[Ol' Dan Tucker]

A metal box is not a Faraday cage unless the metal box is grounded. Otherwise it’s just a metal box

[Bruce Kurtz]

A tightly closed metal box will greatly attenuate electromagentic (EM) waves from coming in the box and prevent EM waves from the leaving the box. EM waves reflect from metal objects (radar)

[FredZarguna]

A metal box is not a Faraday cage unless the metal box is grounded.

This is untrue.

Any conducting material with grid spacing significantly smaller than the wavelength of the signal to be attenuated is a Faraday Cage.

In particular, a metal box with thickness greater than the skip depth of the wave (which is no more than 100 microns or less) is always a Faraday Cage.

Grounding is required to avoid damage to objects outside of the cage, because induction currents induce charge on the outside of the box, which can be quite large if the signal has high energy. It does not affect the effectiveness of the cage as a shield. [I have demonstrated this to undergraduate physics and EE students, many times.]

[St_Thomas_Aquinas]

—— A metal box is not a Faraday cage unless the metal box is grounded. Otherwise it’s just a metal box -—

Does sitting on a concrete basement floor count? Or do you have to run a wire from it into the ground?