Ground resistance tester

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Hello, I have a small old wood fire tower with a 2.5kw portable generator on a rock ledge. the inspector will allow bonding to two of the guy wires support posts in place of ground rods or a ground ring. have six inches of soil to work with. All I need to do it test the resistance, new ground resistance tester are $2,300. a bit out of the range of this job any helpful Ideas? humorous as well.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Locally there are engineering firms and contractors that provide that service and give documentation that is acceptable to the AHJ. You might check with engineers in your area.
 

tkb

Senior Member
Location
MA
Sub out the testing to a testing company.
You won't have to buy the tester or be responsible with the results.
It would also assure the results are accurate.
 

zog

Senior Member
Location
Charlotte, NC
Hello, I have a small old wood fire tower with a 2.5kw portable generator on a rock ledge. the inspector will allow bonding to two of the guy wires support posts in place of ground rods or a ground ring. have six inches of soil to work with. All I need to do it test the resistance, new ground resistance tester are $2,300. a bit out of the range of this job any helpful Ideas? humorous as well.

Call a testing company, not very expensive, typically a minimum service call charge. Where are you located? I can recommend a couple in your area.
 
Thanks all

Thanks all

Thanks all, located in southern Oregon, the clients want to use a small radio antenna (15 watts) for a local radio station. this is a "temporary" means until the main tower can be built. I want to make it safe above all,
even if it for a few months, we are rapidly approaching lighting season here, do not want the responsibility for a smoked station.
 

brian john

Senior Member
Location
Leesburg, VA
Thanks all, located in southern Oregon, the clients want to use a small radio antenna (15 watts) for a local radio station. this is a "temporary" means until the main tower can be built. I want to make it safe above all,
even if it for a few months, we are rapidly approaching lighting season here, do not want the responsibility for a smoked station.

I sent you an IM,

The best grounding in the world may not protect against a stroke of high magnitude and long duration.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110704-1134 EDT

scottp628:

Even if you could get an extremely good ground connection, whatever that might be, what will that do to protect the transmitter?

How is the transmitter powered?
How is the signal being transmitted connected to the transmitter?
Where is the transmitter located relative to the antenna?
How long is the antenna?

Do you have any understanding of what is inside of a transmitter, how the antenna is connected to the internal electronics, and how one tries to protect a transmitter?

Some time ago I did a search with some of the results being:
A search using .... ground rod resistance earth .... on www.google.com produced the following useful sites

http://www.ees-group.co.uk/downloads/LEC-Green- Grounding Manual.pdf
....... good on theory, modification of soil with salts ( NaCl, table salt ), and multi-rod spacing
http://www.eng-tips.com/viewthread.cfm?qid=21381&page=8
....... interesting, to an extent surface area should make a difference, one person claims 1 ohm
http://www.hubbellpowersystems.com/powertest/tips_news/pdfs_best/05-2002.pdf
....... some theory, values, errosion of rod, galvanized
http://www.www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=18;t=004764
....... several different contributers, a question on worms getting fried
http://www.msha.gov/S&HINFO/TECHRPT/GROUND/GROUNDRE.HTM
....... gov report, tough to get as low as 5 ohms, good discussion on how to
....... measure rod to ground resistance

But really this these do not address the lightning problem. Just suppose you could achieve a resistance to somewhere in the earth of 1 ohm. Suppose the direct lightning current is 1000 A. What is the voltage of your grounding electrode relative to that someplace in earth? 1000 V.

Let your transmitter and its power source and any other connections to bring data into the transmitter be totally contained inside a copper box, and only the antenna is exposed outside of the box. If a protective shorting device is placed from the antenna to the copper enclosure such that no voltage will be produced inside the box that would damage anything in the box, then it does not matter what potential the box is raised to relative to any reference point there will be no damage to internal components. I assume this is an AM transmitter in the normal broadcast frequency range. A 15 W transmitter is not much of a transmitter and not very expensive.

A good discussion on transmitter and receiver protection from direct lightning strikes was at:
http://www.harvardrepeater.org/news/lightning.html
....... about ham radio lightning protection
but it is now gone.

What he described was running everything into the house thru a bulkhead at one location with LC filtering on each wire, and I believe some spark gap, and MOV shunts.

But, for a 15 W transmitter in a temporary location is it worth any great effort.

Study Faraday shields, MOVs, LC filters, etc. Ground rods in rock are not your solution.

.
 

Speedskater

Senior Member
Location
Cleveland, Ohio
Occupation
retired broadcast, audio and industrial R&D engineering
An AM broadcast band antenna uses the ground system as one half of the antenna system. Typically the ground consists of a copper mesh under the tower and many very long horizontal radial wires. (maybe 90 wires, each 100 to 300 feet long)
 

zog

Senior Member
Location
Charlotte, NC
An AM broadcast band antenna uses the ground system as one half of the antenna system. Typically the ground consists of a copper mesh under the tower and many very long horizontal radial wires. (maybe 90 wires, each 100 to 300 feet long)

Really? Interesting, what is the typical ground resistance or do they have a spec?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110706-1838 EDT

Speedskater:

Reading the original poster's posts the reference seems to be about a broadcast station, maybe it is something else. See the FCC items at the end.

The original post was concerned with lightning.

You do not have to have a mono-pole antenna system to radiate energy. This application is apparently for very local coverage and is not going to use a 1/4 wavelength vertical radiator in the AM range. Therefore what is the frequency and type of antenna?. They might use a shortened horizontal dipole and tune it.

A ground plane below the antenna does not have much to do with solving the lightning problem.

This is obviously a very short range transmitter at only 15 W. I do not know the rules on unlicensed transmitters in the AM or FM broadcast bands and I don't know how 15 W fits in. See the following on these problems.

From http://en.wikipedia.org/wiki/Title_47_CFR_Part_15
On the standard AM broadcast band, transmission is limited to 100 milliwatts of power (with restrictions on size, height and type of antenna) or, alternatively, under 15.221, if the AM transmission originates on the campus of an educational institution, the transmission can theoretically be any power so long as it does not exceed the field strength limits stated in 15.209 at the perimeter of the campus, 24000/fkHz ?V/m.

Is this 15 W station a licensed station?

http://transition.fcc.gov/mb/audio/lowpwr.html
This provides more information and I do not easily see how a 15 W broadcast station fits this broad outline.

.
 
Thanks again,
The radio station is licensed by the F.C.C. they are building a tower next to the fire tower for their full sized antenna, 195 watts( FM ) at this time, the client has changed plans frequently, I have passed the main job on to a different contractor as I am a one man shop and not able to do a job that large at this time. I am not responsible for the lightning protection, I have told the clients they need to find someone else for that and grounding for the generator IS NOT lightning protection. The person who is installing the tower is doing the bonding, I am only responsible for the grounding / bonding of the generator, Thanks for the links, I will review the links to see what I may have missed, your comments are all greatly appreciated.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110707-1338 EDT

scottp628:

If your only issue is grounding of the generator, then why did you bring up the transmitter?

A frequency in the FM broadcast band has a wavelength of about 3 meters. 1/4 wavelength is 0.75 meters or about 29.5 inches. Thus, a 1/4 wave monopole with a ground plane is about 30" tall, and the ground plane might be 60" or larger in diameter.

If all you are concerned about is grounding the generator, then using the NEC rules you use two ground rods and don't care about ground resistance. I have no direct experience with rock and grounding, but Internet references imply it is very hard to get anywhere close to 25 ohms easily.

.
 
@ Gar I do not have the ability to drive ground rods as this tower is on rock ledge, The AHJ has said to bring two # 6 copper and bond to two guy wire support posts, then test. he wants the test because this is not an approved ground system.

I am using #4, I seen to many temporary installs morf into something permanent

With how this has unfolded I do not believe the new tower will be built on time.

The client had not accurately figured out the cost of a full sized array and are now scaling back.

They want to run the 15 watt antenna because of planning delays over the three year construction window.

Thanks everyone for your patience and help.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
110708-0819 EDT

I used --- ground rod resistance to earth in rock --- in Google and part way down found
http://www.suparule.com/docs/earth_ground_testing_technique.pdf
On quick inspection this appears to be a good well written discussion.
About 1/3 into this paper is a chart on soil resistance.
Rock is the last and its resistivity is very high.

Google has apparently changed its search method and in some cases it is providing very erroneous results.

I do not know how you can satisfy your AHJ. Nor do I have any good idea on you would make a meaningful ground resistance measurement on top of a rock.

Is there any soil?

Suppose you could make a measurement and it was 10,000 ohms. Now what does the AHJ want you to do? There is no significance to my 10,000 ohms, I just pulled it out of thin air, other than it is large relative to the NEC specification and it is not infinite.

.
 

rsorrells

Member
Location
alabama
rsorrells

rsorrells

Hello, I have a small old wood fire tower with a 2.5kw portable generator on a rock ledge. the inspector will allow bonding to two of the guy wires support posts in place of ground rods or a ground ring. have six inches of soil to work with. All I need to do it test the resistance, new ground resistance tester are $2,300. a bit out of the range of this job any helpful Ideas? humorous as well.
we bought some ground resistance testers for @$200.00 and they work really good.
 
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