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Old 20-01-2010, 17:35   #1
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Grounding Against Lightning Strikes

i'm sitting at work, watching a rare electrical storm move through the bay area, and this prompted a thought - curious to see what you guys think.

My mast is well grounded against lightning strikes, but it occurs to me that:
(a) copper is a better conductor than aluminum
(b) the tricolor, strobe, anchor light, anemometer and VHF antenna are all at the top of the mast and all have copper wiring running down straight into my distribution panel, rendering the mast grounding less than fully effective at channeling 100% of the absorbed energy straight to ground. (and as an added bonus, routing all of that power exactly to where i don't want it)

has anyone considered the idea of having a harness at the base of the mast that could disconnect all of these wires, and perhaps recouple them directly to a keel bolt?

sure, the items you disconnected wouldn't work, but if you were willing to forgo those systems, it seems like you'd have a greatly reduced risk of collateral electrical damage should a strike occur. if you needed one or more, you could keep everything coupled per usual, and be in no worse shape than you would be otherwise.

i can't imagine i'm the first person to think of this, but i've never heard of it being done.

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Old 20-01-2010, 17:52   #2
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Not sure how much that would accomplish.

An example is a friend of ours who was caught offshore in an electrical storm; lightning struck the water 15 or 20' from his boat, but still took out all his electronics.

By the way, his explanation of what happened was "everything went orange and then the boom hit."
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Old 20-01-2010, 17:56   #3
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I don't think your logic is flawed. No matter how good the path to ground, in a direct strike some current is still likely to take even less direct paths to ground, e.g. through an electronic instrument. Disconnecting what you can seems one of the best bets.

One big problem is that in even a nearby strike, voltage and current can be induced in almost any length of wire and cause damage in that manner. Think of any piece of wire as an antenna to the lightning strike's electromagnetic field.
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Old 20-01-2010, 18:15   #4
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We are suffering our second strike with in two months. Both times not a direct hit. First one hit a boat two over from us in the marina and this time hit the water a few feet away. Both times major electronics damage. I was told it is the long cable runs that pick up the EMF so maybe your logic is not too wrong however I would also think a switch to cut the lines going to the panel from the batteries would be a good idea as well. In the meantime we are waiting on insurance to give us the go ahead so we can replace some of this stuff and get back sailing.


In Langkawi, Malaysia
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Old 20-01-2010, 19:39   #5
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The true purpose of lightning protections is to keep the boat and its occupants from getting holes burned through them when those nasty electrons start to jump wildly about. Yes, disconnecting electronic gear ups its chances of surviving the hit. However, the last place I would want to be when lightning hits is anywhere near something metal. I especially don't want to be hanging on to a bunch of ungrounded wires coming from the masthead.

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Old 20-01-2010, 21:35   #6
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the only folks i know who were ever hit by lightning were protected--a NASA scientist--engineer--hit 2 times in 4 yrs.....are you sure you want to do this?? there were many other sailboats around his at the times of each strike--in a protected area----but his is the fact.....

we sailed thru a few days of lightning storms off florida this past summer.....only lightning rod in the area---no problem--yes--was frightful--but no want to repeat the feat, but was accomplished--can scratch it off list lol.....
life is an adventure meant to be LIVED!!!!
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Old 20-01-2010, 22:16   #7
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Your point is well taken. Just turning off switches does not do it, as the switch contacts are not physically separated enough to prevent fusing. I am a firm believer in a main disconnect that separates the system hot wires from the batteries and leaves an air gap on at least 4 inches.

I have seen the mess a lead acid battery makes when it explodes and you don’t want to clean up the mess.

Lightning will take the path of least resistance and so the lightning ground should be checked on a regular basis. Add a couple of ohms of corrosion and you no longer have the most direct route to ground potential.

Don’t tie anything else into the lightening ground. It should not be a part of the boat negative or the radio counterpoise.

Um Saudade
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Old 20-01-2010, 22:30   #8
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Outside this ragged darkening cloud
air grows heavy, cringes
while inside, tension builds

a forked tongue flickers
squanders all
in one great leap to earth
or to the heavens

air breathes out
- but inside the cloud
tension builds again.

This story older
than recorded history
our planet's green
- the spark, perhaps
that kindled life.

Looking up, we see it
rip the helpless sky,
we marvel at its power
to scorch forests
wreck cathedrals
put out a city's lights
weave glass serpents
in the desert sand.

Looking down, as now we can,
our globe scintillates
- flurries of dagger-strokes
accompanying great storms
over oceans
jungles, mountains

a fireworks show
blazoning messages
in code
which we must crack.

J. Latham

Here is a link to NASA's pages on lightning:

Lightning and Atmospheric Electricity at the GHCC

Here is a link to NASA global lightning flash data:

The biggest problem is in defining what constitutes "lightning protection". I believe that lightning protection for a sailboat should be defined has:

"Methods and techniques that minimize the risk of injury to crew and damage to the hull. Electronics maybe a cost issue, but if the crew is un-injured and the boat is still afloat then survival is possible".

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