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Externet

Lightning rods downsides ?

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Externet    118

Hi.

Lightning rods, with their properties of attracting and providing a safer electrical path to ground have their advantages to prevent a house to burn or the unpredictable damages caused by a lightning bolt.

Is there any cons; like -say having them installed will promote more hits to the dwelling rod, instead of less hits that may be diverted to nearby trees or other buildings ?  Would the hits be much less destructive, but more frequent ?

For the brutal voltages involved; would a plain reinforcement iron bar as rod and path to proper grounding instead of a 1/2" copper path be reasonably comparable ?   A 1/2" 'rebar' tiny additional resistance to the brutal strike voltage should not be significantly detrimental, is it ?

(This is not about using the structural 'rebar', but as an external path)

 

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John Cuthber    3181

Power dissipation in a conductor is proportional to resistance and I think the resistance of rebar is a lot higher than that of the copper strip they use for lightning conductors round here.
So using rebar might be better than nothing, but it might get turned into shards of red hot metal (Of course, with a big enough strike, that might happen to copper too).

A lightning condctor on a building doesn't mean the building is more liekly to get hit- it means the conductor is more likely to get hit.
Who cares? That's what it's for.

 

On a vaguely related note, why do churches have lightning conductors :)

 

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Externet    118

Thanks.  Yes, iron in 6+ metres long bars can be an ohm or two more resistive than copper.  Which I suspect of little effect in huge voltages, as the high tension transmission lines behave to minimize losses with other than copper.

Yes, the conductor is the one more likely to be hit instead of the building.

So see no downsides ?   Other than more often soiled pants from the much louder and sudden thunder scare originated a few metres away...

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John Cuthber    3181

A lightning strike will deliver something like 10,000 amps or more (the record is 20 times that)

The power dissipated in a resistor is IR2 so  the power is 100 million times the resistance.

You really want to keep the resistance as low as possible. Ten ohms means that it's dissipating a gigawatt. (and the record breaking bolt would mean 400 GW

A hundred microseconds at that power is 100 KJ (or 40 MJ if you are unlucky)

Here's one way to put that in context.

https://en.wikipedia.org/wiki/Muzzle_energy

 

Edited by John Cuthber

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imatfaal    2477
5 hours ago, John Cuthber said:

...On a vaguely related note, why do churches have lightning conductors :)

Other than being the tallest building around often with pointy metal things on the roof (and a huge publicity problem if their god keeps burning their building down) - hmm dunno - Go on I am intrigued

 

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John Cuthber    3181
Just now, imatfaal said:

Other than being the tallest building around often with pointy metal things on the roof (and a huge publicity problem if their god keeps burning their building down) - hmm dunno - Go on I am intrigued

 

Because, when it comes down to it, they have more faith in science than in God.

There are few things more widely regarded as " an act of God" than a thunderbolt, but they can't trust their God to protect the building from Himself.

(The usual excuse is "it's a condition of the buildings insurance"- but why would no insurance company grant them an exception if the evidence didn't show that God destroys churches roughly as often as other buildings?)

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imatfaal    2477

All very well thinking god won't burn us - but not when we might have to pay for a new roof.  And you are right about the blame for and act of god - you will remember the furore which surrounded the York Minster strike and fire - not sure what the Arch-Bish had said but the Daily Fail (or was it the Sunday Excess) blamed it (only sllightly tongue in cheek) on some pronouncement which was a little too liberal for them

Steel's resistivity is at least 6 times that of copper  - so a similarly proportioned (cross sectional area) steel conductor would have to dissipate 36times more heat.  Also copper is much more flexible; lightening conductors are ribbons of copper which arrived on a drum and are unwound in situ.  A steel version would have to be brought to the site in short pieces and welded into a whole.

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John Cuthber    3181

There's another reason for using a ribbon, rather than a rod- lower self-inductance.
You would think a big thick lump of metal would look like a practically zero resistance load, but the rate of change of current  for a thunderstorm is huge and even a tiny inductance creates a significant voltage drop.

Not sure, but I think the magnetic properties of steel would increase the inductance even more than the resistance.

 

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Handy andy    26
On ‎26‎/‎07‎/‎2017 at 7:32 PM, Externet said:

Hi.

Lightning rods, with their properties of attracting and providing a safer electrical path to ground have their advantages to prevent a house to burn or the unpredictable damages caused by a lightning bolt.

Is there any cons; like -say having them installed will promote more hits to the dwelling rod, instead of less hits that may be diverted to nearby trees or other buildings ?  Would the hits be much less destructive, but more frequent ?

For the brutal voltages involved; would a plain reinforcement iron bar as rod and path to proper grounding instead of a 1/2" copper path be reasonably comparable ?   A 1/2" 'rebar' tiny additional resistance to the brutal strike voltage should not be significantly detrimental, is it ?

(This is not about using the structural 'rebar', but as an external path)

 

Lightning is coming from a km or so up normally, and has largely made up its mind so to speak before it comes to ground. A lightning conductor will provide an alternative path for a lightning bolt that is coming down in the vicinity of the conductor. There is however no guarantee the lightning conductor will attract the lightning. I have a sail boat and have had forks of lightning brake on both sides of the boat, with no ill effect except the rigging turning blue with st elmos fire. I have lost electrical equipment due to distant thunderstorms I avoided, whereas my friends have sailed through the same thunderstorm and taken no damage. Lightning comes in many forms with -ve discharge to ground, in the vicinity of a thundercloud, and +ve discharge to ground often known as a bolt from the blue as the positive discharge can travel several miles horizontally before either coming to ground or heading up into the ionosphere.

As for the idea lightning is an act of god that some supreme being should protect churches against, ROFL. :) A basic understanding what this god thing is you are trying to understand and observed past capabilities should be taken into account. If in the event of a thunderstorm and you think you are going to get hit, you could always bend down to pray to said god and kiss your arse goodbye whilst you are at it. If there is a god statistically he she it does not give seem to care about real estate or money, or anything else come to that, so when praying for assistance, one should not be hopeful of getting any.

Lightning is a very interesting subject to study, Plasma from lightning can and does produce positrons. The temperature the plasma reach transiently on implosion matches those of the interior of the sun. A single lightning strike can pack terra watts of energy, etc blah blah.

Some scientists clearly don't know much about what they waffle on about. From Lightning to gravitons with magical properties. :) 

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John Cuthber    3181
20 minutes ago, Handy andy said:

Lightning is coming from a km or so up normally, and has largely made up its mind so to speak before it comes to ground. 

...

Some scientists clearly don't know much about what they waffle on about. 

Lightning starts from the ground and goes up as much as it starts from the clouds and comes down.

https://en.wikipedia.org/wiki/Lightning#Upward_streamers

But you are clearly right about the second part.

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J.C.MacSwell    172
On 7/27/2017 at 0:28 PM, John Cuthber said:

On a vaguely related note, why do churches have lightning conductors :)

 

Not sure, but I think they prefer to use copper...wouldn't want to steel His thunder

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Handy andy    26
19 hours ago, John Cuthber said:

Lightning starts from the ground and goes up as much as it starts from the clouds and comes down.

https://en.wikipedia.org/wiki/Lightning#Upward_streamers

But you are clearly right about the second part.

Ionized or plasma Feeders travel up from the ground, but the main discharge that causes the damage comes down from the cloud, not up. I was keeping it brief, lightning was a hobby of mine.

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John Cuthber    3181
11 hours ago, Handy andy said:

Ionized or plasma Feeders travel up from the ground, but the main discharge that causes the damage comes down from the cloud, not up. I was keeping it brief, lightning was a hobby of mine.

Then you will know that it's the terrain, at least as much as the cloud, that "decides" where the strike is.

So why did you say "Lightning is coming from a km or so up normally, and has largely made up its mind so to speak before it comes to ground. "?

It makes up its mind based on the streamers from the ground (and, where appropriate, from a conductor) so it hardly matters how far up it comes from.

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Handy andy    26
On ‎07‎/‎08‎/‎2017 at 9:03 PM, John Cuthber said:

Then you will know that it's the terrain, at least as much as the cloud, that "decides" where the strike is.

So why did you say "Lightning is coming from a km or so up normally, and has largely made up its mind so to speak before it comes to ground. "?

It makes up its mind based on the streamers from the ground (and, where appropriate, from a conductor) so it hardly matters how far up it comes from.

I didn't mention that positive discharge to ground most likely doesn't follow a streamer, it moves horizontally pauses and moves again.

I didn't mention lightning balls or sprites the make up off which, is an interesting topic in itself.

Just to make sure I have hit most points LMGTFY

https://en.wikipedia.org/wiki/Sprite_(lightning)

https://en.wikipedia.org/wiki/Ball_lightning

I didn't mention anything about the differences observed in low and high altitude ball lightning, or any suspicions I have ref some explosive lightning events.

The reason for this is the thread is about lightning conductors, possibly increasing the risk of lightning strikes.

A shorter path to ground is the most likely path, but when dealing with high voltage lightning it is not the only path. If lightning hits a buildings masonry and a conductor is available to ground hopefully most of the energy will be diverted, without damaging too much masonry.

Ref terrain if you happen to live in the mountains, or like mountaineering lightning doesn't have as far to go to ground. however you do have to be extremely unlucky to get hit. etc

If there is anything I have not mentioned on lightning it may be deliberate, it is a big subject and takes a lot of time to write down. :)

If you would like to speculate on lightning balls, which are more interesting than sprites which I suspect are just a form of st elmos fire, I might be interested to see if they agree with my own ideas.

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