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Lightning has long frightened: in 1425 Capgrave (in his Chronicles) noted: “the moost horribil thunderes and litynnyngis that evry ony man herd.” In reality the chances of being killed or injured by lightning is statistically low (about 1 in 2 million per year – or about 1 in 25,000 over an eighty-year life span). There are about ten such fatalities each year in Australia and 80-100 injured but the risk of being struck is far from random. Of those struck some 80% were using a land-line telephone at the time. Those also and particularly at risk are golfers (particularly if swinging a club), and anyone using an umbrella during a storm. It’s not that hard to reduce the odds!
Some areas of Australia are especially prone to severe thunderstorms: these include the Blue Mountains, the Dandenong Ranges, the Kimberley, and the north of Australia generally during the monsoon season. The risk in these areas is very much higher and it is primarily for those living or travelling in such areas that this article is intended.
Here’s what happens. At all times, the earth’s surface carries a typically negative charge. The upper atmosphere carries a positive charge. As a storm develops the voltage difference builds up to many hundreds of millions of volts. Eventually the atmosphere ‘gives way’ resulting in brief but complex arcing. Currents may be 200,000 amps or more, but only for a fraction of a second. It happens something like this. Once the voltage between the ground and the upper atmosphere exceeds a certain level, the air ionizes (electrons become freer to move). This eases the passage of a lightning strike much as surfacing a road initially eases traffic flow.
Somewhere along the way step leaders reach down toward earth and, like our early pioneers, the one that gets there first tends to set the route for what is to follow.
On earth, objects respond by sending out positive voltage streamers. When such a streamer meets a step leader, a conductive link is formed. The resultant current flow generates so much heat that the surrounding air literally explodes – resulting in thunder claps.
The most dangerous place to be in a thunderstorm is out in the open and there is usually a fair amount of notice. As a general rule seek shelter once a thunderstorm comes within 10 kilometres – that’s when there’s about 30 seconds between seeing the lightning flash and hearing the thunderclap. Stay sheltered for at least 30 minutes after the last lightning is seen.
If you are caught out avoid becoming a positive voltage streamer - such as a golfer in mid-swing. For the same reason do not use an umbrella. You are safer if soaking wet in any case as any current is then more likely to pass through the wet clothing. The instantaneously-formed steam pressure is likely to blow it all off (and often your shoes as well) but being naked is better than thousands of amps flowing through your body. There are many reports of this happening, and people often being physically unharmed.
If the risk seems very high, crouch down with feet together and with your head held low. Do not shelter under a tree. If you have to stand keep your feet as close together as possible because as a nearby strike dissipates its electrical energy it causes voltage gradients of thousands of volts per metre in nearby ground. Having a few hundred volts difference between one foot and the other can leave you very dead. For the same reason, absolutely do not lie down
Almost any form of building is safer than being outside, but keep away from walls, metal plumbing etc. Do not use the loo (water is conductive). Metal bodied motorhomes and coaches are exceptionally safe. They provide what is known as a ‘Faraday cage’ – all of the current flows through the external metal shell. Ideally, lower the TV antenna (disconnect it at least). Physically disconnect external power leads.
Whilst the vehicle’s tyres might appear to insulate the vehicle from earth, rubber tyres contain carbon. They are deliberately semi-conductive to limit static charge build up. At lightning voltages they become good conductors but it is not a good idea to walk out of the vehicle holding the door handle etc whilst touching the ground. It’s an even worse idea to stray outside anyway. If seriously concerned about lightning you may want to earth the chassis to ground via a heavy copper rod and some seriously big cable - but it is illegal to earth an RV if external power is connected as it precludes upstream Residual Current Devices from working.
Bond any large external metal structure (e.g. air conditioners) to the chassis using at least 6 AWG cable.
Fibreglass bodied vehicles are at some risk but can be protected by a conventional lightning conductor with a (sharp) spike well above the roof and earthed to the vehicle chassis via starter motor cable. If you do this you must use crimped joints as the current flow will instantly melt soldered joints. (TV antennas and cables are useless for this.)
Lightning seeks the straightest path so keep any such earthing cable as straight as possible and routed such that is well away from people inside the RV. For those seriously interested in this issue the reference is the just released standard AS/NZS 1768:2007.
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