With our increasing reliance on electronics for navigation, communication and general operation of our boats, lightning is a subject of rather deep concern. In addition to the potential immediate dangers–fire, holes blown through the hull, crew injury–we are now, in the aftermath of a lightning strike, left with a boat that may have no power, no navigation equipment and no means of propulsion.
Today, then, we’ll give some thought to how lightning interacts with a boat and its equipment, and what we can do to mitigate the damage if it does hit.
Anatomy of a Strike
It’s important to note that, while we have a reasonably good idea of how lightning works, our understanding is far from complete and no one has yet developed a reliable predictive model for its behaviour. Here’s a (very) short summary of what we do know about cloud-to-ground strikes, the kind of lightning that we’re worried about when we’re caught out in a storm.
Inside the storm cloud, electric charges become separated—negative near the bottom, and positive at higher altitudes. (The exact mechanism by which this happens is still unclear.) The potential involved is a few tens of millions of volts to about a hundred million volts—impressive, but no more than a few percent of what would be needed to create a spark through several kilometres of air.
Now and then, a bunch of electrons are repelled from the bottom of the cloud, forming a “stepped leader”. These first electrons act as scouts, heating and ionizing the air (and therefore rendering it conductive) as they find their way downward in jumps of about 60 metres at a time. There are plenty of electrons waiting in the cloud to follow the newly ionized trail, keeping it hot and conductive as well as keeping the step leader charged.
At the same time, the charge separation in the cloud has induced a charge separation in the surface, with electrons being pushed down into the Earth, leaving the surface (and, in particular, tall objects) positively charged. Positive “streamers” start to form around convenient attachment points–the preferred ones being high places with sharp tips. (The classic spiked lightning rod is designed to create a very strong streamer.)
When a stepped leader meets a streamer, we have an ionized (therefore, conductive) channel through the air, and the return stroke begins. Propagating upward almost instantly, the return stroke carries tens of thousands of amps across the cloud-to-ground potential, which is many millions of volts. In most cloud-to-ground strikes, electrons are emitted by the cloud towards the surface; using standard sign conventions, the direction of (positive) current flow is from surface to cloud.
Prevention (Maybe) or Amelioration
From this, we can infer two possible defences in our anti-lightning strategy:
- Force a streamer to form at a specific point that we control; this is what a conventional lightning rod does.
- Prevent the streamers from attaching to your boat. This is the idea behind lightning dissipators. If, at what should be the most convenient attachment point for a streamer (the top of the mast), we prevent the electric field from concentrating at a point, streamers–and therefore lightning strikes–should be less likely to form there.
This would be a good time to explore air terminals–the chunk of metal sticking out the top of the mast–in more detail.
The traditional air terminal, “officially” invented by Ben Franklin in 1749 but likely several thousand years older than that, is the lightning rod. It is shaped to concentrate the electric field at a pointed tip, forcing a very strong streamer to form at that point. This streamer is so much more intense than the ones given off by other nearby objects that the stepped leader will almost always choose it, ensuring that the lightning strike is safely conducted through a heavy copper cable between the lightning rod and the sea.
Once we learned how to calculate electric fields around objects, various lightning dissipators were invented. These try to do the exact opposite of a lightning rod. They spread out the electric field, allowing many small currents to flow and dissipate electric charge without producing a streamer. In effect, they try to render the object they’re attached to invisible to the stepped leader, thereby preventing the lightning from striking. Obviously, a lightning dissipator on the mast head will only protect the boat if there are no sharp pointy metal things sticking up that will generate a streamer anyway.
All air terminals need a grounding system that can safely carry the full current of a lightning bolt if they are hit. The choice of which type to fit is up to you. One class says “Hey lightning, go elsewhere, you can’t see me” (it can still be hit, but might at least reduce the probability of a strike). The other class is designed to yell “Here I am, great target, hit me!” and will almost always succeed in that goal.
Path to Ground
The combination of enormous voltage and enormous current leads to some interesting electrical effects that, in more mundane circumstances, we tend to ignore.
In an ordinary electric circuit, the physical layout of the wiring is largely irrelevant; a wire that takes a straight path is pretty much equivalent to one that meanders around like a drugged-up snake. What makes this approximation work is that ordinary wiring has very little voltage drop along its length; any two points on the wire are at approximately the same potential.
That’s not the case in a lightning strike. When we are talking about a lightning discharge of kiloamperes and megavolts, it’s quite possible to have a gradient on the order of a hundred thousand volts per metre in the conductors carrying that current.
If the voltage were provided by an ordinary high-voltage low-current source (like a sheep fence charger) the source would almost instantly short to nearly zero volts. A storm cloud, though, is not so easily depleted, and the high gradient is sustained as fifty thousand amps flow through the boat.
At a hundred thousand volts per metre, a right-angle turn in a wire is a real obstacle. Electrons in one leg of the right angle will feel the much lower voltage in the other leg, and—if conditions permit—some of them might find that jumping through the air is easier than sharing the congested route inside the cable. Once a few electrons try it, the air along their path starts to ionize, and now you have a nice, conductive plasma arc that all that current will find awfully appealing.
Fibreglass, wood, plastic and other insulating materials mean little to a lightning discharge. What’s a thousand or two volts to break down a few millimetres of insulation when you’re a fifty-kiloamp plasma arc with millions of volts to spare? (Pressurized sulphur hexafluoride might stay insulating even under all this, but you really don’t want that stuff in your bilge.)
Lightning wants to take the most direct route to ground, and it packs the punch to make its own way if it has to. Your 4-gauge copper cable will politely ask it “Please consider going this way”, and if the cable follows a nice easy path, the lightning will usually follow it. If you don’t give it a clean, straight route to the sea, though, it’ll take the straight route anyway–even if that means blowing a hole through the mast step in the process.
(Note that if your mast isn’t metal, you’ll likely need a 4-gauge grounding cable running all the way up it.)
So, here’s our next line of defence: Give the lightning a straight, low-resistance path to ground.
Fields and Induced Voltages
The direct discharge is the most dramatic damage mechanism, but it’s not the only one.
There’s an enormous amount of electric charge in a lightning bolt; all that charge creates a strong electric field. That field, in turn, induces electric fields in all sorts of nearby objects, some of them strong enough to generate arcing currents between metal fixtures if the potential between the objects isn’t equalized by bonding them to a common ground.
A large, rapidly changing current also creates a strong magnetic field. The changing magnetic field, in turn, induces a voltage in any conductor that crosses it. Induced voltages are proportional to the rate of change of the magnetic field, which is itself proportional to the rate of change of the current that’s creating that magnetic field.
The rate of change of the AC current that drives your isolation transformer is on the order of a thousand amps per second. The rate of change of current in a lightning bolt is more like fifty million amps per second. Without the multiple windings and iron core of the transformer, the coupling is much less efficient, but it’s still quite adequate to induce a few hundred to a few thousand volts in nearby conductors.
Those conductors include not only the power wires to every device on the ship, but also the data cables, antenna cables, even the traces on the circuit boards.
Thus, we come to the result we all know so well: Any and all devices that include sensitive electronic components are, even if the direct lightning strike misses them, likely to be fried by induced voltages. In addition, any ungrounded metal objects could become part of “side flashes” that arc across the boat.
Our next major rule of lightning protection is: Bond all metal fixtures of appreciable size to a common ground. This helps to discourage side flashes, and is called for by every marine electrical code I know of. That won’t help the electronics, though.
How do we reduce the odds of frying the electronics?
- Surge protectors can help a bit, if they’re very close to the power inlet terminals of each device, but they offer no guarantee of protection.
- Keep all wiring as short as possible, since induced voltage is proportional to length. It’s also not a bad idea (thanks Erik de Jong) to unplug cables from expensive devices when you’re away or when the risk of a strike is high.
- Keep each positive/negative wire pair as close together as possible (ideally, bundled or twisted together) so that any induced voltage is the same on both wires. Electronics are generally more sensitive to the voltage difference between the wires than to the absolute voltage relative to ground.
- Avoid running wires near, or parallel to, bonding/grounding cables. The closer something is to the lightning current, the stronger the fields will be.
Even so, expect at least some of your electrical systems to die in a strike.
If you solve Maxwell’s equations for the inside of a closed conductor that is being bombarded by external electric and magnetic fields, you get zero.
This somewhat odd result gives us a beautiful, elegant defence against lightning: the Faraday cage, essentially just a closed, grounded metal box. Anything inside it won’t feel the fields induced by the lightning. For this to work, the entire surface of the box should be a continuous conductor, unbroken by insulating materials. A gasketed ammunition box won’t do (the lid is electrically isolated from the box by the gasket). A microwave oven can be pretty good – but, as some readers have pointed out, many (or most) recent microwaves will still allow some radio-frequency EM fields to enter. A simple closed box made from aluminum or copper sheet metal is darned close to perfect.
We can, therefore, hide some backup gear—a handheld GPS, a tablet computer loaded with charts, a portable VHF, an autopilot head—in a Faraday cage, and have at least a fighting chance that it will be usable after a lightning strike.
Our lightning defence strategy now consists of:
- Good grounding from mast head to sea, with lightning dissipators up high to try to discourage streamers from forming on the mast.
- A straight, low-resistance path to ground for any lightning that does strike. Conveniently, this can be the same bonding system used to equalize voltage in the previous point.
- Electrical bonding from all metal fixtures to a common ground point, which helps neutralize the potential differences that create side flashes. Conveniently, one bonding system can perform both this role and the corrosion protection role.
- Backup electronics stashed in a Faraday cage so that they’ll have a chance of surviving a strike.