copper boxes and electronics

[kwiac]

Can anyone explain the rationale behind storing spare or back-up electronics in copper boxes to thwart any problems associated with lightening strikes?

[scnicklefritz]

I don''t think there is any (rational).

If the device isn''t wired to the boats electrical circuit, current shouldn''t be able to pass through it in the event of a lightning strike.

If it''s stored on the boat, the best place to keep it would probably be in a tupperware container with some of those silica moisture absorbing bags.

Certainly plastic is going to repel the ''juice'' from a lightning strike a whole lot better than copper.

[colehankins]

If i was going as far as to put spares in a copper box, I would also lead a wire to the ground. I also think that if you dont have a path for the juice to travel, your spares would be spared. I was stationed at Carolina Beach Loran station hundreds of years ago and we had a copper enclosed timer room to keep the emf away from the gear. The timer room was in the ground plane near the base of the antennea. It seemed to work quite well. The only prob we had was lightning strikes on the ant that would trip the XMTRS. We were pumping 440KW @110khz. It made for a fun sleepless night standing watch.

[tsenator]

Wow.... there is alot of Physics behind this...


When the lightning current travels down the mast (or whatever) It creates a HUGE electromagnetic field (typically at right angles to the current)........That HUGE electromagnetic field is then picked up *THROUGH THE AIR" by all different sorts of wiring or metal throughout your boat.

Thus that induced HUGE Electromagnetic field is then turned into current again in the (supposedly) completely isolated wiring of your delicate electronic circuits........Yes all those tiny little circuit boards and electronics are picking up that HUGE magnetic field and converting it to current in those tiny little circuits AND typically it is WAY more current than the electronic circuits can handle.

(This is the same general principal in how alternators/generators work----current in induced into parallel windings of wire......they are not touching.it just is induced across the air gap)

So why do some electronics blow and not others....Well there are many factors but I will list 2 variables.
1) Strength of Lightning -- Obviously the stronger the strike--- the more the current ---- which means stronger EMF (Electromagnetic Field) induced
2) Orientation of Lightning Path to delicate wiring in Electronics. More current will be induced across when these two conductors are parallel. (The source lightning path and the pickup electronic wiring...internal & external wiring) I really don''t think you have any way say in this...the orientation and wiring are in all sorts of different axis and I think its just luck how it might end up . Though in a "perfect" world a wire perpendicular to the conductor for lightning will pick up little to no EMF thus very little current)

That is why some things blow and some things *appear* to be still working.....

PLEASE NOTE: Just because something *seems* to work does not mean it is 100%. The Induced lightning could have caused some slight damage to the instrument that does not "completely knock it out"...There is a very good chance of premature failure being induced.

So this explains why Electronics can blow :
A) EVEN IF your Boat in *completely* bonded and set up to take a lightning strike
B) EVEN IF your VHF Antenna is not connected
C) EVEN IF your electronics are disconnected at the time of the strike
C) EVEN IF your electronics are 20 feet away from the apparent strike....(obviously the further away the less induction of current)



The only way to *really* protect anything in lightning strike is to put the whole device into what people call a "faraday shield"....(Basically a completely enclosed metal box), this way the EMF "rolls" around the outside surface of the box and does not induce current on the inside.

[tsenator]

scnicklefritz and colehankins

"Certainly plastic is going to repel the ''juice'' from a lightning strike a whole
lot better than copper."

No that is not true for EMF. But that is true for Current flow. So there will be no current flowing into the box but PLENTY of EMF .....thus induced current on the delicate instruments circuits as described above. So a plastic tuperware box will do nothing in a lightning strike.


"If i was going as far as to put spares in a copper box, I would also lead a wire to the ground."

It would make no difference. The important thing is that the box was *completely* enclosed with metal. Thats all. Any gaps or wires running inside the box would nullify most of the protection.

Thats why if you enclosed you VHF but kept the DC power leads attached you could *still* blow the vhf, because the EMF would be induced in the DC cable and blow out the VHF power supply if it was strong enough.

[kwiac]

Thanks for the response.

kw

[DuaneIsing]

Great explanation, tsenator!

As Gilda Radner''s character would have said, "It''s always something!"

[colehankins]

Ah yes the left hand rule. I would still ground a metal box. Where would the emf go, with out a ground once it was hit? I didnt say run a power cord to the spare. I was under the impression the spare was unpluged and ready for service. I have also resolved my self to thinking a lightning strike means get new instruments. I also think that most lightning current comes from the ground and up the mast, prompting the strike.
I think i saw it on the TLC channel. Do you think they would lie to me?? You engineer types disgust me. always tooo serious. then blame someone else when the thing breaks. try to design a parking lot where i can open my car door and get out without hitting the ajacent car. design something that works properly for a change. Make it taller, not just for the impared. Are all designers and engineer s 5''5" tall and drive hondas?????

[obiec]

Pay attention He is talking about a magnet pulse not a current that will flow to a ground. A magnetic pulse I do not believe ever truly stops it just dissipates. And if anything by attaching the box to the ground you might convert part of the magnetic pulse to actual current, which could possibly affect something else that is also connected to the ground.