
9. EMC DIRECTIVE AND LOW-VOLTAGE INSTRUCTION
9 - 2
MELSEC-A
9.1.2
Installation inside the control cabinet
Since the PLC is an open type device (device incorporated into another device), it
must be installed in the control cabinet. This has a good effect of not only for
assuring safety but also for shielding noise emitted from the PLC, by means of the
control cabinet.
(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. (22 mm
2
wire or thicker is recommended.)
(e)
Holes made in the control cabinet must be 10 cm (3.94 in.) diameter or
less. If the holes are 10 cm (3.94 in.) or larger, radio frequency noise
may be emitted.
(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 (11.18 in.) 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.
Note)
A long conductor will become a highly efficient antenna at high frequency.
(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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