To clarify some things about Faraday cages and EMP in general :
1. EMP tests have indeed showed that an EMP effect does not guarantee every piece of electronics will be fried. BUT.... your statement is true but worded in such a way as to convey the totally wrong impression to the point we might as well call it a lie. at credible field levels, emp effects caused serious damage in a MINORITY of cars, trucks and other bits of infrastructure when tested while the device was in operation, and in many cases caused NO failures when powered down.
1a : These tests were done with reduced strength pulses, not at the types of pulses an EMP device would typically generate. you are talking about the emp commission report I assume. they did testing up t 50 kv/m, this is well above what MOST of the effected area will receive
1b : New super-EMP devices may generate pulses at least twice as strong as current devices theoretically are capable of. may. but there is no credible information presented that such weapons exist.
1c : These tests were done years ago. Since that time electronic circuits have got smaller with tinier (more fragile) components and smaller distances between them (requiring less voltage to spark across). it wasnt that long ago. the report came out in 2008
So color me unreassured about the EMP testing done to date.
2. A Faraday cage must
NOT be connected to a ground strap. If you do that, you've converted it from a cage to an antenna. Ooops. so? so long as the current is being carried by the antenna and not by your electronics it doesnt matter. thats what a faraday cage DOES
3. The devices in the cage must be insulated from the cage exterior. nope. just like a bird can sit on a bare wire
4. It is true that you can create a perfectly good Faraday cage out of wire netting for some types of 'low frequency' electrical discharge. But to get protection from high frequency EMP effects, you should have a solid metal structure. this is true
5. I don't understand the math/physics behind this point, but my sense is that a Faraday cage
attentuates (ie reduces) rather than eliminates RF and higher frequency fields (can't remember where I read it but what I read, when I did, was convincing). A single cage is therefore good, but for stronger pulses or more delicate circuitry, nested cages are better.
Hope this helps.