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Old 04-11-2007, 00:18   #31
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When current runs down a wire a circular elecromagnetic field extends from the wire.

A moderate lighning strike lasts only a short time but can generate about 10 to the power of twelve watts.

As for transformers wiring--that is a moving magnetic field cutting a closed loop circuit. Yes--transformers have copper windings, but the core is soft iron --with low hysteresis losses. By all means put your goodies in an alloy cabinet. I hope it is able to exclude all of the flux. I am pretty sure your stuff is not safe though.

If you want to try a little experiment--get a powerful welder's magnet and put a sheet of flashing aluminium sheet over it. This is a good conductor. Then see if the magnet with the aluminium over it will stick to a steel object. I just did--and it stuck quite well.

This means that magnetic flux does penetrate non ferrous material--and if moving flux reaches your electronics--they will be have induced currents in accordance with Lenz's law and Farday's Laws.

Lotsa luck
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Old 04-11-2007, 03:01   #32
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Having worked in an aluminum smelter many years ago and having dealt with large magnetic fields and electronic equipment we had done a lot of expermenting to solve the enclosure problem. The enclosures we ended up using were a Mu-metal enclosure. These were made with a heat treating process that made them ferrous but non magnetic. Very effective shield. Very expensive enclosures.
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Old 04-11-2007, 13:07   #33
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Thanks Lancerbye--

I will be on the look out for some Mu metal enclosure material to build a safe space--and if I find one I will get it annealed before using it.

Meanwhile my soft iron box will have to do. Far better than no protection at all.
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Old 04-11-2007, 17:53   #34

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I just want to make two quick points and I'll leave it to you guys to sort out. First, you get hit while in a storm. There is no time to gather up electronics and stow them. Sea keeping comes first, and autopilots tend not to work in 60 mph wind.

My second point is admittedly weaker, but never the less, true. Three handheld electronic devices, all operating and either in the salon or the cockpit were completely unaffected by the lightning hit and are still working today. Two cell phones and a handheld GPS. All were on batteries at the time. By comparison, everything, and I mean everything connected to the DC wiring on the boat was fried. Some of it died immediately and some shortly after.

Enclosures are nice but whatever is in there, you're not using anyway. Just leave it home.
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Old 04-11-2007, 19:49   #35
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Well, usually there is some warning before it hits the fan, but not always. Lightning may precede a stormy wind. Also many electrical storms hit while the vessel is on a berth or lying to an anchor or mooring. I live on the anchor quite a bit, as do many coastal cruisers, and when things start to look ominous I head for shelter and a safe mud bottom anchorage if possible. I then disconnect every expensive bit of kit and stow it in the bin with any hand-held goodies too.

I have had a television ashore fried by induced current from a nearby strike which did not harm anything else in the house--because although there was not a strike there was a large electroststic potential difference between earth and the TV antenna which blew some of the components in the teev including the protection device--so I know the voltege exceeded 400volts, but by how much I have no clue. Made a nice sizzly hissing sound though. So--you need protection when a thunderstorm hits wherever you are--land or sea.

I am sorry to hear you sustained damage but glad to hear that the hand held devices were OK. That was probably because they had no connection with a common sea earth. Did you have an approved lighning collector and if so what kind?

I spent quite a bit of time repairing lightning damage. In all of the cases with which I was involved--proper lighning protection had either not been fitted or had been fitted and then disconnected by some maintenance clown who had not reconnected it after completing their work. Leaving out the gaseous or other type of high voltage arrestors was a common mistake. A very expensive mistake much of the time--since the damage frequently occurred at irregular intervals along the grid and sometimes was time consuming to locate.

The thing about lightning protection is that the subject is not by any means closed and new information is still coming to light. A building with proper lightning protection might not be hit--whereas the sea or land close by may be severely struck--although the building is much taller. A tall building without lightning protection may be struck or may not be struck for years. Sooner or later though--if it is in an area of frequent electrical storms--it will sustain a strike.

All the ground or tree or motor vehicle, or person needs to do to be struck is to be outside the cone of protection afforded by a properly designed and installed protector. You will note I do not use the term conductor--because only a minor strike could be handled whereas continuous discharge of lesser voltages and currents could be conducted to earth safely, reducing the liklihood of a strike ever eventuating.

Quite a few people are killed by lightning each year--and a great many more severely burned--so the subject does need a good airing.

James Wharram recounts an occasion where the surface of the sea close to his catamaran was struck and actually produced a burst of steam, while the tall masts of his vessel afforded a higher point, the lighning instead struck some considerable distance away. It just happened that the air was a better conductor somewhere else--and that was where the lightning strike occurred--as it always will.

The location of a strike is dependant on the dielectric strength of the air between earth and the charged patrticles. Where it is the weakes for whatever reason, a thinner belt of air or hotter and less dense, moister or more more densely ionised--these factors all have a bearing on where the lighning initial "stroke",(usually from the earth upwards) will occur. Once it does--the air is further ionised and the strike develops, the ciurrents often going in either direction as the potential changes with further discharges.

So--by collecting charge in the vicinity of the vessel and conducting it safely into the sea--you greatly reduce the chance of a strike taking place. In fact--you make it far less likely. If it does happen in spite of the protection and you do sustain a strike--it should be of greatly reduced severity--but there are no absolute guarantees where lightning is concerned.
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