lightning conductor

[meldrum]

I have a yacht with a wooden mast and I wish to fit a lightning conductor, what minimum cross sectional area cable do I require to run up the mast, or can I bond the mast track to the spike at one end and bond the other end of the track to a keel bolt.

 

[Salty John]

You need a conductor at least 6" above the mast, then a stranded copper wire of at least size 8 AWG (8.4mm squared) in which no strand is less than 17 AWG. Or a strip of copper at least 0.03" thick and having conductivity of no less than an 8 AWG copper wire.
You can connect to a metal keel or any underwater metal surface of at least 1 sq foot.
Route should be as direct as possible.
The above qualifies the mast as lightning-protective and gives a lightning protective zone around the boat equal to the height of the mast.

 

[oldharry]

<<The above qualifies the mast as lightning-protective and gives a lightning protective zone around the boat equal to the height of the mast.>>

"Lightning Conductors" per se are intended to be just that - a means of attracting the discharge to give it a direct and non-damaging path to ground thus creating a 'safe zone' in the vicinity. This is acheived by ensuring that the conductor produces a strong ground leader above any other ground point which may also be producing a leader so that the descending cloud leader always makes contact with the conductor first, thus creating a 'safe zone' around the conductor.

But it is highly undesireable for a boat to be fitted to attract lightning in the first place. It is not solidly enough built to absorb and disperse the heat and physical shock of a hefty discharge, and would almost certainly be damaged, destroyed or set on fire. The crew, feet away from the discharge would almost certainly be stunned or killed, electrics would be fried by the emf impulse, and the whole thing is to be avoided.

Lightning deterrents are arranged to to try and prevent ground leaders from forming in the first place. Most commercially available ones appear to have very questionable effectiveness, and many seem not to work at all on test!

Just consider a lightning bolt following a discharge cable through the boat at several million amps: the heat alone would blast a hole round the cable where it passes through the hull, quite apart from any secondary shock effects, sinking it fairly promptly. That at least would put out any fire that was started by the heat!

Not a good idea methinks.

But then how DO you protect yourself from lightning? Wish I knew, 'cos it gives the cold grues every time there is a distant rumble, and I look at 10m of metal mast sticking up above me in an otherwise empty area of sea!

 

[Salty John]

Quite right, you don't ever want to be hit by lightning. There is really nothing you can do about it and if you are hit, as I have been, you understand just what an overwheling force you are dealing with.
It makes you realise just how futile it is to stick bottle brushes at the masthead or hang chains from your shrouds. But, if it makes people think they are being proactive and feel less concerned when they are caught in lightning, I don't see any harm in that.
The best advice is, if you are caught in a lightning storm on passage, go below, don't touch any metal and disconnect electronics and put them in the oven.

 

[Danny]

[ QUOTE ]
Just consider a lightning bolt following a discharge cable through the boat at several million amps:

[/ QUOTE ]In the interests of accuracy I think a lightning strike is usually somewhere in the region of 10-20 thousand amps - but you still need a very good conductor to carry this much current for the brief instant of the strike.

 

[mikehibb]

When I went to first view the boat that I now own, there was a big electric storm in the area, passed over just one hour before we stepped aboard.

Whilst we were there we found out that a big Cat with a much higher mast than us (just 30 metres away from us) had recieved a direct hit. Brand new cat ready for new season, just finished commissioning. It blew the radome, auto-pilot and vhf to bits. I feel sorry for the owners, but at the same time thankfull that I own one of the smaller boats in the marina.

 

[oldharry]

Sorry to hear you have been hit -an OH 'worst case scenario'.

And I agree entirely with your advice, having read many accounts of people suffering strikes or near misses, and having been less than 30m from a tree (in the garden) when it was totalled by a lightning bolt.

One of the most amusing accounts I read by an American skipper who was quite badly jolted by a shock from the metal guardrails during a near miss, concluded that crew members should wear rubber boots and rubber gloves, and the male crew ..... well its a family forum /forums/images/graemlins/laugh.gif

The writer also suggested putting a yellow welly over the masthead to deter strikes - but agreed it might be dificult to persuade anyone to put it there in a thunderstorm!

But installing a conductor to attract lightning always struck me as going a bit too far... errrrm, should I rephrase that?

 

[William_H]

I tend to feel that Salty Johns dimensiojns are on the light side. I understood more like 3 square ft of metal contact with the sea and the 8 sqmm wire would be too light.

The problem with yachts is the aluminium mast, a very low resistance pole which should conduct a lot of current without damage. It is the remainder of the path from mast to water that needs a large conductor.

Hopefully the grounding cable will dissipate much or all of the current energy before it fuses into molten copper.

Now with a wooden mast if it were dry you might be better off not trying to attract the lightning however with a brass track up the mast you are back to the same as an Al mast.

What happens in a lightning strike is that a leader starts from the ground it is not very powerfull and can function through fairly high resistance (wet tree). This leader provides a conductive ionised path for the main current to come down.
When it meets any resistance the current will produce heat usually fusing the conductor with the fused part providing ionised air and metal to continue the current flow. Hence trees explode when the moisture boils.

The brass track will provide some conductivity and may be comparable in cross sectional area to a decent cable. It would probably be ruined in a strike but that may not matter so much. So I would suggest connecting the brass track to the keel or other in water metal. You may then consider adding another cable down the mast to help carry the current. But then you ask how much is enough? I think I would not add any more cable just hope for the best. If you want you can fit a conductive rod at the top to a point above the mast. I don't think I would.

Don't use stainless stay wires as conductors as the resistance will just fuse them quickly. Galvanised iron wire may be OK lower resistance)
In most boats there are electrics of some sort in or on the mast. This is connected to the sea via negative of engine. All this wiring will smoke unless it is adequately bypassed by a very heavy cable earthing the mast. (And then may still still get smoked)

The whole plan is to minimise damage to the crew and if possible minimise damage to the boat.
Lets hope it never happens olewill

 

[john_morris_uk]

[ QUOTE ]
<<The above qualifies the mast as lightning-protective and gives a lightning protective zone around the boat equal to the height of the mast.>>

"Lightning Conductors" per se are intended to be just that - a means of attracting the discharge to give it a direct and non-damaging path to ground thus creating a 'safe zone' in the vicinity. This is acheived by ensuring that the conductor produces a strong ground leader above any other ground point which may also be producing a leader so that the descending cloud leader always makes contact with the conductor first, thus creating a 'safe zone' around the conductor.

But it is highly undesireable for a boat to be fitted to attract lightning in the first place. It is not solidly enough built to absorb and disperse the heat and physical shock of a hefty discharge, and would almost certainly be damaged, destroyed or set on fire. The crew, feet away from the discharge would almost certainly be stunned or killed, electrics would be fried by the emf impulse, and the whole thing is to be avoided.

Lightning deterrents are arranged to to try and prevent ground leaders from forming in the first place. Most commercially available ones appear to have very questionable effectiveness, and many seem not to work at all on test!

Just consider a lightning bolt following a discharge cable through the boat at several million amps: the heat alone would blast a hole round the cable where it passes through the hull, quite apart from any secondary shock effects, sinking it fairly promptly. That at least would put out any fire that was started by the heat!

Not a good idea methinks.

But then how DO you protect yourself from lightning? Wish I knew, 'cos it gives the cold grues every time there is a distant rumble, and I look at 10m of metal mast sticking up above me in an otherwise empty area of sea!

[/ QUOTE ]I believe that your thinking is confused. Even a properly designed and installed lightning conductor IS NOT designed to safely conduct a direct hit from lightning. You are right when you talk about 'dissipating ground leaders...' but then you go on to talk about direct hits. The idea of a lightning conductor is NOT to conduct a full on lightning strike, but by dissipating the leaders prevent a full lightning strike occuring. A low resistance path to ground provides the protection. The articles I have read and heard all talk about this confusion.

 

[Salty John]

I quoted the American Boat and Yacht Council (ABYC) recommendations for wooden masts.

 

[Danny]

I think the truth is that lightning conductors should do both: they should hopefully prevent a strike by streaming off ions from the tip of the conductor. This happens because of the opposite change induced on the earthed conductor just before the strike. The tip(s) needs to have a small radius (sharp) to increase the charge density and therefore the ionisation around the tip. In the likely event that this doesn't work then the conductor should be of low enough resistance and thick enough that the thousands of amps in the strike don't vapourise it. Because there's a few million volts available, if the conductor vapourises (increasing the resistance), the amount of heat then generated is enormous and will likely do a lot of damage.

I hope that's fairly accurate - it's what I remember from school physics lessons!

 

[captainslarty]

Salty, that is way undersized. Solid copper bar is needed really.

8.4mm2 is simply a fuse. /forums/images/graemlins/blush.gif