Would a Faraday cage still work if it was part of a circuit?

Question

Faraday cages protect against EM radiation and charges.

As far as I understand, this effect happens through the induction of an electric field, which produces a current inside of it, which then counteracts any incoming current. So, my question:

If I replaced part of a wire in my circuit with a Faraday cage, say + at the feet and - at the top, would the interior still be protected, or would the current, given that the skin effect applies, move through both the outer surface and the inner surface? Are there any differences between DC/AC?

Answer 1

I replaced part of a wire in my circuit with a faraday cage, say + at the feet and - at the top, would the interior still be protected, or would the current, given that the skin effect applies, move through both the outer surface and the inner surface?

Assuming "part of Circuit" meant connected to a battery /generator then you cannot protect(from the electric field due to battery or generator) the material (metals) inside the Faraday's cage because an electric field would exist inside the Faraday's cage to satisfy Laplace's equation and boundary condition , although when connected to battery or generator material inside Faraday's cage still protected from electric field due to outside charges and we can prove it using Superposition theorem

And are there any differences between DC/AC?

In case of DC(battery) you'll get steady electric field inside the cage at steady state while in case of AC (generator) you'll get an alternating electric field at steady state .

Answer 2

Note: Every conductor has resistance. The current in a lightning strike is very large (thousands of amperes). If the Faraday cage is not sufficiently conductive, there may be a voltage drop between where current hits the cage and where it leaves. That voltage drop could cause some current to flow through alternate paths inside the Faraday cage. For the purposes of this answer, I will assume that the Faraday cage is sufficiently conductive to protect it's interior from currents that are likely to strike it. If it is not sufficiently conductive, well, all bets are off.

If a circuit consists only of components completely inside of the Faraday cage, plus the Faraday cage itself, then the components inside of the Faraday cage will be protected from electrical discharges originating outside of, and some distance away from, the cage.

The cage could be used, for example as the "ground" of a circuit, as is often the case in airplanes

However, if the circuit has any part that extends beyond the cage, then this protection will be lost. For example, a circuit in a Faraday cage which has a ground wire that extends outside of the cage to earth, (but is not bonded to the cage itself) would not be protected. Potentially, a discharge could pass from a source (such as lightning) to the cage, then jump to the ground wire (possibly through circuitry inside the cage) and continue to the earth.

On the other hand, a circuit which is entirely enclosed, but contains a battery or other electrical source of power, will have protection. For example, the electronics in airplanes struck by lightning are typically unaffected. (The "Faraday Cage" of a commercial airplane is designed to be highly conductive, and tested for the currents it is likely to experience in a lightning strike.)

Edit: The same principle applies to changing electromagnetic fields. If a circuit contained within a metal box is adequately isolated from external electromagnetic fields by that box, then including that box into the circuit, for example by making it a ground "net", will not compromise the isolation afforded to that circuit.

Nor will driving a current through a Faraday Cage from an external source adversely affect anything inside the Faraday cage (of course always with the proviso that the cage is adequately conductive so that no voltage develops across the cage, and there is no appreciable heating of the cage as the current flows through it.)

Answer 3

Faraday cages are used all the time in RF circuits to isolate the ingress and egress with sufficient attenuation. It also adds some pF from the body as a capacitor plate but this prevents proximity effects of placement from modulating or detuning the resonant LC circuits.

All TV tuners were done this way to output on the IF as well as the demodulator to convert to baseband chroma and lumina signals with sync. Often they use flanged lids for access.

We also used them on 1 GHz ceramic hybrids, so you can expect to see them on WiFi routers. There is a company that specializes in the corner soldered tin plated thin plates. We made them from half etched brass in a PCB shop for prototypes and tinned them in the lab then breakout like a tabbed array of same boards to be folders and soldered onto a thick track with propane microtorch with a lid or just a tuning hole for test point access.

Answer 4

This four page article Shielding Effectiveness of Microwave Cable Assemblies:

https://www.gore.com/system/files/2018-11/MWR-0740-GORE-REPRINT-MAR-2018.pdf

Before delving into shielding effectiveness, let’s define the term. A shield is a conductive barrier that envelops and isolates an electrical circuit. For a microwave coaxial cable, the isolated electrical circuit is the center conductor, dielectric and outer conductor. Because of the skin effect, at microwave frequencies the return current on the outer conductor travels through a thin layer of the inner diameter of the outer conductor. This leaves the remaining portion of the outer conductor as the shield. Shielding effectiveness is defined as the ratio of the RF energy incident on one side of the shield to the RF energy transmitted to the opposite side.

The inner region of the outer conductor provides the circuit function of signal conductor and the outer region of the outer conductor provides the function of shield (cage) for the signal flow in the inner and outer conductors.