How to test lightening strike ground plate?

[Tim Good]

Short question: I have a grounding plate in the hull of my boat. I don't exactly understand it's function and wiring other than that it is design to help in the instance of a lightening strike. Could some explain to me where it is meant to be wired to and how it might check its function with a multimeter perhaps?

More info: Reason is that I've never seen it as much of a risk frankly but I've just met a couple who bought a new boat and in the first 6 months got struck. It destroyed almost all the electrics and cables on their boat. Now they have a new production boat and it doesn't look particularly complex.

However mine is incredibly electrically complex and the cabling is routed deep into the heart of the boat in areas that would be near impossible to re wire. The thought of it being replaced would be almost impossible.

 

[KellysEye]

I don't know about the grounding plate but if lighting is heading your way remove all electrical equipment and put it in the oven even with the glass front it is a faraday cage.

 

[Buck Turgidson]

Unfortunately your grounding plate isn't going to guarantee anything. The idea is it creates a path of least resistance to ground so it should be bonded through the standing rigging to the mast. This type of bonding proved ineffective on aircraft many many years ago which is why they are all now built as faraday cages and the current flows around rather than through. To do this in a composite structure requires a copper mesh just below the outer lamination. Quite an expensive refit if you want to do it properly.

 

[Tim Good]

I don't know about the grounding plate but if lighting is heading your way remove all electrical equipment and put it in the oven even with the glass front it is a faraday cage.

I appreciate your experience on many things but seriously. "Take all electrical equipment and put it in the oven"? I don't have a 10x10ft oven.

 

[sarabande]

I am not sure that Kelly is entirely correct about the "even with a glass front" bit. Ovens are not designed to cope with the sideflash current.

http://hollandshielding.com/135-Faraday cages EMI RFI shielded tents r ooms%20and%20shielded%20enclosures

I wonder if your grounding plate is a relic from some past radio comms equipment ? A full grounding plate for lightning protection would have to be large, and connected to a sort of ring fence round the boat, to collect the strike 's path wherever it went. Otherwise a sink, or a mast heel, might act as the grounding route directly through the side of the boat to the waterline.

 

[Tim Good]

I wonder if your grounding plate is a relic from some past radio comms equipment ? A full grounding plate for lightning protection would have to be large, and connected to a sort of ring fence round the boat, to collect the strike 's path wherever it went. Otherwise a sink, or a mast heel, might act as the grounding route directly through the side of the boat to the waterline.

You could be right. The first owner did installed an insulated backstay, Inmarsat terminal and some sort of mobile booster.. Frankly I have no idea about that nor SSB so you may well be right!

 

[BabaYaga]

Regarding the idea of connecting standing rigging/mast to a grounding plate for lightning protection: From what I have read it is not a good idea to 'invite' the lightning bolt into the boat. This is because there is no guarantee it will exit the hull by the plate, it might well exit elsewhere, creating a big hole.
Better to try leading it outside the hull, that is by copper wires connecting the mast/rigging into the water.

 

[lw395]

Regarding the idea of connecting standing rigging/mast to a grounding plate for lightning protection: From what I have read it is not a good idea to 'invite' the lightning bolt into the boat. This is because there is no guarantee it will exit the hull by the plate, it might well exit elsewhere, creating a big hole.
Better to try leading it outside the hull, that is by copper wires connecting the mast/rigging into the water.

I think that is a fair summary.
Lightning conductors tend to be outside buildings.
Electric charge stays on the outside of objects, I believe the best bet is to trail a bit of chain over the side from shrouds and maybe forestay and backstay.

I also belive there is a random element, whatever yo udo, you may be lucky or unlucky.

 

[geem]

Having recently done some research on lightning conductors on boats the requirement seems to be multiple ground plates just below the waterline all connected to the base of the mast. Almost certain yours is an SSB ground plate.
We cruised, Florida, Bahamas and Panama this season I can confirm that lightning in these places is a serious issue unlike the occasional thunder storm we experience in UK/Europe. We spoke to several people who had been struck in Panama. They had full electrical refits at great expense.
As a precaution during lightning we put electrical items in the oven. We also put spare batteries, handheld vhf and gps in the safe as it is a faraday cage. What wont fit goes in the engine room which is aluminium mesh lined. I am dubious about this as a faraday cage as it has numerous cable penetrations but its seemed better to put laptops and ipads on the bench in the engine room rather than leave them exposed in the saloon

 

[Venus1]

What the OP describes is almost certainly a ground plate for HF radio

 

[William_H]

Excuse me but I have to try this line out."If BS was bitumen this could be a 6 lane highway" Please don't take offence.
However. I think you should avoid any connection to standing rigging as a current path for a discharge. Stainless Steel has a high relative resistance so will get very hot with not so much current and fuse or lose hardness. Ground the base of the ali mast. This has very low electrical resistance so can better carry a lot of current. It is the path from the base of the mast to sea that is the really destructive path. Hence the idea of a heavy cable from base of mast to the sea. A metal keel or grounding plate might be the best bet. Failing those chain from mast base to sea might help bypass some of the current which will other wise travel via wet GRP or wiring to sea ground. (engine prop etc).
Now someone said don't invite a strike. With an ali mast you can not avoid inviting a strike.
There are pointed or brush type projections to fit to the highest point which actually invite a discharge earlier so may dissipate a discharge before it builds up so much.
Regarding electronics damage. First danger as said before is current flowing in antenna cable and mast head wiring through to sea. Remember this can jump largish distances to complete a path. Especially if current starts then conductor melts inviting an arc.
Second danger is the huge current even if it is dissipated without too much damage creates a rapidly rising magnetic field. So any other wiring especially parallel to the main current path will have a voltage induced in it. Like a transformer or radio transmitting and receiving. Putting radios into an oven may see the metal of the oven act as a receiving antenna and shield the gear from this pulse of electrostatic and magnetic power. A little bit like a faraday shield. A steel oven would be even better being able to bypass magnetic field. I think the first thing and most effective is to disconnect all external wiring especially antenna connector and power.
Re Buck Turgidson's comment re aircraft. Regular all aluminium aircraft have never usually had a problem with lightning strikes. The ali conducts the current from nose to tail or wing tip to wind tip easily. I have inspected a B727 which was struck with just a burnt rivet above the cockpit and another slight burn near the tail after a strike. Indeed I did read that in US airliners get struck aprox. once per year. Then they put a GRP radome on the front and had to fit heavy ali strips from nose to fuselage to conduct lightning current.
When aircraft were made with large areas of Carbon fibre it became a real problem. The carbon is a poor conductor but does conduct such that any current through it would over heat quickly. Hence the conductive screen in the resin on the outside. All large aircraft in design testing stage are tested for lightning strike. A bit scary looking for lightning I imagine but not so hard in USA.
So to sum up lightning initiates from the ground (forget aircraft here) with a leader which can flow through relatively high resistance up top the area of high static build up. This leader provides an ionised air path for the real current to strike down. It is of such current and potential voltage that not much stops it.
regards from ol'will who may be just paving our 6 lane highway.

 

[lw395]

What the OP describes is almost certainly a ground plate for HF radio

Indeed.

 

[Tim Good]

What the OP describes is almost certainly a ground plate for HF radio

Well since I'm likely to be half way setup for a HF/SSB in terms of a ground plate and backstay then maybe I should consider getting one.

 

[Old Bumbulum]

Presumably the effects of a lightening plate could be weighed?

 

[coopec]

I did a lot of research into lightning protection a few years ago but I think scientists have refined their theories since then.

Apparently lightning likes sharp edges so I have X4 ss strips approx 1.5 M bolted to the hull below the waterline.

The mast is attached to the main strip and the rigging is also earthed to the strips. The motor, steering, sheet winches, stanchions and fuel tanks will be earthed also.

Cargo ships may be creating lightning at sea
http://www.sciencemag.org/news/2017/09/cargo-ships-may-be-creating-lightning-sea

Lessons in lightning - Ocean Navigator - May/June 2013
www.oceannavigator.com › May-June 2013
Apr 24, 2013 - A voyaging boat can expect one lifetime lightning hit, but the chance increases dramatically in hot, humid places, like the Intertropical ...
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[DownWest]

I looked into this on buildings about 30 yrs ago. The bits I read indicated that a a well grounded rod on the highest part of the building, esp with several spikes at the top, tended to creat an ionised dome over the building, so the lightning went 'next door'. Since the building was an old windmill on the top of a hill and had been struck twice with spectacular results*, I wanted to get it right. Put the spike on the top with a thick copper bonding down the outside to a deep spike in the ground. As far as I know, there have been no further strikes.
While the OP almost certainly has a ground plate for SSB, some sort of earthing outside the hull from the bottom of the mast could creat the ionised dome that keeps the boat clear. Maybe a thick copper strip to the toe rail and something to chuck over when worried.

* One strike blew the top off the mill. The other ran around the electrics and copper pipes, blowing all the sockets out of the walls and all the floor tiles over the pipes in the floor. Quite a mess.