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Old 28-07-2012, 18:02   #16
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Re: Faraday Cage

I'm one of those who puts everything into the oven - BUT - is there any first hand experience or evidence (not just opinion) that this actually does anything (or is it just a maritime urban myth)? Andrew
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Old 28-07-2012, 19:38   #17
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Re: Faraday Cage

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Originally Posted by SunDevil View Post
I thought they used copper to make the mesh?

I would like to make a water proof, lockable, surge protected and lightning resistant case for a USB hard drive I have.
Normal faraday cages work well for electrical currents and RF magnetic fields over 100Khz (Mu-metal - Wikipedia, the free encyclopedia).

Per this post, Lightning Strikes , the high end for lightning strike energy is 1Mhz meaning most of the energy is of lower frequency. This source, 2nd paragraph of item 1.3 pg.6 http://assets.cambridge.org/052158/3...21583276WS.pdf , indicates the peak of the energy is at 5-10kHz.

The way I read this all together is that for protection from the magnetic component of lightning you need a faraday cage made from a ferro-magnetic material. A lightning strike may close enough may have enough energy to overwhelm even that, but you are improving your odds.

Regarding your desire to protect a hard drive. Get a steel cashbox, find a Pelican case it will fit in, voila.

MMF2216194G2 - Tiered Cash Box with Bill Weights, 12 in, Cam Key Lock, Charcoal- STEELMASTER by MMF Industries-Computers & Electronics-Office Products-General Office Accessories
Pelican 1120 Case With Foam - BLACK from Pelican-Case.com

If you wanted to be more sure about your protection, drive in a small cash box, in the Pelican, in a bigger cash box in the oven.
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Old 28-07-2012, 19:40   #18
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Re: Faraday Cage

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Originally Posted by Andrew G View Post
I'm one of those who puts everything into the oven - BUT - is there any first hand experience or evidence (not just opinion) that this actually does anything (or is it just a maritime urban myth)? Andrew
This post on the Lightning Strike thread, Lightning Strikes, indicates his stuff in the oven survived strikes twice when everything else on the boat fried.
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Old 28-07-2012, 19:45   #19
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Re: Faraday Cage

Hot chips?

Another wild idea came to me in the bath....how about a Van de Graaf generator with the static charge sphere located high up...idea being to cancel out the static loading on the boat. Obviously have to get the polarity right or it will do the opposite.
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Old 29-07-2012, 01:18   #20
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The electric field and the magnetic field are effects of the same thing. They are inseparable. So as the Faraday cage acts on a electric field it acts on the magnetic field too.

The magnetic shielding materials such as iron or Mu Metal are used for static magnetic fields such as the earth's or a nearby engine. These static magnetic fields produce no current so the Faraday cage is ineffective. But as they induce no electric current they cannot cause electrical damage except in special cases.
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Old 29-07-2012, 09:00   #21
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Re: Faraday Cage

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Hot chips?

Another wild idea came to me in the bath....how about a Van de Graaf generator with the static charge sphere located high up...idea being to cancel out the static loading on the boat. Obviously have to get the polarity right or it will do the opposite.

Only problem with that is lightning comes in both polarities. Negative strikes dominate, but positive strikes do the most damage (not to say that negative strikes can't do damage as well).

Also listen to Adelie. He is completely correct on the electrical/magnetic fields. I've done a fair amount of work/research in the field and everything he has said is correct, so I won't repeat all it again.

Cheers,
Mike
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Old 29-07-2012, 09:34   #22
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Re: Faraday Cage

Yep, Adelie is sure worth his rum ration, just check out the latest sextant thread.

The silly idea of mine about VdG generator was along the lines of trying to bleed off any charge on the boat...neutralise it, if you will. Don't worry, no intention of any Ben Franklin type experiments...my beard is too flammable.
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Old 29-07-2012, 11:26   #23
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Re: Faraday Cage

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The electric field and the magnetic field are effects of the same thing. They are inseparable. So as the Faraday cage acts on a electric field it acts on the magnetic field too.

The magnetic shielding materials such as iron or Mu Metal are used for static magnetic fields such as the earth's or a nearby engine. These static magnetic fields produce no current so the Faraday cage is ineffective. But as they induce no electric current they cannot cause electrical damage except in special cases.
Here's the way I think about it. Electromagnetic wave are alternating, interlocking loops of electrical and magnetic energy, just like a chain. Straight out of book 2, Halliday and Resnick if I recall my 1st year physics.

If the boat touches the chain, lets ignore diffraction effects, it will touch one or the other type of link more. Any touching by a conductive container of an electrical link will affect the mag link connected to that e-link. Since the boats are all approximately the same size, for this kind of physics question, then it stands to reason that longer wavelengths (lower freq) are more likely to have a lot less of of an e-link touching the container meaning that the more of the energy is magnetic.

Any conductive material will induct an electric current from a changing magnetic field. But it seems to me that not all of the magnetic energy would be inducted some would penetrate the conductive shell and induct currents in anything conductive within the shell unless the shell was ferro-manetic.

The question becomes at what frequency does an em wave start acting like it is static for these purposes and the material of the faraday cage start becoming important. I don't really know the answer for lightning.

I know that for sensitive experiments the cage material starts to become important at about 100kHz and below. It seems reasonable that lightning would be somewhere below that. The peak energy for lightning is at about 5-10kHz. I suspect that cage material has at least some effect on level of protection, just don't know for sure how much and can't find confirming or contradictory info.

In sum, cage material being steel may help and can't hurt.
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