Common mode noise is suppressed by using a common mode choke coil and capacitor (line bypass capacitor or Y-capacitor) installed between each line and the metallic casing.
Noise is classified into two types according to the conduction
mode.
The first type is differential mode noise which is conducted on
the signal (VCC) line and GND line in the opposite direction to
each othe. This type of noise is suppressed by installing a filter on
the hot (VCC) side on the signal line or power supply line, as
mentioned in the preceding chapter.
The second type is common mode noise which is conducted on
all lines in the same direction. With an AC power supply line, for
example, noise is conducted on both lines in the same direction.
With a signal cable, noise is conducted on all the lines in the cable
in the same direction.
Therefore, to suppress this type of noise, EMI suppression filters are installed on all lines on which noise is conducted.
Links
https://www.murata.com/en-eu/products/emc/emifil/library/knowhow/basic/chapter05-p1
https://www.allaboutcircuits.com/industry-white-papers/emc-basics-common-mode-vs-differential-noise/
