To limit the effects of lightning on a pleasure craft, whether a sailboat or a motorboat, you need to provide a quick and easy path for the current, make a high-quality equipotential bonding system and protect sensitive equipment. While the first objective is easy to achieve, the other two are far less so...

The Faraday cage and the protection of people

The basic principle of protecting a ship against lightning is to build a Faraday cage around which the lightning current will flow into the sea, to use a marine analogy, protecting the crew!

On a sailboat, an antenna on the mast, generally made of metal, acts as a receiver. The current flows down from the antenna via a conductor in the mast to the metal keel sail, the keel bolts or a sufficiently large earthing plate. Alternatively, the current can flow down through the forestay or the shrouds, all of which should be connected, via a cable or braid, to one of these three earthing points.

The surface area and number of earth plates vary greatly depending on the quality of the boat. On top-of-the-range boats, for example, there are often several plates: AC, DC and lights; on a houseboat, the same principle applies, with the earthing of an antenna on the superstructure.

On a dayboat or open boat, there is no Faraday cage. In our last article in this series, we'll come back to what to do in the event of a thunderstorm.

The cross-section and continuity of these circuits and the earth contact surface are critical. The ISO TR 10134 standard determines the characteristics of a lightning earthing system, but this standard was only mandatory from 1993 to 2020 (ISO 10134:1993; ISO 10134:2003) on new EC boats less than 24 metres long and intended for pleasure craft.

This ISO TR 10134:2020 text is no longer a mandatory standard, but an informative Technical Report, and its preamble contains a warning: "The presence of a lightning protection device on a boat does not imply that compliance with this Technical Report provides complete protection against material damage or personal injury", which is unlikely to reassure boat builders, who may have to accept responsibility for damage in the event of a lightning strike.

Equipotential bonding

The Faraday cage is capable of distributing the current of lightning falling on the receptor constituted by an antenna or mast. This type of protection applies when lightning strikes an antenna receiver.

In many cases, however, the lightning does not strike the receiver, but the water. In this case, the presence, within a radius that can exceed several dozen metres, of a vessel whose hull contains conductive elements, can cause them to act as entry points for the current into the boat. 

The current must then find the shortest possible route to the ground plate to be dissipated in the water. If this connection does not exist, the boat's electrical system may be taken out of service.

Secondary arcs are generated between the metal entry point and the conductors or components of the DC circuits.

No device, no conductor and no battery, even a lithium battery fitted with a BMS-decoupler, designed for a 12 or 24 V system can withstand a few thousand amperes without total destruction. All the equipment connected to the DC circuit is at risk ECU from the engine, alternator, chargers, converters, lamps, electric winches, windlass, pumps - all equipment that is not protected against such currents and overvoltages. 

Some authors believe that a strip of earth several metres long and just a few centimetres wide would be more effective than a square plate. But using such a strip complicates the installation even further.

If there is an effective equipotential bond between the metal elements, the current, faced with an absence of potential difference, cannot reach the DC circuits, which remain protected.

It is interesting to note that the American standards for equipotential bonding on pleasure boats are much stricter than the European ones...

The latter simply require large metal objects to be connected to the equipotential bonding system, whereas the American standards require all electrical components, even small ones, to be bonded to earth.

Protection of electrical and electronic equipment

The power supplies for the electrical and electronic equipment on board are all protected, if properly fitted, by a fuse commensurate with the maximum current that can be carried by the cable supplying them. But even in this case, and without additional protection, none of these fuses is capable of withstanding the thousands of amperes represented by a lightning strike. 

Radio and communications equipment are the most exposed, their aerials acting as lightning current receivers. But the communication of the lightning current to the DC circuit endangers all the equipment connected to it. 

The most important (engine ECU, alternator, radio, electronics) should be protected by an individual surge protection device. 

Large yachts have these anti-surge devices on all their DC circuits, very similar to the lightning protection boxes that protect the inverters of photovoltaic installations. The protection, installed upstream of the line and the device to be protected, diverts the current flow in the event of a surge towards the earth plate and protects the devices located downstream.

Surge protection devices are also available for all coaxial cables, such as aerial cables.

Equipotential bonding

The Faraday cage is capable of distributing the current of lightning falling on the receptor constituted by an antenna or mast. This type of protection applies when lightning strikes an antenna receiver.

In many cases, however, the lightning does not strike the receiver, but the water. In this case, the presence, within a radius that can exceed several dozen metres, of a vessel whose hull contains conductive elements, can cause them to act as entry points for the current into the boat. 

The current must then find the shortest possible route to the ground plate to be dissipated in the water. If this connection does not exist, the boat's electrical system may be taken out of service.

Secondary arcs are generated between the metal entry point and the conductors or components of the DC circuits.

No device, no conductor and no battery, even a lithium battery fitted with a BMS-decoupler, designed for a 12 or 24 V system can withstand a few thousand amperes without total destruction. All the equipment connected to the DC circuit is at risk ECU from the engine, alternator, chargers, converters, lamps, electric winches, windlass, pumps - all equipment that is not protected against such currents and overvoltages. 

Some authors believe that a strip of earth several metres long and just a few centimetres wide would be more effective than a square plate. But using such a strip complicates the installation even further.

If there is an effective equipotential bond between the metal elements, the current, faced with an absence of potential difference, cannot reach the DC circuits, which remain protected.

It is interesting to note that the American standards for equipotential bonding on pleasure boats are much stricter than the European ones...

The latter simply require large metal objects to be connected to the equipotential bonding system, whereas the American standards require all electrical components, even small ones, to be bonded to earth.

Protection of electrical and electronic equipment

The power supplies for the electrical and electronic equipment on board are all protected, if properly fitted, by a fuse commensurate with the maximum current that can be carried by the cable supplying them. But even in this case, and without additional protection, none of these fuses is capable of withstanding the thousands of amperes represented by a lightning strike. 

Radio and communications equipment are the most exposed, their aerials acting as lightning current receivers. But the communication of the lightning current to the DC circuit endangers all the equipment connected to it. 

The most important (engine ECU, alternator, radio, electronics) should be protected by an individual surge protection device. 

Large yachts have these anti-surge devices on all their DC circuits, very similar to the lightning protection boxes that protect the inverters of photovoltaic installations. The protection, installed upstream of the line and the device to be protected, diverts the current flow in the event of a surge towards the earth plate and protects the devices located downstream.

Surge protection devices are also available for all coaxial cables, such as aerial cables.