[TowerTalk] FW: Tower Grounds

[jimlux]

On 10/17/17 7:53 AM, Clay Autery wrote:
> A good question, Jim...  I suppose the though is that the stainless 
> steel through bolts that bolts the powdercoated steal clevis to the 
> foundation rod and the aluminum tower legs, are a) not copper/aluminum 

Who cares if it's a "good" conductor as long as it's not an insulator or 
semiconductive - that is, wouldn't steel work?  After all, the tower is 
steel, the bolts for a UFER ground are steel, etc.




> and b) may not have the best of electrical interfaces with the tower 
> leg. 

Well, for *lightning* you're not concerned about RF losses - what you 
worry about is physical destruction (I'd not put a layer of bronze wool 
in there to catch fire<grin>) and durability.  So something that 
clamps/welds well is good.

Broadcast towers are a different story - the ground system is part of 
the radiating system, so RF loss is important.


And for electrical safety ground (what the NEC is worried about) you DO 
worry about resistance, sort of - after all, it is perfectly acceptable 
to have the bonding path be galvanized steel conduit all the way from 
the load end back to the connection to the grounding electrode system.

The primary purpose of the NEC bonding system is to ensure that the 
circuit breaker will trip (or fuse will blow) if there's a line to case 
short.  So for 120V, if the resistance is greater than a few (single 
digit) ohms, you won't draw enough current to trip the 15-20 A breaker.

(this is ultimately what caused the MGM Grand Hotel fire in Las Vegas - 
a short from line to poorly bonded electrical box got hot and ignited 
all the wires in the above ceiling area, which was the AC return plenum.)


  But since you mention it, I will consider doing some testing to
> see what the actual measurements are...  Not sure I want any arcing 
> between the bolts and aluminum through holes in my base section 
> though...  I'd prefer no arcing at all if possible.  But I get your 
> point.  Those bolts are very attractive...  and the steel threaded rods 
> protruding 6 foot into the concrete foundation also look mighty 
> attractive for additional bonding points assuming they were properly 
> bonded to the rebar cage and the rebar cage were properly welded 
> together...  Things to think about.  Would be much easier to construct a 
> brazed lug end from the strap end than to make it conform to a round 
> tower leg, etc...

You bet - you can get a crimped on lug, for instance.


> 
> Feedline grounding to the tower is a whole 'nuther animal...  I won't 
> pollute this thread by drifting to that.  <grin>
> 
> 73,
> 
> ______________________
> Clay Autery, KY5G
> MONTAC Enterprises
> (318) 518-1389
> 
> On 10/17/2017 9:31 AM, jimlux wrote:
>> Why not just a lug on one of the bolts? Good clamping force, good 
>> electrical connection, etc.  If you've already bolted the base to the 
>> rods, is there enough to put a lug and a second nut on top?
>>
>> This brings up an interesting point, why is everybody talking about 
>> making "clamps" of some sort to clamp onto a structure which has 
>> bolted joints?  I can see needing a clamp when you're grounding a long 
>> length of hardline that you don't want to break for a connector.  And 
>> I've seen clamps on things like chain link fence posts (typically so 
>> that the gate is bonded to the rest of the fence - the fence posts are 
>> set in concrete in the soil, so they *are* a grounding electrode)
> 
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