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Author Topic: Lightning Grounding  (Read 5203 times)
W3KC
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« on: July 29, 2013, 08:20:49 AM »

My antenna feedline enters the house at the rear of the house
where I have installed 3 spaced ground rods. 
The power enters the house at the front corner of the house. 
I'm planning to run a 2" wide ground strap from the rear ground rods around the house (outside) and connecting it to the utility ground.
The breaker box ground wire exits through the foundation and disappears into the ground on its way down to the utility ground rod. 
I dug down a good ways without locating the top of the utility ground rod.
I had planned to connect my strap to the rod with a Polyphaser clamp (58R-112S).
My options seem to be to keep digging to find the top of the rod (it may be quite deep due to backfill), or simply connect my strap to the ground wire between the breaker box and ground rod. 
My question is if connecting the strap to the wire and not directly to the rod would be sufficient, and if so, what kind of clamp would I use to make the strap to ground wire connection?

73 Chas W3KC

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K5LXP
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« Reply #1 on: July 29, 2013, 10:04:00 AM »

Quote from:  link=topic=91282.msg689397#msg689397 date=1375111249
what kind of clamp would I use to make the strap to ground wire connection?

You shouldn't use strap to connect to your utility ground.  Per NEC it should be #6 solid copper and it should terminate at the rod, not the wire going to the rod.

Mark K5LXP
Albuquerque, NM
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WB4SPT
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« Reply #2 on: July 29, 2013, 12:56:41 PM »

Quote from:  link=topic=91282.msg689397#msg689397 date=1375111249
what kind of clamp would I use to make the strap to ground wire connection?

You shouldn't use strap to connect to your utility ground.  Per NEC it should be #6 solid copper and it should terminate at the rod, not the wire going to the rod.

Mark K5LXP
Albuquerque, NM

I'll argue this.  The NEC might say solid copper, but copper strap is, well, solid.  Does NEC say round copper??  Also, the actual connection to the rod is secondary to the service entrance bonding.  The key item is bonding to the service.  The 4 to 6' of the OP delta to the rod is not particularly of significance.  What is significant is the low Z presented to the entire home neutral.  The rod is many ohms to dirt.  The ohms to the house neutral should be <<.1 ohm.   
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K5LXP
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« Reply #3 on: July 29, 2013, 02:12:39 PM »

The NEC might say solid copper, but copper strap is, well, solid.  Does NEC say round copper?? 

They do specify #6 AWG copper.  AWG is round solid wire.  Doesn't say "or equivalent".


Quote
The 4 to 6' of the OP delta to the rod is not particularly of significance. 

Well, the issue is current capacity but it turns out that only applies to separately derived systems.  I did confirm that in the case of supplemental grounds it is permissible to bond to the grounding electrode conductor (wire from the meter/disconnect to the ground rod). 

There's no advantage to using strap in this application.  It's a safety ground, not a lightning ground.  Save the strap for the lightning ground portion of the system, closer to the feedline entrance. 


Mark K5LXP
Albuquerque, NM
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NN4RH
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« Reply #4 on: July 29, 2013, 04:47:44 PM »


They do specify #6 AWG copper.  AWG is round solid wire.  Doesn't say "or equivalent".


I assume that specification would be the minimum bonding conductor size.

The question then is whether the copper strap we're talking about is more than the minimum.

In terms of surface area, definitely. In terms of cross sectional area probably if the 2 inch wide copper strap is thick enough.

So I think the 2 inch copper strap probably exceeds the minimum requirement.
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NN4RH
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« Reply #5 on: July 29, 2013, 04:53:02 PM »

 
Quote
It's a safety ground, not a lightning ground.

The 2 inch strap he's talking about is not a "ground". It's a bonding conductor between the utility ground and his ground at the entrance to this shack.  

Bonding the shack ground to the utility ground is done for lightning protection.

Making it low impedance by using strap is a good thing.
« Last Edit: July 29, 2013, 04:57:53 PM by WN9HJW » Logged
NN4RH
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« Reply #6 on: July 29, 2013, 04:56:49 PM »


The breaker box ground wire exits through the foundation and disappears into the ground on its way down to the utility ground rod. 
I dug down a good ways without locating the top of the utility ground rod.


The ground rod  might be horizontal and buried about about 3 feet down, which used to be (maybe still is) an allowed grounding conductor.  Or if a dishonest contractor built the house, there may just be a wire stuck in the ground and no ground rod at all.


Rather than dig that deep to find out and connect to it,  it might make more sense to just drive another rod at that point.

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KD0WDH
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« Reply #7 on: July 29, 2013, 10:06:54 PM »


 Or if a dishonest contractor built the house, there may just be a wire stuck in the ground and no ground rod at all.

Rather than dig that deep to find out and connect to it,  it might make more sense to just drive another rod at that point.

Funny you should mention this, could not locate a rod myself. Finally I called the power Co. And they could not locate one as well. For my set-up, I installed 3 rods. One rod for the mains, another with a copper plate and my coax lightning arrestors(power Co. advised to use strap to bond this rod to the mains rod), rod 3 is bonded to the mains with #6 awg copper and is connected to a busbar in the shack for component ground. I don't have a tower, so this is sufficient.
Owned this house for 20 yrs. and never realized there was no ground rod installed. Hope this helps.

73
Cheers,
Shawn
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NN4RH
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« Reply #8 on: July 30, 2013, 02:39:44 AM »



Or sometimes they use the re-bar in the foundation as the grounding electrodes. That's a possibility too if you can't find a ground rod.
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N8BOA
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« Reply #9 on: July 30, 2013, 04:11:59 AM »

NEC knows that Joe Electric won't find a copper strap at the hardware so they reference standard electric wiring techniques.
Mr. Electron is not going to stop at the ground rod and start to recite the NEC. There is much more surface area in a strap then a wire and due to the fast rise and decay time of a Lighting strike it behaves like AC ( You hear it on your radio don't you) The Strap is superior solutin but with that said it is the inspector that has the last word and yes the NEC even says so. So if the inspector say used the wire cause he thinks that Mr. Electron reads the NEC then you use wire i.e. NEVER GET IN A STINK FIGHT WITH A SKUNK.
   
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W6RMK
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« Reply #10 on: July 30, 2013, 05:26:55 AM »


The breaker box ground wire exits through the foundation and disappears into the ground on its way down to the utility ground rod. 
I dug down a good ways without locating the top of the utility ground rod.
I had planned to connect my strap to the rod with a Polyphaser clamp (58R-112S).



The wire coming out of the foundation is likely a concrete encased grounding electrode, or Ufer ground.  There is no ground rod. Your other grounds should be bonded to this with the required conductor.   I wouldn't waste money on strap for the bonding jumper. AC resistance at radio frequencies isn't a real issue for this.  Round wire is preferred for mechanical reasons and it's just less hassle with the inspector, should it come to that.
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WX7G
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« Reply #11 on: July 30, 2013, 05:29:33 AM »

Let's quantify this installation.

The strap or wire between the shack and the AC power ground is to bypass some of the lighting current around the house wiring. The house wiring from the shack to the AC power ground consists of three #14 or #12 wires and is equivalent to a #9 or #8 wire. Compared to the NEC specified #6 wire the resistance of the house path is 2X and the inductance is about 1X. The frequency content of the high current portion of a lightning strike is such that it is the inductance and not the resistance that dominates current sharing between these parallel wires. So, the house wiring and a #6 wire will just about equally share lightning current.

The 2" strap has about one-half the inductance that a #6 wire does. Using 2" strap will reduce the current through the house wiring by 1/3, from a total of 1/2 of the current to just 1/3 of the current.

To satisfy the NEC requirement a #6 wire can be run in parallel with the 2" strap. Or, two #6 wires can be run spaced 2" or more to both satisfy NEC and to provide inductance as low or lower than a 2" strap.
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WB4SPT
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« Reply #12 on: July 30, 2013, 08:59:28 AM »

Look up NEC 810.20.;  It calls for not smaller than 10awg Cu or 8AWG Al, etc.   The 6AWG reference is used for the actual service grounding conductor to electrode path, depending on service amperage rating.  NOT, the antenna to service grounding conductor.  The OP issue is bonding the antenna ground path to the service, not making up the service conductor to electrode.  Strap is best, as noted before for lowest inductance.  The Mike Holt series has great instructions on this.  

"In addition, the lead-in cable from an "outdoor antenna" must be provided with a listed antenna discharge unit (grounding block) located as near as practicable to the entrance of the conductors to the building [810.20]. The discharge unit must be grounded to an acceptable location in accordance with 810.21(F)(1)(a) through (f), with a 10 AWG copper conductor (bare or insulated) that is run in as straight a line as practicable [810.21(E)] to the electrode. Figure 2"

http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/GroundingSatelliteDishandLead-InCables~20020303.htm

http://www.reeve.com/Documents/Articles%20Papers/AntennaSystemGroundingRequirements_Reeve.pdf

I use 2" strap from GA Cu, myself.  
« Last Edit: July 30, 2013, 09:12:50 AM by WB4SPT » Logged
K1CJS
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« Reply #13 on: July 30, 2013, 09:36:56 AM »

The point that most are overlooking is this--that is NOT a discharge path.  It is an equalization path for safety sake--so that the potential between the two ground points will always be zero.  The strap is fine, and even as one person put it--a ten gauge copper wire would be adaquate--but possibly not as safe as a #6 cable. 

It is again for safety sake that the #6 cable be used so that if the charge did follow it, it wouldn't be blown apart as easily as the 10 gauge wire would, and THAT is why most people interpret the bonding requirements as meaning ANY connection between ground rods be a #6 cable.
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WX7G
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« Reply #14 on: July 30, 2013, 09:53:49 AM »

The potential between the two grounds points will not be zero unless the current is zero.

Let's say a lightning strike causes a current di/dt of 5000 A/us between the ground points and that they are tied together with a #6 wire having a length of 50 ft. The inductance of this wire is approximately 6 uH.

V = Ldi/dt = 6 uH (5000A/us) = 30,000 volts

30,000 volts is certainly not zero potential. So, the wire between the ground points is not to maintain zero potential, it is to reduce the current through other paths, such as the AC wiring in the house. As calculated in my earlier post this additional wire reduces the current though the house by a modest amount but does not reduce it to zero.
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