Lightning?

[hellosailor]

Gord-
So, if an aircraft effectively is grounded while in midair...a cookie tin on the countertop isn't better grounded?<G> I mention cookie tins because butter cookies in ornamental tins, and booze or amaretto biscuits, pasta, etc, are available in tins now, sometimes nice ones at a price equal to that of a tin alone. In theory they meet the qualifications for a good faraday cage for a vhf or gps, magnetic metal, tight sealing metal contact on the lid. No?

Joe-
"I have also read in similar discussions that the better an object is grounded the more likely it may be to attract the lightning in the first place." That's what I believe. I've felt and heard the ground charge boiling "up" towards the clouds, I have to think (maybe wrongly<G>) that if I'm better conducted to where that upstrike is coming from, I'm more likely to be in the returning groundstrike.
I've always thought the best thing would be to get in the habit of good seamanship, i.e., when you SECURE THE VESSEL to go ashore for some time, also secure the elctronics by physically disconnecting the antennas and power lines, and running them (antennas at least) directly into a ground that is not near the electronics, so the electronics aren't a target and you just have to replace "wires". I know land-based radio ops who routinely do that. PITA to walk around pulling plugs, yes, and some of them actually aren't meant to be unplugged on a regular basis (ie. the duty cycle on a common PC printer plug can be only 300 uses!) but what else an you do?

Wheels-
"But there a few stories that suggest one or two aircraft may have gone down due to strikes, " Well, I should HOPE SO. I mean, after all the times I've been told my lowly cell phone can bring down an aircraft, I'd feel out of place in the universe if Jove's own blasts couldn't do what my little cell phone could. (When the pilot says he doesn't wanna fly in that soup, I don't argue with him either.<G>)

[ssullivan]

Digressing to science. I have a quesiton regarding the grounded Faraday cages (and I use this in the sense of modified "Faraday cage" such as a sphere of copper).

Can anyone tell me why a grounded copper sphere would have any different effect on the EM waves on its interior vs an ungrounded one, floating free in space?

I seem to remember solving Maxwell's equations in a sophomore class in college whre we did just that. We solved for a Faraday cage in the form of a sphere (math was more simple) with a point EM source outside this sphere. I remember the equations basically zeroing out for regions inside the sphere. No discussion on if it was grounded, since it didn't matter.

Anyone?

[soul searcher]

H.S.

The hinges and latch tie it all together.

I was thinking of an economical way of storing a spare hh gps and vhf.
It would be easy enough to ground the can to a thruhull. but i would think the main concern wold be EM.

Matt

[Rick]

Sean,
Maxwell's equations can be used to show that a uniform charge on a space enclosed by a conductor (the shape does not need to be spherical yet the "proof" utilizes closed integrals which are relatively trivial when considering a sphere). Gauss's law explains that the divergence of the e-field is zero when there is no charge enclosed by a closed surface and this is much easier to show than to begin with Maxwell's equations. Although Gauss's law is easy to explain the way that a Faraday cage works with any external charge it also implies that a charge CAN be enclosed by a closed surface and everything changes in that condition.

Uniform (and steady-state) electromagnetic waves will not cause a voltage gradient inside a Faraday surface yet lightning phenomenon is not restricted to the generation of uniform electromagnetic waves. Think of "uniform electromagnetic waves" as being "static" (yeah, "steady-state" is the correct term here) in that they were generated at the beginning of time and vary not for eternity. Think now of reality. A "real" electromagnetic wave is not static or uniform in that it contains information of convergence or divergence, no matter how small, of each component (i.e. electric and magnetic) As WELL as the relationship between the two which is NOT necessarily two vectors at right angles to each other (wow! now you begin to get the idea of a lack of uniformity and "dynamic" in that the vector phenomenon is relatively easy to analyze when "static" even though the wave moves near the speed of light).

For non-uniform electromagnetic waves Maxwell's equations come into play (because the others are not so generalized) and become very combersome in that they may be challenged with a method of handling the math to work a solution. We "like" to use linear calculus equations yet non-linear ones must be utilized in some cases to "work" a solution for just what a non-uniform, non-rectilinear electric and magnetic field might do to penetrate a conductive sheet.

In short: Faraday's law summarizes the way that voltages can be generated by time-varying uniform magnetic fields (has little to do with the Faraday cage concept); Lenz's law focuses on Faraday's law in that there is a negative sign on the "right" side of the equation indicating that a current will be generated which will make a magnetic field equal to and opposite to the one indicated by Faraday's law; Gauss's law is much more interesting to this discussion here in that the electric flux through an area is defined as the electric field multiplied by the surface area projected on a plane perpindicular to the field. this is a general law applied to any closed surface. To an engineer the applicable statement of Gauss's law is that the total of the electric flux out of an enclosed surface is equal to the charge enclosed divided by the permitivity of the media that the flux passes through. NOW one can realize that ONLY a material which has permitivity (air and non-ferrous materials have a permitivity of one, obviously...and not zero else the result is indeterminite) is capable of protecting an electronic device, because if a charge gets generated INSIDE an enclosed surface (that is not made inside that surface) there MUST be an enclosing surface which either is not totally conductive and enclosed or does not contain a material with sufficient permitivity so as to completely contain the magnetic flux.

How's that for an explanation?

[Tropic Cat]

Ummmm..... Rick, does this mean I can, or can not protect my electronics if I stuff them inside of my oven prior to a lightning hit?

Rick in Florida

[Rick]

Yes, your oven will help. All that explanation merely points out that the oven enclosure materials need to be both conductive as well as of a "magnetic material". It further points out that whatever material one uses mother nature may generate a condition so horrendous that the protection may fail.

The good news is that approximately 90% of lightning discharges recorded generate less than 10,000 Amps. This means that there are practical materials which will protect agains those levels. I doubt that 20 gauge steel is thick enough for that level yet still not bad.

[hellosailor]

Soul searcher, my owm meter and pyro can (a 30cal. can that I store flares & such in) disagree with you. Granted, I cleaned the can and put three or four good coats of paint on it before using it for pyros, but the GI paint that they start out with is going to be there too.

On a 20megohm scale, with both probe tips spiked into the can, I read *infinite* resistance. Both across the seal (lid to can) and on the same surface (lid to lid, an inch apart).

Which tells me that I didn't spike the probes in far enough to reach metal, and also, that a simple coat of paint may totally insulate whatever contact you were expecting from the hinge and clip. You may need to take the can down to bare metal to get that contact. Even then...I'm not sure the contact is good enough, being only at the opposite ends of the can. When some of the online resoures talk about differential voltages of 50-100 kilovolts per meter...that still allows a real voltage buildup between the two ends of the can, doesn't it?

On the other hand, speaking of electricity and magic, did anyone catch David Bowie's portrayl of Nicholas Tesla in "The Prestige" ? I suspect moviegoers will have no idea that most of what was said about Tesla was simply true.

[GordMay]

Originally Posted by hellosailor

Gord-
So, if an aircraft effectively is grounded while in midair...a cookie tin on the countertop isn't better grounded? ...

You got me there.
After about 10 years of study & research, and writing 10's of thousands of “draft” words, I’ve given up trying to write an technically accurate and generally comprehensible tutorial on lightning protection for boats.
It’s beyond my capabilities.

[ssullivan]

Awesome, Rick (from Seattle)!!! Sorry for the drift, guys, but it was a really nagging question. Rick has more than summed it up - thanks so much. It took me a while to digest, but it definitely refreshed some memory I am losing now that I'm out of the science field and into computers again. Computer programming makes you dumb... ha ha ha. You tend to forget a lot of other, more interesting things.

The concept of non-linear electrodynamics hadn't occurred to me - nor had the possibility of imperfect conductors. How naive of me. That's the difference between a guy with a little undergraduate Physics training and a full blown Electrical Engineer (who I am starting to suspect might hold some graduate degrees).

I did do some work in non-linear systems as they related to plasma physics, but it was quite limited in scope. Plus, I'm old and I forget everything.

Thanks again. Very interesting thread. I really enjoy reading Rick's posts.

[Tropic Cat]

Great post on lightning and aircraft. It sure compelled me to read up on this.

Aircraft lightning protection relies on "skin effect" and the insulating properties of the interior cabin liner. The explanation I read about Skin effect is that lightning sees the fuselage as a hollow conductor and and only travels on the outside of it.

When you think about this, it can't possibly work. As has been pointed out earlier, the aircraft isn't even grounded. Yet, it apparently works perfectly well, as a major airline hasn't lost a plane due to lightning, since a 707 blew up from a lighning discharge inside of it's vapor filled almost empty fuel tanks 40 years ago. And to my knowledge, no one onboard an aircraft which has been struck by lightning, has been electrocuted. And if these planes land OK, it means their electronics are intact.

So, apparently an ungrounded aircraft is in fact a Faraday Cage, and protects occupants and equipment from both high voltage gradients and the huge EM wave.

The $64 question is if they can do it, why is it so difficult for us to protect our plastic boats?

Rick in Florida

[Rick]

Gord's recommendation to ground Faraday (whether they be only conductive or both conductive as well as magnetic) protective devices is VALID in a confined space (as always) because if one contacts an isolated conductive device the device MUST be grounded. Yes, grounding alone does not guarantee that one might not be able to place oneself across an instantaneous voltage it vastly helps. Without the ground there is not only NO guarantee, there is an increased probability that a damaging voltage may be developed.

Skin effect has relatively NO advantage regarding aircraft safety. If anything, it is the Faraday cage concept that works here. Keep in mind that there is a DISTINCT difference between an aircraft in the vicinity of a lightning discharge and a vessel LAYING TO AN ALMOST PERFECT CONDUCTOR, THE SEA. Current plays a succinct part in differerentiating between a "strike" in the air and a "hit" in the sea. For sure, probability dictates that a particular aircraft will find itself in a large current lightning discharge path that obliterates the internal electronics. That same probablility is huge when considering a surface vessel.

Again, aircraft lightning protection does not depend upon skin effect. Skin effect, by the way, is most easily understood as a steady-state analysis of an electromagnetic wave with a conductor. The concept is that the higher the frequency content the closer to the outer surface (due to the initial wave incident to that "outer" conductive surface) the more compact are the current carriers distributed from the surface inward. What is not commonly known is that the "skin effect" works right down to zero frequency with an attendant decrease in the carrier distribution from outer to inner conductance. Lightning discharges are by no means "steady-state" in their energy distribution as well as frequency distribution and although skin effect still plays its phenomenon, it is a minor role in the overall play of potential damage.

[Tropic Cat]

Is it the concensus that after grounding the mast and chainplates to form a protective cone, and tossing your electronics in a grounded oven that you're as protected as you can get? Or would it be prudent to keep that oven out of the path of the discharge current?

Rick in Florida

[Alan Wheeler]

After about 10 years of study & research, and writing 10's of thousands of “draft” words, I’ve given up trying to write an technically accurate and generally comprehensible tutorial on lightning protection for boats.
It’s beyond my capabilities.

I think it is beyound anyones capabilities. I know Rick has given us some outstanding info, and I know I said that it still follows laws of physics, but when ever some scientist seems to go Eureka! I found it, writes an artical on the "new theory" of how lightening shall/should act, some dude somewhere gets toasted like a marshmellow by a strike and the theory is out the window.

[hellosailor]

"Or would it be prudent " In the Chicken Soup Theory ("It can't hurt!") even if your goodies were in a sealed armored box...Damn straight, keeping the box out of the discharge path would be a good idea.

I'm also told that sinking the boat, or at least inverting it, and not raising it again until after the storm works pretty well.

[Rick]

From a purely physics standpoint of view it would be better to ensure that a current does not pass through the oven ("keeping" the electronics). In addition, it is advisable that a current does not pass through you or a loved one and that is best ensured by grounding the oven.