bonding and lightning

[walt]

I don’t think a coating of epoxy or paint on metal keel means much at all to lightning. You have to consider how easily the charge jumps between ionized air (or ionized paint) and metal conductors.

Its really electric field that matters with lightning as this is the number that’s important for ionization. Electric field is volts per distance and when distance gets very small (like the thickness of epoxy or paint), it does not take much voltage to achieve a very high electric field (or volts per distance) and then ionization breakdown.

 

[Sailvayu]

Valid point. I was not thinking in those terms but yes you are correct.

 

[walt]

edit.. my post was a little rude so I deleted it.. My apologies..

 

[Stu Jackson]

Why?

And so far, a farily good ratio of useful posts to worthless.. (the ratio just went down a little)

Sorry, Walt, but this topic has been beaten to death. Have you tried a search on "lightning" on this forum? BTDT.

 

[Sailvayu]

Walt, thanks for the information and discussion. I fear we might be getting a bit over my physics knowledge when we get into discussing the behavior of electro magnetic fields and how they react under varying physical conditions. It has given me some things to think about and perhaps when I have more time I would like to revisit this.

I was thinking it might be cool to get an old boat and use rockets with leader wires to have the boat hit like they do at the University of Florida. This of course would only be useful to study the effects of being hit and not preventing hits but would still be interesting and could help develop better systems for protection. http://www.iclp-centre.org/pdf/Invited-Lecture-Rakov-2010.pdf

As for those that think this has no relevance and that all the answers are known I would suggest they simply not visit or participate in the discussion. Even the scientists agree that there is much to learn on this subject and that we do not have all the answers.

Man has been reinventing the wheel since the day it was first invented. Good thing too or we would be driving cars with stone wheels like the Flintstones!

 

[Maine Sail]

Sorry, Walt, but this topic has been beaten to death. Have you tried a search on "lightning" on this forum? BTDT.

Pretty impossible to beat a topic to death that has ZERO consensus even amongst the foremost experts in the field...... About the only consensus I have seen from the experts are:

*Fuzzy Bottle Brushes = Snake Oil

*Lightning Hits What It Wants To

*You Can Not Prevent A Strike

Even amongst that consensus there is still debate...

 

[walt]

Yep, another lightning thread.. I started this wondering if there was any new information particuarly from the insurance industry regarding grounding or not grounding.. looks like there probably isnt. I think this particular forum is probably the best place to find out and think that was true again for this thead - much appreciated to everyone.

I saw this on another forum (Hobie - but the poster also has a sailboat) regarding fresh water

. I was sailing my 32 ft monohull on Lake Superior. At the end of a bad thunderstorm, we were at anchor, when the bolt hit the VHF antenna at the top of the aluminum mast. The light blinded a crew member who just happened to look up at the masthead when the bolt hit. The bolt vaporized 3 feet of the antenna, then traveled down the mast to the chain plate which joins the mast to the iron keel. But the bolt was so powerful that it also jumped to the grounding wire(s) for the boat's electrical system and traveled through the wire conduits burning out essentially all the wired-in equipment! Unknown to us, the current also traveled through the fiberglass hull and exited the gelcoat. The captain of an adjacent anchored boat happened to see the bolt exit through the sides of the hull. He called over that he saw 'fingers' of light traveling outward through the water. I put on a diving mask and dove into the frigid water. There were more than a dozen 'pockmarked' craters in the gelcoat. My crew had sprayed the electronic equipment with a fire extinguisher and then proceeded to check the hull for any holes. We didn't find any water coming in. The strike had taken out the diesel engine, but with a good breeze we were able to sail back to the marina. The boat was immediately lifted out of the water for inspection. It had to be hauled to Duluth for wiring and hull repairs. Luckily, my boat insurance covered most of the $20K repair.

I think there are some folks here who would have a better idea than I but its my impression that there is a good chance that this same strike in salt water would have had the electronics damage but likely only dielectric stress marks on the keel instead of the pockmarked craters in the hull gelcoat. Interesting that someone on a different boat actually observed the "fingers of light" traveling outwards through the water (which I think possibly happened because this was in fresh water).

 

[Sailvayu]

Walt, as a surveyor I have never had an insurance carrier comment one way or the other about lighting protection or grounding related to lightning protection. Most insurance companies have little or no knowledge of boats in general and none when it comes to a subject like this. Boat US keeps some statistics but not much detail. I think part of the problem is most surveyors (Myself included) really do not have the skills required to fully analyze what may have caused the strike in the first place. We are paid by the hour by the insurance company and all they want to know is what is the damage and what is the repair going to cost. Boaters are sort of on their own to figure this one out at the moment. Interestingly you would think the military would have some info on this but I have yet to see anything if they do.

 

[shemandr]

Regarding the salt water vs. fresh water post: A friend's boat was hit in its salt water slip and the lightning exited the hull in multiple places including through the fiberglass hull. The boat would have sunk if the owner didn't go back to it late that night to check it.

 

[mauruuru]

This conclusion is counter-intuitive and does not agree with my experiences in HV lab or more than 33 years of electric utility experience. The determinant is the electric field strength at the point that the air begins ionization- partial discharge-and the geometry as well as moisture content of the air govern whether an arc will flow and where it will go.
Spherical probes generally tolerate a higher field strength than pointed ones for this reason.
Having said that I know many cases where lightning completely avoided grounded pointed masts, passed through energized HV lines and struck some lower groundedor energized part. You can not guarantee the path of io ization and power follow through current. You can only try to induce the strike to hit a grounded part to shunt the current and collapse the field.

 

[skygazer]

Extremely interesting discussion, thank you to all that contribute. Walt, I found the photos of lab discharges spreading out over the surface of fresh water very helpful to understanding - not to mention actual instead of purely imaginative/theoretical.

I've seen some weird lightning results in the real world. I try to read all the threads on lightning (it scares me ). I found this one to be excellent and it gave me new insights.

 

[walt]

The link below that Sailvayu posted has something interesting and sort of related to grounding or not grounding

http://www.iclp-centre.org/pdf/Invited-Lecture-Rakov-2010.pdf

On page 6, there is a figure of a induced strike where they shot up a 150 meter conductor 400 meters high when it triggered a strike. Before the strike, the conductor has essentially no current flowing so it has a constant single voltage over its entire length and this is what concentrates the electric fields at either end and causes the leaders to form. Leaders form on both ends - even though there is no direct connection to "ground". This is important to note.. you dont need a conductor to be grounded to form leaders.

The negative leader is easier to understand as its electrons flowing in the direction of the leader. You might think the positive leader must be positive ions but really the positive current is still electrons - but they are flowing in the opposite direction of the progressing leader. The strong field at the very end of the leader ionizes the air out of ahead by pulling electrons from atoms which then travel downwards (assuming a negative sky charge). This leaves a positive charge in the direction the leader is traveling so looks like a propagating positive charge. As long as the field is high enough, this can continue by pulling the electrons off the atoms and the positive leader heads on upwards.

Another way to look at this is that when an air atom is ionized, there is an electron with a negative charge and the rest of the atom now with a positive charge. Both "parts" have the same charge so see the same force from the electric field. However.. the electron is something like 60 thousand times less mass so given the same force on both the electron and the ion (because they have the same charge), the electron sees a huge higher acceleration. Conductivity is proportional to the number of charge carriers and their mobility and the electrons are much more mobile - so make up the current flow in both positive and negitive streamers.

There is another picture in that link which is both interesting and "maybe" slightly humorous.. On page 12, there is a picture of an induced strke to a car "with a live rabbit inside". I didnt see in the paper if the rabbit lived.. but Im pretty sure its bunny brain probablly had to do a reboot shortly after the strike. (photo credit - page 12 of the link above)