line Ground, FG : Frame Ground) with
the thickest and shortest wire possible. (The wire length must be 30 cm
(11.81 inch) or shorter.) The LG and FG terminals function is to pass the
noise generated in the PLC system to the ground, so an impedance that is
as low as possible must be ensured. As the wires are used to relieve the
noise, the wire itself carries a large noise content and thus short wiring
means that the wire is prevented from acting as an antenna.
(b) The earth wire led from the earthing point must be twisted with the power
supply wires. By twisting with the earthing wire, noise flowing from the power
supply wires can be relieved to the earthing. However, if a filter is installed
on the power supply wires, the wires and the earthing wire may not need to
be twisted.
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