Nec800.100(a)(5)

[erickench]

I looked at the above section NEC and could not believe my eyes. It states the following:

(5) Run in Straight Line. The grounding conductor shall be run to the grounding electrode in as straight a line as practicable.

In Mike Holt's "Understanding the NEC" an Author's Comment following this section state's:

Lightning doesn't like to travel around corners or through the loops, which is why the grounding conductor must be run as straight as possible.


This is nonsense! When lightning strikes a conductor it becomes current traveling through a wire. It follows the path of least resistance which in this case is the conductor. It does'nt jump from the wire when it makes a turn. I don't know why they put this in the NEC. It does'nt make a bit of sense.

 

[chris kennedy]

When lightning strikes a conductor it becomes current traveling through a wire. It follows the path of least resistance

Wouldn't a straight conductor have less resistance than one run with a bunch of bends? Less material, less resistance.

FWIW, current will travel along all available paths.:smile:

 

[erickench]

The bend's don't matter. It's the length that adds resistance. Sometimes it can't be helped. You may have to run the wire around a corner. A wire traveling through the air offers the path of least resistance. The current running on that wire is not gonna overcome the resistance of the air.

 

[Twoskinsoneman]

The bend's don't matter. It's the length that adds resistance. Sometimes it can't be helped. You may have to run the wire around a corner. A wire traveling through the air offers the path of least resistance. The current running on that wire is not gonna overcome the resistance of the air.

It makes perfect sense to me. There are characteristics of conductor bends that will facilitate the arcing of high voltages. This is a fact. Do some research on the shape of spark plug electrodes. The arc will jump off of an edge. It seems perfectly logical to me to assume the high voltage of the lightening will be more lightly to jump from the wire if it has a sharp bend in it than if it were a straight run.

 

[erickench]

Spark plugs have a very small gap that is close enough to cause an arc. This small gap produces a smaller resistance. But not in the case of a grounding conductor running around the outside perimeter of a building. Besides that grounding conductor is required to be insulated. No current is gonna get through that. Webster's dictionary defines lightning as "a flash of light" not current. Now if you wanna talk about fiber optic cable, that has light traveling through it. That has to be kept as straight as possible. That's why fiber optic cables come with a metal structural member in the center. The only exception being patch cords which are very short.

 

[Twoskinsoneman]

that grounding conductor is required to be insulated. No current is gonna get through that.

Yeah a billion volt bolt of lightning is certainly not going to get through a 600v insulated wire.

Look I'm not lightning expert...but when your talking that kind of voltage I highly suspect that the distance the lightning pulse WILL arc will significantly smaller if the conductor has a bend in it.

 

[don_resqcapt19]

The lightning protection standard, NFPA 780, sets an minimum bend radius for lightning protection conductors. As I recall it is 9". Lightning tends to jump off the conductor at tight bends.

 

[nakulak]

I looked at the above section NEC and could not believe my eyes. It states the following:

(5) Run in Straight Line. The grounding conductor shall be run to the grounding electrode in as straight a line as practicable.

In Mike Holt's "Understanding the NEC" an Author's Comment following this section state's:

Lightning doesn't like to travel around corners or through the loops, which is why the grounding conductor must be run as straight as possible.


This is nonsense! When lightning strikes a conductor it becomes current traveling through a wire. It follows the path of least resistance which in this case is the conductor. It does'nt jump from the wire when it makes a turn. I don't know why they put this in the NEC. It does'nt make a bit of sense.

are you an expert on lightning ? have you invested a significant amount of time researching this notion ?

I'm not an expert on lightning, but when installing downcomers for an installation the engineer, who had significant experience in this area, was adamant that we run the conductors as straight as possible off from the air terminals down the poles, and had me adjust two of them. I could only guess that he had some experience in this area, since some of the buildings there do occasionally blow up due to the contents. If you have some evidence to support what you are saying, please post it. At the voltages present, lightning can jump right back out of the conductor and flow through the air again, so I don't see why the NEC instructions are counter-intuitive, but I would like to keep an open mind . . .

 

[erickench]

Yeah a billion volt bolt of lightning is certainly not going to get through a 600v insulated wire.

600 Volts plus the high resistivity of air. The wire still provides the path with least resistance. I would have to see hardcore evidence of charge jumping from a wire through the earth. Perhaps an article on the internet and some photos to back it up?

 

[cadpoint]

... This is nonsense! When lightning strikes a conductor it becomes current traveling through a wire.

Wait where's ohm law in all this ?

It follows the path of least resistance which in this case is the conductor. It does'nt jump from the wire when it makes a turn. ...

OK, call it a controled release...

I don't know why they put this in the NEC. It doesn't make a bit of sense...

That'd be an Personal opinion, wouldn't it be ?

Spark plugs have a very small gap that is close enough to cause an arc. This small gap produces a smaller resistance. But not in the case of a grounding conductor running around the outside perimeter of a building. Besides that grounding conductor is required to be insulated....

And the DC power is an amplified application, the wire in the ground has no potentioanl as it sits there!

No current is gonna get through that. Webster's dictionary defines lightning as "a flash of light" not current.

Well You can't have it both ways ...

And what KW help make up that Light?

Now if you wanna talk about fiber optic cable, that has light traveling through it. That has to be kept as straight as possible. That's why fiber optic cables come with a metal structural member in the center. The only exception being patch cords which are very short.

And I wonder why!

 

[erickench]

Okay. lets look at this as a parallel circuit. What is the ratio of the insulation and air resistivity combined to that of the wire resistance?

If it's 10:1 then about 91% of the current is gonna flow through the wire.

If it's 100:1 then about 99% of the current is gonna flow through the wire.

The wire shunts out any current that will jump through the air. In the 1980's I remembered reading about something called shield wire. It was a bare wire installed above Electric Utility conductors. This shield wire was grounded to earth. Lightning would strike the wire before hitting the power conductor and then shunt it out to the earth.

 

[cadpoint]

And who or what knows what exactly comes down per a strike or releases upward to the atmospheric conditions of any or all conditions that creat lighting ? I believe they know of it, but haven't exactly qualified nor defined it...

Point is, in MO your statements don't qualify or are a stretch of this application to ease a prospective and
there exist to many unknowns!

 

[erickench]

An average lightning bolt is about 20,000-30,000 Amps. I'm sure someone has determined the resistivity of air. The conductor insulation resistivity should also be known. If anyone could produce a photo showing burn marks on the side of house caused by charge jumping the wire then that would be something to see.

 

[iwire]

For what it's worth the installation instructions of any decent TVSS (transient voltage surge suppressor) also ask for as few bends as possible in both the line conductors and the grounding conductors.

Me thinks it's not as ridiculous as you think, or is everyone else just been wrong for many years waiting for you to straighten them out. :grin:

 

[erickench]

Like I said I have to see definite proof of this phenomena happening. I might be able to understand installing the conductor as short as possible thus reducing the resistance of the wire. But for a one-two family dwelling it's not gonna make much difference. The grounding conductor is limited to 20 feet.

 

[Twoskinsoneman]

600 Volts plus the high resistivity of air. The wire still provides the path with least resistance. I would have to see hardcore evidence of charge jumping from a wire through the earth. Perhaps an article on the internet and some photos to back it up?

??? I don't have hardcore evidence. I only offered what I thought seemed reasonable based on logic induced from the points I already brought up...(spark plug electrode engineering research) Also the fact that it IS a pretty-much universal requirement with any conductor installation involving lightning suggest that YOU should be the one to provide hardcare evidence to the contrary.

 

[hillbilly]

I may sound ignorant, but this is my opinion.
I believe that it's been discussed here before.

I believe that a high current flow at elevated voltages has inertia.
If that inertia becomes great enough, the current flow will tend to travel in a straight line.

If the current flow encounters a tight enough radius in the conductor, it will exit the conductor and travel in as straight a line as possible onto the next path having the lowest resistence.

That's just my opinion, and I'm sticking to it until I see some concrete evidence that proves me wrong.

steve

 

[K8MHZ]

The bend's don't matter. It's the length that adds resistance. Sometimes it can't be helped. You may have to run the wire around a corner. A wire traveling through the air offers the path of least resistance. The current running on that wire is not gonna overcome the resistance of the air.

It is impedance, not resistance that we are concerned with. In addition to that there is this ionization thing going on that makes air and copper come close to having the same impedance at the 100 Mhz pulsating DC frequency that lightning tends to generate.

Corona effect will occur at bends and will be more pronounced with sharper bends due to the above. The effect is well known by lighting arrestor folk.

Did you know that a 1 1/2" copper strap 20 thousandths of an inch thick will carry a direct hit without fusing open while a #4 copper conductor will pretty much vaporize when hit direct?

You need to spend some time at the PolyPhaser site which is currently not working. The tech notes explain all of this very well.

Here is the site. You are looking for the customer support link, then the technical notes link to the lightning protection area. There is great stuff there and no one knows it like PolyPhaser.

www.polyphaser.com

 

[K8MHZ]

I may sound ignorant, but this is my opinion.
I believe that it's been discussed here before.

I believe that a high current flow at elevated voltages has inertia.
If that inertia becomes great enough, the current flow will tend to travel in a straight line.

If the current flow encounters a tight enough radius in the conductor, it will exit the conductor and travel in as straight a line as possible onto the next path having the lowest resistence.

That's just my opinion, and I'm sticking to it until I see some concrete evidence that proves me wrong.

steve

It's not inertia, really, but that does make a good analogy.

What happens is that the air becomes ionized around and ahead of the bolt. This ionized air presents a very low impedance to the bolt which by it's nature has increased the impedance of the copper due to reactance. A straight run presents less reactance than a bent run. At the bend where the reactance is higher there becomes a point where the low impedance of the surrounding ions becomes lower than the impedance of the metal conductor and the bolt takes the path of less impedance exiting the metal conductor at that point.

It's not rocket surgery, really. As a radio operator I kind of have to know this stuff. Or have really good insurance. Or both. Both is better.

 

[K8MHZ]

Some clarification

Some clarification

I may not have been as clear as I intended in my above posts.

Ionization is caused by the high voltage potential of lightning, over 100kV and sometimes a half meg or so.

Reactance is caused by the frequency of the pulsating DC that can be around 100 Mhz.

Ionization makes the air more conductive while reactance makes the metal less conductive.

Reactance increases with the length of the conductor and with asymmetry. Bends are the most common form of asymmetry.

Skin effect is very pronounced at 100Mhz. That's why the strap works better than a round conductor, for one reason anyway. The strap also dissipates heat better due to a higher surface to mass ratio.

Lightning is DC, not AC but has AC characteristics. A common example is an automotive ignition coil that has pulsating 12 VDC. Like 12 VAC we can use a transformer to step up the voltage to around 50kV in some cases.