How much of a concern is a lightning strike to a yacht and what can we do about it? Nigel Calder looks at what makes a full ‘belt and braces’ lightning protection system
Most sailors worry about sailing in lightning to some extent, writes Nigel Calder.
After all, going around with a tall metal pole on a flat sea when storm clouds threaten doesn’t seem like the best idea to most of us.
In reality, thunder storms need plenty of energy, driven by the sun, and are much less frequent in northern Europe than in the tropics.
However, high currents passing through resistive conductors generate heat.
Small diameter conductors melt; wooden masts explode; and air gaps that are bridged by an arc start fires.
On boats, radio antennas may be vaporised, and metal thru-hulls blown out of the hull, or the surrounding fiberglass melted, with areas of gelcoat blown off.
Wherever you sail, lightning needs to be taken seriously.
Understanding how lightning works, will help you evaluate the risks and make an informed decision about the level of protection you want on your boat and what precautions to take.
Most lightning is what’s called negative lightning, between the lower levels of clouds and the earth. Intermittent pre-discharges occur, ionising the air.
Whereas air is normally a poor electrical conductor, ionised air is an excellent conductor.
These pre-discharges (stepped leaders) are countered by a so-called attachment spark (streamer), which emanates from pointed objects (towers, masts, or lightning rods) that stand out from their surroundings due to their height.
This process continues until an attachment spark connects with a stepped leader, creating a lightning channel of ionised air molecules from the cloud to ground.
The main discharge, typically a series of discharges, now takes place through the lightning channel.
Negative lightning bolts are 1 to 2km (0.6 to 1.2 miles) long and have an average current of 20,000A.
Positive lightning bolts are much rarer and they can have currents of up to 300,000A.
Preventing damage when sailing in lightning
A lightning protection system (LPS) is designed to divert lightning energy to ground (in this case the sea), in such a way that no damage occurs to the boat or to people.
Ideally, this also includes protecting a boat’s electrical and electronic systems, but marine electronics are sensitive and this level of protection is hard to achieve.
Lightning protection systems have two key components: First, a mechanism to provide a path with as little resistance as possible that conducts a lightning strike to the water.
This is established with a substantial conductor from an air-terminal to the water.
This part of the LPS is sometimes called external lightning protection.
Second, a mechanism to prevent the development of high voltages on, and voltage differences between, conductive objects on the boat.
This is achieved by connecting all major metal objects on and below deck to the water by an equipotential bonding system.
Without this bonding system high enough voltage differences can arise on a boat to develop dangerous side flashes.
The bonding system can be thought of as internal lightning protection.
Rolling ball concept
Lightning standards, which apply ashore and afloat, define five lightning protection ‘classes’, ranging from Class V (no protection) to Class I.
There are two core parameters: the maximum current the system must be able to withstand, which determines the sizing of various components in the system, and the arrangement and number of the air terminals, aka lightning rods.
Let’s look at the arrangement of the air terminals first. It is best explained by the rolling ball concept.
A lightning strike is initiated by the stepped leaders and attachment sparks connecting to form the lightning channel.
The distance between the stepped leader and the attachment sparks is known as the breakdown distance or striking distance.
If we imagine a ball with a radius equal to the striking distance, and we roll this ball around an object to be protected, the upper points of contact define the possible lightning impact points that need to be protected by air terminals.
The air terminal will theoretically provide a zone of protection from the point at which the terminal connects with the circumference of the rolling ball down to the point at which that circumference touches the water.
The shorter the striking distance, the less the radius of the rolling ball and the smaller the area within the protection zone defined by the circumference of the rolling ball.
The smaller the protection zone, the more air terminals we need. So, we use the shortest striking distance to determine the minimum number and location of air terminals.
Class I protection assumes a rolling ball radius of 20m; Class II assumes a rolling ball radius of 30m.
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Boat building standards are based on a striking distance/rolling ball radius of 30m (Class II).
For masts up to 30m above the waterline, the circumference of the ball from the point at which it contacts the top of the mast down to the water will define the zone of protection.
For masts higher than 30m above the waterline, the ball will contact the mast at 30m and this will define the limit of the zone of protection.
If Class I protection is wanted, the radius of the ball is reduced to 20m, which significantly reduces the zone of protection and, on many larger recreational boats, may theoretically necessitate more than one air terminal.
With most single-masted monohull yachts, an air terminal at the top of the mast is sufficient to protect the entire boat to Class I standards.
The circumference of the rolling ball from the tip of the mast down to the surface of the water does not intercept any part of the hull or rig.
However, someone standing on the fore or aft deck might have the upper part of their body contact the rolling ball, which tells us this is no place to be in a lightning storm.
Some boats have relatively high equipment or platforms over and behind the cockpit.
These fittings and structures may or may not be outside the circumference of the rolling ball.
Once again, this tells us to avoid contact with these structures during a lightning storm.
Ketch, yawl, and schooner rigged boats generally require air terminals on all masts, except when the mizzen is significantly shorter than the main mast.
The external LPS
The external LPS consists of the air terminal, a down conductor, and an earthing system – a lightning grounding terminal.
The down conductor is also known as a primary lightning protection conductor.
All components must be sized to carry the highest lightning peak current corresponding to the protection class chosen.
In particular, the material and cross-sectional area of the air terminal and down conductor must be such that the lightning current does not cause excessive heating.
The air terminal needs to extend a minimum of 150mm above the mast to which it is attached.
It can be a minimum 10mm diameter copper rod, or 13mm diameter aluminum solid rod.
It should have a rounded, rather than a pointed, top end.
VHF antennas are commonly destroyed in a lightning strike.
If an antenna is hit and is not protected by a lightning arrestor at its base, the lightning may enter the boat via the antenna’s coax cable.
A lightning arrestor is inserted in the line between the coax cable and the base of the antenna.
It has a substantial connection to the boat’s grounding system, which, on an aluminum mast, is created by its connection to the mast.
In normal circumstances, the lightning arrestor is nonconductive to ground.
When hit by very high voltages it shorts to ground, in theory causing a lightning strike to bypass the coax – although the effectiveness of such devices is a matter of some dispute.
A down conductor is the electrically conductive connection between an air terminal and the grounding terminal.
For many years, this conductor was required to have a resistance no more than that of a 16mm² copper conductor, but following further research, the down conductor is now required to have a resistance not greater than that of a 20mm² copper conductor.
For Class I protection, 25mm² is needed. This is to minimise heating effects.
Let’s say instead we use a copper conductor with a cross-sectional area of 16mm² and it is hit by a lightning strike with a peak current corresponding to Protection Class IV.
The conductor will experience a temperature increase of 56°C. A 16mm² conductor made of stainless steel (for example, rigging) will reach well over 1,000°C and melt or evaporate.
Shrouds and stays on sailboats should be connected into a LPS only to prevent side flashes.
The cross-sectional area of the metal in aluminum masts on even small sailboats is such that it provides a low enough resistance path to be the down conductor.
Whether deck- or keel-mounted, the mast will require a low resistance path, equivalent to a 25mm² copper conductor, from the base of the mast to the grounding terminal.
Metal hulled boats can use the hull as the grounding terminal. All other boats need an adequate mass of underwater metal.
In salt water this needs a minimum area of 0.1m². In fresh water, European standards call for the grounding terminal to be up to 0.25m².
A grounding terminal must be submerged under all operating conditions.
An external lead or iron keel on monohull sailing boats can serve as a grounding terminal.
In the absence of a keel, the cumulative surface area of various underwater components – propellers, metal thru-hulls, rudders – is often more than sufficient to meet the area requirements for a grounding terminal.
However, these can only be considered adequate if they are situated below the air terminal and down conductor and individually have the requisite surface area.
Metal through-hulls do not meet this requirement.
If underwater hardware, such as a keel, is adequate to be used as the grounding terminal, the interconnecting conductor is part of the primary down conductor system and needs to be sized accordingly at 25mm².
Propellers and radio ground plates
Regardless of its size, a propeller is not suitable as a grounding terminal for two reasons.
First, it is very difficult to make the necessary low-resistance electrical connection to the propeller shaft, and second, the primary conductor now runs horizontally through the boat.
The risk of side flashes within the boat, and through the hull to the water is increased.
An engine should never be included in the main (primary) conducting path to a grounding terminal.
On modern engines, sensitive electronic controls will be destroyed in a lightning strike, and on all engines, oil in bearings and between gears will create resistance and therefore considerable heat which is likely to result in internal damage.
However, as it is a large conductive object, the engine should be connected to the internal lightning protection system.
Internal lightning protection
On its way to ground, lightning causes considerable voltage differences in adjacent objects – up to hundreds of thousands of volts.
This applies to boats with a functioning external lightning protection system but without internal protection.
Although the lightning has been given a path to ground along which it will cause as little damage as possible, dangerous voltages can be generated elsewhere, resulting in arcing and side flashes, threatening the boat and crew, and destroying electronic equipment.
We prevent these damaging voltage differences from arising by connecting all substantial metal objects on the boat to a common grounding point.
The grounding terminal is also wired to the common grounding point.
By tying all these circuits and objects together we hold them at a common voltage, preventing the build-up of voltage differences between them.
All conductive surfaces that might be touched at the same time, such as a backstay and a steering wheel, need to be held to the same voltage.
If the voltages are the same, there will be no arcing and no side flashes.
The bonding conductors in this internal LPS need to be stranded copper with a minimum size of 16mm².
Note that there can be bonding of the same object for corrosion prevention, lightning protection, and sometimes DC grounding.
We do not need three separate conductors.
Electronic Device Protection
With lightning protection systems, we need to distinguish electric circuit and people protection from device protection.
Even with an internal LPS, high induced voltages may occur on ungrounded conductors (such as DC positive) which will destroy any attached electronics.
A mechanism is needed to short high transient voltages to ground.
This is done with surge protection devices (SPD), also known as transient voltage surge suppressors (TVSS) or lightning arrestors.
In normal circumstances these devices are non-conductive, but if a specified voltage – the clamping voltage – is exceeded they divert the spike to ground.
There are levels of protection defined in various standards depending on the voltages and currents that can be handled, the speed with which this occurs, and other factors.
This is a highly technical subject for which it is advisable to seek professional support.
Most SPDs are designed for AC circuits.
When it comes to DC circuits there are far fewer choices available to boat owners although there are an increasing number for solar installations that may be appropriate.
There is no such thing as a lightning-proof boat, only a lightning-protected boat, and for this there needs to be a properly installed LPS.
Even so, in a major strike the forces involved are so colossal that no practical measures can be guaranteed to protect sensitive electronic equipment.
For this, protection can be provided with specialised surge protection devices (SPDs).
The chances of a direct lightning strike on a yacht are very small, and the further we are north or south of the equator, the smaller this chance becomes.
It’s likely your chances of receiving a direct lightning strike are very much higher on a golf course than at sea.
‘Bottle brush’-type lightning dissipators are claimed by sellers to make a boat invisible to lightning by bleeding off static electrical charge as it builds up.
The theory rests upon the concept that charged electrons from the surface of the earth can be made to congregate on a metal point, where the physical constraints caused by the geometry of the point will result in electrons being pushed off into the surrounding atmosphere via a ‘lightning dissipator’ that has not just one point, but many points.
It is worth noting that the concept has met with a storm of derision from many leading academics who have argued that the magnitude of the charge that can be dissipated by such a device is insignificant compared to that of both a cloud and individual lightning strikes.
It seems that the viable choices for lightning protection remain the LPS detailed above, your boatbuilder’s chosen system (if any), or taking one’s chances with nothing and the (reasonable) confidence that it’s possible to sail many times round the world with no protection and suffer no direct strikes.
Whichever way you go, it pays to stay off the golf course!
Enjoyed reading Sailing in lightning: how to keep your yacht safe?
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