Re[2]: [TowerTalk] New Tower Construction - Questions
R. Otto
R. Otto" <N8NGA@one.net
Thu, 31 Aug 2000 11:51:37 -0400
Hello K7LXC,
Couple of comments regarding the rebar and ground connections. I am
NOT an expert on this topic <G> so I'll simply quote from the
Polyphaser publication "Lightning Protection & Grounding Solutions for
Commercial Sites, copywrite 2000 - Polyphaser. These are excerpts,
and anyone interested should read the entire text for the complete
story..........
"A Ufer ground could be made by routing a solid wire (#4 AWG) in the
concrete and connecting to the steel reinforcing bar (rebar).
Theoretically, the outermost sections of the rebar structure should be
bonded together, not just tied, If tied, a poor connection could
cause an arc. Because arc temperatures are very high and very
localized, they could cause deterioration of the concrete (cracking
and carbonizing) in that area.
Although possible, this has NOT been the case in practice. The wire
ties are surprisingly effective electrical connections. "one might
think that the ties would fail under fault conditions. However, it
should be remembered that there are a large number of these junctions
effectively in parrellel, cinched tightly." (IEEE Seminar Notes
1970)"
>From this, I take a couple of thoughts:
A) Better get a good clinch tool rather than twisting the tie wires
with my fingers.
B) Better make LOTS of connections, and make them good and tight.
There is a sketch on page 34 of the same publication that shows really
well how the "J" bolts (or tower legs), rebar cage, Ufer ground, and
ground rods should be connected. It suggests using #2/0 stranded
copper pigtails to interconnect Tower Ufer ground to equipment
building ground, rods, radials, etc.
Lastly, on the subject of whether an electrical lighting hit can cause
concrete to explode.....(I was told I was NUTS when I first posted
this thought <G>).......there is evidence that this condition
can occur........but doesn't under normal circumstances if proper
grounding methods are used including tieing the rebar to ground, Ufer
grounds, etc.. Here's another quote from the Polyphaser book:
"A Ufer ground should always be used to augment the lightning
grounding system and not be used alone. For those who are afraid to
use the Ufer, think about this: The heating of the concrete is more
likely if the current is high or concentrated in a given area. Your
towers anchor bolts are already in concrete. If the ground system is
poor, the current density surrounding the bolts could be high, turning
any ambient moisture to steam, and could blow apart your concrete. If
the rebar is tied in to the "J" bolts, the area is increased and the
current density is reduced. (corrosion will be reduced as well.)"
The lesson here as I see it is that many hams have no rebar cage, and
only one to three ground rods. Where this occurs, my logic tells me
that the concrete can in fact be exploded in a direct hit, and that
the ground system is inadequate. My recommendation, after reading the
Polyphaser data is to make a rebar cage in ANY tower base, whether required or
not, and use it as part of a complete grounding system. Rebar is
CHEAP compared to having a tower come down when the base explodes, or
not having the best ground possible for my station.
73's from.......
R. Otto
N8NGA@one.net
Cincinnati, Ohio
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Thursday, August 31, 2000, 11:07:27 AM, you wrote:
Kac> In a message dated 08/30/2000 3:43:45 PM Pacific Daylight Time, N8NGA@one.net
Kac> writes:
>> A) It is advisable to use the rebar cage as PART (not all) of a grounding
>> system. Lightning will in fact go through the tower to the rebar and
>> the concrete for a ground. (interesting note, most building
>> departments now require the rebar in concrete swimming pools to be
>> grounded).
>>
>> B) The concrete, under certain conditions can, in fact, explode
>> exactly as I described and for the reasons I described. When a
>> proper system is done, though, the chance of this happening is very
>> slim.
>>
Kac> The central issue in using the rebar/base as part of a grounding system
Kac> is that it all has to be bonded properly. Simpy using wire ties won't do it -
Kac> everything will still arc. It has to be an exothermic bond like CadWeld or
Kac> similar or it needs to be brazed or welded somehow. I've seen discussions of
Kac> why you shouldn't braze/weld rebar but IMO it's still better than wire-tying
Kac> everything. Most hams won't spend the time or money to use CadWelds and don't
Kac> know how to braze/weld so I would not recommend doing it that way. Installing
Kac> ground rods is easier and can be done by just about any ham so that would be
Kac> my suggestion.
Kac> Cheers, Steve K7LXC
Kac> Tower Tech
Kac> --
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