When lightning strikes

Christopher Murman | VOLUME 24, ISSUE 5

Lightning is one of nature's most powerful forces. Although a thunderstorm may only last for an hour or so, its enormous power is something to be feared.

There are more than 2000 thunderstorms in progress in any given hour throughout the world. And of course, Australia certainly has its fair share of activity; thunderstorms are common as far south as Melbourne and as far north as Darwin, so all vessels should take precautions.

Many people wrongly believe that only yachts, with their tall masts, are at risk, but the truth is any vessel out on the water is vulnerable due to the simple fact that they will be the highest point for some considerable distance, even in a harbour.

PROTECTING YOUR VESSEL

The most up-to-date research has confirmed that installing a grounding system is the best way to protect a vessel. The grounding system, which must be designed to provide a very low resistance path to ground (see Figure 1), is made up of a number of components: an air terminal, a down conductor and an external grounding plate.

Air terminal

The air terminal should be made of copper rod of between 10mm and 19mm in diameter. Some research suggests that a rod of 16mm diameter is optimal. The rod should be installed so that it’s at least 150mm above all other objects on the boat. On a yacht, this is typically at the top of the mast. On a power boat, a mast structure of some sort is required (see Figure 4).

Here’s how it works: positive electrons from the surface of the ocean are attracted to the negatively-charged electrons that accumulate in the bottom of the storm clouds, so they accumulate at the tip of the air terminal. The accumulating positive electrons naturally repel each other, pushing some of the electrons into the surrounding atmosphere, thus ionising the air.

Research shows that when the charge density approaches that required for a lightning strike, the electrons will accumulate on any surface, so it’s better to provide a proper grounded path for the flow, rather than let it accumulate on an unprotected surface (with the associated serious risk to life and property).

Down conductor

The main purpose of the down conductor cable is to provide a low-impedance path between the air terminal and the external grounding plate. This cable should have a minimum cross sectional area of 21mm2. An aluminium mast is usually of sufficient cross-sectional area to facilitate the necessary lightning flow, but it should be connected by the cable at its base to an external grounding plate in order to complete the circuit. External grounding plate

The external grounding plate should be located so that it can never be out of direct contact with the water, and should be made of either copper, bronze or Monel (a predominantly nickel and copper alloy). A long strip is more effective at dissipating the electrical flow than a square plate of the same area.

Electricity prefers to exit the plate via an edge, so the plate should not be faired into the hull. In order to maximise the surface area of the edges, many recommend that grooves be cut along the length of the grounding plate. If the hull is constructed of steel or aluminium, many recommend using it as the grounding block. This may seem like a good idea, however the problem is that the metal hull is protected by coatings (i.e. paint) so the electrical path to the water is impeded.

In the past, a sinter block (a composite metal/ceramic device) was used as the grounding plate. This is not a good idea. The surface area of a sinter block is insufficient to effectively dissipate the electrical energy involved in a lightning strike. Also, the water trapped in the pores of the rough metal surface boils so violently (due to the extreme heat generated) that the block often explodes, thus putting the water-tight integrity of the hull at risk.

ZONE OF PROTECTION

A well-grounded air terminal provides a good measure of protection from lightning strikes inside the zone, however it’s important to realise that the area is by no means absolute. The area of protection is governed by the height of the air terminal. The radius inside the zone of protection is equal to the height of the air terminal (see Figure 2).

Yachts with tall masts easily fall inside the zone of protection (see Figure 3), while power vessels usually have areas well outside the zone, typically including the foredeck (see Figure 4). During thunder stormactivity, it’s imperative that all persons remain well within the zone of protection.

Another aspect to consider is secondary paths of conductivity, because all conductive materials have some measure of conductivity and impedance. This means that lightning can follow an alternative route to the sea. If these secondary paths are also of high impedance, there will still be a very high electrical flow, but the final path to the ocean will become highly unpredictable. Thus, in the event of a strike, the result could still be substantial damage to property and serious risk to life.

Protecting passengers during a thunderstorm

The first task is to get everyone below decks and located as follows:

• Up as high in the cabin as possible (i.e. away from the waterline)

• Away from the sides of the vessel

• Well away from the mast

• As far as possible from the lightning protection system

• Away from all electrical cabling and electronic instruments

Avoid all contact with, or proximity to, any large metal objects, such as an engine or the helm (especially if it’s made of stainless steel). Also, remember the steering system is usually made of many metal components installed behind a bulkhead.

People should sit with their legs up to minimise their frontal surface area, thus minimising the risk of conducting a side flash. This position also affords some additional protection to the heart.

If a person is wet, they should dry off and change as soon as possible. In the case of a power boat, it’s also imperative that everyone remains well within the zone of protection (see Figure 4).

If a person must remain outside (for example, at the helm), he should avoid touching large metal objects and also stay well away from any likely secondary paths, such as standing rigging.

Anyone who is caught in an open boat should sit in the bottom of the boat (but not in any water), with legs folded up against the chest. It’s also imperative to remain as still as possible, as there is some evidence to suggest that moving objects may increase the chance of a lighting strike. While maintaining this position may become uncomfortable, it could well save your life.

Side flashes occur wherever the electrical flow incurs high impedance. This includes many types of electrical connections, rapid changes in direction of the electrical path, poor or loose connections or corrosion in the lightning protection system.

Side flashes can do substantial damage to a vessel. They can blow a hole through the side or bottom of the boat anywhere from 2mm to 100mm in diameter, depending on the location and the impedance to the electrical flow. Side flashes are also often the cause of damage to onboard electronics.

PULSE PROTECTION

Any electrical pulse, especially one with a fast rise time and decay rate such as a lightning strike, will generate an electromagnetic pulse field. Given the magnitude of a typical lightning strike, it’s easy to see that a substantial electromagnetic pulse will be generated as the electrical flow passes through the grounding system. This electromagnetic pulse will be transmitted into any electrical conductor that is inside the field as it spreads and decays. The magnitude of these electromagnetic pulses will typically seriously damage or completely destroy all wiring and electronics in the proximity.

While it’s impossible to eliminate these resultant electromagnetic pulses, there are some measures that a good lightning protection installation can achieve to minimise the risk to onboard wiring and electronic equipment.

Keep all ship-board wiring as far away from the down conductor and the external grounding plate as is practical. In addition to this, all down conductor cables should be installed as near to the vertical as is practical, and all shipboard wiring should be installed as near to the horizontal as practical. This minimises the amount of the impressed electromagnetic pulse into the wiring system.

It’s common for people to say that lightning hit their onboard electronics. This is most unlikely, because if there was a direct hit the electronics would not just be ‘fried’, they’d probably explode and burst into flames due to the extremely high energy of a lightning strike. What usually happens is that the electronics are badly damaged or completely destroyed due to an electromagnetic pulse and/or a side flash.

Electronics typically require a 12v supply, but the internal circuit boards usually only use 3v, so it’s easy to see how an energy burst resulting from a lightning strike will overpower these circuits and destroy them. There are a few measures that can be taken to protect these expensive pieces of technology.

When a thunderstorm is likely (and where it’s practical), physically remove all connections from the back of the instrument. Then, if you can, remove the instrument and store it in a closed metal box such as an oven (ensure the oven is also disconnected from the electrical supply).

For essential instruments, always install a lightning arrestor. There are a number of different types of lightning arrestors available, so it’s important to consult a professional. Only high quality arrestors will do the job properly.

One final note: designing and installing a good lightning protection system is a task for a professional, who thoroughly understands the physics of, and the risks associated with, lightning strikes. The energy involved in lightning strikes can easily kill and/or sink a vessel, so it’s not an issue to be taken lightly. Cheap installations and poor practices are, at best, a waste of money, and, at worst, deadly.


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