One good reason is that copper is anodic to zinc and steel. You will get
preferential corrosion of the zinc and eventually of the steel - eventually
corroding the steel until it is unable to support its weight. At least that
is what I was thinking....
Scott, AKA KB0FHP
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of jimlux
Sent: Sunday, August 02, 2009 7:39 PM
To: John Kemker
Cc: Pete Stark K4OM; towertalk@contesting.com
Subject: Re: [TowerTalk] bonding to Rohn tower
John Kemker wrote:
> Why? Prime Directive: Always follow the Manufacturer's instructions.
Hmm... what mfr are we talking about here? I can see using the bolts that
the mfr of a tower recommends to hold sections together. But here, we're
talking about a system design recommendation.
>
> The manufacturer recommends using strap and clamps. Polyphaser and
> ICE both, that is. Polyphaser is out to sell things, sure. However,
> they'd probably sell the non-strap solution for just as much as the
> strap solution, so I'd tend to trust their engineering over guesswork
> about what a few microhenries might or might not mean to my house and
gear.
But, then, why would the electrical code and NFPA780 (the lightning rod
code) not mention the use of copper strap. I don't see any *published*
engineering analysis on ICE's website nor on PolyPhaser's, while there is
copious engineering analysis published in connection with the electrical
codes.
I'd note that most electrical codes require the use of only "listed"
components in a system, and those parts from ICE and PP aren't listed.
PP's copper strap listed online doesn't even tell you how thick the ribbon
is. Is it 0.01", 0.1", etc.
In one note, they say: "Where any wire ground path is used, the copper strap
will have lower inductance. Radials on bare mountain tops -- the strap's
sharp edge will allow a lower voltage side flashing to the bare rock than
wire."
The usual lightning protection specs for structures actually have a lot of
detail on why you want to suppress side flash. That sharp edge is actually
counter to the good engineering in that situation. Sure, in a case where
you're running from the base of a tower to a ground rod a few feet away (the
case in the original post), I'll bet it makes no difference.
I do see that PP has a 58R-112S clamp designed to attach their strap to a
ground rod, which would replace the need for an exothermic weld, although it
would mean you'd have to periodically inspect it to make sure the bolts are
tight.
And, how many hams follow PP's instructions and make sure that their copper
strap is routed with bends at least 8" in radius (right out page
6 of their ham radio ap note). How would you support that bend so that it
is mechanically secure.
> That way, when the insurance agency comes around to settle a claim, I
> don't have to justify why I didn't follow the manufacturer's
> recommendations for ground system. ICE doesn't sell the strap at all,
> but discussions on the phone with them indicate that they strongly
> recommend using strap over wire or braid. Therefore, the probability
> of there being a conflict of interest in their case is pretty much nil.
You're going to tell the insurance company that you followed the
manufacturer's "discussions on the phone", or the manufacturer's "printed on
paper and we stand behind it legally" instructions?
Actually, I doubt either makes a heck of a lot of difference to whether the
insurance company will settle the claim, unless you've done something
particularly egregious. In fact, I'd imagine that nobody can cite a case
where there was *no lightning protection at all* on an antenna and the
insurance company used that as a justification to not pay. If you're in a
situation where the insurance company is looking to see whether you properly
torqued the bolts to mfr spec and did your annual inspections with
independent QA, you're in a world of hurt.
>
> Besides, a coworker used to design ground systems for broadcast towers
> before he discovered he enjoyed programming and the higher paycheck it
> brought more than he enjoyed designing ground systems. He recommended
> copper straps over wire or braid, based upon the skin-effect of the
> high-frequency components of a lightning strike. You have way more
> surface area on the strap than you do on any of the braids or wire.
I'm not sure this is really the case.
1) ground systems for commercial installations (e.g. broadcast towers) have
different requirements than those for ham installations. Yes, they have
*some* similarities, but for instance, I think the FCC 120 radials style
ground is driven not by lightning protection (for which it would be
overkill) but by RF radiation efficiency.
2) Surface area of a 1" strap isn't much different than that of a 4/0 cable.
A thin strap will be cheaper than a 4/0 cable, but now you have the "how do
I connect the strap to my ground rod using an exothermic weld" problem
(which was the original question).
> More surface area means less resistance/impedance to the energy,
> meaning more gets diverted to ground and less makes its way into your
shack.
> Considering it's a relatively inexpensive addition to the overall cost
> of the tower project means I'll stick with the Manufacturer's
> Recommendations.
I'd just like to know the origin of this whole "thin copper ribbon"
grounding thing. It doesn't stand up very well to analysis, so I'm thinking
that it was borrowed from somewhere else, driven by other considerations,
and happened to work, so became "common knowledge".
If you were worried about impedance at 100 MHz, then your ribbon vs cable
starts to be more relevant. If you are trying to make your lightning and RF
ground the same, maybe that's more relevant.
But from a straight lightning protection or electrical safety bonding
standpoint, either seems that it would work, and I'd go with the
mechanically more secure approach, which means cadwelds, round cable, etc.
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