Gary,

As far as the NEC requiring grounding and bonding of structures, They
want to be sure that there is a continuous bond on things they are
concerned with. They don't always consider what they are grounding.

Do you really think that placing a down conductor of #6 wire on a
tower with a 6 or 8 foot face (big tower) is going to make any
difference in the impedance path that the lightning is going to see.

I never suggested such a silly thing! You mistake an earlier reference I
made to a bonding conductor. Down conductors are sized according to code
standards which provides a minimum for given elevation categories. 3/0 wire
which is close to 1/2" in dia is the largest reqirement in NEC sizing table
250.66. In telecommunication applications, down conductors are normally
sized to equal or exceed the size of the feed lines, and this means larger
sizing yet. It's bonding conductors that burying usually serves no purpose
except protection. One bond that is an exception is the station ground to
utility entrance ground - that bond will carry ground potential rise current
that hopefully bypasses the power connection at the back of station
equipment. It must remain low impedance and high current capability, so
additional ground rods are required along that bond if farther than 20'.
Don't sell the NEC short. NFPA, which oversees the National Electrical Code,
doesn't specify maximum protection, it specifies minimum protection
standards. Industry does better where it sees cost benefit from doing so.

Even if the added down conductor did carry a large part of the current
it would get coupled to the tower anyway before it reached the bottom.
That is what happens to the coax lines in reverse. Any energy that the
tower is carrying is coupled to the coax lines whether they are
grounded to the tower or not. You ground them at multiple points to
prevent flashovers between the lines and the tower.

Actually this is bonding Gary, an important distinction to understand. A
device is only grounded at a grounding electrode. All other connections are
bonding for equipotential, to minimize voltage differences. The principles
of bonding are not taught in the amateur radio or any other communications
hobby. Only electrical enginners, electricians, and anyone who studies the
electrical code and reference materials on bonding and grounding will
understand this. It's hard to even communicate about lightning protection
until the basics of protection by equipotential are understood, and this
doesn't come from a casual read or anything learned in ham-world postings.
At least not from what I've seen, which is about everything a search engine
can find.


You can not keep the lightning energy off the coax lines or any other
lines coming down the tower. They are all mutual. When those lines
leave the tower at the bottom they are going to have some energy on
them unless you have a perfect ground at the bottom of the tower.

A grounded antenna will keep voltage levels on the center conductor at
a safe level. No need for a protection device at the top.

I didn't write the specifications that major companies are using, and
neither did the companies selling lightning protection, although I agree
there is influence there. Professional Engineers write these to protect
equipment, personnel, and maintain operations, maybe not in that order. If
they specify arrestors at tower tops, they must be trying to avoid damage
that was not protected without them. It is no guarantee that damage is
eliminated by their presence, I'm just relaying their usage - now.


I agree that there is a lot of mis-information on lightning floating
around. But don't cut all the hams short either. Some of them have
lots of experience in this area.

I don't cut all hams short. Richard Harrison (resident guru here on RRA) and
others are thanked on my website for great information, their experiences,
and helpful answers to questions. Many hams provided me with details about
damage to equipment, and their humble honesty cannot be thanked enough. I
have written many the best of the amateur websites discussing grounding and
offered suggestions and references to improve their quality .

Here are a few excerpts you or others may have missed from the NEC:

Article 810.51 of the current NFPA 70, which states in part...

"810.58 Grounding Conductors - Amateur Transmitting and Receiving Stations.

Grounding conductors shall comply with 810.58(A) through (C).

(A) Other Sections. All grounding conductors for amateur transmitting and
receiving stations shall comply with 810.21(A) through (J).

(B) Size of Protective Grounding Conductor. The protective grounding
conductor for transmitting stations shall be as large as the lead-in but not
smaller than 10 AWG copper, bronze, or copper-clad steel.

(C) Size of Operating Grounding Conductor. The operating grounding conductor
for transmitting stations shall not be less than 14 AWG copper or its
equivalent."

and Article 810.15, which states

"810.15 Grounding.

"Masts and metal structures supporting antennas shall be grounded in
accordance with 810.21."

finally, 810.21, which states

"810.21 Grounding Conductors - Receiving Stations.

Grounding conductors shall comply with 810.21(A) through (J).

(A) Material. The grounding conductor shall be of copper, aluminum,
copper-clad steel, bronze, or similar corrosion-resistant material. Aluminum
or copper-clad aluminum grounding conductors shall not be used where in
direct contact with masonry or the earth or where subject to corrosive
conditions. Where used outside, aluminum or copper-clad aluminum shall not
be installed within 450 mm (18 in.) of the earth.

(B) Insulation. Insulation on grounding conductors shall not be required.

(C) Supports. The grounding conductors shall be securely fastened in place
and shall be permitted to be directly attached to the surface wired over
without the use of insulating supports.

Exception: Where proper support cannot be provided, the size of the
grounding conductors shall be increased proportionately.

(D) Mechanical Protection. The grounding conductor shall be protected where
exposed to physical damage, or the size of the grounding conductors shall be
increased proportionately to compensate for the lack of protection. Where
the grounding conductor is run in a metal raceway, both ends of the raceway
shall be bonded to the grounding conductor or to the same terminal or
electrode to which the grounding conductor is connected.

(E) Run in Straight Line. The grounding conductor for an antenna mast or
antenna discharge unit shall be run in as straight a line as practicable
from the mast or discharge unit to the grounding electrode.
/clipped

73,
Jack Painter
Virginia Beach VA