Re: Preventing a REALLY expensive mistake.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>" <jimlux-at-earthlink-dot-net>



> >
> 
> Ok....several people have addressed wire in the walls, etc so I'm going to
> see if I can pass/fail that one right now. I'm hoping I'm wrong....
> 
> Doesn't a Faraday cage only work if it's a COMPLETELY ENCLOSED room? I don't
> mean airtight (but there is a specific maximum size hole for a given
> frequency protection limit)?

Yep..  Fortunately for you, most of your power is going to be down below 30
MHz, so for electrical propagation, the holes can have perimeters up to 5
meters or so, and still not impair the shielding (that means that windows
are OK).  Note that this doesn't mean that magnetic fields won't pass
through, and it IS a significant problem.

> 
> And if I do spend the time/money in caging my High Voltage Lab, Computer
> labs, or both the second I plug the computers in to the AC, Ethernet, or any
> other antenna (wire) that passes outside the cage (into the wall or another
> room) haven't I just defeated the cage?

Exactly... any wire or conductor passing through the cage must be properly
filtered (fiber optic network links are popular for this reason.. and
remarkably reasonable in price these days, too.)


> 
> If there is a way around the wire issue and a secure cage can be built. I
> need a DEFINATE size maximum hole. Chicken wire is great, but the HOLES in
> chicken wire vary considerably. I know the math Gods can help on this. :)
> 
> ok, on to the next one. Sorry to play Devils advocate guys, but I need to be
> SURE on this one.

Radiated power (which is what cages are for) isn't likely to be your
problem.  Conducted emissions, particularly in the form of shared grounds,
is where you should concentrate your effort.