Faraday cages

oldbilbo

oldbilbo

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A number of boaties have had lightning strikes, and damage to all e-equipment. Subsequent suggestions include keeping certain vulnerable devices - such as handheld VHF, GPS, PLBs, smartphones, and the like - inside a Faraday cage such as the boat's steel oven.

Qwessie. Might the grab bag, lined with metal foil, perform much the same purpose?

Or.....?
 
oldbilbo said:
A number of boaties have had lightning strikes, and damage to all e-equipment. Subsequent suggestions include keeping certain vulnerable devices - such as handheld VHF, GPS, PLBs, smartphones, and the like - inside a Faraday cage such as the boat's steel oven.

Qwessie. Might the grab bag, lined with metal foil, perform much the same purpose?

Or.....?
Click to expand...

Don't know but some forum members have been adamant that even my steel hull and deck would not protect my electronics. If that is the case why would a steel oven or metal foil lined grab bag be better.

I think the said forum members are not totally correct but I don't have any experience on the boat but I do at home when I lighting strike melted mt TV aerial but did nothing to the TV or my radio
 
oldbilbo said:
Qwessie. Might the grab bag, lined with metal foil, perform much the same purpose?
Click to expand...
Possibly not much metal and might get vaporised by the strike leaving aluminium vapour condensing in your cabin, it'd be very shiny after though.

(caveat: obviously with naff all technical knowledge on the subject.)
 
A faraday cage is a mesh of conducting material which acts like a transformer winding in taking a changing magnetic field and turning it into current flow which is shorted out. The faster the rate of change of magnetic field the more current.
Stable or slow changing magnetic field can be shunted by iron. The magnetic field being conducted and so an iron stove can give some protection from the magnetic field of a lightning strike. This magnetic field can magnetise iron (engines etc) and can ruine compasses or cause permanent errors.
The magnetic field from the current of a lightning strike rises rapidly and continues as DC until the bolt finishes.
The foil will tend to protect from that fast rising magnetic field which can induce destructive currents into electronics, however I think the main danger is actual lightning current in ships wiring. ie the lightning current from the top of the mast to the water. So primary move to protect electronics is to disconnect from antenna and ships DC and data wiring if you think there is a danger of strike.
While on the subject news this morning tells of a surfer struck by lightning in New South Wales. He has to be on the one hand unlucky to be struck but on the other hand lucky to survive. I imagine the wet suit carried the current but boiled the water in the process. He got burns to the top of his head and cardiac arrest. Don't know if he had a wetsuit hood on. Probably not.
The answer for surfers is clearly a lightning arrestor sticking up on a hat with heavy concutors down to feet and water to bypass/ carry the current unimpeded. A new fashion.
 
No I don't think so. The moisture in the wet suit or even bare body is enough of a conductor to instigate the initial up leader (low current) which provides an ionised path for the main strike. So good conductor or poor conductor are equally likely to invite a strike. olewill
 
Rogershaw said:
Don't know but some forum members have been adamant that even my steel hull and deck would not protect my electronics. If that is the case why would a steel oven or metal foil lined grab bag be better.
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William_H gives a good explanation. The problem with wiring connected to an aerial/windvane/masthead lights is that it will conduct the charge and destroy whatever is on the end of the wire as well as other items connected to the same loom or nearby.

The oven or foil bag isn't connected to anything electrical so is isolated but, should it be earthed to provide Faraday cage protection?
 
I have worked on some faraday cages for MOD london under ground orange phones. French electric board and many more the cage dose not have to be a mesh it is a enclosed area a submarine or a battle tank are some of the best
We used copper. Or Aluminium foil or plate steel stailess steel for EMP. Or RFI protection Different metals for different frequencies and price lead was used mainly for radiation
For instance the uk has concrete data bunkers deep under ground in them Welsh hill lined with copper foil and all our Library' of magnetic tapes the cage has to be grounded
in London offices near the point the tube trains change from bottom rail live picup to over head as thay go from under to over ground a ELecto Magnectic Paluse is emitted and could interfere with computors

http://www.mpe.co.uk I used to work for these on encloser construction and testing

Plus

Not who I worked for but some info

http://hollandshielding.com/135-Faraday cages EMI RFI shielded tents rooms and shielded enclosures
 
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GrahamM376 said:
The oven or foil bag isn't connected to anything electrical so is isolated but, should it be earthed to provide Faraday cage protection?
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No expert, but I think that for lightning protection it need be earthed in exactly the same way as an aeroplane: i.e. not at all.
I believe that cages intended to protect delicate equipment from EMPs (electromagnetic pulses, including solar flare activity) often are grounded.
 
macd said:
No expert, but I think that for lightning protection it need be earthed in exactly the same way as an aeroplane: i.e. not at all....
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Isnt the trick with aircraft to make the resistance between entry and exit points as close to 0 as possible ? The metal skin acts as a conductor and, iirc, with high voltages the flow is mostly on the outside of the conductor (hence HT grid cables with steel core and ally outer).

From my own experience of lightning near misses, not actually been hit but have had a couple of strikes within 200m or so; the equipment taken out can be very random. On both occasions I've had the chip that drives NMEA interfaces fail in different units so I now think these specific parts are susceptible to EMP. I do put phones, laptops, handheld GPS etc in the oven and not had failures - on the other hand I dont always disconnect all the equipment and I've not had a direct strike but I've not lost any of the radio equipment.

There seem to be two failure modes - the EMP which can take out vulnerable stuff at a distance, then the high energy current of a direct strike that fries any stuff in its path. It does seem that the closer one is to the strike location the more damage will be caused.

Love the idea of surfers with some sort of pointed helmet but what about kite surfers emulating Faraday himself ;)
 
William_H said:
A faraday cage is a mesh of conducting material which acts like a transformer winding in taking a changing magnetic field and turning it into current flow which is shorted out.
Click to expand...

There is an awful lot of confusion about Faraday Cages.

A closed metal surface will shield its contents from electric fields, and therefore from electromagnetic radiation because it is a Faraday cage. It will give no shielding from magnetostatic fields unless it is also a magnetic material. It will give some shielding from varying magnetic fields; the degree of protection depends on the the geometry of the enclosure and on the material used. Only the electric field shielding is a consequence of its being a Faraday Cage.

If you make holes in the surface it is no longer a true Faraday Cage, but will still provide reasonable shielding from electric fields as long as the holes are not too big and as long as you are a reasonable distance (typically the hole diameter) away from the hole. It will shield from electromagnetic radiation with a wavelength more than the hole diameter, which is why you can see into a microwave oven through the holes in the door although the microwaves can't get out through them.

A VHF in a boats oven will be protected from electric fields in a lightning strike only if the oven has a metal door. It will be protected from transient magnetic fields only if the whole casing, including the door, is made of a magnetic material. If the door is glass there will be no significant shielding from either. However, the casing will provide a relatively low resistance route for any stray currents flying around the cabin and should therefore stop damage to the radio from that source. There may still be damage from induced voltages appearing the radio resulting from changing magnetic field.

The bottom line, though, is that a Faraday Cage is ONLY a device to shield from electric fields.

Incidentally, the Wikipedia article on this is almost unbelievably bad. This is just gibberish:

wikipedia said:
If a charge is placed inside an ungrounded Faraday cage, the internal face of the cage becomes charged (in the same manner described for an external charge) to prevent the existence of a field inside the body of the cage. However, this charging of the inner face re-distributes the charges in the body of the cage. This charges the outer face of the cage with a charge equal in sign and magnitude to the one placed inside the cage. Since the internal charge and the inner face cancel each other out, the spread of charges on the outer face is not affected by the position of the internal charge inside the cage. So for all intents and purposes, the cage generates the same DC electric field that it would generate if it were simply affected by the charge placed inside. The same is not true for electromagnetic waves.
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Having been hit by lightning twice, we now put laptops, GPS's, handheld VHF, memory sticks, etc in the oven at first hint of a storm.

Our son recently gave us a beautiful metal box from Best Made Company (https://www.bestmadeco.com/products/the-strongbox). This holds a Garmin GPS and handheld VHF perfectly. Hopefully this will provide even better protection. Shame you can't do the same with all the other electronics that festoon a modern boat.
 
TheCharteringClasses said:
Wouldn't that also make them more likely to be struck though?
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My recollection is that the main purpose of a lightning conductor is to create a strong beam of ions which reduces the electrical charge in the sky above the conductor. It's a kind of controlled as opposed to an uncontrolled strike. The reason for the sharp point is to intensify the electric field and hence the ionisation process.
 
Lightning strike protection on a small boat is virtually impossible. Perceived wisdom used to talk about chains wrapped around the mast and trailing into the sea. Ever tried it on a 35 footer?

Lightning strike prevention is more likely to be both practical and possible. We have one of those Lightningmaster wire brushes riveted to the top of the mast and the rigging well earthed. From one close encounter and based on previous experience, we think that it works – sometimes, at least. I doubt it is 100% but am pretty sure that it can help to give some protection.

I do not know the answer to the OP but will continue to put movable electronics in the oven.
 
alan_d said:
You're not thinking of Benjamin Franklin's kite and key experiment, are you?
Click to expand...

:redface-new:
I did type 'faraday lightning' into Google which suggested farady's kite experiment but when actually using that search it goes to Ben Franklin - teach me to do my research properly!
 
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RobbieW said:
Love the idea of surfers with some sort of pointed helmet ;)
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The wetsuit with pointed helmet appears already to be standard equipment in some circles. Now we know why.

dipsy.jpg


Of course some sort of protection is required for the point (in non thundery conditions).

dipsy2.jpeg
 
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