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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