[MrB]

And that ground needs to be CLOSE to your cage, or else it will act as an antenna for EMP.

The problem with this setup is, there is really no way to test it. We don’t know if it will work at all.

[Bobalu]

Easy to test.
Place a small am radio within.
Turn radio up loud so you can hear only static.
Make some sparks near the cage and see if the radio picks up the static pops.

Or perhaps place a cellphone within and see if you can
call it.