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Author Topic: What is the best way to ward off a lightning strike?  (Read 6145 times)

Offline daniescholtz

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Danie Scholtz  asked the Naked Scientists:
   
Hi Chris,

I want to start by saying newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive]. I listen to your newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] as much as I can. Although I am far behind on the newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive], I will catch up one day. I am from South Africa and notice you are on Talk Radio 702 every Friday and listen to all your answers.

I have a question which causes a lot of debates and I am not sure what the answer is. I am 24 years of age and I am working at an electrical engineering firm. So I am fairly familiar with most of the electrical "stuff".

Q: let's say for instance I got a TV antenna on the roof of my house. I stay in town and if I stand on my roof, I can see other houses with antennas higher than mine and some other buildings in the city which is about 4Km away line of sight. So my antenna is not in that big danger of lightning if I can put it that way, there are many other potential around my house for lightning to strike. But as we all know, lightning may strike anywhere.

If I want to minimize my chances of a lightning strike damaging my electronics etc, what would be the way to go?

Scenario 1: with the antenna on my roof, I take a thick copper wire and connect it to my pole holding the antenna and to a strike that is about 2m in the earth. Together with that, I earth my coaxial cable every 3m.i also ensures my TV is correctly grounded.

Ok, so let's say the lightning does strike my antenna... Will my appliances (TV) be damages by the lightning strike?

Scenario 2: with my antenna on the roof, I use a plastic/ rubber pipe and mount my antenna on that. The rubber/plastic pole mount to my brick wall and I ensure that it is isolated from earth as much as possible. I take the coaxial cable from the antenna and put it in an optically isolated device and the output from that device I take to my TV. So bear in mind, I want to isolate the antenna part from earth as much as possible.
So if the lighting does strike my antenna, will it still damage my appliances?

To my understanding, people say you must ground you antenna as good as possible. Wouldn't that "attract" the lightning in a way because it is a good path to ground? Or will the isolated antenna "avoid" the lightning?

If I can give you another example, let's say my house is in the desert and there is nothing else in the open for miles. Considering the 2 scenarios and a storm would be coming over my house, which one will the lightning be likely to strike? Or will there be no significant difference?

Many thanks.

Best regards,

Danie Scholtz

What do you think?


 

lyner

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What is the best way to ward off a lightning strike?
« Reply #1 on: 10/06/2009 19:45:02 »
You can reduce the probability of a lightning strike considerably by using a lightning conductor. A sharp, pointed conductor, elevated above the building will generate a corona discharge very readily and produce a conducting 'umbrella' of ions over the roof. This umbrella will have a large radius, which will limit the field strength locally (It is inversely related to curvature) and the lightning will tend to go elsewhere.

This is the main action of a lightning conductor. It takes the majority of the current if there should happen to be a strike but, the rest of the time, it is an inhibitor.

To prevent damage to electronic equipment you need to have good, thick earths and good coaxial cable. I think spark gaps on lines can help, too. If the lightning spike is above your antenna, it will protect it. However, you can still get inconveniently large currents induced in wires when there is a nearby lightning strike. 'Crossed diodes' across lines can limit the effects of surges but can introduce distortions into signals. So you pays yer money and you takes yer choice.
 

Offline Ophiolite

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What is the best way to ward off a lightning strike?
« Reply #2 on: 10/06/2009 21:32:46 »
Buy a set of golf clubs. During a storm stand on your roof holding the number two iron in the air. Not even God can hit a number two iron.
 

Offline turnipsock

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What is the best way to ward off a lightning strike?
« Reply #3 on: 11/06/2009 00:11:52 »
Using a lighting conductor on your aeriel will certainly minimise the chances of a lightning strike...not to sure what it will do for your telly reception though.

When you get to studing Electric Fields you will see that pointy things are more likely to be hit by lightning. Favorites for strikes are churches, people swimming and golfers in the middle of a backswing (usually they are on the round of their life when they get struck down, otherwise they would have walked off).

Sophiecentaur might have some info on yachts at sea. They would seem to be a prime target, ...pointy...nothing else around to hit. When I was mid atlantic and a lighting storm started, the skipper went and got some chain and wrapped it around the leeward stay so the other end dragged in the water. The idea was that it would earth the rig.

« Last Edit: 11/06/2009 11:08:25 by chris »
 

lyner

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What is the best way to ward off a lightning strike?
« Reply #4 on: 11/06/2009 23:36:32 »
A vertical pole near a telly aerial can have an effect but if you choose where you put it (or the spike) you can minimise the effect.

Yes, pointed / high buildings will have a high field strength near them but,  a sharp spike  will effectively 'round them off' and lower the potential gradient.

I don't know what 'expertise' has been put into yacht protection but, even with a fully functioning spike at the top of the mast, the effective radius is less than that of the surrounding sea. This still makes it the most desirable part of the sea for lightning to strike.

Perhaps a yacht should tow a sacrificial raft behind with a mast to 'draw' the lightning from it. OR follow a bigger boat!

The skipper was probably doing the right thing. I've never been out in a thunderstorm and I've not been aware of any local boats being struck whilst mored, either.
 

Offline Shadec

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What is the best way to ward off a lightning strike?
« Reply #5 on: 12/06/2009 10:03:57 »
an enourmous silver (Ag silver, not just painted silver) cage built around your house (a few hundred metres should do it) with a huge silver, very sharp point or spike on top, reception might be limited, and it would be constantly hit by lightning, but your aerial would be safe :P
 

Offline westom

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What is the best way to ward off a lightning strike?
« Reply #6 on: 21/08/2009 23:35:47 »
Yes, pointed / high buildings will have a high field strength near them but,  a sharp spike  will effectively 'round them off' and lower the potential gradient.
  Research conducted in NM suggests that a blunt rod was more effective than a pointy rod.  But that is arguing trivialities.  A lightning rod will only be as effective as its earthing - both the electrode and how connections are made to that electrode.

  Protection is about where current goes. That surge is a connection from a cloud to earthborne charges some maybe four miles away.  The shortest electrical path is not five miles to those charges.  The shortest current path is 3 miles down to earth and four miles over to those charges.   Anything in a structure that becomes part of that path can be harmed.

  Even a strike down the street on AC mains can be a direct strike to household appliances.  Any incoming path to earth means surge damage.  We connect every incoming conductor (even buried wires) to single point earth ground BEFORE it enters the building.  Every wire in every cable.

  For cable TV and the antenna, a ground block connected by a short wire to earth is sufficient (assuming the main surge from antenna has its own direct connection to earth).  For AC electric, every wire connects to earth via a protector.

  Either surge energy gets dissipated harmlessly in earth. Or it enters the building to hunt for earth destructively via appliances.

  As noted, nothing stops or blocks surges.  A surge voltage will increase as necessary so that the current will flow.  We have direct strikes with no damage when the current has a low voltage path to earth.  That is not a low resistance path.  That is a low impedance path - which means even sharp wire bends can compromise surge protection.  More important than wire thickness (lower resistance) is wire length (lower impedance).  But most important is the quality of that single point earthing electrode.

  Too many see the lightning rod and assume that defines protection.  What they do not see is the only and most critical component in protection - single point earth ground.

  Antenna must have a direct connection to earth.  Then the antenna lead must connect to the single point earth ground before entering the structure.
 

lyner

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What is the best way to ward off a lightning strike?
« Reply #7 on: 23/08/2009 17:31:47 »
Interesting. How blunt is the optimum?
They certainly use spherical ends on purposely placed spark gaps on EHT equipment. I always thought that was to define the striking voltage

Spot on about the impedance thing. Bends and loops are a no no. Every microhenry counts.
 

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What is the best way to ward off a lightning strike?
« Reply #7 on: 23/08/2009 17:31:47 »

 

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