Faraday Cage For Lightning Protection

[jim hardy]

as i said I'm neither a lightning expert nor a good mariner. and I should have said 'plate' not keel...

I'm under the impression it takes ~10*9 amps/m^2 to explode a wire in a millisecond
for your half inch bronze bolt that would be 190 kiloamps which is a HUGE lightning bolt... i doubt your bolt is the weak link.
but i'd run a big wire over to the mast...

sorry for butting in - just it's interesting. I like to learn something every day.

Thanks for letting me play in your sandbox, guys...

old jim

 

[anorlunda]

Your butting is always welcome Jim. You have goo instincts.

I wasn't worried about the stud melting. I can't see fitting 50 of these connectors on a one inch long stud. Even 2 or 3 is tight. I also can't imagine one of these carrying 20 KA.

 

[Jeff Rosenbury]

The purpose of the "keel plate" is to allow the lightning to ground directly under the mast. It was added when, in many cases, lightning didn't follow the bends in the conductors toward the sides of the boats but instead went straight through the hull. It should be noted that the current "keel plate" designs are usually sufficient for salt water, but not fresh water. Boats struck by lightning in fresh water typically have a hull blow through even with a plate. (source)

This source also suggests upping the down wire from 8 to 4 gauge, since the 8 gauge apparently came from a study on rural buildings in Poland with only a 95% safety factor. It indicates the "keel plate" would need to be unreasonably large in fresh water (bigger than the keel).

Baluncore, what you think I might know many years from now depends a lot on your beliefs in the afterlife. I'm not some wet behind the ears kid. I'm also not recommending ignoring established standards.

 

[nsaspook]

as i said I'm neither a lightning expert nor a good mariner. and I should have said 'plate' not keel...

Thanks for letting me play in your sandbox, guys...

old jim

Don't worry Jim, you won't be Keelhauled.

 

[Jeff Rosenbury]

Your butting is always welcome Jim. You have goo instincts.

I wasn't worried about the stud melting. I can't see fitting 50 of these connectors on a one inch long stud. Even 2 or 3 is tight. I also can't imagine one of these carrying 20 KA.

I always enjoy Jim's posts. It is kind of him to make the effort.

I agree your plate's lug isn't big enough. And I suspect the connector is seriously undersized, particularly if you go with the IEEE recommended 4 gauge wire. Remember these plates aren't required by shipbuilders and are only put into make insurance companies happy. Given that about 3% of boats are struck each year in South Florida, a clever man would figure something out.

 

[anorlunda]

I too like Jim's posts.

The 4AWG connectors are slightly better.

But this whole discussion stretches my imagination by several orders of magnitude. Especially when the ABYC standard wants me to give all large metal objects above and below decks the same treatment as the mast. (I think of a sauce pan in the cupboard or a spare anchor stored under the floor as absurd examples).

I'm an old power plant guy. To safely handle 20 KA, I think of things like isolated phase bus ducts as shown below.

Add to that the opinion of a knowledgeable engineer like Baluncore that I should eliminate the ground plate entirely, and you see how far ranging the advice is.

I believe these questions can only be answered statistically. My insurance company, BOATUS, publishes a safety magazine. I asked them to publish an article based on their claims database about what works and what doesn't for lightning protection.

But I must say, this has been one the most enjoyable PF discussions I've seen. Divergent opinions, but all rational. Thanks all.

 

[jim hardy]

Remember these plates aren't required by shipbuilders and are only put into make insurance companies happy.

Look at a copy of National Electrical Code. Publisher isn't IEEE or some such august academic outfit, but NFPA - National Fire Protection Association.

I guess experience is the best teacher. It teaches us which fundamentals to apply for given situations.

Thanks again, guys .

old jim

PS that Isophase photo is nostalgic. I measured flux in vicinity or ours, 0.1m² gave 0.3 volts per turn. ~8 milliTeslas ?

 

[Jeff Rosenbury]

I started a thread on the general physics forum about how predictable lightning is.

 

[anorlunda]

My hopes that the BOAUS insurance company could shed more light are dashed. But the reply I got does add some interesting stuff.

On the ABYC, lightning protection is no longer a standard, just a “technical bulletin” which means it is there to provide information to interested parties. I wasn’t part of the discussion, but I would guess that objections like this had a lot to do with that decision.

Unfortunately, our claims files can only be of limited assistance on the topic of lightning protection. First, there’s only a very few boats (relatively speaking) that get hit, so the data is thin to start with. Historically, our files did not include information on the grounding/bonding system in the boat even after a lightning strike, and our application doesn’t ask about it, so we definitely don’t know anything about lightning protection when it comes to boats that did NOT get struck. We’re working on all of this, so we might have really good data on how lightning protection affects claims in a decade or so…​