Nec800.100(a)(5)

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pfalcon

Senior Member
Location
Indiana
K8MHZ is following the correct logic for lightning strikes.

It's not just the current but the voltage in lightning that does the damage. The corona and ionization is created when the voltage exceeds the dielectric breakdown of the air. At that point the air becomes the best conductor.

DC tends to use the conductor cross-section while high frequency AC uses the surface. When the lightning ionizes the air (actually turns it to plasma) the lightning flows through the ionized air (plasma).

Plasma is attracted to the conductor because plasma carries a charge. The charge naturally would like to flow through the conductor rather than push through non-ionized air. There just isn't enough "space". So the plasma uses its mass to push through non-ionized air but is attracted to the copper pathway. A sharp bend can cause the inertia of the plasma to overcome the attraction to the conductor.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
K8MHZ is following the correct logic for lightning strikes.

It's not just the current but the voltage in lightning that does the damage. The corona and ionization is created when the voltage exceeds the dielectric breakdown of the air. At that point the air becomes the best conductor.

DC tends to use the conductor cross-section while high frequency AC uses the surface. When the lightning ionizes the air (actually turns it to plasma) the lightning flows through the ionized air (plasma).

Plasma is attracted to the conductor because plasma carries a charge. The charge naturally would like to flow through the conductor rather than push through non-ionized air. There just isn't enough "space". So the plasma uses its mass to push through non-ionized air but is attracted to the copper pathway. A sharp bend can cause the inertia of the plasma to overcome the attraction to the conductor.

Lightning, although DC by definition can be considered AC and travels on the surface of a conductor in the very same manner.

I, like most on this site, are not versed in plasma physics, although I know someone that is. He worked on HIPAS in Alaska. I am aware of corona effect only due to a working knowledge needed to design to mitigate it.

I tried to explain the phenomena using basic electrical references as that is what most of us are comfortable with.

I wish the PolyPhaser site was up, the stuff there is state of the art when it come to explaining lightning protection.
 

erickench

Senior Member
Location
Brooklyn, NY
Well first of all lightning being DC does not have a frequency in the MHZ range. Lightning frequency is just how often it strikes the earth which is definitely not MHZ. Second, because it's DC on discharge there is no frequency and therefore no impedance which includes both reactance and resistance in its definition. It is pure resistance that lightning is opposing. Third, the DC caused by a lightning strike travels through a wire which has resistance only and there is no inertia. Current is just the flow of electrons, it's not mechanical. There cannot be inertia.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Well first of all lightning being DC does not have a frequency in the MHZ range.
My understanding is that the lightning waveform is not a simple step function or constant function (classical DC), but something quite complicated, whose Fourier transform will have large components at high frequencies.

Cheers, Wayne
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Well first of all lightning being DC does not have a frequency in the MHZ range. Lightning frequency is just how often it strikes the earth which is definitely not MHZ. Second, because it's DC on discharge there is no frequency and therefore no impedance which includes both reactance and resistance in its definition. It is pure resistance that lightning is opposing. Third, the DC caused by a lightning strike travels through a wire which has resistance only and there is no inertia. Current is just the flow of electrons, it's not mechanical. There cannot be inertia.

In a nutshell you are so wrong.

Before you continue with your opinions, give the PolyPhaser site enough time to get back online and READ the technical notes.
 

hillbilly

Senior Member
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.


Yeah, yeah, that's what I meant.:grin:

steve
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I would like to see you prove that. I am not saying you are wrong, it's just that the proof (or not) would be so much fun to watch.
Okay. What kind of short-term current do you think I'd get from my service conductors on a 200a underground feed of about 45' from the pad-mount?
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Okay. What kind of short-term current do you think I'd get from my service conductors on a 200a underground feed of about 45' from the pad-mount?

I think that MAY work for a half cycle or so. Get the video camera and some 18 ga speaker lead and let the fireworks begin. However, I am wagering that with speaker wire of that diameter you won't get more than 46,234 amps before the conductor fuses open and that will happen in .3658 cycles.

Oh, and don't forget some way to measure the MCBMC (Max Current Before Molten Copper).

This reminds me of experiments I liked to perform as a youngun' in rural Michigan.
 

Speedskater

Senior Member
Location
Cleveland, Ohio
It just occurred to me that the original paper may have had 18 gauge and sixty Amps. Then through a combination of converting to a spread sheet and crossing the Atlantic Ocean it became 60.000 Amps which then was read as sixty thousand amps. But I'm sure that the home theater poster doesn't have a clue about electricity or lightning.
 

digitap

Member
In very high frequency pulses, the electricity traveling along anything becomes radio. There are extremely low limits to this phenomenon. Lightning is W.E.L.L. documented as jumping from point to point rather than traveling only along conductive planes.

As a wireless electromagnetic telecommunications technician - the foundation industry being two-way radio communications, satellite up & downlinks, and whatever other electromagnetic particle-wave functions come along like light, I had to be thoroughly familiar with the potential paths of lightning's jumping potential.

A lot of the infrastructure used by the radio and all two way communications really, is stuck up into the air in the place it can cover the most, or most of a specific, area.

This means big radiative metal objects beings stuck out on top of everything from the local buildings to the mountains around. Understanding of the way lightning responds to all that specifically conductive equipment is part of the deal.

My initial entry into the various communications fields was preceded by finishing two private electronic vocational schools, the first in analog & digital electronics with basic software principles, the second was called ''Advanced Wireless Electronic Communications'' for a total of about 2400 class hours in the theories and applications associated with various electrical and electronic phenomena.

While lightening consists of a d.c. pulse, it's initial strike speed after the plasma leader fills out is pretty high; I don't remember the precise speeds, it varies obviously: but, here's an idea of what's happening:

ever listen to an a.m. radio? Ever notice how bad it gets interfered with, by lightning? This interference isn't caused by the massive number of electrons passing around; people pass massive voltages, and massive currents through this & that, all the time; it's the frequency, with which the flow of electrons snaps from one place to another, across space: from cloud to cloud, cloud to whatever else...

Notice how everyone relies on FM communications in both their broadcast listening, and their work site walkie talkies etc? The reason for that is that FM signal processing is effectively impervious to the interference that lightning emits. That's just said incidentally;

but in any case, the way that lightning overwhelms your radio-frequency AM receiver, is by putting off an enormous radio-frequency spike, when the air path along the plasma leader finally sets off the massive jump of the large number of electrons across the space between where it starts, and where it ends.
 

Skokian

Member
Location
Skokie, Illinois
Bends in Lightning Wiring

Bends in Lightning Wiring

The requirement makes sense to me.

Lightning strikes have a very, very high rise time. Bends in conductors add inductance. This inductance, although small at power frequencies, is significant in the microsecond and nanosecond regions.

Regards . . .

Jim
 

srblx

Member
Location
Ohio
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.

would burn marks in the ground leading to the hole where the house stood be meet your request?
 

tom baker

First Chief Moderator
Staff member
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.:cool:

Not all FO comes with metal structural member, some does not, that is considered non conductive and article 770 allows it in a conduit or junction box with power wiring. See the definition of Non conductive FO and 770.113
 
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