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Author Topic: How do you measure lightning ground resistance?  (Read 1667 times)
AG4DG
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« on: June 09, 2005, 10:36:50 AM »

Is there an instrument one can buy to measure the resistance of one's lightning ground?  I've read a few articles that refer to ground resistance values.  An instrument that can measure this would tell you how good your ground is, so you can decide if you need to rearrange things, use wider copper strap, add another ground rod, etc.
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WA6BFH
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« Reply #1 on: June 09, 2005, 11:03:13 AM »

I suppose you could use a “Megger” but, it is probably sort of a useless exercise!

If you design and install a “Ground System” so as to be a good “RF Ground”, it should be more than adequate for lightning protection!
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K5LXP
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« Reply #2 on: June 09, 2005, 11:11:21 AM »

As a primer, I would recommend reading the engineering documents on Polyphaser's site.

Lightning protection is as much about low impedance, as well as low resistance grounds.  Because of the rise time of a strike, inductance in ground connections becomes a significant factor.  I'm not aware of a 'meter' that can be used to judge a ground's lightning protection effectiveness.  This is different than the ground resistance measurements one would do to test an AC safety ground.  They are two separate systems.  For lightning protection one installs a minimum configuration of equipment considered to be standard operating practice.  There is no way to know what is 'enough', and nothing will protect against a direct hit.  Surge and spike mitigation is all one can hope to achieve in most station installations.

Mark K5LXP
Albuquerque, NM
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K5DVW
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« Reply #3 on: June 09, 2005, 11:17:36 AM »

You put a 1 micro-ohm resistor in series and wait for the flash. Each volt you measure is 1 million amps. Rearrange things between flashes and go for the lowest reading.

But seriously, it's too difficult to measure and make anything out of the data. I dont think commercial tower guys measure it. At least not that I've seen.

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K6AER
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« Reply #4 on: June 09, 2005, 11:28:17 AM »

“If you design and install a “Ground System” so as to be a good “RF Ground”, it should be more than adequate for lightning protection!”

WRONG, DEAD WRONG. The RF counterpoise system (mirror image of the antenna radiator) has nothing to do with the ability to couple lightning surge energy into earth. And a good earth (read low resistance) will not provide a proper mirror counterpoise system to a RF radiator.  Most counter poise systems rarely are connected to earth other than for convenience. The issues are not the same.

Surge energy grounding is a separate issue. A good lightning ground needs an earth coupling lower than 5 ohms and lower than one ohm is even better. Your antenna system, when struck by lightning, sets up a voltage divider between the surge strike and ground. The lower the ground coupling resistance the lower the voltage at your grounding demarcation point. This is generally the base of the tower or antenna entry point. The ground demarcation point is where you ground your coax shield, RF surge protectors and controller protectors such as rotor wires or remote coax switch lines.

Ground resistance must be measured with a ground resistance meter. These are complex impedance meters that measure deep ground resistance from several points at once going out from the ground system over 100 feet.  A simple volt ohm meter will not work. Some older resistance measurement meters were called Meggers and had a side crank to produce the high voltage necessary to measure ground resistance. These have been replaced by low voltage units with sophisticated built in amplifiers and metering.

Look at the Lyncole Grounding and PolyPhaser web sites for more information on proper earth grounding and system grounding techniques. Ground resistance meters can be found at Tucker instrument on the web.
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WA6BFH
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« Reply #5 on: June 09, 2005, 11:40:20 AM »

AER, nothing I said made reference to antenna “counterpoises”. You are really confusing the issue, and rather humorously screwing up the concept!

Let me put it this way, if I have a 10 Meter Ground Plane antenna (with a resonant frequency at 28.400 MHz), mounted at a height of 32.96 feet,  what is its impedance relative to ground?
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K6AER
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« Reply #6 on: June 09, 2005, 12:15:23 PM »

John,

In you statement you mentioned, “ If you design and install a “Ground System” so as to be a good “RF Ground”, which can be construed as a RF counterpoise system.

You then followed with the statement, “it should be more than adequate for lightning protection!” which is a reference to lightning surge protection. At that point you have brought the two subjects together.

In answer to your last question, the DC ground impedance of the antenna above ground is the resistance of what the conductor path to ground or that cable resistance of a ground cable which includes the earth coupling resistance of the ground system. In addition you have the inductive impedance of a resonate cable, 32.96 feet which will present a high impedance for the lightning RF energy at about 7 MHz. You antenna resonate frequency has nothing to do with ground resistance which was the question of the original post.
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GHOSTRIDERHF
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« Reply #7 on: June 09, 2005, 12:54:49 PM »

With your tongue!!!
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GHOSTRIDERHF
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« Reply #8 on: June 09, 2005, 12:55:40 PM »

With your tongue!!!
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K0BG
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« Reply #9 on: June 09, 2005, 03:55:06 PM »

Once again, we get confused with DC ground, RF ground, and groundplane. Three different animals altogether. And let's not forget there's a safety ground too, which may or may not provide any of the aforementioned.

A good DC ground could be a poor RF ground, and thus provide little or no lightening protection.

A good RF ground could provide good lightening protection, and usually a good DC ground, but not always.

A groundplane may or may not be a good DC ground, but better be at least a good RF conductor. If it is mounted above ground, it's worthless as an RF ground and a DC ground; a requirement for adequate lightening protection.

A Megger could be used on ground rods to measure the resistance between them, but that  proves nothing with respect to RF ground. If you know how to use it, a good antenna analyzer will give you an indication of an RF ground, a DC ground, and maybe even a groundplane.

I'm with Mark on this. Go to Polyphaser's site and read the data.

Alan, KØBG
www.k0bg.com
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WA4DOU
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« Reply #10 on: June 09, 2005, 05:15:36 PM »

In land mobile radio installations we like for towers to have ground systems of 5 ohms and under. This generally requires extensive ground systems. It isn't unusual for such ground systems to provide complete protection of the equipment installed at such towers, running 24 hours a day for years on end.  
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WA6BFH
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« Reply #11 on: June 09, 2005, 08:23:55 PM »

AER, you're last sentence made sense anyway (he said still chuck'ln from the original responses)!

In the RF ground as I was trying to state it (and I accept that I guess I was deficient in this), I was thinking of a situation such as that for a shunt-fed tower, where you would put at least several ground rods around the base of the tower, or even a more extensive 'earthed' grounding system.

Myself, I would design and lay out both the RF aspects of the grounding system, as well as safety aspects. I would probably not bother with the Megger!
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K9KJM
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« Reply #12 on: June 10, 2005, 12:13:40 AM »

Lots of good replies, I would spend my money on adding more ground wires and ground rods and not worry about trying to "test" the system.........     Great advice to read the notes on the Polyphaser tech notes website, and also check out this one:
http://members.cox.net/pc-usa/station/ground0.htm
After you have about a dozen or so 8 foot deep 5/8" copper clad rods in the ground, All spaced twice the distance apart as the depth (8 foot deep rods spaced 16 feet apart) You will start to have a good low resistance ground system.......
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KB9CRY
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« Reply #13 on: June 10, 2005, 07:22:50 AM »

Here at work we use a special instrument made by Biddle (Fluke also makes one) that uses the "Fall Of Potential - 3 rod" method of measuring grounding systems including soil resistance.  It's actually quite easy to do if you have the correct instrument and a Megger is not one of them.  Go to the following and check out the ground/soil measuring page and you'll find pdf files that describe exactly how to measure ground resistance.

www.biddlemegger.com

I'm in agreement with others that you should thoroughly read and understand the Polyphaser (as well as ICE Products at Array Solutions) technical literature and then do as much as you can to follow them to the letter.  I personally never disconnect and have had surges and nearby super strikes and all equipment keeps ticking.  Don't be fooled by those who are just really lucky and don't do anything for lightning.  Phil  KB9CRY
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AA4PB
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« Reply #14 on: June 10, 2005, 10:11:36 AM »

E = I x R so 100,000A x 5 Ohms = 500,000V across your ground.
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