September, 2011 Lightning Protection - Marine Standards IV Up until recently, lightning protection has been mainly considered for land based applications. However, lightning strikes at sea as well as on land and it is becoming increasingly accepted that protection should be provided for the marine sector as well. Countries around the world are moving to introduce and mandate lightning protection standards for vessels at sea, if they have not already done so... The ISO standard includes metallic rudder surfaces, struts, metal centreboards and keels or radio earths, provided that they have the required surface area and are in the correct or suitable location. However, you will need to bear in mind the possibility of introducing electrolysis. When it comes to communications equipment, the antenna is an obvious target for lightning attack. Therefore it is best erected about 1-2 metres below the chosen point of air termination, although this is often not possible, particularly on smaller vessels. For situations where the antenna feed impedance varies widely, such as with a whip or wire HF system, protection can be provided by a horn gap discharge system, such as the Moonraker LPU, placed directly in the feed line to the antenna from the antenna tuning unit (ATU), with one side connected directly to ground. HF antenna system ground plates should not be connected to the earth system. However, Moonraker earth plates may be used as separate lightning earth. Coaxial surge suppressors can protect communications systems using HF whip or wire antennas by being placed in the 50 ohm coaxial feeder, just where the cables enter the vessel cabin rooftop etc. This is to discharge from the cables all over voltages before they enter the vessel and radiate within. Moonraker CSS units are designed for this purpose. As most ATUs provide little resistance to high reverse voltages, it is wise to fit a suppressor in the coaxial cable between the ATU and the transmitter in HF installations. They should also be placed in the feeds from the VHF, UHF, TV and other communications equipment that is connected to an external antenna. Various coaxial connections are available to accommodate this and should be chosen to suit the frequency range of the equipment. For a VHF antenna that is matched to 50 ohms, it would be placed directly in the coaxial line between the antenna and the transceiver where the cable enters the vessel. 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. Similarly line protectors can prevent damage to equipment connected to the DC or AC power supply. DC and AC external cables feeding navigation lights, wind speed and direction indicators, etc., can also carry transient over voltages to other instruments and electronic devices such as chart plotters, computers and the like, to cause damage. Moonraker DCL and ACL line protectors act to prevent this happening by clamping the voltage and conducting surges safely to ground. The Australian/New Zealand standard recommends the use of these commonly available materials listed below, which are readily available, as suitable for air terminals, downconductors and main current carrying bonding conductors. 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. For this reason it is not recommended.
| |||||||||||||||||||||||
