Faraday cage

Re: Faraday cage - biscuit tin

But isn't it a fact that aircraft are regularly hit by lightning (commercial aircraft about once every 1,500 flight hours, which for most commercial aircraft means on average at least once per year) - in fact I have been in commercial aircraft so hit and I suspect that not one passenger on board realised what had happened. Not often they fall out of the sky because of it or suffer any damage at all (may rarely happen in cases of design being inadequate on small aircraft eg non metallic wing tip tanks).

If you are hit on a small boat I don't think you can do much about it. I am not even convinced by the conductors used for lightning bonding on some small vessels as out of necessity those conductors are of much smaller x-sectional area than is used on larger vessels (or on land) so is really just toying with hits of any magnitude.

I, personally, would be more interested in a lightning storm in protecting the crew if in a non metal boat (and even then keep off deck) than worrying about protecting loose electronic equipment). In any event the risk is very low.

I do agree with the biscuit tin approach but I personally think that there are more important things to concern oneself over, but actually turns out I don't need to as our boat is a big metal "biscuit tin".

One good idea passed on to me by my old granny is to not have any mirrors, spoons or knives on the boat. All those things are famous for attracting lightning /forums/images/graemlins/smile.gif.

John
 
Re: Faraday cage - biscuit tin

Probably agree with ships cat that the chances of taking a strike this side of the pond are very low indeed!

The other side I have met 3 boats who have been hit and had everything wiped out - so it is a consideration.

I was given a HH gps by Doyle's sails as a little gift when they messed up the cut of my new sails. It is really basic and slow but would get you home - it lives in a biscuit tin stowed away in the bows simply because there was space there and it needed to be forgotten.

The problem with being hit for a boat is that it is so well grounded into the sea - aircraft not at all and cars are sitting on rubber tyres. I would not bother in European waters but would probably keep a gps with spare batteries if I was cruising the northern Caribbean or East Coast USA...

These safety issues are odd - how many people, do you personally know, who have taken to a life raft - set off an Epirb - put out a may-day?
Very very few I bet.
 
Re: Faraday cage - biscuit tin

When sailing in the Caribbean I used to put the computer in the oven and the small hand helds in the pressure cooker. Fortunately we never found out if it would have worked or not!
 
Re: Faraday cage - biscuit tin

Getting a little nervous reading this thread! Eight years in one of the most active lightening places in the world and so far OK. Just a passing thought. If you were able to extend all your rigging attachment points to below the WL on the exterior of the hull would you, in effect, be inside a cage?
 
Re: Faraday cage - biscuit tin

Not a scientist but it does not work like that - the 'power' wants to go to earth (or water!) so it travels down the mast and the shrouds - stays and into the boat system - it then exits via engine - shaft - sea cocks - etc - If you put a chain onto the shrouds that would probably facilitate a better 'exit' - sometimes the skin fittings are destroyed - damaged. In my case the water around was full of dead/stunned fish and a boat 50mtrs away had problems with diodes being blown out.

A Faraday cage is surrounded by metal - biscuit tin - oven et al - Thats how it works but I am not sure why!
 
Re: Faraday cage - biscuit tin

Its because the charged 'particles' repel each other, the denser they are packed the more they repel each other. So the charge expands to occupy the largest surface area - the outside of a conductor.

Aircraft are struck by lighting fairly frequently, damage is rare and lighting induced crashes are very rare - I half remember that one plane crashed after being struck on take-off or landing.

The plane's electronics are hardened to resist the effect, but this is the effect of induced currents caused by the charge flowing around the skin. There is absolutely no way any electronics would survive a lighting charge flowing through it. Fortunately, the charge has no interest in flowing into an aircraft's interior.

A boat is very different, that charge is going to earth one way or another, in a plastic boat the route chosen is almost random, there is so much current flowing and so much ionisation of the air and so many bits of metal and wire to provide bits of a path that anything can happen, and probably will.

Disconnected bits of kit like a hh gps lying on a table are probably relatively safe, but putting them in a biscuit tin seems like an excellent idea.
 
Re: Faraday cage - biscuit tin

Thank you - live and learn -

............"Disconnected bits of kit like a hh gps lying on a table are probably relatively safe...................",

Not sure about that - a domestic world band battery operated radio was in the saloon on a table and brewed up as well.....
 
Re: Faraday cage - biscuit tin

I just had to add something here....as many posts seem to be missing the supposed advantage of a cage for removeable items.

The danger to unconnected electronics is the large magnetic fields generated around areas of high voltage and current, (emf's) bombs have been designed to generate these fields to knock out electronics.
In the proximity of a strike there will be these magnetic fields and any device containing transistors or chips will probably be rendered useless. (the magnetic field upsets the delicate balance of charged particles in the devices a trsnsistor works because one half has more charged particles than the other put very simply))

The metal cage shields items from this magnetic field...so its possible the oven might work, its what we did.

The other problem is the route to earth for the electrical current in a strike this will do electrical current related damage, or heat. It will melt metal and generally fry anything it touches.

I was always worried about a strike to chain plates that then wants to exit to the nearest water through the hull, as the chain plate supports were below water level....big hole and probably wobbly mast.
 
Re: Faraday cage - biscuit tin

Failed O level physics so bit of a blank spot here - if a lightening strike exits through such as skin fittings in a plastic boat, what happens in:
a) a steel boat
b) an aluminium boat
c) a ferro-cement boat?

and which of all these is safest/lowest impact on boat integrity and survival of electronics?
 
Re: Faraday cage - biscuit tin

I agree with TigaWave, It's the EMF that can kill semiconductors, and there's a massive amount of it in the vivinity of a strike.
A farady cage will in theory block this EMF, however how effective an oven or a biscuit tin is depends on its construction. I have read of a skipper who put all his electrical gear in the oven during a storm, was hit by lightening which took out all of the boats electrics, but the kit in the oven survived. I would do the same.
I'm just checking/replacing all my earthing and finding it very difficult to work out an earthing strategy that satisfies the needs of Electolysis, SSB, Lighting and Marina safety. What I can't understand is why I should Earth the mast via the cap shrouds, seems to me that's asking for trouble in a lightning storm
 
Hi All.
Thanks for all the replies on this post, very interesting. Definately biscuit tin inside the oven and the whole lot parceled in tin foil!
I am heading for the Caribbean where as already mentioned the risk of strike will be higher, and although the risk may be still very small, like any other risk I shall do all I can (touch wood) to minimise its effects and having some navigation tools that still work seems like a good idea. If the boat is sinking then having an epirb that still works, also seems good idea.
Big tin of bickies here I come.
Regards.
Norman Brett
 
A bit of wooly thinking, I fear.
1. Lightning conductors DON'T conduct lightning, the last thing you want is several thousand amps flowing through the conductor path. They generate a stream of opposite charged ions that neutralise locally the charge in the cloud, hence the 'brush' type things seen on masts - the charge density is high on a point so ions spray off it nicely.
2. Your biscuit tin will only work if it can carry the current flowing over it so get one made of 5 mm copper
3. The effect is EMP, electromagnetic pulse, not EMF (em force i.e. voltage)

I was struck by lightning while driving a tracked excavator - can't get much better earthed than that and, fortunately, the bodywork is all 1/3" plate. Almighty bang and a huge blue flash left me blind for a few minutes. Apart from paint blown off at the entry point there was no electrical damage so the hypothesis works in practice but I'm not convinced the thing could be made to float - although didn't they invent sommat for tanks ?
 
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2. Your biscuit tin will only work if it can carry the current flowing over it so get one made of 5 mm copper

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If your biscuit tin is struck directly you are in serious trouble, and I don't just mean your biscuits will be singed. But I think the principle of a tin to add to the protection of electronics from minor stray tracks and induced currents is sound.

Bambola's description of a radio on a table that died but was presumably not vapourised by Mega Amps of current is a good indication of the usefulness of the approach.

Can I change the subject? Has anyone ever seen ball lighting ? I have, it was awesome. I believe that some authorities doubt its existence. I have no idea how it sustains itself.
 
Tell me more re your balls of fire.
The locals here claim all sorts of amazing incidents from ball lightening but in all the storms I've been in (including the direct hit above) I've never seen it.
What were the circs and description ?
 
This was about 12 years ago in the highlands of Kenya, a place that can be surprisingly cold and wet in winter. There was no storm going on at the time but there had been lots of lighting storms over the previous week.

Early evening, dusk outside, sitting in the house with curtains drawn, an intense white light came through the curtains and at the same time there was the characteristic sound of a major electrical arcing. I stepped outside to investigate and saw a ball of lighting about the size of a football, suspended in space about a foot from the trunk of a tree, not moving at all, but just a fizzing ball of sparks throwing off a very bright light and making that very alarming buzzing/arcing sound. After a few seconds (10 at a guess) it just vanished instantly. Interestingly there was a TV on in the room at the time and it was totally unaffected. Even more interestingly a few hours later exactly the same thing happened again. I have never seen anything like it since.

I find it very hard to understand what I saw and can quite see why experts are sceptical.
 
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