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I was discussing high-quality twisted pair cables that go from a DSL modem to a wallsocket and have an overall shield.

A friend has pointed out that shielded cables ought to have the shield connected to earth. My understanding of this point is that it prevents the local potential inside the Faraday cage possibly bouncing up and down relative to the outside environment. Is this an accurate interpretation? I am wondering how much benefit you get anyway in an un-grounded setup, in this case where the conductors inside are a twisted pair. Since the inside of the shield is a faraday cage the potential is flat and the electric field is zero i.e. grad V = E = 0 everywhere so this surely makes life say for the twisted pair. Maybe the charge redistribution response time of the shield is inadequate or it has internal resistance so that the interior is not an effective Faraday cage up to sufficiently high frequencies. (Superconducting shields are out of my price bracket.)

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You ground these because Faraday cages are not perfect. One near field effect which matters in this case is capacitive coupling between the shield and the wires. If the shield is ungrounded, it's effectively an antenna, and thus acts as coupled antenna to your wires. Grounding the shielding fixes the antenna like effect.

In theory you could also resolve this issue with an ungrounded shield that is further away from the wires, but that takes up more space.

Of course, you have to be careful. If you ground it on both sides, you get a ground loop, which can cause all sorts of problems for some analog hardware.

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  • $\begingroup$ So would a superconducting ungrounded Faraday cage work as desired? $\endgroup$ – Cecil Ward Mar 26 '17 at 1:21
  • $\begingroup$ I would think so, but in my experience, superconductors surprise me all the time, so I'd be wary taking my word for it. I do think you'd still have to deal with inductive coupling though. $\endgroup$ – Cort Ammon Mar 26 '17 at 2:02

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