Mitsubishi Electric MELSEC-Q Series User Manual page 46

(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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