I was wondering if anyone built some sort of ground resistance tester? I
read that the NEC requires 25 Ohms or less.. But I don't understand how to
measure that. I certainly could use a ohm meter between a ground rod and the
test subject, but then the ground rod I use as a reference could be faulty..
It seems to me it must be measured in some other manner. Maybe low frequency
signal and then measure how much it's absorbed in the earth?.. I know they
sell a clamp on ground resistance tester for big bucks. Considering it's a
clamp on, it certainly don't have any external reference.. Anyway, I was
wondering if anyone had any experience with these testers and maybe building
one?

Joe AG4QC

Ground resistance varies considerably with frequency, and at HF it can
be very different from the DC or 60 Hz value. (For example, pure water
is virtually lossless at DC and low frequency, but very lossy at HF.)
And in order to get a decent representation of ground, you also have to
know its dielectric constant. The only reasonably accurate method I know
of to make these measurements is a system using a buried transmission
line, called an OWL tester. I've never seen or used one, just read some
papers where it was used.

If your need is for an NEC model, you should realize that the skin depth
of current in soil in the HF range is on the order of 10 - 20 feet,
meaning that significant current exists to well below this depth. Soil
is seldom homogeneous, so to make an accurate representation of real
soil, you'd have to test the soil at various depths down to perhaps 30 -
50 feet or so, then make up some sort of average value for the program
to use. I don't believe any single average value will really be a good
representation of a typical stratified ground.

So the bottom line is that even if you could accurately measure the
surface soil conductivity, you wouldn't have enough information for an
accurate model. And in fact, even if you could measure the conductivity
and dielectric constant to a great depth, the model still wouldn't be
very good because of its assumption that the soil is homogeneous.

About the best you can do with currently available software is to
estimate the best and worst likely soil characteristics which might
occur from the surface to a few tens of feet depth. Run simulations with
both. The antenna performance will probably end up somewhere between the
results of the two. Even that isn't guaranteed, though, since any single
value probably isn't adequate to represent a stratified ground.

Ground modeling is the weak point of all NEC-type programs. Don't put
too much weight on results which are strongly dependent on ground
characteristics.

Roy Lewallen, W7EL

AG4QC wrote:
I was wondering if anyone built some sort of ground resistance tester? I
read that the NEC requires 25 Ohms or less.. But I don't understand how to
measure that. I certainly could use a ohm meter between a ground rod and the
test subject, but then the ground rod I use as a reference could be faulty..
It seems to me it must be measured in some other manner. Maybe low frequency
signal and then measure how much it's absorbed in the earth?.. I know they
sell a clamp on ground resistance tester for big bucks. Considering it's a
clamp on, it certainly don't have any external reference.. Anyway, I was
wondering if anyone had any experience with these testers and maybe building
one?

Joe AG4QC

Apparently Joe is talking about contact resistance between a grounding rod
and the soil. Is that correct, Joe? I also saw those ground rod clamp
testers in the Inet, and is still puzzled how they claim accuracy down to
0.01 Ohm.

Thks,
Ivan

Well, yes that's true.. In this case I was talking about the NEC (National
Electrical Code).. Sort of a basic electrical question, from a ham radio
prospective. They put up a Satellite Internet dish on my roof and it was
too far away from the 'Service' protective ground so it's not grounded. So I
ran my own bonding ground wire to tie it into the rest of my single point
grounding system. It started me wondering just how affective all these
ground rods were, even though they are all bonded together. So that started
me looking around for a way to measure the resistance to ground, as I figure
the less the better. Of course, in this case I am talking about 'protective'
ground as required in the NEC handbook. The electricians around here eyes
glaze over when I ask about the 25 ohm or less requirement.

So I figured maybe someone on here had experience measuring the required
ground.. It would be my luck the house would take a minor lightning hit and
the Insurance company would try to weasel out of there responsibilities by
saying the ground didn't meet the NEC requirements not to mention I rather
not have issues. While unlikely, I hear all sorts of horror stories on how
they are trying to disallow claims for all sorts of reasons after Katrina.

Joe AG4QC

"Ivan Makarov" wrote in message
...
Apparently Joe is talking about contact resistance between a grounding rod
and the soil. Is that correct, Joe? I also saw those ground rod clamp
testers in the Inet, and is still puzzled how they claim accuracy down to
0.01 Ohm.

Thks,
Ivan

On Sun, 4 Dec 2005 21:11:23 -0500, "Ivan Makarov"
wrote:

Apparently Joe is talking about contact resistance between a grounding rod
and the soil. Is that correct, Joe? I also saw those ground rod clamp
testers in the Inet, and is still puzzled how they claim accuracy down to
0.01 Ohm.

Thks,
Ivan


Accuracy as in reading + or - 0.01 vs reading of 0.01 ohms absolute -
very big difference in technology involved. ;-) You have to define
what they are doing.

So I figured maybe someone on here had
experience measuring the required ground..

Well, not a lot of experience, but the keywords you're looking for are
"fall of potential".

Depending on which spec you're meeting, you drive multiple ground rods
in at distances that are large compared to the length of the rods.
Then you measure the resistance between the rods.

Just sticking your ohmmeter leads between two ground rods is not good
enough. There is a substantial chunk of 60Hz AC and probably some DC
current flowing in the ground and the little battery inside the
ohmmeter will generally add nothing. Fall-of-potential meters use much
larger currents (and usually measurement techniques, like using an AC
frequency unrelated to 60Hz and harmonics) to overcome this.

My impression with antenna installations that I've been peripherally
involved in is that NEC requirements (25 ohms) are generally easy to
meet unless you've got dry rocky soil. And that the 25-ohm requriement
is nowhere near good enough to protect your equipment/buildings from a
direct hit.

Tim.

If a 60 Hz measurement is the only physical test that is available, that is
better than nothing. The experts say that is useless, but I do not agree
100%.

Once upon a time I used a 12.6 V 60 Hz transformer and an AC ammeter to
measure the current between some ham radio ground rods and the building
utility ground and then calculated a resistance value of about 16 ohms. The
substantial current flow helped to overcome an initial high value of
resistance. The ground path did not seem to be perfectly linear at very low
current values.

What the 2 to 30 MHz values were I have no idea. A high power (100 W ) HF
signal and an RF ammeter would have been a better approach. This test has
to be carefully designed to reduce HF impedance effects.

The best approach, I believe, is to use an antenna design that does not
utilize a ground path return impedance. In a balanced system the return
current is through a transmission line, not the earth. A multi-wire ground
plane with a transmission line return path is also a good approach that is
very often used. Common-mode currents that try to return on the outside of
the transmission line must be attenuated using a current balun.

Bill W0IYH

"AG4QC" wrote in message
...
I was wondering if anyone built some sort of ground resistance tester? I
read that the NEC requires 25 Ohms or less.. But I don't understand how to
measure that. I certainly could use a ohm meter between a ground rod and
the test subject, but then the ground rod I use as a reference could be
faulty.. It seems to me it must be measured in some other manner. Maybe low
frequency signal and then measure how much it's absorbed in the earth?.. I
know they sell a clamp on ground resistance tester for big bucks.
Considering it's a clamp on, it certainly don't have any external
reference.. Anyway, I was wondering if anyone had any experience with these
testers and maybe building one?

Joe AG4QC

It sounds to me like you are looking for legal advice, not technical
advice.

With that in mind, and a lot of bad experience with lightning, I take
the attitude that the NEC establishes the minimums for protection.
After all, what can an extra ground hurt?

Last summer I had lightning rods installed on my house. I live on a
low ridge and have taken several (expensive) hits over the last few
years. They installed ground rods on two corners of the house and tied
one of them to the service ground. They also tied the Antenna tower to
the opposite ground rod. I added another ground (#6 wire) from the
Cable TV ground rod to the service ground. I also tied the Cable TV
ground to the ground rod on the invisible fence (dog containment) with
#6 wire.

With all of that I still don't have a good RF ground. I hope to put up
a vertical antenna soon and the first thing I need to do is establish
a system of radials. Of course, the first wire will be a #6 to the
service ground.

BTW, I don't understand "too far away from the service ground to be
connected."

John Ferrell W8CCW

On Sun, 4 Dec 2005 21:06:16 -0600, "AG4QC"
wrote:

Well, yes that's true.. In this case I was talking about the NEC (National
Electrical Code).. Sort of a basic electrical question, from a ham radio
prospective. They put up a Satellite Internet dish on my roof and it was
too far away from the 'Service' protective ground so it's not grounded. So I
ran my own bonding ground wire to tie it into the rest of my single point
grounding system. It started me wondering just how affective all these
ground rods were, even though they are all bonded together. So that started
me looking around for a way to measure the resistance to ground, as I figure
the less the better. Of course, in this case I am talking about 'protective'
ground as required in the NEC handbook. The electricians around here eyes
glaze over when I ask about the 25 ohm or less requirement.

So I figured maybe someone on here had experience measuring the required
ground.. It would be my luck the house would take a minor lightning hit and
the Insurance company would try to weasel out of there responsibilities by
saying the ground didn't meet the NEC requirements not to mention I rather
not have issues. While unlikely, I hear all sorts of horror stories on how
they are trying to disallow claims for all sorts of reasons after Katrina.

Joe AG4QC

"Ivan Makarov" wrote in message
...
Apparently Joe is talking about contact resistance between a grounding rod
and the soil. Is that correct, Joe? I also saw those ground rod clamp
testers in the Inet, and is still puzzled how they claim accuracy down to
0.01 Ohm.

Thks,
Ivan



John Ferrell W8CCW

AG4QC wrote:

I was wondering if anyone built some sort of ground resistance tester? I
read that the NEC requires 25 Ohms or less.. But I don't understand how to
measure that. I certainly could use a ohm meter between a ground rod and the
test subject, but then the ground rod I use as a reference could be faulty..
It seems to me it must be measured in some other manner. Maybe low frequency
signal and then measure how much it's absorbed in the earth?.. I know they
sell a clamp on ground resistance tester for big bucks. Considering it's a
clamp on, it certainly don't have any external reference.. Anyway, I was
wondering if anyone had any experience with these testers and maybe building
one?

Joe AG4QC




I think others have touched on the key requirement of testing at least
three grounds rods. Then
from the three (or more) readings you can determine each resistance
value. As others have pointed out
you probably need a high current to mask the other currents in the soil.

Cheers.
Bill K7NOM

Here is a good overview technical article that discussing earth grounding
and how to measure it. This info is for 60Hz AC system and equipment
grounding saftey and protection grounding, not antenna ground systems.
Be sure to look at the pictures and figures.

http://www.duncaninstr.com/Gr_article.htm

Here is another useful paper. Item number 14, Ground System Testing.

http://www.kilowattclassroom.com/ArchiveCatalog.htm

Ronnie, N5CSE

"AG4QC" wrote in message
...
I was wondering if anyone built some sort of ground resistance tester? I
read that the NEC requires 25 Ohms or less.. But I don't understand how to
measure that. I certainly could use a ohm meter between a ground rod and
the test subject, but then the ground rod I use as a reference could be
faulty.. It seems to me it must be measured in some other manner. Maybe low
frequency signal and then measure how much it's absorbed in the earth?.. I
know they sell a clamp on ground resistance tester for big bucks.
Considering it's a clamp on, it certainly don't have any external
reference.. Anyway, I was wondering if anyone had any experience with these
testers and maybe building one?

Joe AG4QC