Need A Better Explaination Why Grounding Protects Electrical Systems from Lighting


by Prince Rilian
Tags: electrical system, grounding, lighting, lightning, wiring
Prince Rilian
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#1
Dec6-12, 08:11 PM
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About two weeks ago, I wanted to find out more about the design of lightning protection systems. In particular, I wanted to figure out what gage of wire would be necessary for linking a lightning rod to earth-ground. What I had learned is that the gage of the wire would have to be extremely thick to absorb a direct stroke of lightning, causing the cost of the lightning protection system to quickly become more expensive than most of the equipment that it would be protecting. Therefore, most lightning protection systems in place today are designed to only protect against "indirect" lightning strikes.

Exactly what is meant by the "direct" stroke and the "indirect" stroke?

Also, how do you protect against the EMP produced by the lighting bolt? (I know that Faraday's law demands that a voltage gets induced in conductors nearby the lightning strike due to the impulse of magnetic flux created by the strike. Taking this into account seems to complicate the discussion of lightning protection quite a bit.)
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Naty1
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Dec7-12, 02:54 PM
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lightning protection remains a somewhat empirical science.....no matter the theory, it usually seems something can happen to circumvent it. I would not agree that protection is more expensive than equipment protected. My own experience involves boat and telecommunication systems; in the latter even buried cables often use a conductor above them buried in the ground to divert lightning.
In boat protection, I believe recommended conductor size is now #8AWG. That's likely because insurance surveyors found melted smaller size czbles rather than new theoretical results.

A direct strike means the preponderance of strike current is present; in an indirect strike only fractional portions of the main current are present....the latter are the type people can possibly survive. That doesn't guarantee anything, but improves one's chances of survical.

Check here for some interesting background. http://en.wikipedia.org/wiki/Lightning_protection
yungman
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#3
Dec7-12, 03:38 PM
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Also, you should use multiple smaller gauge wires in parallel. At the lighting strike, the dv/dt is very high, skin effect comes into play. Current flow on surface only, one big wire don't have that much surface. Multiple insulated wire give you the max surface even though the total cross sectional area is the same.

I don't know anything about lighting, but it is very hard to imagine using like multiple 18 gauge insulated wires to make up an 8 gauge wire is not enough.

Just a thought.

mrspeedybob
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Dec7-12, 05:10 PM
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Need A Better Explaination Why Grounding Protects Electrical Systems from Lighting


One thing I have not seen mentioned is the reason that lightning rods are pointed. It's not just to attract the lightning but to induce it. The point gives the charge a place to concentrate. This results in a lightning strike that happens sooner but at lower energy then would otherwise have taken place.
Naty1
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Dec7-12, 05:13 PM
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I have read and observed a lot of lightning theory in practice....because it is scary when boating to see lightning bolts splashing all around. I have never seen any recommendation suggesting multiple wires.....this are vaporized in an instant I would suspect. Surface current means there is more current flow in outer regions of an conuctor...not that all the current resides there.
yungman
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Dec8-12, 02:49 AM
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I have never work with lighting, but I worked with a lot of HV arc of 10+KV. The rise time is very fast, it's in sub nS range. It is definitely RF. I have no idea why no body mention about multi wires and skin effect. Theory should be the same.

In HV arcing, all the energy discharged in matter of a few nS, the instantaneous current is very high. Let's just use an example of a 100pF cap charged up to 10KV. Then discharge in 1nS.

[tex]Q=CV= 10^{-10} X 10^4=10^{-6} \hbox { coulomb.}[/tex]

If discharge in 1nS

[tex] CV=It \Rightarrow \; I=\frac {10^{-6}}{10^{-9}}=1000A[/tex]

That is a lot of current running through a conductor for 1nS. Just imagine a lighting strike, the amount of energy discharged can easily exceed tens of thousands of amp for very short period of time.

I have seen power wire wound resistors or even metal film resistors burned open by arc. Particular the big wire wound resistors opened up by just small arc!!! We had to use bulk resistors. This is because the instantaneous current is so high that things melt before it even have a chance to dissipate the heat out. We know arcing and lighting is RF, we know skin effect have a play, it's just logical to use ways to provide maximum surface area.
davenn
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Dec8-12, 03:14 AM
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Quote Quote by Naty1 View Post
I have read and observed a lot of lightning theory in practice....because it is scary when boating to see lightning bolts splashing all around. I have never seen any recommendation suggesting multiple wires.....this are vaporized in an instant I would suspect. Surface current means there is more current flow in outer regions of an conductor...not that all the current resides there.
yup likewise, when I worked for a major communications company, installing lightning rods on towers and earthmats in the ground around the tower became a regular part of the job.

we ALWAYS used heavy solid (not stranded) copper strip from the lightning rod at the tower top down the tower and to the earthmat. The strip was usually ~ 1 to 1.5 inch wide and ~ 1/4 inch thick. Depending on ground type ... burial difficulty, the buried earthmat would either consist of sections of steel mesh welded together, similar to building reinforcing mesh. Or multiple buried 4ft long earthstakes radiating out from the base of the tower and all connected together.
The principle of the lightning rod at the mast top was to provide a umbrella of protection to the antennas mounted lower down the mast. Li9ghtning will generally strike the highest part of the object ( the top of the lightning rod) but that doesnt always happen.

see this pic of the Washington Monument, by a fellow stormchaser, it gets struck a third of the way down its side
even tho there is a good leader travelling up from the tower top



cheers
Dave
Attached Thumbnails
monumentstruck_web.jpg  
sophiecentaur
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Dec8-12, 05:56 AM
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When it comes to lightning protection, I understood that the sharp spike at the top of a 'lightning conductor' readily produces a corona, which forms a large radius conducting sphere, reducing the local field strength, making the building less likely to get a direct strike. I guess that would apply to a boat mast, too. I would need to lower my radio antenna, I suppose. Now which is more important to me - long range radio communication in the event of any emergency, of reducing the possible effect of a lightning strike???? Decisions, decisions!
Prince Rilian
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Dec9-12, 02:49 PM
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Again, back to the question of what is meant by "direct" and "indirect" strokes. Davenn conveniently posted a picture two posts back which may help answer my question.

Would the "direct stroke" be the thick bolt striking the side of the Washington monument and an "indirect stroke" be the thin bolt striking the top of the monument?
Naty1
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Dec9-12, 06:34 PM
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The Wikipedia article on Lightning Rods has some interesting information including here:

http://en.wikipedia.org/wiki/Lightni..._system_design

I have absolutely no idea what the 'rolling sphere model' means. What I do know is that when outside aboard a boat watching a thunderstorm approach, and you feel static electricity building...as on clothes and hair...its time to make a hasty retreat indoor!!


and in the following section, this [not the final word]:

Calculations of the relative strengths of the electric fields above similarly exposed sharp and blunt rods show that while the fields are much stronger at the tip of a sharp rod prior to any emissions, they decrease more rapidly with distance. As a result, at a few centimeters above the tip of a 20-mm-diameter blunt rod, the strength of the field is greater than over an otherwise similar, sharper rod of the same height. Since the field strength at the tip of a sharpened rod tends to be limited by the easy formation of ions in the surrounding air, the field strengths over blunt rods can be much stronger than those at distances greater than 1 cm over sharper ones.
The results of this study suggest that moderately blunt metal rods (with tip height to tip radius of curvature ratios of about 680:1) are better lightning strike receptors than sharper rods or very blunt ones.
Regarding the thin and thick lightning bolts in the posted photo......I do not know how scientists qualify strikes versus leaders.... It's possible, I think, that the thin bolt at the top of the building might be considered a leader and a major portion of the strike will follow it's path. This would seem to depend in part on the charge accumulation...if it is mostly dissipated at the moment of the photo, no such significant strike would likely follow.
Bobbywhy
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Dec9-12, 10:17 PM
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“Direct stroke – In the direct stroke, the lightning discharge (i.e. current path) is directly from the cloud to the subject equipment.

Indirect stroke – Indirect strokes result from the electrostatically induced charges on the conductors due to the presence of charged clouds.”

http://www.circuitmaniac.com/2009/03...tning-strokes/

See the above website for details and diagrams.

Cheers,
Bobbywhy
Bobbywhy
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Dec9-12, 10:48 PM
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“Should a lightning rod have a point?

This was a controversy as early as the 18th century. In the midst of political confrontation between Britain and its American colonies, British scientists maintained that a lightning rod should have a ball on its end. American scientists maintained that there should be a point. As of 2003, the controversy had not been completely resolved.[25] It is difficult to resolve the controversy because proper controlled experiments are nearly impossible in such work; in spite of the work of Moore, et al. [described below] most lightning rods seen on buildings have sharp points. Work performed by Charles B. Moore, et al.[26], in 2000 has helped this issue, finding that moderately rounded or blunt-tipped lightning rods act as marginally better strike receptors. [described below] As a result, round-tipped rods are installed the majority of the time on new systems in the United States. To quote:

Calculations of the relative strengths of the electric fields above similarly exposed sharp and blunt rods show that while the fields are much stronger at the tip of a sharp rod prior to any emissions, they decrease more rapidly with distance. As a result, at a few centimeters above the tip of a 20-mm-diameter blunt rod, the strength of the field is greater than over an otherwise similar, sharper rod of the same height. Since the field strength at the tip of a sharpened rod tends to be limited by the easy formation of ions in the surrounding air, the field strengths over blunt rods can be much stronger than those at distances greater than 1 cm over sharper ones.

The results of this study suggest that moderately blunt metal rods (with tip height to tip radius of curvature ratios of about 680:1) are better lightning strike receptors than sharper rods or very blunt ones.”
http://en.wikipedia.org/wiki/Lightning_rod

and,

Pointing out the advantages of being blunt!
Oct 262009, By Frank Ross

The classic Franklin point design, and the more blunted version of modern lightning rods.

In any lightning protection system, the soldier manning the front line is the air terminal. This simple component is engineered to intercept the downward-moving stepped leader by launching an upward-reaching attachment spark that will serve to close the circuit and direct the flow of current to a solid grounding device, away from a structure and expensive electronics.
Even if you’ve never seen an air terminal in person, more than likely you’ve seen pictures of them on tall buildings, barns, and homes. Most likely the end of the air terminal you’ve seen was very sharply pointed. The theory behind the sharp point design is that it causes the largest electric field and hence is more likely to launch the attachment spark. Sharp pointed air terminals, originally designed by Ben Franklin are a time-honored concept; however, recent research has revealed that blunt rods are slightly more effective in accomplishing this job.

Many modern physicists have shown that, under strong electric fields, the air around sharp rods becomes ionized. This ionized air creates space charges that act to weaken the fields of electricity building up and flowing from ground to components atop roofs, towers and similarly susceptible structures. In both laboratory and field experiments scientists have established that critical field strengths for lightning interception develop more quickly around moderately blunt or blunt lightning rods that those with sharp points.
The most effective air terminal is one with a radius of curvature between 3/16 and 1/2 inch. This is not to say that pointed terminals should be avoided or replaced. They are effective, but moderately blunt or blunt terminals are just more effective.
http://site.electrical-insulators-an.../pointed-rods/

Cheers,
Bobbywhy
davenn
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Dec10-12, 01:58 AM
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Quote Quote by Bobbywhy View Post
................
Indirect stroke Indirect strokes result from the electrostatically induced charges on the conductors due to the presence of charged clouds.

Cheers,
Bobbywhy
indirect stroke is just that, its a stroke/strike close by, as apart from a direct strike. A stroke IS a discharge, its not just the presence of charged clouds.

A storm that went throuigh the home area a week ago produced a strike within ~ 300 metres from home. It was one of those big flash-bangs and one of the very rare times that I have heard the static "crash" on the TV receiver audio.

cheers
Dave
tiny-tim
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Dec10-12, 03:55 AM
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Quote Quote by Prince Rilian View Post
… What I had learned is that the gage of the wire would have to be extremely thick to absorb a direct stroke of lightning … most lightning protection systems in place today are designed to only protect against "indirect" lightning strikes.

Exactly what is meant by the "direct" stroke and the "indirect" stroke?
Quote Quote by Bobbywhy View Post
“Direct stroke – In the direct stroke, the lightning discharge (i.e. current path) is directly from the cloud to the subject equipment.

Indirect stroke – Indirect strokes result from the electrostatically induced charges on the conductors due to the presence of charged clouds.”

http://www.circuitmaniac.com/2009/03...tning-strokes/
yes, so far as i can see, "indirect stroke" is of no relevance unless you're managing a transmission system

according to that link, direct stroke is where the lightning hits the object

indirect stroke is where no lightning hits the object …

the lightning goes from one cloud to another (or from the cloud to some other object), and charge already on the object is released, and surges along it (which can damage a transmission line)
Quote Quote by Prince Rilian View Post
Again, back to the question of what is meant by "direct" and "indirect" strokes. Davenn conveniently posted a picture two posts back which may help answer my question.

Would the "direct stroke" be the thick bolt striking the side of the Washington monument and an "indirect stroke" be the thin bolt striking the top of the monument?
Quote Quote by davenn View Post
… see this pic of the Washington Monument, by a fellow stormchaser, it gets struck a third of the way down its side
even tho there is a good leader travelling up from the tower top

Quote Quote by Naty1 View Post
A direct strike means the preponderance of strike current is present; in an indirect strike only fractional portions of the main current are present....the latter are the type people can possibly survive. That doesn't guarantee anything, but improves one's chances of survical.
no, i think that's the difference between leader stroke and return stroke

a leader comes down from the cloud, splitting into several branches

another leader comes up from a building (etc), and may or may not connect to the downward leader

if it does connect, that connected path (from the cloud to the building) becomes the path of least resistance, and allows a huge return stroke, see http://en.wikipedia.org/wiki/Lightni..._return_stroke
Once a channel of ionized air is established between the cloud and ground this becomes a path of least resistance and allows for a much greater current to propagate from the Earth back up the leader into the cloud. This is the return stroke and it is the most luminous and noticeable part of the lightning discharge.
so the very bright stroke in the above picture is the return stroke, and (look carefully! ) the leader from the top of the building failed to connect to the downward leader (ie the lightning rod didn't work in this case )

here's another picture of a failed leader (the commentary says "Lightning hits a tree. See the streamer on the left coming up from a pole. It is not connected to the main leading channel.") …
davenn
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Dec10-12, 01:45 PM
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Quote Quote by tiny-tim View Post
yes, so far as i can see, "indirect stroke" is of no relevance unless you're managing a transmission system

according to that link, direct stroke is where the lightning hits the object

indirect stroke is where no lightning hits the object …

the lightning goes from one cloud to another (or from the cloud to some other object), and charge already on the object is released, and surges along it (which can damage a transmission line)
NO definately not. An indirect stroke/strike can cause huge amounts of damage when its close enough to be induced into equipment. I have spend many years repairing/replacing damaged equip caused by indirect strikes. Large voltages can still be induced that easily take out comms equip.

Dave


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