Use of cable ferrties on screened three core cable?

[treez]

..is it worth doing this?
I mean, we all know cable ferrites work as a common mode choke when applied to a 2 core "go" and "return" type cable.
But what about when a third "Non current carrying" conductor is in there, which simply connects to the screen, and gets srewed to chassis?

 

[KlausST]

Hi,

The shielding is not a magnetic shielding like ferromagnetic materials.
Thus magnetic field caused by common mode current noise will "go through" the shielding and may be attenuated by a ferrite.

Klaus

 

[FvM]

Thus magnetic field caused by common mode current noise will "go through" the shielding and may be attenuated by a ferrite.

They effectively don't at higher frequencies. Shield blocks magnetic fields according by generating eddy currents. Skin depth can be considered as a rough estimation of magnetic field penetration.

Blocking sheet currents of a shielded cable by ferrite toroids can make sense nevertheless. Not if all cables entering or leaving both cases are perfectly silent and don't conduct any interferences, but that's rarely the case.