Electrostatic protection

[lw395]

"Some people advocate trailing a stout wire or even chain from the shrouds to the sea. It probably needs to be stout though. This hopefully takes any discharge straight past your electronics keeping electric fields in the boat low."

It would be far better to connect wire or chain to the (ali) mast than rigging wire. The rigging wire is of such a electrical resistance that it will conduct some current but get very hot either melting or losing its temper. So subsequently likely to fail under strain. The mast on the other hand is very low electrical resistance so hopefully will dissipate power unscathed.
....

The current in a lightning strike doesn't really follow Ohm's law. It will probably favour travelling through the shrouds as they appear to be 'outside' the mast.
If it does go through the mast, what's going to happen at deck level?

 

[AntarcticPilot]

The current in a lightning strike doesn't really follow Ohm's law. It will probably favour travelling through the shrouds as they appear to be 'outside' the mast.
If it does go through the mast, what's going to happen at deck level?

That's why the chain from the mast to the sea is a good idea - to provide an "easy" path for the current to take. Personally, I'd try and wrap the chain round both mast and the base of the shrouds; as JumbleDuck says, it is likely that the charge will "prefer" the shrouds. On my boat, though, I suspect that the "easy" path is down the mast, down the (iron tube) mast support pillar and into the iron keel. Little things like less than an inch of fibreglass breaking the path aren't likely to be noticed by a lightning strike!

I've seen it reported that on boats without an "easy" path to the sea, the hull below the chain plates has been seriously damaged by a lightning strike, as the charge jumps from chain plate to the sea.

 

[JumbleDuck]

It would be far better to connect wire or chain to the (ali) mast than rigging wire. The rigging wire is of such a electrical resistance that it will conduct some current but get very hot either melting or losing its temper. So subsequently likely to fail under strain. The mast on the other hand is very low electrical resistance so hopefully will dissipate power unscathed.

A modestly sized C211 Selden mast section has a mass of 5.34kg/m, which suggests a cross-sectional area of 2 x 10^-3m³, which in turn suggests an electrical resistance of 2.65 x 10^-8/2 x 10^-3m³ = 1.32 x 10^-5 ohms/metre.

A typical lighting strike is 30kA, so the I²R Ohmic heating would be 30,000² x 1.32 x 10^-5 = 12kW/m, which with a specific heat capacity of 0.91 kJ/kgK would give a temperature rise of just 2.5^oC per second.

Golly. Less than I expected, and so I agree with you - a mast ought ought to be able to take the current. As AntarcticPilot says, though, you need to be able to get it out of the mast and into the sea as well.

 

[solent clown]

I have a souvenir from three decades ago from Garmisch Partenkirchen, up on the hills, and the mountain there are lots of crosses with little roofs on them. Many have lightening conductors, solid aluminium rod about 13mm thick on them, We cam across one that had taken a hit, and the metal was a twisted series of tortured looking bits on the ground. We each nicked a bit as hiking up a hill with a ton of scuba gear, climbing equipment, and provisions was clearly not enough weight. I still have it somewhere.

 

[halcyon]

Golly. Less than I expected, and so I agree with you - a mast ought ought to be able to take the current. As AntarcticPilot says, though, you need to be able to get it out of the mast and into the sea as well.

When I bought or current boat, it has a grounding plate on the hull that was connected to the mast, it also served as radio grounding.

Brian

 

[duncan99210]

On my boat there's a hefty (think bigger than 50sq mm) cable running from the mast base to the keel bolts to provide an exit path for a lightning strike.
I do think that there's not much that can be done to protect electronics from a strike. Certainly switching them off has little or no effect: a friend of ours had his HR on the hard when it took a strike. The boat electrics were all switched off but the strike took out every item of electronic and electrical equipment on board. Much of the wiring loom was also damaged, along with some of the through hull fittings which became the escape route for some of the energy from the strike.
Whilst some of the damage might have been amplified by the lack of an escape route for the strike because of being on the hard, another acquaintance has twice lost all his electronics to lightening strikes whilst afloat.
This all suggests to me that whilst there might be some mileage in sticking handheld electronics in the oven, there's not much to be done for the rest of the boat.

 

[Lomax]

For protecting electronics from surges caused by nearby lightning strikes a gas discharge device can be used: http://www.l-com.com/surge-protector These would need to be mounted on a (copper) ground plane with a low resistance connection to the sea - and they are unlikely to save anything from a direct strike. Amateur radio operators protect their gear this way, see for example Lightning Protection for the
Amateur Radio Station (PDF).

 

[jiris]

A modestly sized C211 Selden mast section has a mass of 5.34kg/m, which suggests a cross-sectional area of 2 x 10^-3m³, which in turn suggests an electrical resistance of 2.65 x 10^-8/2 x 10^-3m³ = 1.32 x 10^-5 ohms/metre.

A typical lighting strike is 30kA, so the I²R Ohmic heating would be 30,000² x 1.32 x 10^-5 = 12kW/m, which with a specific heat capacity of 0.91 kJ/kgK would give a temperature rise of just 2.5^oC per second.

Golly. Less than I expected, and so I agree with you - a mast ought ought to be able to take the current. As AntarcticPilot says, though, you need to be able to get it out of the mast and into the sea as well.

Sorry, you maths is correct, but applied to a wrong case . As mentioned before, lighting strike doesn't quite follow basic Ohm's law. Because of its short duration its behaviour has to assessed with high frequencies behaviour in mind. Skin Effect (the tendency of current to travel only on the surface of a conductor) would be one of the most important ones and it is the one responsible for most of the damage to masts and rigging - and conversely, for some otherwise inexplicable survivals of people directly struck by lightning.

 

[William_H]

Sorry, you maths is correct, but applied to a wrong case . As mentioned before, lighting strike doesn't quite follow basic Ohm's law. Because of its short duration its behaviour has to assessed with high frequencies behaviour in mind. Skin Effect (the tendency of current to travel only on the surface of a conductor) would be one of the most important ones and it is the one responsible for most of the damage to masts and rigging - and conversely, for some otherwise inexplicable survivals of people directly struck by lightning.

I still reckon that ohms law is your best guide to trying to deal with lightning. Agreed lightning can take some peculiar paths. However if we look at tree damage it is the core of the tree which takes the current and explodes with boiling of the moisture right through not just on the surface. Likewise churches seem to be protected by low resistance copper strap from top to ground. So I would rather be under a well grounded (to sea) ali mast. Certainly not under any conductor with a bit of resistance like ss wire. olewill

 

[JumbleDuck]

Sorry, you maths is correct, but applied to a wrong case . As mentioned before, lighting strike doesn't quite follow basic Ohm's law. Because of its short duration its behaviour has to assessed with high frequencies behaviour in mind. Skin Effect (the tendency of current to travel only on the surface of a conductor) would be one of the most important ones and it is the one responsible for most of the damage to masts and rigging - and conversely, for some otherwise inexplicable survivals of people directly struck by lightning.

Sorry, I'm not convinced. If you look at the return strike current profiles in http://www.lightning.ece.ufl.edu/PDF/Electrostatics/Pavanello_et_al_2007.pdf you'll see some fairly rapid initial transient effects, but plenty of time for a quasi-DC flow to become established.

Anyway, a hollow mast is a skin!

 

[jiris]

Sorry, I'm not convinced. If you look at the return strike current profiles in http://www.lightning.ece.ufl.edu/PDF/Electrostatics/Pavanello_et_al_2007.pdf you'll see some fairly rapid initial transient effects, but plenty of time for a quasi-DC flow to become established.

Anyway, a hollow mast is a skin!

O.K., I don't want to get into some semantics or nite-picking. I didn't say the current in a lightning bolt is governed ONLY by Skin Effect. No question, there is a lot of DC component in it as well, what may account for the split trees etc mentioned in other posts. But the HF effects definitely play a major part in the behaviour of the lightning bolt, for which probably the most scientific description would be "totally unpredictable" ). As for using the cross section of a hollow mast (a "skin", as you say) in your calculations consider this: "At 60 Hz in copper, the skin depth is about 8.5 mm. At high frequencies the skin depth becomes much smaller." (Wikipedia). An average duration of a lightning bolt is about 30 microseconds - what pushes the frequency of our quasy-AC component somewhere in the region of MHz. Without searching for the exact formula (which would be returning precise numbers based on imprecise data) it is obvious, the thickness of the "skin" in this case will be in fractions of a mm. So, I still believe using the total cross section of the mast material leads to grossly distorted results.
Your calculations prove (correctly) that masts shouldn't (a long way) fail due to lighting strike. The fact they do indicates something fishy is going on ;-). I believe the anatomy of the failure is as follows: a huge current passes through a very small cross section of a metal (the very surface of the mast) causing high and sudden local overheating of it (everybody is familiar with scorch marks on metal parts struck by lighting, I guess). The underlying cold metal doesn't take it kindly and develops cracks that quickly spread through the whole thickness of the material. And there we go...

 

[JumbleDuck]

O.K., I don't want to get into some semantics or nite-picking. I didn't say the current in a lightning bolt is governed ONLY by Skin Effect. No question, there is a lot of DC component in it as well, what may account for the split trees etc mentioned in other posts. But the HF effects definitely play a major part in the behaviour of the lightning bolt, for which probably the most scientific description would be "totally unpredictable" ).

It's sufficiently interesting that I may try to dig up some of the literature on lightning bolts. Do normal lightning conductors, which have a much smaller cross-sectional area that masts, normally survive a strike?

 

[lw395]

If you have rise times putting the most significant components in the MHz, then those wavelenghts are possibly going to push the current into the shrouds rather than the mast.
There are a lot of HF aerials about where a web of a few wires behaves like a solid shape, so long as you are talking about currents travelling in the direction of the wires.

Also basic lightning conductor theory is about the presence of the conductor affecting the voltage gradient in the air. This dictates where the air ionises. By the time big currents start flowing, all the decisions have been made.

 

[jiris]

It's sufficiently interesting that I may try to dig up some of the literature on lightning bolts. Do normal lightning conductors, which have a much smaller cross-sectional area that masts, normally survive a strike?

I don't have any experience specifically with lightning conductors, but I was responsible for the technical aspects of running a power station for some time. We had a few lightning strikes in the overhead distribution lines during the time and I think it is a very similar set of circumstances. No damage to the lines caused in any of these cases - could be due to action of lightning arresters too, but the current had to get to it first through the line. True, all the protection circuitry went bananas and disconnected everything each time - but I don't believe it changed anything on the outcome - it was just closing gates after the horses bolted. I never studied the effects of lighting in details, all what I am saying here comes from my general education and experience in related fields, but it is certainly a fascinating subject .

 

[JumbleDuck]

Also basic lightning conductor theory is about the presence of the conductor affecting the voltage gradient in the air. This dictates where the air ionises. By the time big currents start flowing, all the decisions have been made.

I think - perhaps I'm wrong - that one of the purposes of a lightning conductor on a building is to dissipate charge from the top before the initial upwards stroke can happen. That's the small one, which ionises a path for its big brother to come down.

No damage to the lines caused in any of these cases - could be due to action of lightning arresters too, but the current had to get to it first through the line. True, all the protection circuitry went bananas and disconnected everything each time - but I don't believe it changed anything on the outcome - it was just closing gates after the horses bolted.

Thanks - that's very interesting. I started researching superconducting fault-current limiters for power stations at one time, but got distracted - the way you do - and ended up studying non-linear diffusion (heat flow in the superconductors) instead.

 

[jiris]

I think - perhaps I'm wrong - that one of the purposes of a lightning conductor on a building is to dissipate charge from the top before the initial upwards stroke can happen. That's the small one, which ionises a path for its big brother to come down.



Thanks - that's very interesting. I started researching superconducting fault-current limiters for power stations at one time, but got distracted - the way you do - and ended up studying non-linear diffusion (heat flow in the superconductors) instead.

You are 100% right about the prime purpose of lighting conductors on structures, that's why the tips are spiky and not spherical. But they do, obviously, get hit sometimes too. And, as I said before, I don't know what happens in these cases. But my experience with powerlines gives some indication. The cross section of those I was talking about wasn't as big as you may think. The distribution from our 10 MW baby was protected by a measly 50 A overload.
BTW; After reading you posts, I did have the feeling you have something to with heat on a professional level ;-).