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Here is the scenario:

  • You are in an environment where there are sources of EM signals that your body will pick up as though it were an antenna via capacitive coupling, i.e. what happens with the 50/60 Hz mains when a person is inside their home.
  • There is a hollow cube of metal serving as a Faraday shield with a hole you can stick your lower arm into. The fit is snug enough for no signals to enter through any kind of air gap. Also, although I'm using an arm as an example limb, I'm posing this under the lens of whether or not this would work for any limb in general. If the results would change for whatever reason if it was the head instead, for example, please let me know.
  • Within the Faraday shield there are sensors attached to your lower arm that measure how much of the interference signal is able to be picked up via electrodes attached to your skin.

My question is this: if the shield is grounded, would the flush contact between the metal of the enclosure and your skin draw out all the incoming interference travelling along your upper arm so that the sensors inside register none of it? And if not, would placing bracelet-like bands of grounded metal along the length of your upper arm serve to at least lessen the extent to which the picked up signal is able to travel into the enclosure?

The context is that of biopotential sensors, and whether or not a setup like this, however absurd, could be used fruitfully to mitigate mains interference without having to use a shield the size of a whole room.

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    \$\begingroup\$ Any kind of opening in the faraday shield will allow certain frequencies through, regardless if these openings are "plugged" with a limb. This is because electromagnetic (RF) energy has a wavelength, and wavelengths smaller than about 1/10th the opening distance pass right through. (The holes in a microwave oven door are smaller than the wavelength of the microwaves, else they'd come pouring out of it.) \$\endgroup\$
    – rdtsc
    Commented Feb 20, 2023 at 20:14

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Skin has some resistance, but on the other hand, humans are just big blobs filled with conductive saltwater. If you capacitively couple 50 Hz to a conductive element, you have skin resistance plus some capacitive coupling to your grounded metal tube.

So in theory, the amplitude of 50 Hz signal measured at the human subject in the system you described depends on three things, how well 50 Hz capacitively couples to the subject, how much there is skin resistance to grounded Faraday shield, and how much capacitive coupling there is from subject to Faraday shield.

If you have approximations of capacitances and resistances, their relative values can tell how much of the disturbance signal is left in the measurements, but as you see, the disturbance won't be completely eliminated, as skin resistance is not that close to zero, and even capacitive coupling to the Faraday shield is not infnitely large, or even much larger than capacitive coupling to mains.

But it may still help. The effect of just the subject touching or having a grounded strap may reach same levels, but then the Faraday cage won't protect the sensitive measurement probes.

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