Copper Coat as faraday cage ?

[RobbieW]

Having had some equipment failures following a lightning near miss on Sunday, a surveyor from our insurer came for a look today. One question I asked was whether the copper coat would increase the chance of an 'up strike'. His answer was the reverse, he thought that the copper would provide some protection through a faraday cage effect.

Valid point or complete tosh ?

 

[rob2]

As I understand it, the metallic particles, being suspended in a resin substrate are discontinuous and also therefore unearthed with the exception of those exposed on the surface. This is the explanation given as to why it doesn't interact in a corrosive couple with other underwater metals. Straining the memory a bit here (I can only just remember which school...) a Farady Cage is a continuous structure, that is any voids above a tuned size for frequency will allow RF to penetrate through them. On that basis I suspect there may be some minimal effect from the Coppercoat, but at the energy levels of a lightning strike, I doubt that would count for much!

Rob.

 

[Graham_Wright]

As I understand it, the metallic particles, being suspended in a resin substrateare discontinuous and therefore unearthed with the exception of those exposed on the surface. This is the explanation given as to why it doesn't interact in a corrosive couple with other underwater metals. Straining the memory a bit here (I can only just remember which school...) a Farady Cage is a continuous structure, that is any voids above a tuned size for frequency will allow RF to penetrate through them. On that basis I suspect there may be some minimal effect from the Coppercoat, but at the energy levels of a lightning strike, I doubt that would count for much!

Rob.

I think cage holds the clue. All the ones I have seen looked like chicken wire. Even the boat's rigging forms something of a Faraday cage. However, all the components are electrically connected.

 

[AntarcticPilot]

As I understand it, the metallic particles, being suspended in a resin substrateare discontinuous and therefore unearthed with the exception of those exposed on the surface. This is the explanation given as to why it doesn't interact in a corrosive couple with other underwater metals. Straining the memory a bit here (I can only just remember which school...) a Farady Cage is a continuous structure, that is any voids above a tuned size for frequency will allow RF to penetrate through them. On that basis I suspect there may be some minimal effect from the Coppercoat, but at the energy levels of a lightning strike, I doubt that would count for much!

Rob.

+1

The copper particles in Coppercoat are electrically insulated by the surrounding epoxy resin, so they cannot form a Faraday cage. This has been repeatedly reported on here, by people who ought to know, in response to questions about Coppercoat interacting with other metallic fittings.

You could easily test it by checking the resistance between two points on a Coppercoated hull - the expected resistance would be infinite (or very, very large!).

Even if it was a conductor, I'd imagine that Coppercoat would have absolutely no effect on the results of a lightning strike, as it is directly in contact with seawater, a much better electrical conductor. As far as I know, there has never been any effect reported for copper sheathing (used for centuries on wooden warships), and there will have been plenty of oipportunities for any effect to be observed.

 

[EwanClark]

As others have mentioned, the copper particles in a treatment of Coppercoat are isolated and encapsulated within epoxy. Consequently it is a non-conductive coating and will not act like a Faraday cage.

 

[RobbieW]

OK - thanks all - utter tosh then

 

[rob2]

Giving it some more thought, particularly as you were originally interested in whether protection was offered from acharge coming in from a nearby sea strike rather than a downward air strike. High voltage discharges tend to flow across surfaces rather than through a conductor, in fact a wet hull is attributed to be instrumental in the conversion of a strike into St Elmos fire within the vessel. Not sure how all this fits together, but I suspect that any wet surface would react much the same above a critical voltage.

Rob.

 

[Sailingsaves]

OK - thanks all - utter tosh then

I wonder if that says anything about that particular surveyor? I don't think it would act as a cage either, but wouldn't say either way to a customer unless I was sure.

 

[Sailingsaves]

Giving it some more thought, particularly as you were originally interested in whether protection was offered from acharge coming in from a nearby sea strike rather than a downward air strike. High voltage discharges tend to flow across surfaces rather than through a conductor, in fact a wet hull is attributed to be instrumental in the conversion of a strike into St Elmos fire within the vessel. Not sure how all this fits together, but I suspect that any wet surface would react much the same above a critical voltage.

Rob.

Interesting points. Would love to have time to look into it more. I do miss research.

 

[RobbieW]

Giving it some more thought, particularly as you were originally interested in whether protection was offered from acharge coming in from a nearby sea strike rather than a downward air strike. High voltage discharges tend to flow across surfaces rather than through a conductor, in fact a wet hull is attributed to be instrumental in the conversion of a strike into St Elmos fire within the vessel. Not sure how all this fits together, but I suspect that any wet surface would react much the same above a critical voltage.

Rob.

Gosh - you just reminded me of stuff I learned about HVAC as an apprentice. That HVAC distribution systems, mostly overhead I guess, can use a steel cored cable for strength with a thin aluminium coat as the actual power travels along the outside of the cable. Nothing like the voltage of a lightning strike but based on the same theory.

 

[AntarcticPilot]

I wonder if that says anything about that particular surveyor? I don't think it would act as a cage either, but wouldn't say either way to a customer unless I was sure.

Actually, even if it were conductive - which it isn't - it wouldn't make a Faraday cage. A Faraday cage is a complete enclosure, with no holes larger than the longest wavelength it is to block. The Coppercoat - or copper sheathing or whatever - on a hull has a dirty great hole in the top!

 

[RobbieW]

I wonder if that says anything about that particular surveyor? I don't think it would act as a cage either, but wouldn't say either way to a customer unless I was sure.

Maybe, maybe not - he was Italian, as thats where we are, and although his English was good it wasnt perfect so things may have got lost in the translation.

 

[savageseadog]

I would argue that with a high voltage discharge a coppercoat surface would act as a continuous conductor and therefore will act as a Faraday cage. Although the copper grains wouldn't conduct at low voltages, at higher ones conduction would occur from copper grain to copper grain, the surface would appear as a conductor. The value of this under the waterline would be doubtful I would have thought.

 

[Billjratt]

Surely if it started to conduct - ie, the epoxy dielectric between the copper particles broke down, it would destroy itself- at least partially.
Although in doing so it may have sacrificed itself to create a discharge path and save the boat.

 

[William_H]

A Farady cage is used to effectively block electrostatic radiation. This may be roughly described as conduction of a high voltage through the air as in a capacitor.
The other mode of electrical transmission in a lightning strike is the magnetic pulse.Now the Farady cage will to some extent act like a transformer coil turn and so dissipate some of the power by acting as a shorted turn this only occurs as the lightning current rises and falls. While the current is steady the magnetic field can penetrate the farady cage. Only a magneticaly conductive material like iron can shield from a powerful magnetic force.
So a Faraday cage may shield you from some of the lightnings induced current. All this assuming that the lightning strikes near the boat.
If any part of the boat becomes part of the current discharge circuit then it is a whole different story. The current will flow probably initially down the Ali mast then probably find a resistive path to the sea. There will be a combination of fusing (with excess current) of any slightly conductive material and of the high voltage jumpng across any gap with destructive arcing. Most of your boat electronics will be connected by wiring to this conductive path. Hence is highly likely to be damaged.
So while it is perhaps a good idea to put electronics in the oven I think the primary protection comes simply from disconnecting the gear from boat wiring.
Many boats now are built of carbon fibre reinforced plastic. This is quite conductive. Not low resistance but low enough to require insulation for any electrical connections made by mounting bolts etc if you don't wan that connection. The carbon fibre would include cabin top. However I have never heard of any peculiar or beneficial protection caused by carbon fibre.
So it is all a bit mysterious. Forgive my own waffle on the subject. Certainly coppercoat would have no effect whatsoever. good luck olewill

 

[prv]

I think cage holds the clue. All the ones I have seen looked like chicken wire.

+1

I was lucky enough to get a tour of the old British Embassy in Moscow before it got refurbished as the Embassador's Residence (the actual Embassy had already moved to a purpose-built modern block elsewhere). Up in the attic where they used to do Cold War sneaky-beaky radio stuff were cages that were exactly that. A rough structure knocked up out of random bits of wood, covered with chicken-wire. It looked like a dilapidated back-garden aviary, or the sort of thing someone might knock up to keep birds off his soft fruit.

I'm sure they have much more sophisticated countermeasures now.

Pete

 

[jdc]

...One question I asked was whether the copper coat would increase the chance of an 'up strike'. His answer was the reverse, he thought that the copper would provide some protection through a faraday cage effect...

His answer was certainly tosh because that it didn't answer the question you asked. There are two factors: the probability of being struck, and the damage a strike is likely to do. A Faraday cage - were copper coat to provide one, which it wont anyway as others like AntarcticPilot have said - is irrelevant to the probability of initiating the strike even 'tho it might help reduce damage in the event of one.

It was certainly a reasonable question for you to ask whether the increased metal area on the bottom of the boat due to the copper coat would increase the probability of a (up)strike. I believe that this is a really difficult and little understood subject, but as I understand it the probability is related to the 'footprint' of the hull in electrical contact with the water, ie approx 1/2 beam x LWL. In that case it's difficult to see that there is a very significant increase in effective area of the boat from applying copper coat, and this seems to be borne out as there is little or no evidence that metal boats get struck any more often than do grp ones of the same size. There is apparently evidence that catamarans get struck more frequently than monohulls, postulated to be due to the larger effective footprint (they're wider!). But as I said it's a little understood subject so this post may be tosh itself!

 

[JumbleDuck]

I think cage holds the clue. All the ones I have seen looked like chicken wire.

Two separate things are often called Faraday Cage.

The original is simply a closed conducting surface which will not allow external electric field to penetrate: any field which tries produces charge distribution on the surface which exactly cancels out the applied field inside. Or, as we James Clerk Maxwell fans say, div D = rho.

There is a related but rather different effect which says that EM radiation sees a conducting mesh with aperture of a similar size to its wavelength or less as a conducting surface and hence cannot penetrate it. That, for example, is why mesh reflectors on radar or TV antennae work and why it's possible to look safely into an operating microwave oven, typical wavelength 10cm, through 2mm diameter holes in a metal sheet.

So ... if you make a closed mesh surface, you have something which will not allow EM radiation with wavelengths above the mesh size[1] to penetrate, and that is often called a Faraday Cage too, although it isn't really what Faraday had in mind.

So ... will coppercoat act as a Faraday cage? In the electrostatic sense, definitely not. The particles are insulated from each other, so no significant charge movement is possible. And, in any case, as AntarcticPilot has pointed out, there's a ruddy great hole on top of the coppercoat surface.

From the EM point of view there will be an effect, since EM radiation doesn't like moving through conducting media. It'll be strongly frequency dependent, though.

[1] It's not a rigid cutoff. Anything with wavelength ten times as big as the mesh will be well blocked, anything a tenth of the size will mostly get through.

Caveat: my area of expertise is magnetostatics.

 

[RobbieW]

...It was certainly a reasonable question for you to ask whether the increased metal area on the bottom of the boat due to the copper coat would increase the probability of a (up)strike. I believe that this is a really difficult and little understood subject, but as I understand it the probability is related to the 'footprint' of the hull in electrical contact with the water, ie approx 1/2 beam x LWL. In that case it's difficult to see that there is a very significant increase in effective area of the boat from applying copper coat, and this seems to be borne out as there is little or no evidence that metal boats get struck any more often than do grp ones of the same size. There is apparently evidence that catamarans get struck more frequently than monohulls, postulated to be due to the larger effective footprint (they're wider!). But as I said it's a little understood subject so this post may be tosh itself!

hmm, interesting thoughts. I guess that because each copper particle is separated by the epoxy you wouldnt be creating something like a sintered bronze ground plate, which would certainly increase the surface area.

 

[CreakyDecks]

Surely, unless you copper coat the whole boat, the part that is copper coated is sat in seawater.