..Lightning at Sea
- Don't be a target!
As incidence of lightning varies throughout the world, so logic dictates will the risk. However, while you may live in a region where lightning is relatively scarce, your sailing career may take you into areas of much higher risk. It is not unusual for the novice inshore sailor to become far more ambitious once the sailing bug takes hold.
By installing a lightning protection system , however, you will not be making your vessel lightning-proof, only lightning-protected. Lightning protection systems do not prevent lightning strikes. On the contrary, the aim is to attract the lightning and conduct it safely to ground (the sea), so that damage to the boat and the possibility of injuries or death is reduced.
So, while you may not be able to avoid a strike, you can take steps to minimise the dangers associated with one or from dangers that may occur from one in the vicinity of the vessel. Essentially, there are two main situations to be dealt with. The first is that of a direct strike. The second is that of a nearby strike creating an overvoltage situation.
In the case of a direct strike, there is, of course, a very large amount of energy trying to find a way to ground (the sea in this case). Most strikes occur in less than half a second and each charge contains up to 30 million volts at 100,000 amperes. In a complete discharge there can be a sequence of strokes following the same path and lasting up to one second or more. These high voltages can cause intense heating if they pass through a bad joint in a metal conductor or poor insulators, and can result in fire.
Usually, but not always, direct strikes will be taken by the highest fitting on the ship that is, relatively speaking, close to ground or earth potential. Often the most attractive path to ground is provided by the mast where there is one. Protection involves ensuring that the mast can conduct the lightning safely to ground and that it is more attractive than other parts of the vessel.
For the mast to provide a suitable air termination, it must be of sufficient height to provide a zone of protection for the whole vessel. It must also be made of metal or, if timber, have metal fittings such as a metal sail track and metal stays, to conduct the energy to ground.
If a mast is of insufficient height to give adequate protection, more than one air termination will be required. On larger ships, it is often wise to provide protection for key areas or the whole vessel. Where an area is to be protected, a number of metal rods can be placed around the upper perimeter of the area in question or in key locations like the main mast or highest point and at the bow and stern. Lightning protection experts can calculate what is required to ensure the safety of the vessel
Lightning rods can normally be bolted direct to metal superstructures, even when painted, as the bolt threads provide a suitable connection. It is wise to use a suitable corrosion resistant paste between the two surfaces and on the bolt connection. In the case of collapsible masts, the mast mount may not be a suitable conductor and earth strap (as below) used for the connection.
To make good connection with the deck or superstructure, welded lugs (or similar) should be used to attach the strap(s). Bolting the strap will facilitate replacement if it becomes necessary. You can, of course, use welded connections.
Earthing can be provided by the metal superstructure of a vessel or providing an earth connection to any metal surface that is normally under water. This includes metal hulls, earth plates, metal keels, propellers and metal rudder surfaces.
Because lightning tends to take the path of least impedance (resistance and inductance), it is essential that a low impedance path to ground is provided with no high impedance joints. Equally important, the conductor should follow as direct a route as is possible to earth, especially if it runs through the accommodation section of the vessel. Any bends in the conductor should have a minimum radius of 200mm (8 in). In the case of metal vessels, there should be a direct connection to the metal structure.
If the vessel construction is other than metal, it will be necessary to run the conductor, by the most direct route to a point of contact with the sea. Donít be tempted to run the lightning conductor in the cable duct or alongside any other cabling.
The Australian standard for lightning protection, AS1768, recommends 50sq mm. The most common material used is 25 x 3 (75sq mm) copper strap or aluminium T5 temper. This is normally available in strips and needs to be butt welded together to provide the required length and can be painted if required.
It is important to remember that the connectors must be able to carry as much electrical current as other components of the systems and all connections must be secure and non corrosive. Braided strap needs to be fully sealed to inhibit corrosion.
The strap should be welded to the deck. It may be necessary to make provision for a hole at the top end to fit the connection stud where a lightning rod is used. Preferably run the flat on the outside of the superstructure with no sharp bends, and well away from any other metal objects. If the boat has a lead keel it is best to bolt it on in some place that is reasonably dry and it can be inspected from time to time. There are a number of brackets available to secure this strap onto rods, wire and the like. HF antenna system ground plates should not be connected to this earth system.
Another consideration when installing the strap is side flashes. Where there is the possibility of a permanently installed metal object providing an alternative route to ground or bridging out a substantial length of the down conductor, it is recommended that they be electrically interconnected with the down conductor, unless their correct operation is adversely affected by grounding.
Smoothing sharp points makes metal objects at risk less attractive to the lightning leader. These factors become more important where the chosen path to ground is not sufficiently of low impedance.
Of course, when designing your lightning protection system, it is also important to make sure that in bonding dissimilar metals and electrical equipment that you are not inviting electrolysis to occur and, once you have your lightning protection systems, it is a good idea to carry out a yearly inspection of all bolted connections, lightning rods, etc.
The importance of a well designed and installed earth system cannot be over stressed. Poor connections and neglecting to protect equipment that can offer an alternative path to ground may negate all your efforts to provide protection.
When it comes to communications equipment, the antenna is an obviously target for lightning attack. Therefore it is best erected about 1 metre below the chosen point of air termination.
This is placed directly in the feed line to the antenna from the ATU, with one side connected directly to ground.
However, because very large instantaneous voltages and currents flow during a direct lightning strike, only a very good lightning rod installation will survive. More often damage to equipment is caused by voltage or transient currents in antennas and mains wiring and by static charges rather than by direct strikes, as electrical and magnetic fields generated from lightning kilometres away can be induced into power lines and cause equipment damage. For this reason all radio and navigation equipment with exposed transducers (radar, wind speed/direction indicators, etc.) require protection.
The suppressor is placed in the 50 ohm coaxial feeder, preferably close to the receiver or a grounding position, between the ATU and the HF transceiver.
Where there is an internal risk from over voltage from spikes, etc., such as on coaxial feeds to computers, monitors, coaxial surge suppressors should also be fitted.. With some TV distribution systems there may be a personnel risk from electrocution via faulty TV sets feeding AC mains back into the distribution network unless line isolators are built in, as in the Moonraker TV Distribution System.
If your vessel has suffered a lightning strike, you will need to check compasses and navigation instruments for calibration and inspect protective coatings on steel hulls and fibreglass sheathing over ballast keels for damage. All standard and running rigging and associated fittings also need to be carefully checked.
When you find yourself in a lightning storm, there are a number of obvious precautions you can take, although faced with the real thing things often get very interesting in a hurry!: