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Old 19th Oct 2017, 5:19 pm   #1
Biggles
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Default Lightning protection zones

When I used to work a lot on radio towers, I was always wary of working on equipment when a lightning storm was in progress directly overhead. In fact in most cases I used to stop work and go and sit in the truck till it passed, having seen lightning damaged equipment before, and not wishing to become an integral part of the charred mess. However, I was informed that the metal tower provides a perfectly safe zone of protection, in a sort of cone shape with the tower in the middle. Is this right? I was just a bit concerned that a surge of one form or the other may affect me if I happened to be touching equipment racks or test equipment during a nearby strike. Saying that, the earthing arrangements on site equipment and feeders, cabins etc was always very good, maybe for this reason. What do you think?
Alan.
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Old 19th Oct 2017, 5:38 pm   #2
dave walsh
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Default Re: Lightning protection zones

Do you mean like a sort of Faraday Cage effect. I would imagine it might be a "shocking" experience on the inside even if you were totally protected. Given the current Tesla thread I'm reminded of that famous photo of him sat inside a discharging coil. Even if you'd been safe from the voltage/current during a direct hit there is the possiblity of a physical consequence to the structure I suppose. St Andrews Church in Ramsbottom has one of those square Norman type towers with a conical section at each corner. A few years back one of these took a direct hit at 6-30 in the morning, a hell of a bang from the best part of half a mile away. Part of the tower collapsed into the Church putting it out of action. The children from the associated Junior School were due to be in there three hours later! I think the truck was a good option. Vehicles in in general have a "skin"affect/affect as I understand it.

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Old 19th Oct 2017, 5:47 pm   #3
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Default Re: Lightning protection zones

The lightening conductor is to prevent a strike by equalising the static potential so that the ground primary trail goes up from somewhere else.
If it receives a direct hit, it will be destroyed with the huge current.
I don't think a lattice under a tower would constitute a proper Faraday Cage.

We always feel safe in our steel boat when there is lightening about but its probably false security.
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Old 19th Oct 2017, 6:28 pm   #4
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Default Re: Lightning protection zones

I wouldn't trust anything more open than a substantial metal box to protect me from lightning. The peak currents can be terrifyingly large and can set up considerable potential differences even as they flow through quite good conductors. There's quite a well known phenomenon where a farmer turns up to find multiple dead cattle in a field. The bodies are arranged in dartboard formation, all lined up radially around a central point. Other cattle in the field seem unharmed, if distressed. What happens is that lightning strikes a point on the ground and the current then flows radially outwards through the soil. Any cattle unfortunate enough to have their front legs close to the strike point and their rear ones much further away (or vice versa) are electrocuted by the voltage difference between their front and rear legs. Any who who are standing sideways to the direction of current flow only get a relatively small voltage applied between their left and right legs. They might end up deaf in one ear though.

I was in a building which was struck by lightning once. It's not something you forget in a hurry.

Cheers,

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Old 19th Oct 2017, 7:14 pm   #5
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Default Re: Lightning protection zones

Hi you are right a radio tower or in fact any high structure with a conductive path to ground forms a safe cone around it I think if you draw a line from the top down at a 45 degree angle the area under the line back to tithe structure is safe from strike having said that if your standing there at the moment of a strike you may feel something for the same reason cattle/sheep and other animals do although its not so severe for humans because you are wearing socks and shoes which provide a degree of protection. Although you may get hit by side flash if you are particularly unlucky
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Old 19th Oct 2017, 7:41 pm   #6
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Default Re: Lightning protection zones

The National Lightning Safety Institute [US] has some interesting things to say about the idea of the "cone of protection" around high objects:

http://lightningsafety.com/nlsi_pls/...tion-myth.html

There's a lot more useful info on the associated site.
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Old 21st Oct 2017, 6:03 pm   #7
Biggles
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Default Re: Lightning protection zones

Thanks for all the input. It seems fairly conclusive after reading that link. I will continue to respect all naturally generated voltages and currents while working out and about. It's a good excuse to sit in the truck and read the paper anyway. Nowhere have I ever seen in our health and safety bumph at work any mention of the dangers from lightning. The danger of insect bites and too much sun (!), but not lightning. Maybe it's time to give them a nudge.
Alan.
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Old 21st Oct 2017, 8:23 pm   #8
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Default Re: Lightning protection zones

Having R4 LW on in the background gives advance warning of regional lightning activity....
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Old 22nd Oct 2017, 11:28 am   #9
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Smile Re: Lightning protection zones

Hi,
I remember hearing advice to keep your feet close together to avoid, or minimise, the potential difference between them, and to crouch down to minimise your target area.
If there's a threat of lightning here, I tune a small pocket radio to a quiet part of the LW band to monitor the activity.
This is a good website to see lightning activity anywhere in the world:

http://en.blitzortung.org/live_lightning_maps.php

Cheers, Pete.
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Old 22nd Oct 2017, 12:17 pm   #10
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Default Re: Lightning protection zones

Once upon a time I was involved in the development of lightning protection devices for mobile telecoms base station equipment. I seem to remember that a typical current in a direct lightning strike is 20kA, but may be over 100kA. I wouldn't want to be anywhere near that field: the induced voltages and currents nearby are themselves frighteningly destructive.

We once experienced the effects of a direct strike on a steel fence post some 30m from our house. For some fortunate reason it chose the garden fence rather than the multi-element J-Beam FM antenna on the house itself!

It immediately melted the lighting circuit fuses and damaged any electronic equipment with more than a few feet of wire connected to it. We were watching the Grand National on our CRT TV at the time, some 70m from the strike and the effect on the shadowmask was enough to turn the green grass purple, requiring some pretty serious degaussing work afterwards.

An interesting side effect was the destruction of the steel cored washing line some distance from the fence itself. It was chopped into 50cm long pieces, all neatly laid out on the lawn - a nice illustration of a high power standing wave.

Alan - you were wise to retreat to the truck Faraday cage.

Martin
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Old 22nd Oct 2017, 12:20 pm   #11
Terry_VK5TM
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Default Re: Lightning protection zones

Good lightning site.
I use that to keep an eye on thunderstorms in my area.

I recently did a forklift course and one of the things taught was that if you managed to knock down high voltage power lines you basically shuffled you feet to avoid being electrocuted when departing the area.

Same principle as in thunderstorms.

Terry
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Old 22nd Oct 2017, 4:01 pm   #12
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Default Re: Lightning protection zones

I always thought that Farady only did an electrostatic sheild, i.e. an unshorted turn of copper (or other conductor) in a transformer. To prove it was only the magnetism doing the work.
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Old 22nd Oct 2017, 4:25 pm   #13
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Default Re: Lightning protection zones

Martin(Hartley118)'s comment about a lighting strike chopping a steel cored washing line in pieces reminded me that, back in 1971, the instructor on my Electronics Training Course told us not to unplug the TV aerial from the set during a thunderstorm, since, in the event of a strike, there would be a path to earth via the TV chassis, whereas unplugging aerial would render the coaxial cable liable to be chopped into pieces for the same reason. Since then, I've never unplugged the TV aerial. What a direct strike on the aerial would to to the electrnics in a modern TV, I don't know.
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Old 22nd Oct 2017, 4:49 pm   #14
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Default Re: Lightning protection zones

If you unplug the aerial and leave it floating it will be less likely to get hit than if you earth it because then you're bringing earth potential high up on your roof and making the aerial a more attractive target. Also I don't think the mains neutral would provide much of a path for lightning, it's far too inductive.
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Old 22nd Oct 2017, 5:11 pm   #15
Biggles
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Default Re: Lightning protection zones

When I lived in Penrith many years ago, I was watching TV one afternoon when there was an indirect strike, probably on a power line nearby and it fried a lot of our electrical stuff. The TV went up with a bang and a mushroom cloud came out of the back vents. The dog which was sitting beside me quietly licking it's paws shot off like a bullet in fright. When I went outside there was an acrid burning smell in the air and every neighbour for as far as I could see was outside looking up and down the street in a sort of daze. The insurance companies would have quite a big bill that day. We lost about 1500 of electrical gear. The local electrical shops must have had a field day, as it was before internet shopping.
Alan.
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Old 22nd Oct 2017, 5:45 pm   #16
Peter.N.
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Default Re: Lightning protection zones

I went to a customer who had a lightning strike, it hadn't done the TV a lot of good but the most fascinating thing was that the coax was welded to the radiator behind the set which should have protected it but it obviously wasn't quick enough.

We have an 11kv line going across the fields from our house, it only feeds us, we have a transformer on the pole in the garden. I have quite an array of ham radio antennas so always feel a bit nervous when we have a storm but as I was looking out of the window one day lightning struck the second pole down which is probably about 50' lower than us and a lot lower than the antennas. Strange stuff lightning!

Peter
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Old 22nd Oct 2017, 7:00 pm   #17
Biggles
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Default Re: Lightning protection zones

I have seen burnt out poles which have (had) the local transformer mounted on them. Not quite sure what the exact mechanism is but the poles seem to burn well due to being impregnated with some kind of creosote or tar. When you think of a lightning strike as a very short transient, I expect any kind of inductance in the path will affect the most direct electrical route to earth. With regards to aerials and thunderstorms, I sometimes connect neon bulbs across them to see them light up. It's a sort of morbid fascination with the danger I suppose.
Alan.
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