lutfulPremium
join:2005-06-16
Ottawa, ON Reviews:
 ·TekSavvy DSL
| ARRL and Polyphaser lightning protection links ARRL published a 3-part series on "Lightning Protection for the Amateur Radio Station" some time ago that has excellent information for WISPs too.
part 1: »www.arrl.org/tis/info/pdf/0206056.pdf
part 2: »www.arrl.org/tis/info/pdf/0207048.pdf
part 3: »www.arrl.org/tis/info/pdf/0208053.pdf
End of part 3 actually has a little bit on non-tower sites too:
Numerous Amateur Radio stations will be in locations that make it impractical or impossible to follow some or all of the advice given so far in this article.
For those stations there are some things that you can do to achieve a reasonable level of protection for your equipment.
First, establish an SPGP for your equipment by creating a box-level schematic of your radio station. Identify and procure the appropriate protectors for all of the I/O connections to the radio station. Mount them on a common conductive surface.
Where your installation varies from the ideal may be that
the SPGP cannot be connected to an external ground. As a substitute for an external ground, locate an alternative conductive path. Here are some recommendations for potential sources.
These are in the order of most to least desirable.
• Building steel
• Stand pipe
• Metal cold water pipe
• Metal building skin
• Electrical system safety ground
Even though many of these ground choices are highly inductive and will not function as a good RF ground, the goal is to survive a lightning strike event by ensuring no current flows on the radio equipment I/O connectors.
Polyphaser has a good article for grounding urban rooftop radio sites.
" Special considerations for connecting roof top sites to “earth ground” "
»www.polyphaser.com/NR/rdonlyres/···1024.pdf
Same conclusion as ARRL that building steel is best path for lightning surge current. Same goes for tower steel of course.
Finally this Polyphaser article explores if tower down conductors are useful at all. Conclusion is valid for rooftop down conductor too.
»www.polyphaser.com/NR/rdonlyres/···1027.pdf |
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 UHFAll static, all day, ForeverPremium,MVM
join:2002-05-24
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| Polyphaser published a book about grounding and lightning protection. I bought mine from Amateur Electronic Supply in Milwaukee for about $20. It is worth it's weight in gold and anyone that needs to do grounding should get a copy of this book.
I can't recall the title off the top of my head, it's sitting on my desk at work where I was using it as a reference for a large site grounding project I was spec'ing today. |
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2 edits | said by UHF:Polyphaser published a book about grounding and lightning protection. I bought mine from Amateur Electronic Supply in Milwaukee for about $20. It is worth it's weight in gold and anyone that needs to do grounding should get a copy of this book. I can't recall the title off the top of my head, it's sitting on my desk at work where I was using it as a reference for a large site grounding project I was spec'ing today. Funny, I am actually reading the book. I thought I'd hit up DSLR for a minute inbetween chapters.
"The "Grounds" for Lightning and EMP Protection"
Second edition, Polyphaser Corporation
This one has 22.95 stamped on the cover.
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"No job is so important, and no service is so urgent that we cannot take the time to do it safely."
-- AT&T
--Safety One Tower Rescue Certified
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lutfulPremium
join:2005-06-16
Ottawa, ON Reviews:
·TekSavvy DSL
| reply to lutful
Ufer method of tower grounding Anyone building a new tower should really ground it using only the rebars in concrete base using the "Ufer" method developed by an US military engineer.
Ufer grounding works better and costs less than using copper grounding rods connected to the steel tower with wire or strap.
Mike Holt site has a good article on this method. »www.mikeholt.com/documents/groun···ufer.pdf
Google provides many other sites: »www.google.com/search?sourceid=n···r+ground |
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 John GaltForward, MarchPremium
join:2004-09-30
Happy Camp
kudos:3 | said by lutful:Anyone building a new tower should really ground it using only the rebars in concrete base using the "Ufer" method developed by an US military engineer. From your own posted source...
"As well as the Ufer ground works, it should not be used alone. We always recommend that radials or radials with ground rods be used as the main ground system and that the Ufer ground be used to further reduce the ground resistance of your system. Many tests have been done, dating back to 1968, which prove that the Ufer is a safe and very effective way of augmenting a ground system."
His point being since it is there...why not use it.Ufer grounding works better and costs less than using copper grounding rods connected to the steel tower with wire or strap. But it does nothing to lower the surge impedance of the grounding system. Ground resistance alone is not the only criteria for an effective grounding system.
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LLigetfa
join:2006-05-15
Fort Frances, ON
kudos:1 | I don't think rebar in concrete should be used as ground. There is something about the chemicals in concrete and electrolytic corrosion. |
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John GaltForward, MarchPremium
join:2004-09-30
Happy Camp
kudos:3
2 edits | said by LLigetfa:I don't think rebar in concrete should be used as ground. There is something about the chemicals in concrete and electrolytic corrosion. Actually, in many states it is a mandatory electrode and must be installed and connected to the grounding system.
Now, as to whether that applies to "tower bases" is a question that must be answered by the local inspection authority. But, as stated, it cannot hurt a thing to use it as an additional electrode.
EDIT: Additional plus emphasis
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LLigetfa
join:2006-05-15
Fort Frances, ON
kudos:1 | Actually, there is a difference between grounding and "using as a ground".
I came upon this document on this forum.
»www.itrainonline.org/itrainonlin···sion.pdf |
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| reply to John Galt
And I would like to present the following, straight from our site construction bible: Motorola R56.
"6.3.1.4
CONCRETE-ENCASED ELECTRODES
Though concrete-encased electrodes (also known as Ufer electrodes) are not required by this standard, they should be used in new construction as a method of supplementing the grounding electrode system. Concrete-encased electrodes (Figure 6-7) enhance the effectivity of the grounding electrode system in two ways: the concrete absorbs and retains moisture from the surrounding soil; and the concrete provides a much larger surface area in direct contact with the surrounding soil. This may be especially helpful at sites with high soil resistivity and/or limited area for installing a grounding electrode system. Requirements for a concrete-encased electrode, if used, are listed below (per NFPA 70, Article 250-50 and NFPA 780, Section 3-13.2).
• Concrete-encased electrode shall be encased by at least 5 cm (2 in.) of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth.
• Concrete-encased electrode shall be at least 6.1 m (20 ft.) of bare copper conductor not smaller than 25 mm2 csa (#4 AWG) or at least 6.1 m (20 ft.) of one or more bare or zinc galvanized or other conductive coated steel reinforcing bars or rods at least 12.7 mm (0.5 in.) in diameter.
• Concrete-encased electrode shall be bonded to any other grounding electrode
system at the site."
Disclaimer: Motorola "Standards and Guidelines for Communications Sites."
Copyright Motorola 2000
FWIW: We rely solely on R56 when we construct sites for people, in addition to our many years of experience.
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"No job is so important, and no service is so urgent that we cannot take the time to do it safely."
-- AT&T
--Safety One Tower Rescue Certified |
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| reply to LLigetfa
Interesting.
Motorola's blurb:
If the guy wire anchor is encased in concrete, keep copper rods a minimum of 61 cm (2 ft.) away. A galvanized rod can be used if desired without any minimum spacing requirements. Ground rods used for grounding guy wires should be galvanized to prevent galvanic corrosion of the guy anchor (See ANSI/TIA/EIA-222-F). This is especially important when the resistivity of the soil is at or below 2,000 ohm-cm (See ANSI/TIA/EIA-222-F, Annex J for more information).
Disclaimer: Motorola "Standards and Guidelines for Communications Sites."
Copyright Motorola 2000
I am however forwarding your link to our engineering department for review.
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"No job is so important, and no service is so urgent that we cannot take the time to do it safely."
-- AT&T
--Safety One Tower Rescue Certified |
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lutfulPremium
join:2005-06-16
Ottawa, ON Reviews:
·TekSavvy DSL
| reply to LLigetfa
IAEI which claims to be "the definitive magazine for electrical inspectors" has a good interpretation of concrete-encased electrodes. »www.iaei.org/subscriber/magazine···ston.htm
My point is that even the fattest ground wire explosion-bonded to a tower leg ( »www.highenergymetals.com ) will still present a higher impedance path to ground than the very straight tower leg at the Khz frequencies in a lightning strike.
Many recent experiments have shown that very little surge current branches off into the grounding straps - most keep going straight down the tower legs into the concrete base.
The CN Tower in Toronto, built in 1973, is protected by a very effective traditional grounding system but reseachers have observed that the skypod attracts significant streamers (as seen in the photo here: »www.news.utoronto.ca/bin5/040316a.asp ) which travel all the way down through only the concrete/rebar structures.
Building and tower concrete foundations are good enough for dissipating all the lightning surge current into using a properly bonded rebar cage. Some horizontal rebars could protrude out of concrete to eliminate traditional ground radials, or bond to existing radials.
This is a different philosophy that goes against advice from last 50 years, but it actually works even with direct strikes with less effort than building an elaborate traditional grounding system. |
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robbinPremium,MVM
join:2000-09-21
Leander, TX
kudos:1 | said by lutful:...Many recent experiments have shown that very little surge current branches off into the grounding straps - most keep going straight down the tower legs into the concrete base... Please provide links to references |
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| reply to lutful
I have a hard time believing that one should ONLY rely on a ufer style ground system. |
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LLigetfa
join:2006-05-15
Fort Frances, ON
kudos:1 | There is repeated mention of concrete-encased electrodes and of rebar as if they were one and the same! They are not. Copper or galvanized electrodes are designed to resist corrosion. Rebar generally does not although sometimes they are epoxy coated but epoxy being an insulator would not enhance the grounding.
Also, it has been said that copper and concrete are not chemically compatible. Copper plumbing that is embedded in concrete is usually protected with a dielectric jacket. |
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John GaltForward, MarchPremium
join:2004-09-30
Happy Camp
kudos:3 | reply to robbin
said by robbin:Please provide links to references. Yes...please do. I am patiently waiting to reply...
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lutfulPremium
join:2005-06-16
Ottawa, ON Reviews:
·TekSavvy DSL
1 edit | reply to robbin
»www.astrosurf.com/luxorion/qsl-l···ion4.htm
It seems this ham has bonded radials to tower leg inside the concrete base? He also has some thoughts on Ufer grounding.
robbin and JohnGalt, original post second Polyphaser link has some numbers already.
we placed a 10-foot (3 m) section of Rohn 25 tower over PolyPhaser's “Big Bertha” lightning simulator. We bonded a 10-foot section of insulated #2/0 cable to the surge input side of the tower. The other ends were monitored for the amount of surge current for each path. The setup was 'shot' with 5000 volts. The results show the tower section had four times the #2/0 cable current! Only 19.7% went down the #2/0 cable and 80.3% traveled on the tower
The article sort of ridicules down-conductor(s) clamped up high but the theory and observations apply at any height along the tower.
LLigetfa, steel re-bars are good conductors even with the very common thin layer of rust which actually works to protect against further corrosion by design ...
The IAEI NEC "concrete-encased electrode" analysis link above goes on to answer a few re-bar related questions from the building industry like this one:
The Code establishes the minimum requisite, which means one is required to do at least that much. The answer is yes; a single section of rebar in a length of 6.0 m (20 ft) satisfies the requirements of Section 250.52(A)(3). The minimum length of 6.0 m (20 ft) can be accomplished by multiple reinforcing bars being bonded together using the usual steel tie wire as indicated in the last sentence of this section.
I also just noticed that article starts with a good summary : "The goal when establishing a grounding electrode or grounding electrode system for a building or structure should be to strive for the best possible connection to the earth. Concrete-encased electrodes have a proven record of providing this." |
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