Here is the way a shield is SUPPOSED to work.
Each shield end needs to be GROUNDED.
The shield needs to be low resistance to the FREQUENCY it is shielding.
When a signal hits the OUTER surface (ingress) or the INNER surface (EGRESS) it tries to get to the other side of the shield. The design of the shield ATTENUATES that signal based on the designs capability.
Since both ends of the shield are GROUNDED, and if the ground is good the shield ends looks like the same “point”. We don’t generate a current in the shield in theory.
The problem is, we do see resistance. As the shields Transfer Impedance increases with frequency, transfer impedance is a description of a shields resitance to frequency, we generate a current in the shield. Better shields generate less current. The induced current trying to get through the shield inductively couples “interference” to the signal wire inside. The graphs will show a shield’s performance will get worse with frequency as the transfer impedance goes up.
The magnitude of that inner signal that isn’t completely attenuated is measured in a RATIO called dB. The higher the better and means a smaller and smaller signal level on the opposite side of the shield.
When we DO NOT ground the shield, we creat a resonance in the shield. The energy isn’t REMOVED and basically bounces around in the shield or travels along the shield, inductively coupling energy as it goes. Not good.
PLEASE GO HERE
"SPG resonates so that the induced signal is amplified, not attenuated, at the shield’s resonant frequency. This model is a low frequency model however all of the relevant parameters are included."
With BOTH ends of the shield “open” or shorted to each other, we also have a resonance system, not an attenuator to RF. Removing a shield to break a ground loop is not right, the “broken” ground potential was already there and needs to be fixed. A ground should see the same potential at any ground point in a system such that a PROPERLY gounded shield is essentially the same potential at each end. I = E/R and if R is zero we have no current to induced into the inner pairs after the shield attenuates the signal.
Most of our audio devices are not needing a power cable to shield RF. First the RF is many, many decades smaller than the 50 or 60 Hz “signal”. Second, the power supply inductors look like an open circuit to RF and block it. Second, after the power supply we have filter caps that look like shorts to ground for RF in each circuit blocks DC supply traces.
Will adding RF “filtering” to a power cord help? Let’s say it won’t hurt and leave it at that. We use an EDPM dielectric for BAV power cords and it passively filter RF. We get it at a reasonable cost in a durable 600 V cord. It is NOT the major benefit to the cords, though. We don’t have shielded power cords as the dB relationship to the noise and signal don’t merit a shield and the device SHOULD consider RF removal right at the circuit blocks AFTER the power supply and well away from anything the power cord is trying to add. RF gets in AFTER the power supply and good RF filtering on the DC rails is a necessary design step. TED knows more about that than I do…your turn Ted.