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CM emissions  ( conducted )  are affected by capacitance to earth  -  because the  currents /   voltages in the 2 lines are  common to earth  -  hence the term.

Modelling this in Spice is really a waste of time  -  as physical elements tend to dominate  -  for example above 450kHz  the length of the attached wires determines how effective a radiator the system is  -  and therefore whether there would be a tendency to exceed radiated limits.

We have to meet RFI/EMC standards for the DC lines on telecom power supplies  -  we never model this   -  testing  is the only way.

Obviously a lot of  CM currents in the DC lines above 450kHz  requires us to test for radiated in a typical system setup.