Mitsubishi Electric MELSEC A Series User Manual page 24

(d) Earth the control cabinet with a thick wire so that a low impedance
connection to ground can be ensured even at high frequencies.
(22mm2 wire or thicker is recommended.)
(e) Holes made in the control cabinet must be 10cm (3.94in.) diameter or
less. If the holes are 10cm (3.94in.) or larger, radio frequency noise
may be emitted.
(2) Connection of power and ground 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 30cm (11.18in.) or shorter.) The LG and FG terminals
function is to pass the noise generated in the PC 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 more 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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