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Old 17-03-2014, 18:18   #16
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Re: Faraday cage for hand held GPS

More to the facts, it is basic physics that accelerating charged particles radiate away energy in the form of electromagnetic waves.



"The rapidly changing currents also create electromagnetic pulses (EMPs) that radiate outward from the ionic channel. This is a characteristic of all electrical sparks. The radiated pulses rapidly weaken as their distance from the origin increases. However if they pass over conductive elements; i.e., electrical wires, communication lines or metallic pipes, they may induce a current which travels outward to its termination."[/QUOTE]

Electronic circuits are effectively all protected against this type of EM field by nature of the fact that all PCBs have a ground plane that is a reference to ground will then tie out to some common system ground. The caveat in the wikipedia article above is a big If they do and they MAY induce a current. But this is logarithmically small as the distance from the point charge decreases, and does so very quickly.

If your electronics components fail during a lightning strike, the chances of them failing because of the proximity of an electromagnetic wave is incredibly statistically unlikely as opposed to a faulty ground system, separate ground schemes, or some other commonality of that device and a faulty circuit. If faraday cages were effective at all in protecting against lightning strikes...then don't you think some engineers would have some product on the market?

Just because lighting strikes and you have devices fail doesn't mean it was because of an EMP. Yes, very small magnetic fields arise from ESD fields. And yes, you can induce a voltage or current from magnetism. These are basic principles, but the relative magnitude of of a magnetic field is miniscule in an ESD, and naturally occurring non nuclear EMPs of any consequence are pretty rare. When raymarine engineers start pushing out faraday cages to protect from lightning strikes, let me know.
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Old 17-03-2014, 22:34   #17
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Re: Faraday cage for hand held GPS

A Faraday cage will protect the electronics stored within them in most but not all cases. Here is how it works.

First you need to know what and how the damage is done to the device you want to protect. The damage is due to overloading components in the device with ether high voltage or high currents or both. The high voltage and current is created through a process called induction. A lightning strike will create a rapidly changing magnetic field around the core of the strike or streamer. This is a large magnetic pulse if you will and will induce a current into all the small wires and circuit board traces in a device. That potential or voltage across the wire or trace will rise to a voltage which may be higher or have more current available than the device, a transistor or ic chip can take and poof the magic smoke is let loose.

A Faraday cage is a metal box, it does not have to be solid and it can actually be made of metal screen or other conductive material , unfortunately screen may not be able to handle the currents generated by a lightning strike so solid metal might be better choice.

How it works... well current has a funny characteristic, current flows on the surface of a wire or piece of metal, now if you crate a metal box the current or the electrical charge will flow on the outside of the box and cancel any charge or current flow on the the inside. Its a hollow wire.

So lightning strikes and the strike is close to your Faraday cage, the magnetic pulse induces a current into all kinds of metal including your Faraday cage, now if that cage (Box) is grounded the induced current will travel around outside surface of the box to ground. If its not grounded it may maintain a charge for a short period of time so don't touch it. By the way the induced charge is why you should never hold on to any thing metal during a storm, not even a long bolt as a very high voltage and current can be induced on to the object by a lightning strike near you, does not have to be a direct strike.

Because the current flow or charge stays on the outside of the box the device inside is not effected ( Usually ) This is why there is no or little damage to an airliner struck by lighting. The current flows around the outside and does nor effect what inside.

Now if you have a direct strike most likely nothing is going to survive, Faraday cage or not. The induced currents will be so high it may blow a hole in the cage.
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Old 17-03-2014, 22:52   #18
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Re: Faraday cage for hand held GPS

I had to steer during an electrical storm. Metal wheel. I got zapped a good one. No other damage to vessel.
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Old 17-03-2014, 22:57   #19
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Re: Faraday cage for hand held GPS

Yup, I learned the hard way to. Up on stands no mast but had a frame and cover on the boat, cover frame was made out of conduit and I was holding on to it when lightning hit a house down the block. Through me across the boat and scared the $%&$ out of me. Actually burned my hand a little. And the frame was not connected to anything else. Shock was through induction.
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Old 17-03-2014, 23:11   #20
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Re: Faraday cage for hand held GPS

My backup epirb, ipad with inavx, handheld vhf and handheld gps are all separately wrapped in thick foam and a few metres of aluminium foil.
Never tested. Hope it works...
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Old 17-03-2014, 23:17   #21
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Re: Faraday cage for hand held GPS

It cant hurt and I think it will help, but a grounded box, microwave is the best, might be more effective.

Buy the way the reason a microwave is best is because it has been designed as a Faraday cage to keep the microwave radiation in. Its also grounded if the power plug ground is grounded. (Mine is not but thats another story)
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Old 17-03-2014, 23:21   #22
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Re: Faraday cage for hand held GPS

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Its also grounded if the power plug ground is grounded.
Does a good faraday cage needs to be grounded? What about the advise to get in a automobile during lighting? A car is not grounded? The same for the oven in my boat, not a microwave oven.
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Old 17-03-2014, 23:25   #23
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Re: Faraday cage for hand held GPS

No, it does not need to be grounded, but don't touch the ungrounded one for some time (sec to mins depending) after its been charged as it will shock you.

Oven is good, but microwave is better. Door has conductive seals and stuff like that.
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Old 18-03-2014, 06:41   #24
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Re: Faraday cage for hand held GPS

My understanding is that a Faraday cage is only good down to about 100MHz and is ineffective against anything below including static magnetism, for example a compass would still work inside a Faraday cage.

The Faraday cage works by enveloping whatever it is you want to shield with a continuous conductive surface. Ideally this is a solid surface, however mesh or screen material can work as long as the hole size is sufficiently small enough to NOT function as a slot antenna - Slot antenna - Wikipedia, the free encyclopedia

So chicken wire, probably not the best choice. Microwave oven is good, as mentioned they are designed to be a Faraday cage to keep the microwaves in.

Your gas or electric oven not so much, particularly if it has a big window shaped hole in it, but also remember the slot antenna - that gap between the door and the rest of the oven is basically a slot antenna. Ammo can like a garbage can good, but can be made much more effective by sealing the lid with a conductive aluminum or copper tape.

What might be best would be to put the item inside a couple anti static bags of the kind used to protect sensitive electronics (I got a good chuckle when ionic said there were no products to provide protection so it must not be possible when in fact these bags are quite common) and then place them inside an ammo can.

No guaranty, but few things in life are, that said might be worthwhile to protect an cheap GPS as a hedge against a rainy day. Nice thing about the lunchbox is that you can tape up the lid easy.
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Old 18-03-2014, 07:02   #25
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Re: Faraday cage for hand held GPS

alum foil covered shoe box, size 9 is best. I can prove it if you guys send cash for new monkey shoes.
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Old 18-03-2014, 08:19   #26
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Re: Faraday cage for hand held GPS

These bags are not all created the same, I think the ones you are looking for are called "conductive grounding bags" and are handy because they are also ziplocks and will keep your stuff dry. Put one bag inside of another and then put them inside a metal box. I think that about as good as it gets. If I recall correctly 3M makes some really neat metalized plastic boxes with foam lining if you want to burn money.

Again, I don't think it's any guaranty, but it's better than nothing. Obviously you don't know when you are going to get hit by lightning so some might consider it prudent to have a back up GPS device protected in this manner.

Yes, I have taken a lightning strike before. It was back in the LORAN days and before cell phones so I can't comment from experience regarding the LEMP effect caused by lighting, which is really just a big electrostatic discharge, which is what the bags are designed to protect against, however a couple of plastic bags seem like cheap insurance to me and should satisfy your wife.
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Old 18-03-2014, 08:23   #27
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Re: Faraday cage for hand held GPS

"My understanding is that a Faraday cage is only good down to about 100MHz and is ineffective against anything below including static magnetism, for example a compass would still work inside a Faraday cage."

Yup you are right except the freq is more like 100 KHz and lower not MHz think pentagon. That entire place is a big Faraday cage to stop radio activity (Including low freq AM) from getting out to the outside world.

Anyways, the pulse caused by a lightning strike is so rapid it would be considered a single wave with a freq in the MHz and is all that is needed to induce a voltage and current into metal or wires. A compass will work inside a Faraday cage as the field is continuous and not rapidly changing and being continuous it can not induce energy onto another object. Its the induced energy the cage will protect you from.
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Old 18-03-2014, 08:45   #28
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Re: Faraday cage for hand held GPS

Quote:
Originally Posted by ionic View Post

Electronic circuits are effectively all protected against this type of EM field by nature of the fact that all PCBs have a ground plane that is a reference to ground will then tie out to some common system ground. The caveat in the wikipedia article above is a big If they do and they MAY induce a current. But this is logarithmically small as the distance from the point charge decreases, and does so very quickly.

If your electronics components fail during a lightning strike, the chances of them failing because of the proximity of an electromagnetic wave is incredibly statistically unlikely as opposed to a faulty ground system, separate ground schemes, or some other commonality of that device and a faulty circuit. If faraday cages were effective at all in protecting against lightning strikes...then don't you think some engineers would have some product on the market?

Just because lighting strikes and you have devices fail doesn't mean it was because of an EMP. Yes, very small magnetic fields arise from ESD fields. And yes, you can induce a voltage or current from magnetism. These are basic principles, but the relative magnitude of of a magnetic field is miniscule in an ESD, and naturally occurring non nuclear EMPs of any consequence are pretty rare. When raymarine engineers start pushing out faraday cages to protect from lightning strikes, let me know.
Perhaps you can explain how these unplugged items were fried in our strike?

*EPIRB, in its wall bracket
*Garmin 176 GPS, unplugged on nav desk
*Magellan Hand Held GPS wrapped in foil inside ditch bag
*iPod, unplugged under nav table
*Garmin 76 Hand Held GPS, unplugged under nav table
*Laptop computer on nav desk not plugged in

None of these items were physically connected to anything yet all were fried in the strike. The Garmin 176 & laptop are the only ones that even powered up but you needed to remove and re-insert the battery on the GPS to make that happen. None of the keys worked however so the unit was toast..

The computer reacted to the power button, making a noise, but nothing really happened and the screen would not even light.

Can you elaborate what fried all those unplugged electronics in our strike..?

I do lightning strike damage estimation and repairs as part of my business. About 6 out of 10 compass compensators are also wiped out. Our compass was professionally swung just the year before and after the strike was blown way out of calibration. My compass adjuster told me I could wait and see if they self corrected over time, or just get a new set of compensators. I went for new compensators because even after three weeks it was still way out..
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Old 18-03-2014, 09:20   #29
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Re: Faraday cage for hand held GPS

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I do lightning strike damage estimation and repairs as part of my business. About 6 out of 10 compass compensators are also wiped out. Our compass was professionally swung just the year before and after the strike was blown way out of calibration. My compass adjuster told me I could wait and see if they self corrected over time, or just get a new set of compensators. I went for new compensators because even after three weeks it was still way out..
Same here. Lightning screwed up our compass so I guess I can comment from experience regarding the LEMP effect. The result of this put us on the rocks in the fog while the depth sounder, which had seemed to be working prior, was showing us in sixty feet of water. Saying this makes me think I should throw a hockey puck in my bag as well, again for what it's worth.

I wonder if the problem with the aluminum foil approach is that simply lining a cardboard box with tin foil means that you have gaps which function as slot antennas. Maybe better to make foil packets like you are cooking fish by folding over all the sides but still, aluminum foil isn't exactly tear resistant any any hole or puncture is a problem.

I would stick with the bags, that's what they are made for. Put them in a metal box and hope for the best.
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Old 18-03-2014, 09:35   #30
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Re: Faraday cage for hand held GPS

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Same here. Lightning screwed up our compass so I guess I can comment from experience regarding the LEMP effect. The result of this put us on the rocks in the fog while the depth sounder, which had seemed to be working prior, was showing us in sixty feet of water. Saying this makes me think I should throw a hockey puck in my bag as well, again for what it's worth.

I wonder if the problem with the aluminum foil approach is that simply lining a cardboard box with tin foil means that you have gaps which function as slot antennas. Maybe better to make foil packets like you are cooking fish by folding over all the sides but still, aluminum foil isn't exactly tear resistant any any hole or puncture is a problem.

I would stick with the bags, that's what they are made for. Put them in a metal box and hope for the best.
An old timer told me he wrapped his stuff in foil, and in a strike it survived. Figured I would try it, failed... The GPS was also in a ziplock bag with a desicant pack...

I find it interesting that researchers such as Dr. Ewen Thompson, who has spent his entire life studying lightning, believes EMP from lightning occurs but our expert here does not...??

The most amazing things I hear from folks who were hit are "Good thing I still had my compass & paper." I then ask for their deviation card? "Huh, deviation card?"...

How do you know your compass is accurate if you don't have a deviation card and don't know it was accurate to begin with?? For those customers I try to assume the compass was at least within 10 degrees and I have found them off by 70 degrees or more.... Yeah, that "compass and paper" are not going to help much after a strike either.... All strikes are different, and some compasses are not affected at all, but you really should assume it has been 'whacked" and take steps to ensure it is accurate.
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