lightning storms

[Trim50]

Here's my thinking with respect to lightning strikes and sailboats...now that I've been thinking about it.

Electrical Conductivity (S•m-1)
Silver 63.01 × 10^6 Highest electrical conductivity of any metal
Copper 59.6 × 10^6
Aluminium 37.8 × 10^6
Seawater 5
Drinking water 0.0005 to 0.05
Deionized water 5.5 × 10-6

So, Seawater is approximately 7 orders of magnitude less conductive than your typical mast and as such it provides excellent insulation. The surrounding air can achieve a high level of electrical conductivity through ionization to form a plasma when in the presence of a strong enough electric field. Therefore, it would seem that the only reason lightning would strike a boat would be that it has provided a less resistive path to ground than the surrounding water. Such a case would be in shallow water, at anchor or operating RF equipment (or all the above).

My understanding is that boats in a marina typically have a much lower likelihood of being struck by lightning because there are a large number of sailboat masts, electrically grounded by shorepower, all acting like lightning rods. The large number of lightning rods in a given area reduce the local electrical potential by slowly drawing current from the atmosphere and preventing the sudden discharge of electricity through a single ground source and lightning strike.

Therefore, I would theorize, that being at anchor in shallow water without neighboring anchored sailboats would greatly increase the probability of being struck. So, if you find yourself in a strong electrical storm, either find your way to a marina, or sail out to deep water and turn-off all of your RF equipment…don’t drop anchor.

Of course, I could very well be completely wrong .

[coot]

Originally Posted by Reality Check

COOT: The do not use Mesh... a series of elevated lines ( some 25 feet or so) that are spaced some 10 to 15 feet apart run over and adjacent to areas being protected.

If I follow your description correctly, I would have described it as a mesh with 10-15 feet betwen conductors. Or do you mean to say that there are only a large number of parallel conductors, like a comb instead of a screen?

[Rodz]

Originally Posted by GordMay

Isolating Spark Gaps electrically connect the grounds, at higher lightning potentials, that are otherwise isolated*.

* The lightning down conductor may be isolated (at lower voltages, under several hundred volts) in order to reduce galvanic corrosion and eliminate electrolysis.

I would rather say that the reason to include spark gap is to avoid circulating current inside litghtning protection grid. This is not a "farady cage" but commonly used lightning wires that are supposed to protect what is under.

Look at the transmission towers, they have one or two lightning protection wires on the top of each structure runnng parallel to the conductors below. They are meant to protect them from direct stricking. They are grounded at every tower. If we look at our boats standing rigging they resemble almost exactly the protection that electrical engineers use for transmission line or substations protection. They are lack of grounding only. I found a site that claim they resolved the problem. I doubt, but it is worth to look at it:
Marine Lightning Protection Inc.

Chris

[Tropic Cat]

I went to that site and am now wondering why they advocate the exit terminal 1/4" above the waterline? Current electical specs call for a copper strip below the waterline with a matching strip in the interior of the boat.

In looking at that trawler photo they have marked up, that's going to look like one weird contraption with the lightning protection installed!!

[GordMay]

Overhead shield wires at a U.S. Navy munitions storage site (left), and a Peruvian Mine Site (right).

Some light reading, which may help dispell some of the misconceptions about lightning, and lightning protection:

A CRITICAL REVIEW OF NONCONVENTIONAL APPROACHES TO LIGHTNING
PROTECTION
bY Martin A. UMAN and Vladimir A. Rakov
http://www.lightningsafety.com/nlsi_lhm/Uman_Rakov.pdf


Conventional and Un-conventional Lightning Air Terminals: An Overview
by Z.A. Hartono and I. Robiah
http://www.lightningsafetyalliance.c..._terminals.pdf

The Scientific Basis for Traditional Lightning Protection Systems
Report of The Committee on Atmospheric and Space Electricity
of The American Geophysical Union (June 2001)
http://www.lightningsafetyalliance.c...ific_basis.pdf

AMS Statement on Lightning Protection Systems
American Meteorological Society
http://www.lightningsafetyalliance.c..._statement.pdf

And many other publications from the Lightning Safety Alliance
|| Lightning Safety Alliance - Education ||

[c.spots]

I recommend reading these articles before wasting any time or money on an unproven lightning protection system.

In theory it sounds like a huge down conductor would be needed to convey the large current/voltage of a direct strike. However, the reality of over 250 years of experience suggests that such may not be the case. Many very successful down conductors were made of iron, which is a much poorer conductor than copper. And these where on structures much taller than our sailboat masts.

Even if the lightning system is destroyed during a strike (while protecting the structure as described), this is far better than antennas, electronics, or the boat.
What I did not find was a recommended wire size for a particular length. Any advice? Clearly bigger is better, but what would give a reasonable confidence level.

[GordMay]

Originally Posted by c.spots

In theory it sounds like a huge down conductor would be needed to convey the large current/voltage of a direct strike…
… What I did not find was a recommended wire size for a particular length. Any advice? Clearly bigger is better, but what would give a reasonable confidence level.

#6 AWG Cu. is the recommended minimum size conductor.

[Mike Banks]

Most of the lightning protection systems I have put in place related to outside electrical plant and buildings--but the principles are the same as any lighning situation--so I can give you my take on it.

First--the strike itself.
In the Land of Oz and the Southern States of the USA a full-on lightning strike would vapourise half-inch thick copper conductors. I have seen lightning vapourise telephone multiconductor cables--all that remained was a pink stain in the surrounding soil.

So how come lightning protection works as well as it does?

Because the idea as I understand it is to prevent the air from becoming ionised and providing a conductive path from the cloud to earth. If the conductor is working properly, as the air becomes charged because of the difference in potential between the earth and the charged particles in the cloud, this charge is "bled off" by the lightning collector and passed to ground before it can build up enough potential to fuilly ionise a path down which a strike can come--so the air in the vicinity of the collector has less charge density than elsewhere--therefore a higher resistance path than other areas where the air is more strongly ionised.

The streamers one sees rising from trees etc are the precursors to a discharge which may be severe--or may not develop into a severe strike or even a strike of any kind. The streamers may be in themselves be sufficient to safely discharge the air--but one can not count on them to do so all of the time or even any of the time.

A severe strike can occur when a discharge begins and then several charged areas of sky all unload their energy into the established discharge path. This can do real damage.

Now--to the wire in the plastic conduit idea.

It would be better than no protection at all and may work--but I think this idea of the pvc tube is only a way of preventing the copper wire from corroding the mast. Other than that it serves no protective purpose--and it would not matter if it got covered in salt. I would not even use copper if the mast was alloy--the mast itself can be quite effective and all it needs is a half-inch thick aluminium rod--sharpened to a point---or perhaps crowned with aluminium wires fanned out and tig welded to the top of it.

The rod can be clamped to the mast or even pass through it. The earthing plate in the ocean can be a large aluminium plate, pivoting down or lowered through a centrecase on deck, connected to the mast by flexible aluminium cables.

If you have wooden masts you may elect to use copper--then it must be fastened to the mast via a length of heavy copper cable and any jonts should be silver brazed or fused copper. If silver brazed the joint must be kept dry and free of salt otherwise severe corrosion might take place. For a wooden mast I would go all copper--and for a catamaran I would rig a copper-clad plywood centreboard type discharge plate exactly under the mast and able to be lowered to a depth of at least half a metre.

The only way you can protect expensive electrical and electronic equipment from a magnetic pulse or strong inductive field is to have it mounted in a ferrous cabinet. This can be painted or even galvanised--but as there is no field inside a ferrous container your precious gear will be safe--provided you disconnect any wires leading outside the cabinet before the storm hits.

By and large your lightning rod needs to extend a metre or so above the mast. As a rough rule, the zone of protection extends within a right cone with base angles of sixty degrees and as high as the collector. On a ketch you may need two of them.

The idea of not using the elecrical equipment earthing system is to prevent the equipment providing a shunt path for induced currents--and frying the circuits.

Lightning earths need to be separate and preferably directly beneath the masts if possible--but if not possible then two plates should be provided either side and below the chainplates, if you wish to continue sailing. I would suspend them like lee boards. In the case of an all copper system 1 or 2mm copper plate could be fastened on the outside of heavy plywood and connected to the collector by straps soldered or better still--silver brazed to the sheet copper before it is fixed to the epoxy-saturated plywood. Do not paint the copper, and use it only on the outside of the plywood, not both sides as in the catamaran single plate.

In the case of aluminium plate--the same applies but aluminium strapping can be used, tig welded to the plates and bolted to the conductor.

The idea of dangling a chain overboard or dropping a circular discharge plate over the side makes it impossible to keep sailing--and sometimes you need to get to heck into a safe anchorage if you can--or if you can not--then get well out into the deepest water you can reach. You need protection while thios is happening.

This is just something for you to comment on or think about. There are proprietary systems available--but they are not as cheap as offcut marine aluminium plate or copper water service metal bashed flat and recycled.

[GordMay]

Originally Posted by Mike Banks

”... I have seen lightning vapourise telephone multiconductor cables--all that remained was a pink stain in the surrounding soil.
So how come lightning protection works as well as it does?
Because the idea as I understand it is to prevent the air from becoming ionized and providing a conductive path from the cloud to earth ...”

Lightning rods, along with a good lightning protection system of grounded cable, only serve to divert lightning current safely to ground, should lightning strike.
There is no way to 'discharge' or 'drain' the charge from a storm with lightning rods, or with "charge transfer", “lightning elimination” or “lightning dissipation” devices.
The various theories based upon “Charge Dissipation” have been thoroughly de-bunked by the Engineering & Scientific community.

Originally Posted by Mike Banks

”... The only way you can protect expensive electrical and electronic equipment from a magnetic pulse or strong inductive field is to have it mounted in a ferrous cabinet ...”

I think you mean a Conductive Metal* cabinet (Farraday Cage)
* Non-Ferrous Conductive metals: aluminum, tin, copper, zinc, brass, silver, gold, and platinum are all conductive. Ferrous metals contain Iron, and are also Conductive.

[cal40john]

Originally Posted by GordMay

Lightning rods, along with a good lightning protection system of grounded cable, only serve to divert lightning current safely to ground, should lightning strike.
There is no way to 'discharge' or 'drain' the charge from a storm with lightning rods, or with "charge transfer", “lightning elimination” or “lightning dissipation” devices.
The various theories based upon “Charge Dissipation” have been thoroughly de-bunked by the Engineering & Scientific community.



I think you mean a Conductive Metal* cabinet (Farraday Cage)
* Non-Ferrous Conductive metals: aluminum, tin, copper, zinc, brass, silver, gold, and platinum are all conductive. Ferrous metals contain Iron, and are also Conductive.

Not that I know the answer, but if the magnetic field created is strong enough to create enough current flowing in your electronics to kill your electronics, then ferrous metal is required for protection.

John

[GordMay]

Originally Posted by cal40john

Not that I know the answer, but if the magnetic field created is strong enough to create enough current flowing in your electronics to kill your electronics, then ferrous metal is required for protection.
John

Nope, you don’t need ferrous metals, just conducive shielding material.
Search Faraday Cage, R.F. Shielding, Electromagnetic Shielding, etc.

[Alan Wheeler]

Hence the same principle of computer spares coming wraped in a plastic bag. The bag is a conductive plastic. Not Ferrouse metal.

[Mike Banks]

You are quite correct John. There is a difference between electrostatic fields and magnetic fields. Both can be a problem--and a ferrous metal container protects from both whereas a non-ferrous container protects only from the electroststic field--which is the one that can damage computer spares.

The killer for boat electrics is the magnetic surge--hence ferrous.

Lightning conductors do not bleed the clouds of their charge--only the air immediately surrounding the conductor and for a few metres at most. The idea is yto PREVENT a discharge. Yes it would be nice if it could divert a strike to earth--a lovely idea and one which is still offered as an explanation for how they work. I have seen streamers from an operating lightning conductor only once--and no severe strike occurred although this was a fairly tall building.

Go have a look at some improperly protected steel structures which have sustained a severe strike and the realisation dawns that a severe strike would never pass down half inch copper rod given the current squared times resistance factor for power consumption of several million amps.

A Faraday cage will only protect you from electrostatic fields, which is why lightning conductors on buildings are divided into as many walls as has the building as a minimum protection. More are better.

Lotsa luck

[Tropic Cat]

Originally Posted by Mike Banks

The killer for boat electrics is the magnetic surge--hence ferrous.

Mike,

I heartily disagree. The electronics killer is thousands of volts pulsing through mastwiring down into the boat. Anything connected to it is fried.

It's an expensive lesson... Ask me how I know...(sigh)

[Alan Wheeler]

whereas a non-ferrous container protects only from the electroststic field

Not true. It only has to be a conductor in a magnetic field. Anything that conducts. It dose not have to be ferrous. That is why copper and Aluminium can be used in transformer windings. A farraday cage is doing much the same thing as a transformer winding. The resistance in the conductive material acts like a winding and allows a Potential difference to be created because it has a resistance. But because the conductive material (what ever it is) is a continouse connection to itself, it simply shorts the current out.