Here are some references re resistance goals for tower grounding systems:
From the Motorola "Standards and Guidelines for Communications Sites"
"Motorola recommends a design goal of 5 ohms or less (also see the
International Association of Electrical Inspectors publication, “Soares
Book on Grounding and
Bonding” - 9th Edition, Appendix B and the United States National
Weather Service Manual 30-4106-
2004, “Lightning Protection, Grounding, Bonding, Shielding, and Surge
Protection Requirements”). If
the design goal of 10 ohms (or recommended 5 ohms) cannot be achieved
with the minimum accepted
grounding electrode system, reasonable efforts shall be made to achieve
the design goal using
supplemental grounding techniques."
and regarding where there is substantial agreement among the posters to
this thread
"The resistance of the grounding electrode system is only a general
measure of
merit. Proper design and installation of the grounding electrode system,
installation of ground rings,
ground rods, radial grounding conductors, and the bonding of systems and
equipment, is as important as
the resistance to earth."
Times lightning protection brochure
http://www.timesmicrowave.com/cms/products/times-protect-lightning-and-over-voltage-protection/index.shtml
"There are ground “resistance” test sets available to give you a
measurement value. For
example, a residential ground is considered acceptable at 20 Ohms, and 5
Ohms is thought to be an
adequate tower ground measurement."
This brochure provides a straightforward analysis of the grounding RF
design issues with towers and is the best short guide I have found.
MIL handbook 419A GROUNDING, BONDING, AND SHIELDING FOR ELECTRONIC
EQUIPMENTS AND FACILITIES section 2.2
..With proper design and installation of the EES, the design goal of 10
ohms should be attained at reasonable cost.
...Experience has shown that a grounding resistance of ten ohms gives a
fairly reliable lightning protections to buildings, transformers,
transmission lines, towers, and other exposed structures.
....The lower the resistance, the greater the protection; therefore,
attempts should be made to reduce the resistance to the lowest practical
value.
From ERICO brochure
"IEEE std. 142-1991 recommends system resistance values of 1 to 5
Ohms." (perhaps others have access to the relevant sections and how
they apply to towers)
Grant KZ1W
On 1/10/2016 8:56 AM, Roger D Johnson wrote:
On 1/9/2016 9:38 PM, Grant Saviers wrote:
Roger,
Do you propose that it is not necessary to achieve the recommended 5
to 10 ohms of resistance in a tower ground system?
Recommended by who and for what purpose?
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