While EMP is an often discussed threat, I think most here consider it lower on the spectrum of potential disasters.

That being said, there are still non-disaster related reasons to understand Faraday cage construction.  RFID wallets or electronics shielding for automotive applications are two examples.   I did some experiments following what the "internet experts" declared would work without fail and they failed.  One experiment involved putting a cellphone, a FRS radio and then a WiFi device in a microwave.  The microwave attenuated the signal quite a bit, but didn't fully block/disrupt/shield any of the devices.  I repeated the experiment with tin foil, ammo boxes, etc.  None were fully effective.  Now I know you need perfect metal seals, hole sizes related to frequency/etc.  BUT I couldn't explain why my experiments didn't seem to follow the conventional wisdom.

It turns out that convention wisdom may not be as solid as believed.
Are Faraday cages less effective than previously thought?

One thing, microwave ovens operate at 2,4 ghz band and the RF 'seal' is designed for that center frequency...

Any other frequency 'band' may not work.

As far as "Perfect" seals being necessary... That's silly.

All that is necessary is to attenuate the EMP 'signal' below the damage level for a particular device.

A few 10's of db's should take care of most issues, --my estimate...

I read a few paragraphs of your linked article and I have to say, it appears the writer has little practical knowledge of RF propagation, shielding, etc...

buttons...

Yeh, there's a good reason for that!

Microwave EMP has a range too small to be useful against anything other than small, specific military targets.

It is directional (has to aimed in a specific direction in order to be effective), and becomes exponentially weaker as the distance away from the target increases.

HEMP (which is EMP produced by detonating a nuclear weapon at high altitude) is essentially non-directional, and is capable of affecting large areas of the continent. However, it is very low-frequency (produces almost no energy at microwave frequencies) - which means that it doesn't have much effect on anything small enough to fit inside a "Faraday Cage".

BOTTOM LINE: Unless you live very close to a high-value military target, constructing "Faraday Cages" to protect against EMP is a complete waste of time.

A Faraday cage does not completely block EM radiation. It simply attenuates the signal to a hamless, low level.
You probably need at least 50 db. of attenuation for a Faraday Cage to be effective. A metal garbage can with a metal lid will probably provide 10-15 db of attenuation. Seal the gaps in the lid, with a wide ,sticky aluminum tape and it will double the attenuation level.
I seriously doubt any electronics can be damaged by EMP if they are not connected to large antennas or cables. I also doubt that the electrical grid will be completely damaged. There are a lot of protection devices installed to prevent the grid from a major damage. It's not easy to damage the large power transformers either.
My 2 cents. I'm an EE.

Did you ground the objects you used as faraday cages and were all panels and doors physically connected to each other with conducive materials?

Lots of reasons why faraday cages fail.

While seemingly simple, they are actually a little more complicated than just a metal box.

Other than safety...

Why would one need to "GROUND" a Faraday cage????

That makes NO SENSE...

Yes, microwaves are 2.4Ghz, that's why I used a 2.4Ghz WiFi device to test it.

No, perfect seals are not necessary, but there are conditions that should be followed.



Correct, but there are practical applications such as isolating or "hiding" a radio device, preventing it from communicating with the outside world.  



Yes, and isolated the target object inside from the faraday cage.  It was interesting and I did get a fair amount of attenuation, but not 100% what I was hoping for.

I can tell you that I've tested faraday bags and EMP bags at work for blocking cell phone frequencies. They all failed testing. The best object we found which would block them enough to prohibit communication, was an arson can. Of course, the devices will burn through the batteries when they go full power to try an connect to a network....

There is no "block", only "attenuation" or reduction of signal. So even something that may seem impenetrable to radio signals is not, it only reduces a certain amount.

Radio signal in the real world tend to be measured on logarithmic scales, because of the massive range of signal strengths that are encountered and useful.

Consider the following:

Cutting a signal level in half, in logarithmic terms, is a 3dB reduction. Cutting it in half again (so now 1/4 of the original signal) would be 6dB.

The signal meter on a typical amateur radio transceiver, from strongest to weakest signal, covers roughly 100dB.

A basic duplex repeater in two-way radio has a filtering system to isolate the transmitter and receiver from each other, which typically have an isolation of 80 to 100dB.

So even what seems like a good shield, and even if it is a good shield, may still not be enough to prevent a device from being able to communicate through that level of attenuation. To accomplish that amount of attenuation would take a lot of attention to detail. Without sophisticated equipment the best bet would be to use multiple layers, as the attenuation of each layer is additive to other layers. Actually the best bet would just be to remove power from the device so it can't function.

Grounding a Faraday Cage will not improve it's effectiveness. Interconnecting parts of a metal box with a wire will not greatly improve it's effectiveness either. All the gaps in doors, covers or any other parts must be electrically sealed by means of special conductive gaskets or covered by special sealing tape. A regular Air Duct, aluminum sealing tape will work too. Small holes will not let as much RF energy in as even the thinnest gaps between the parts of the box.
Hoffman sells special caskets for RF enclosures. They also publish RF attenuation data for their pre-made RF sealed boxes. It's not cheap.

Many people fail to understand a difference between static electricity and RF radiation. Some make money on crap that does very little to protect electronics from EMPs.  I've seen them testing their "faraday cages" by zapping them with stun guns and calling it good. This is so misleading.
Some preppers even store a spare starter and battery in their Faraday cages thinking that EMP will fry everything electrical too. Believe me, you will have a lot more to worry about if you live (highly unlikely) through an EMP strike, strong enough to damage a car starter or a battery.

I will buy some of what you're selling, sir.

i found a seabag when I was moving that is lined with anti static shit for carrying rockets onto helos... would that be worth a damn as far as blocking signals?

What practical application are specifically envisioning?

A large market ----for companies making shielded RF  'enclosures'...

Is, enclosures for forensic preservation of evidence WRT electronic devices.

Since we're on the subject, this is an interesting document for HAMs or technical types.

EMP for HAMs Part1

EMP for HAMs Part2

^^^This is has always been my thought and why I've never put much stock in an EMP strike from another nuclear power. Too risky. You might as well just launch a first strike.