Sunday, 29 May 2011

Lightning Strike Protection for Sailboats

When at sea at night and the lightning is erupting all around your sailboat you feel extremely fragile and exposed. The dread of the lightning actually striking your boat is very real with the possible results not something you wish to contemplate when you are on passage in the middle of an ocean.

That is when you take some comfort in the knowledge that your lightning prevention system you have installed on your vessel will protect you and your boat in the event of a strike. If you don't already, then it is something you need to look at and have professionally installed before you leave port on your 'adventure of a lifetime' sailing odyssey.

Here is an article written recently by Kurt Kopper for 'Afloat Magazine' and is well worth studying:       

'Lightning protection systems do not prevent lightning strikes. The aim is to attract the lightning and conduct it safely to the water, so that damage to the boat and the possibility of injuries or death is reduced.

A good protection system has three components – grounding, bonding and electronics protection.

The purpose of the grounding system is to provide a safe conducting path from a point of lightning attachment to conductors in the water, without producing side flashes (sparks that form between the protection system and ungrounded conductors or the water).

The point of attachment can be a metal mast or a lightning rod. The zone of the boat protected by the air terminal is in the shape of a cone with a 90° apex. If that does not fully cover all parts of the boat, the height of the lighting rod must be increased or additional rods installed at different points of the boat. If there is electronic equipment such as antennas or transducers mounted on the mast, a lightning rod can be fitted onto the mast to reach above these. The equipment must all be within the protected cone.

Lightning rods should have a sharp metal point. The brush type terminals once used have been found to be less effective.
If no suitable metal pathway such as an aluminium mast or metal superstructure exists, it is essential that a low impedance path to ground is. The Australian standard recommends at least a 75mm² conductor, e.g. 25mm x 3mm copper strap. The conductor should follow a direct as possible route to the water. Any bends in the conductor should have a minimum radius of 200mm, as the current can otherwise “jump out of” the conductor. If the base of the mast is mounted onto a fibreglass or wooden structure, a conductor must be used to carry the current further downward.

The grounding system must conduct the current to a point of contact with the water. A lead keel is adequate, and it may be painted. The conductor must be bolted onto the keel in some place that is reasonably dry and where it can be inspected from time to time. Alternatively a grounding plate with a surface area of about 1000cm² can be fitted. This must be separate to any HF radio grounding plates.

The bonding system provides conductors to connect large metal fittings to the grounding system, in order to protect the crew from side flashes. Whereas the grounding system requires heavy conductors to carry the full current of the strike downward, the bonding system consists of mainly horizontal conductors to short out any voltages that might otherwise develop between the metal objects and the path of the lightning current. Lighter gauge conductors are used for these connections, e.g. 8mm² copper cable. Good electrical connections are essential. Items that should be bonded include chain plates, toe rails, steering wheels, engines, engine controls, railings, and the ground connections of electronic components.

Electronics protection limits power supply and transducer voltages to equipment in order to minimise or prevent damage to that equipment. Due to the extreme sensitivity of electronic components to over voltages from DCpower and transducer cabling, merely grounding these components is not enough to prevent damage.

DC electronics are best protected by placing surge suppressors immediately before the DC power feed to each component.
Radio antennas can be protected using coaxial surge suppressors, like the Moonraker type CSS.

For HF antennas, protection can be provided by a horn gap discharge system. This is placed directly in the feed line from the antenna to the ATU, with one side connected directly to ground.
Great care must be taken when making all grounding and bonding connections to minimise the potential for galvanic corrosion.

As with all aspects of lightning protection, 100% effectiveness cannot be guaranteed. Disconnecting equipment in advance of a storm helps isolate it from voltages induced by lightning. Use disconnects in preference to knife switches, and these in preference to switch panels.

Every boat is unique, and expert advice must be sought to ensure that any lightning protection system is correctly designed to ensure the safety of boat users and best protection of assets.'

Article and images courtesy Kurt Kupper and Afloat Magazine

You can read more about lightning and effects whilst cruising in my ebook  'Voyage of the Little Ship Tere Moana' downloadable from my website

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