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Author Topic: Antenna GROUNDING...solid or stranded?  (Read 12016 times)
W0DV
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« on: March 29, 2012, 03:19:30 PM »

I had an antenna install business install a 10 meter 5/8 wave vertical on my roof the other day. When I got home from work, I noticed that the ground wire from the tripod to my house ground is stranded #10 copper wire with a green jacket. I was surprised they used this. Shouldn't it be solid wire? I've been doing some research on the web and It looks like this is common practice?

Opinions?

Dave
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AA4HA
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« Reply #1 on: March 29, 2012, 04:03:17 PM »

Green jacketed, stranded wire is very common. I used to work for a company that made industrial control panels and it is all we used. In fact, other than the really small wire (telephone type) the only thing we used was stranded conductor.

The only time I ever saw the problem was in attaching the wire to an acorn nut on a ground rod.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
W9KDX
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« Reply #2 on: March 29, 2012, 06:22:56 PM »

I've never heard anything mentioned here for lightning grounding but #6 solid copper or copper sheeting with a larger surface area.  What you are looking at is what I see for grounding higher amp AC wiring.  

I would be concerned.

http://www.rainmaster.com/lightning.htm

These guys specifically say it is not appropriate.
« Last Edit: March 29, 2012, 06:28:06 PM by W9KDX » Logged

Sam
W9KDX
K0ZN
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« Reply #3 on: March 29, 2012, 07:46:56 PM »

It is a whole lot better than no ground, but that wire is inadequate for a serious lightning hit. It will be vaporized!  Also, what you have for the ground termination
will make a lot of difference too. i.e.  one ground rod is rock bottom minimum....more is better etc.

Heavy gauge stranded copper cable is OK, such as # 2 or heavier stranded.  Current lightning protection practice appears to be leaning towards flat straps, but there
are a ton of lightning protection systems out there that have worked perfectly well with heavy gauge copper cable too.

My guess is that if the installer had told you it was going to cost $200 for the ground WIRE you would not have had them do it.....but good ground cable is expensive!

They are kind of in a heck if they do and heck of they don't situation. Give them credit for using #10 instead of #14 !  ....but yes, you really want a minimum
of # 6 if you want real lightning protection......and #2 or heavier if you can afford it.  The statistical average lightning stroke is about 18,000 amperes !! That takes some
serious wire to handle that and survive.

73,  K0ZN
« Last Edit: March 29, 2012, 07:52:28 PM by K0ZN » Logged
W6RMK
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Posts: 657




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« Reply #4 on: March 29, 2012, 09:03:47 PM »

it depends on which section of the NEC they were wiring in accordance with.  CATV (Art 820), for instance can use a different gauge than radio and TV antennas  (art 810) which is different than amateur antennas (also 810) which is different than telephone wires (art 800 and 830)

AWG 10 is what's required for an antenna support.

telephone - insulated AWG 14 (800-40)
antenna structures - bare or insulated AWG 10 (810-15) - TV, Radio, Amateur
discharge unit/leadin cable - bare or insulated AWG10 (810-21)
CATV - *insulated* AWG 14 (820-40)

stranded or solid doesn't make much difference.

One thing to remember is that all earth grounds have to be interconnected with AWG6.. so if you drive a new rod (for whatever reason), bond it to the system ground with #6.

the primary purpose of that ground wire is to reduce the number of deaths and injuries when a powerline hits the antenna or structure (one of the most common causes of death by electrocution, as it happens), it's not really a "lightning grounding conductor".

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W0DV
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« Reply #5 on: March 29, 2012, 09:38:37 PM »

it depends on which section of the NEC they were wiring in accordance with.  CATV (Art 820), for instance can use a different gauge than radio and TV antennas  (art 810) which is different than amateur antennas (also 810) which is different than telephone wires (art 800 and 830)

AWG 10 is what's required for an antenna support.

telephone - insulated AWG 14 (800-40)
antenna structures - bare or insulated AWG 10 (810-15) - TV, Radio, Amateur
discharge unit/leadin cable - bare or insulated AWG10 (810-21)
CATV - *insulated* AWG 14 (820-40)

stranded or solid doesn't make much difference.

One thing to remember is that all earth grounds have to be interconnected with AWG6.. so if you drive a new rod (for whatever reason), bond it to the system ground with #6.

the primary purpose of that ground wire is to reduce the number of deaths and injuries when a powerline hits the antenna or structure (one of the most common causes of death by electrocution, as it happens), it's not really a "lightning grounding conductor".


Besides the house ground, I have 2 other 8' copper poles in the ground, all are interconnected with solid #6 copper wire. My radio, tuner, power supply are all connected to a ground hub which is then connected to the earth ground. My antennas all have a surge protector installed on the coax before they enter my shack and are also connected to the same earth ground. Nothing is daisy chained. I put together this ground for my station as the arrl antenna book described. I was dissapointed when I came home from work and seen my 10 meter vertical was installed on my roof with #10 copper stranded wire, connected to my house ground.. But from what I am hearing is that this is standard for grounding roof mounted antennas these days.

Dave
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K9KJM
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« Reply #6 on: March 29, 2012, 10:07:33 PM »

#10 is really pretty light......    #6 copper is considered the minimum gauge to use, But as pointed out, The larger you go, The more expensive it gets! 
Even #6 is pretty light if you feel your antenna could take a direct strike (Many years ago power companies studied lightning damage and arrived at a compromise: #6 as being heavy enough to handle about 96% of all lightning strikes, And it was simply more economical to replace damaged equipment than to use heavier ground conductors at all poles....  Note that critical locations like power substations and commercial towers use much larger conductors)
 Flat copper strap, Or even hollow copper tubing can be used as a good ground conductor with good results. (More surface area= low inductance)  In all cases, NO sharp bends in the conductor on it's path to ground!

Tips on doing it on a lower budget:   http://www.scribd.com/anon-849269/d/14868226-lightning-protectiontaming-thors-thunderon-a-budget

(Give that site plenty of time to load)
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W0DV
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Posts: 200




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« Reply #7 on: March 30, 2012, 02:46:54 PM »

As it turns out, #10 copper wire meets code for grounding an antenna. I assumed too much when I hired these guys to put up my antenna. I would have used #6 copper if I had done it myself. I just didn't feel like climbing on the roof. Laziness doesn't pay Sad
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K0ZN
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« Reply #8 on: March 30, 2012, 03:30:58 PM »


Well, the odds are that at some point, a situation will arise where it is convenient to add some # 6 in parallel. That would be very easy to do, fortunately.
Then you would be in pretty decent shape.

73,  K0ZN
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W6RMK
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« Reply #9 on: March 30, 2012, 10:30:19 PM »

#10 is really pretty light......    #6 copper is considered the minimum gauge to use, But as pointed out, The larger you go, The more expensive it gets! 
Even #6 is pretty light if you feel your antenna could take a direct strike (Many years ago power companies studied lightning damage and arrived at a compromise: #6 as being heavy enough to handle about 96% of all lightning strikes, And it was simply more economical to replace damaged equipment than to use heavier ground conductors at all poles....  Note that critical locations like power substations and commercial towers use much larger conductors)
 Flat copper strap, Or even hollow copper tubing can be used as a good ground conductor with good results. (More surface area= low inductance)  In all cases, NO sharp bends in the conductor on it's path to ground!


AWG 10 has a fusing current for a lightning waveform (short pulse some tens of microseconds long) very much higher than the stroke current, except perhaps for a 300kA megastroke, and even there, it's probably good enough.  Larger conductors are used for mechanical reasons, not electrical.

Upon what basis do you recommend #6?  where did the 96% number come from?   If there is a published study I'd like to read it, because I've not run across an economic analysis like that.  Most recommendations basically copy someone else's recommendation, which in turn isn't based on physics or analysis, but more a "it worked, it wasn't too expensive, so we keep doing it" strategy.

Electrical substations use heavy grounding because they have very high fault currents from the power line.  The flashover may be triggered by lightning, but when you have a transmission line carrying 10kA, if it breaks down, it's going to burn for a long time before the breaker trips and interrupts the current.  A very, very different scenario from a lightning impulse which lasts a fraction of a millisecond.  (google for "Lugo station arc"... that's why they have heavy grounding at substations)

More surface area does NOT markedly decrease the inductance.  It *does* reduce the RF resistance for high frequencies, but for lightning, the inductance dominates.  The difference in inductance between a square bar that is 1x1 cm and a flat strip that is 20cm wide by 0.05 cm thick (i.e. same 1 sq cm area) is about 40%. 

Sharp bends don't increase the inductance.  The concern is with arcs from "pointy things"
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K1CJS
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« Reply #10 on: March 31, 2012, 08:26:08 AM »

It is a little light for a lightning strike ground, but perfectly adaquate for draining static charges from the antenna.  Of course, that depending on the kind of antenna that's up there.  Static discharging was probably what the installers were considering when they did the install. 

Since you know a little better, you should replace that wire with what is recommended--a heavier wire--#6.  Solid or stranded doesn't make that much difference since both have their advantages.
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K9KJM
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« Reply #11 on: March 31, 2012, 10:07:00 PM »

[
AWG 10 has a fusing current for a lightning waveform (short pulse some tens of microseconds long) very much higher than the stroke current, except perhaps for a 300kA megastroke, and even there, it's probably good enough.  Larger conductors are used for mechanical reasons, not electrical.

HA HA!  Good one!  So all the commercial tower companies have been just wasting money on solid #2 copper (And much heavier) "For mechanical reasons" to protect from lightning on tall towers?   That is so silly an answer it is a joke.

Upon what basis do you recommend #6?  where did the 96% number come from?   If there is a published study I'd like to read it, because I've not run across an economic analysis like that.  Most recommendations basically copy someone else's recommendation, which in turn isn't based on physics or analysis, but more a "it worked, it wasn't too expensive, so we keep doing it" strategy.

I told you where, The commercial power companies did large studies, I think back in the 30's or 40's.  DO YOUR OWN RESEARCH if you cannot accept that!   I have seen #6 solid copper wire on power poles fused open by lightning strikes on more than one example.  I doubt those were all "Mega" strikes.

To say that #10 is large enough is just bad advice. 



Electrical substations use heavy grounding because they have very high fault currents from the power line.  The flashover may be triggered by lightning, but when you have a transmission line carrying 10kA, if it breaks down, it's going to burn for a long time before the breaker trips and interrupts the current.  A very, very different scenario from a lightning impulse which lasts a fraction of a millisecond.  (google for "Lugo station arc"... that's why they have heavy grounding at substations)

More surface area does NOT markedly decrease the inductance.  It *does* reduce the RF resistance for high frequencies, but for lightning, the inductance dominates.  The difference in inductance between a square bar that is 1x1 cm and a flat strip that is 20cm wide by 0.05 cm thick (i.e. same 1 sq cm area) is about 40%. 

Looks like you also need to do some basic research.

Sharp bends don't increase the inductance.  The concern is with arcs from "pointy things"

That statement makes it very clear you know very little about lightning protection.

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K1CJS
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« Reply #12 on: April 01, 2012, 06:32:16 AM »

Number 6 cable is what is specified in the NEC.
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W6RMK
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« Reply #13 on: April 01, 2012, 02:47:05 PM »

[
AWG 10 has a fusing current for a lightning waveform (short pulse some tens of microseconds long) very much higher than the stroke current, except perhaps for a 300kA megastroke, and even there, it's probably good enough.  Larger conductors are used for mechanical reasons, not electrical.

HA HA!  Good one!  So all the commercial tower companies have been just wasting money on solid #2 copper (And much heavier) "For mechanical reasons" to protect from lightning on tall towers?   That is so silly an answer it is a joke.
I suggest you look up the Onderdonk and Preece equations. 
Quote
Upon what basis do you recommend #6?  where did the 96% number come from?   If there is a published study I'd like to read it, because I've not run across an economic analysis like that.  Most recommendations basically copy someone else's recommendation, which in turn isn't based on physics or analysis, but more a "it worked, it wasn't too expensive, so we keep doing it" strategy.

I told you where, The commercial power companies did large studies, I think back in the 30's or 40's.  DO YOUR OWN RESEARCH if you cannot accept that!   I have seen #6 solid copper wire on power poles fused open by lightning strikes on more than one example.  I doubt those were all "Mega" strikes.
Which commercial power company?  A name.  I've got most of the reports generated by, for instance, the Rural Electrification Administration, which would be a likely candidate for that time span.  You don't think perhaps that economics have changed in the last 80 years?  Copper and labor costs might have changed?

Fused #6 on a *power pole* is a very different situation from an antenna.  For the fault current reason.  On a power pole, a lightning strike can provide an initial conductive path from an energized line to the ground wire, so the high current can persist much longer than from an antenna.  Grounding practices in electrical utilities are different from those for antennas, for good reasons.
Quote
To say that #10 is large enough is just bad advice. 
But hey, it's in the NPFA 70/National Electrical Code.  And the code making panels do know their stuff. There are hundreds of pages of discussion around changes in the code and why they make them.  Someone claiming, as you do, that the code requirement is insufficient has a higher burden of proof.   So show us.
Quote


More surface area does NOT markedly decrease the inductance.  It *does* reduce the RF resistance for high frequencies, but for lightning, the inductance dominates.  The difference in inductance between a square bar that is 1x1 cm and a flat strip that is 20cm wide by 0.05 cm thick (i.e. same 1 sq cm area) is about 40%. 

Looks like you also need to do some basic research.
Hmm, I would point you to the work of E.B. Rosa, in 1907, at the National Bureau of Standards.  He did the analysis of inductance, based on fundamental physics.

You, like many, are confusing AC resistance (flat strips better than round conductors) with inductance (flat strips not that much better than round conductors, for the same cross sectional area).  Hey, up until a few years ago, I thought, too, that flat strips had low inductance.  Then I went out and started calculating it, and gosh, it's not that big a deal.   

Simple example.. Consider a wire AWG10... it has some inductance per unit length.  Now split that into two AWG13 wires (same total cross sectional area). Lay the wires next to each other, and hook them in parallel. What's the inductance?   say that AWG 13 has *exactly the same* inductance as AWG10 (it's slightly more, but let's assume for now).  Two inductors in parallel.. you might think, at first, that the total inductance is 1/2 (two parallel inductors).  but it's not.. The magnetic field from one wire is tightly coupled to the other wire: it's right next to it.  Turns out, the *mutual* inductance is almost the same as the inductance, so what you have is two inductors with twice the inductance, in parallel, so the net is the same as before.

In reality, the inductance is sllightly (emphasis on slightly) less.  AWG 13 has slightly more inductance than AWG 10, and the mutual coupling isn't perfect, but it's still remarkably close.

Go do the math.  Read the book from people who spent their life doing this stuff 100 years ago.
Quote


Sharp bends don't increase the inductance.  The concern is with arcs from "pointy things"

That statement makes it very clear you know very little about lightning protection.
I'm going to say that the inverse is the case.  I can cite the basis for my statements.  Go calculate the inductance or AC resistance of a 90 degree bend. You'll find it's not much different from that from the same straight length of cable.  That is, say I've got a 1" radius 90 degree bend.  That is, the wire in the bend has a length of 1.57 inches (roughly).  The inductance of the 1/4 turn coil is pretty close (as in less than a few % different) from a straight length of wire 1.57 inches long.

Even 180 degree turns don't have that much inductance, beyond the length of the wire involved.  You need to start getting towards 360 degrees before the inductance starts to rise.  It's because inductance is all about magnetic field coupling, and the field from a wire doesn't couple well to a wire that's not parallel (why we put antennas at right angles to avoid interaction).

On the other hand, a sharp bend dramatically reduces the breakdown potential. Breakdown voltage for a curved surface is roughly 70kV/inch for the radius of curvature.  A fairly short wire with a fast rising edge from lightning (rise times of 1-2 microseconds) can have a fairly high voltage induced on it.  (1uSec @ 20 kA @ 1 meter of wire is about 20kV, from L*di/dt).. it's easy to get a potential difference of several hundred kV on a long wire carrying a lightning impulse, and if the surface next to it is at ground potential, then breakdown from the sharp edge is likely.

This, by the way, is why the NFPA 780 (lightning protection) and NEC require lightning down conductors to be spaced from other wiring and metallic things (downspouts, plumbing, window frames). 

But I'm always willing to look at new (to me) analyses and calculations, perhaps that cover some aspect I've overlooked and that hasn't been covered in the literature I've seen.  So if you have that analysis from the 40s.. tell me where I can find it, and I can figure out why what they recommended then is different from what is recommended now.  It very well could be relevant to ham installations (which are different than power stations, FAA control towers, and commercial radio stations: all of which have widely disseminated grounding handbooks and standards)
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K9KJM
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« Reply #14 on: April 01, 2012, 09:09:54 PM »

Quote: "Even 180 degree turns don't have that much inductance, beyond the length of the wire involved.  You need to start getting towards 360 degrees before the inductance starts to rise.  It's because inductance is all about magnetic field coupling, and the field from a wire doesn't couple well to a wire that's not parallel (why we put antennas at right angles to avoid interaction)."

Anyone who talks 180 degree turns while talking about lightning protection proves how little they know about lightning protection!

Same with the "Sharp bends" etc.

Real life lightning protection at high voltage and high power is different from "paper" calculations at low power levels.......



  



« Last Edit: April 01, 2012, 09:13:12 PM by K9KJM » Logged
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