Understanding what a Faraday cage blocks starts with knowing that it works on one mechanism only. Charges in a conductor rearrange to cancel an external electric field inside the enclosure. That handles electrostatic fields and radio waves very well. It does almost nothing against a steady or slowly varying magnetic field, and a single opening can undo the whole thing.
The mechanism is charge rearrangement
Place a conductor in an electric field and its free electrons shift until the field inside the metal is zero. That redistribution also cancels the field in any hollow interior, which is why a metal box protects sensitive electronics from static charge and why a car body offers occupants meaningful protection during a lightning strike. For time-varying fields the same idea applies through induced currents: an incoming wave drives current in the shell, and that current re-radiates a field that largely cancels the incoming one inside. The effectiveness depends on the shell conducting well enough to carry that current without significant voltage developing along it, which is why continuity and joint quality matter more than thickness for radio frequencies.
Magnetic fields are a different problem
A static magnetic field passes through copper and aluminum as though they were not there, because there are no magnetic charges to rearrange. A slowly changing magnetic field induces some opposing current, but at power frequency the skin depth in copper is millimeters, so a thin enclosure provides very little attenuation. Blocking low-frequency magnetic fields requires a high-permeability alloy that offers the flux an easier route around the protected volume, and those materials must be annealed after forming and can be saturated by a strong field. This is the reason a simple metal box does nothing about hum from a nearby transformer, and why shielding a magnetometer or a sensitive instrument is a specialist job rather than a matter of wrapping foil.
Apertures set the real limit
In practice, most enclosures are limited by their holes rather than their metal. A gap radiates efficiently once its longest dimension approaches a half wavelength, so a long thin seam is much worse than a round hole of equal area. Screens and meshes work when their openings are small compared with the wavelength of interest, which is exactly the principle behind the perforated window in a microwave oven door. Cable entries are the biggest offender of all: a wire passing through the wall carries common-mode current from outside to inside, bypassing the shield completely, unless the shield is bonded around the full circumference at the entry point or the conductor is filtered there. Fix the openings before you upgrade the metal.
Grounding a cage is optional for shielding and required for safety
A common misconception holds that a Faraday cage must be earthed to work. For excluding an external field it does not: the cancellation happens through charge redistribution within the conductor regardless of what it is connected to, which is why a fully enclosed metal box protects its contents while sitting on a wooden bench. Grounding matters for other reasons. It keeps the enclosure from floating to a hazardous potential if something inside fails, it provides a defined reference for filters and cable shields, and in equipment it is usually a legal requirement rather than a design choice. So ground the enclosure, but ground it because of safety and reference needs rather than in the belief that shielding depends on it.
Improvised cages usually disappoint
Foil-lined boxes, microwave ovens used as storage, biscuit tins and mesh pouches all get recommended for shielding phones or keys, and their performance ranges from useful to negligible. The usual failure is continuity: foil creased at a corner, a lid resting on a painted lip, a zipper that leaves a slot, a seam held by tape rather than conductive contact. A simple test is to put a transmitting device inside and try to reach it, though passing that test at one frequency proves nothing about another. If the requirement is serious, use a purpose-built enclosure with conductive gasketing and a specified attenuation figure at the frequency you care about, and remember that even a good enclosure does not address low-frequency magnetic fields.
What to check before you act
- Identify whether your problem is an electric, magnetic or radiated field.
- Close long seams and bond cable shields at the point of entry.
- Use high-permeability alloy, not copper, for low-frequency magnetic shielding.
- Ground enclosures for safety, not in the belief that shielding requires it.
Common questions
Does a car protect you in a lightning strike?
A closed metal-bodied vehicle gives useful protection by conducting current around the occupants. Convertibles and fiberglass bodies do not offer the same path.
Will a foil pouch block a phone signal?
Often partially. Performance depends on continuity of the conductive layer and the closure, and a pouch that works at one frequency may leak at another.
The practical takeaway
Match the shield to the field. A conductive enclosure handles electric fields and radio waves if its seams and cable entries are controlled, while low-frequency magnetic shielding needs a different material entirely. Test the finished enclosure at the frequency you actually care about.
References and further reading
- FCC: electromagnetic compatibility and shielding related guidance
- FCC: radio frequency energy fundamentals
- NFPA: electrical safety and enclosure grounding resources
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
