A microwave oven actually uses a type of Faraday Cage however it is optimized for keeping the microwave radiation
inside the oven. This does mean however, that it
will block the same frequency of radiation from getting in.
As I understand it, the magnetron does not need to be disabled but unplugging the oven from the socket is probably a good idea to prevent power surges possibly causing the magnetron to activate (and frying whatever you had inside!) because the magnetron controls etc. etc. are on the outside of the Faraday Cage and they will be effected by any electrical or radiation surge.
Keep in mind that I do not know for a fact that EMP blasts could do this, it was an unsupported claim I read somewhere that an EMP induced power surge could cause your electrical devices to activate for a few seconds before they short out.
The word cage gives the idea that there are openings in the cage material but for a Faraday Cage, this is not actually necessary and if they do have any openings, they need to be related to the frequencies you're trying to block. Hole size will block certain frequencies and I believe it's a case of smaller hole size is better. With a microwave oven, the hole size in the Cage is not particularly small - look at the viewing window on the oven and you'll see the embedded mesh that makes up one side of the Faraday Cage.
To get an idea of what I mean by all of this, try the following: -
1. turn off & unplug your microwave oven
2. make note of the reception strength of your mobile/cell phone (how many bars)
3. put your mobile/cell phone inside the oven
4. check the reception strength again and see how many bars are blocked by the oven
5. try the same thing with another mobile/cell phone running on a different phone network and compare the results
A microwave oven is optimized to stop a small range of radiation frequencies, some phone networks will be blocked by the oven while others using different transcieving freqs will suffer reduced connection but not be entirely blocked because
the oven is not optimized to block those specific freqs.
A typical man-made EMP will spew out a massive range of freqs so a microwave will stop some, reduce others and probably not stop the rest.
The best defence from EMP is to have your devices in
nested Faraday Cages but the cages
must be insulated from each other.
If you are only trying to stop radiowave type radiation, even something as simple as wrapping your devices in heavy duty aluminium foil (make sure there are no gaps!), enclosing that package in any non-conductive material (e.g. rubber, plastic, wood) and then securing that inside another Faraday Cage (such as a microwave oven) will allegedly protect the device.
For maximum effect, apparently the best thing to do is place your aluminium foil wrapped items inside a heavy duty plastic bag, wrap that in more foil, place that package inside another heavy duty plastic bag and then wrap that in another layer of foil. Then place all of that inside your Faraday Cage.
Keep in mind that the outside Cage must be grounded if you wish to protect against electrical surges but it is not necessary to ground the Cage if you are only trying to stop radiowave type radiation.
Now...
Keeping in mind that radiation travels in direct line of sight, a modern safe with inner flanges to prevent the door from being jimmied open will apparently work as an ad hoc Faraday Cage as long as you block up any gaps with suitable radiation blocking material and again, if you're trying to stop electrical surges, the safe would need to be effectively grounded.
However, always keep in mind that the outside Cage should be made of the best electrically conductive material you can get - the steel of a safe is helpful but not as useful as copper sheeting.
Resources:
These are the websites I trawled through to get some idea of all this - Warning! It entails lottsa reading!
http://modernsurvivalblog.com/emp-el...-faraday-cage/
http://www.thenakedscientists.com/fo...?topic=41755.0
http://www.futurescience.com/emp/emp-protection.html
http://www.thehighroad.org/archive/i.../t-344280.html
http://www.preparednesspro.com/many-...-faraday-cages
http://boards.straightdope.com/sdmb/...d.php?t=557413