Slightly OT, but I went to a test lab demontration a few years ago showing the difference between STP and UTP Cat 5e cables. The UTP cables worked OK up to a field strength of just over their design limit of 3V/m and comms then stopped, but the STP cables (shielded) worked happily at their design limit of 10V/m and carried on until the strength increased to 17V/m. 10V/m is really only encountered in industrial environments with very heavy currents or standing next to powerful radio transmitters.

OK, you only need to distinguish between a 0 and a 1 for comms circuits, but it does demonstrate how much more robust a shielded cable is in a noisy environment given the same internal cable structure.

To be fair to the UTP cable, we also wrapped it (100m length) around fluorescent lights, operated walkie-talkies next to it, stuck transformers nearby and ran an electric motor next to it (all at the same time) and the comms still worked quite happily, showing that a good balanced cable is pretty good at rejecting noise on its own.

I'm not trying to say anything here except show by an analagous example that shieding can improve the noise resistance of even the best balanced circuits.

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