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Old 12-10-2005, 00:58   #1
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Best Lightning Insulator?

I want to protect a backup handheld GPS from a possible lightening strike. Any suggestions on wrapping/stashing it to keep it safe?
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Old 12-10-2005, 04:28   #2
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A Faraday Cage is used to protect items vulnerable to EMI & EMP. Utilizing the "skin effect", the cage is actually a good conductor (not insulator). A Faraday Cage can be something as simple as wrapping the vulnerable item in aluminum foil, metalized mylar, a coffee or cookie can, or an anti-static/anti-magnetic mailer bag. Sometimes the cage is grounded with a short wire & alligator clips.
See:
http://www.hmcelectronics.com/cgi-bi...duct/8490-0002
http://www.lpsind.com/MetallicAnitStaticBags.htm
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Old 12-10-2005, 05:07   #3
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many thanks! i think i've seen these at my local computer market here in thailand, but didn't know what they were.
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Old 12-10-2005, 11:39   #4
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Protecting items from lightning

Sorry to say that a Faraday cage can protect an enclosed item from an external charge yet it may not protect the enclosed item from a magnetic field.

In the case of a lightning current path that generates both electric and magnetic (expanding and contracting) components at right angles to each other in order to protect a sensitive piece of electronic equipment both field components must be shielded from the item.

The proper manner to protect a small item is to enclose it in a ferromagnetic material comprised of two components. The outer layer consists of a non-saturating magnetic material (like bits of ferrous material encapsulated in an epoxy) an an inner layer composed of a ferrous material which does have a magnetic saturation point. The idea is that the outer layer shunts the greatest part of the magnetic field around the inside item. The part which "leaks" past the non-saturating material gets shunted by the saturable inner layer so that the item inside receives a magnetic field insufficient to develop electric potential across conductors within the device to be protected.

If the layers are reversed then the saturable material quickly saturates and passes All of the rest of the magnetic field to the inside layer which passes more of the field to the inside part with much less attenuation than the proper layering would do.

With proper layering the inside layer also attenuates the electric field as well (with good choices of materials).

There have been many documented cases of radios, gps devices, and computers which have been damaged by nearby lightning strikes EVEN THOUGH they were "protected" by a layer of conductive material only. BTW: static bags will do very little to protect items from lightning.

Now I realize that most of us do not have easy access to a box properly designed to do what is outlined above. If nothing else put the gps or radio or laptop in a steel box as thick as practical. Obviously this will be heavier than an electric field only box.

Just thought that this subject should be clarified so that someone doesn't mistakenly believe that some item might really be protected with aluminum foil. It will not.
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Old 12-10-2005, 16:18   #5
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Cone of protection....

My understanding is that a properly mounted and connected lightning rod on the top of the mast provides a "cone of protection" around the boat.
The energy of the lightning is conducted to (or from?) the seawater via the mast, rigging and grounding points.
I would be more worried about myself being fried than a piece of replacable equipment.
If your boat is properly protected then it is possible that all you may need to do is make sure that sensitive equipment is disconnected and out of the way of the conductive path of the lightning.
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Old 12-10-2005, 16:37   #6
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I’m once again ashamed at offering an incomplete, and thereby inaccurate reply.
Both Rick & Chris31415 are correct .
Rick is technically accurate, as always - but Chris... has pointed out a practical consideration.
While the (so-called) cone of protection offers some safety, it’s not nearly infallible - and neither is a simple magnetic barrier, such as I described.
On the other hand, an electromagnetic pulse, such as would penetrate the protection of a basic Faraday Cage, is beyond the capability of most of us to defeat. In practice, there is no ultimate protection against lightning, so “bag” your spare GPS (whatever) and don’t worry about it. If it gets ‘zapped’ - you’ll probably have bigger problems.

Certainly, disconnect you electronics during a lightning storm - but don't expect much more than minimal protection. Remember, the lightning strike that “gets you”’ travels through several miles of air to strike - much greater separation than the 24" (or so) that you’ve distanced your antenna cable from the radio (etc).
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Old 12-10-2005, 20:40   #7
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Magnetic protection

A few points must be made to be outstanding:
1. A cone of protection will do nothing to inhibit a magnetic pulse from a strike adjacent to the vessel which CAN and HAS destroyed electronics, disconnected or not from external wires.

2. The reason that disconnecting a piece of electronics from external wires does not guarantee protection from a magnetic pulse is because the physics phenomenon is that a voltage will be produced across the ends of any conductor which has a perpendicular component to an increasing or decreasing magnetic field and is proportional to the rate of change with time of that field. Once the voltage is developed it may be across a solid-state device internal to your goodie and exceed its voltage rating thereby destroying it or even stressing it so that its life is radically decreased.

3. One non-ideal solution is to buy a steel box like at any electrical supply house (even Lowes or Home Depot) used to house electrical switches, breakers or whatever. Get one that has no holes in it and one which the cover overlaps the opening when closed. Put your backup GPS or whatever in it.

4. There are two general types of discharge: static and dynamic. A cone of protection works only as geometrically described to prevent static buildup in the viscinity of the vessel. The cone may do NOTHING to prevent a dynamic discharge. Understanding this difference explains why some people decry the usefullness of a "so-called cone of protection" when being hit anyway and say that it does not work. Well it does, but NOT for dynamic discharges which can be extremely rapid movements of charges carried by air masses and/or induced from other layers of moving charged air masses.

Hope that this helps clarify the muddy air.
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Old 12-10-2005, 22:02   #8
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I wouldn't dare try to pretend I had the knowledge Rick has. But if I may add, scientists once thought that ligthing went to the highestand best conductor ont eh ground. But a two knowen situations occured that have since made them have to rethink this. It seems a lightening strike may not always go to the highest point or best conductor. The scariest for us situation, was that a lightening rod on top of a tall building or tower, was not struck. The tower recieved the direct hit instead. Lots of test were carryed out to see that the Rod was infact well founded and heads are still being scratched as to why this occured. The other situation was that of a Woman being struck by lightening. She was in a Stadium packed with people and she was 1/3rd of the way down the seating stands. No one can work out to this day as to why she wore the strike and not someone standing up the top, nor anyone standing around her. So I guess it is also possible that a strike may not neccessarily be at the mast head.
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Old 12-10-2005, 23:30   #9
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ok, so we're talking about a galvanized metal junction box with cover, right?

many thanks,
scott neuman
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Old 13-10-2005, 01:10   #10
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Two issues...

I think that we are talking about two issues here.
The first is that a high voltage arc (the lightning) does not have to travel in a straight line. I assume that its path can be affected by the movement of moist air, temperature, its own inductive affect etc. so we can possible assume that from time to time lightning will do some very strange things. Anecdotal evidence would seem to support this.
I would assume that one would need to use some kind of probability analysis to evaluate strategies for dealing with a lightning strike.
The worst case seems to be that the electronics get fried, but provided basic protection is is place and the crew are below then injuries can be minimised. I do not recall seeing a report of any crew being killed by a lightning strike, but I do have some recollection of injuries.
I found a discussion on this at :-
http://www.stewartfam.net/arlyn/lightning2.html
with a more formal aproach at :-
http://www.thomson.ece.ufl.edu/lightning/
with a more practical (insurance) aproach here :-
http://www.boatus.com/seaworthy/swlightning.asp
It does not seem to be a good idea to be in an aluminium dinghy in the middle of a thunderstorm!
Given the number of lightning strikes that occur it would be possible that noticable damage may not occur in many cases.
The second point is the screening of electronic gear from the electromagnetic impulse produced by the lightning strike.
Most electronic equipment is has a metal case which is grounded, and provided the lighning path is sufficiently distant from the gear one could hope for a positive outcome.
For equipment that does not have a metal case and is not grounded I would guess that putting it in a metal box that is grounded and as far from the probable grounding path of the lightning (ie away from chainplates, mast etc,) could possibly give some protection.
However given the voltages and the energy in a major strike I do not think that complete protection would be possible.
I should note that there does appear to be a case for not grounding the mast in trailer /sailors under some circumstances.
Members in Florida would have a lot more practical experience than me!
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Old 13-10-2005, 01:44   #11
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well, if i read these links correctly, they seem just as confusing as everything else i've seen on the subject. i have a deck-stepped wooden mast with no antenna mounted, but do have an inboard engine - so am i more or less likely to be struck?
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Old 13-10-2005, 11:36   #12
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so am i more or less likely to be struck?
Yes
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Old 13-10-2005, 11:59   #13
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Grounding "the box"

This point may be important to those attempting to protect electronics. Grounding a metal box does NOTHING to protect what is inside the box. The idea of grounding is ONLY to possibly protect YOU from being a current path between an ungrounded piece of metal that has achieved a high potential due to lightning (or fault) conditons WHEN you are ALSO touching a different piece of metal at a different potential.
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Old 13-10-2005, 16:06   #14
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Protection ????????

Your best protection is good insurance. Let the stuff fry and buy new and updated equipment. My only concern is whether I will be around to file the claim and select new equipment.
Jim
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Old 13-10-2005, 21:06   #15
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i'll be sure to get my insurance man to express replacement GPS, nav lights, etc. address: south of hainan island, s. china sea.
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