[TowerTalk] Back of desk grounding buss

[Jim Lux]

On 3/22/14 1:34 PM, Patrick Greenlee wrote:
> Sorry I was unclear. Let me put it more simply. First I am in full
> agreement with your conventional wisdom comment.  Much of the
> conventional wisdom is not wise.  My group grope comment and angels on
> the pin apparently didn't communicate as well with you as the humorous
> agreement I got in a couple PMs. I was indicating that a lot of folks
> with a lot of misconceptions, not unlike the folks debating the number
> of angels on the head of a pin, could profit by the equivalent of
> counting angels with a microscope to get at the reality of the situation
> which in this analogy would be to read the Motorola pub.
>



One thing to keep in mind when reading documents like the Moto R56, or 
the FAA manual, or looking at broadcast industry recommendations is that 
they are making recommendations/requirements that can be flowed out to a 
contractor and which are universally applicable and cover worst case 
scenarios.

The recommendations in those documents are also based on typical 
commercial building practices, including relative costs of labor and 
materials.   From a commercial contracting standpoint, the cost 
difference about whether you put in AWG2, AWG6, or AWG 10 wire probably 
doesn't make as much difference as the cost of the guy or gal doing the 
installation.

They also make recommendations for sites that are typically more complex 
than the usual ham station. And they have to be concerned about 
longevity, maintainability, etc. with a widely varying set of people 
over the coming years.  How many of you have cable trays around your 
shack to carry the dozens of cables above your racks of equipment? 
There's probably a dozen or so list members that have something like that.

There may be requirements in there that are driven by "operational" 
needs rather than a careful optimization based on science.  For 
instance, maybe they have a practice that everything should be 
serviceable with a 7/16, 1/2, 9/16, and 5/8" wrench: that's what's in 
the standard utility toolbox or something. So when the standard is 
calling out bolts, they use ones that can use those wrenches, and not 
smaller or larger.  A friend of mine worked for Briggs and Stratton for 
a while, and they had a design principle that engines destined for the 
third world had to be able to be assembled and disassembled with a pair 
of pliers and a screwdriver.


there's also, often, a "stickyness" to requirements that may have made 
sense in the past, but may not anymore, but nobody is willing to bet a 
potential lawsuit on the change.

The hot activity in the NEC code making panels these days has to do with 
things like solar panels, electric car chargers, and the like.  I doubt 
anyone is reviewing the NEC requirement for copper clad steel of a 
particular size for transmitting antennas.  Those kind of  requirements 
have probably been in the code since the days of King Spark. Since 
nobody is putting up commercial broadcast antennas like that anymore, 
nobody is clamoring to change the code.

The Atlantic Magazine, this month, has an interesting article that talks 
about playgrounds and "recommendations" for safety as the result of an 
unusual event that turned into de facto requirements. It turns out that 
the fatality rate hasn't materially changed over the years since the new 
requirements came in: it was always really low and mostly freak unlikely 
occurrences.

The same is true for official recommendations.. A single bad experience 
can trigger a "lets make sure that never occurs again" even if it is a 
one in a billion chance.

I think we sometimes spend too much time worrying about lightning 
safety, and not enough about power line safety. I've seen ham shacks 
with all kinds of transient suppression on the coax, and automatic 
grounding relays for coax, and all manner of stuff.  But if there were 
to be a medium voltage to low voltage short on a neighborhood 
distribution transformer, the shack would essentially be destroyed.  How 
much of your gear can take a 10-16kV overvoltage on the power cord?
I've had first hand experience of the latter kind of overvoltage twice! 
with substantial damage to appliances and wiring in both cases.  Once 
was an underground transformer with an internal fault, once was a wind 
induced failure of a MV feeder falling across the drop to the house. 
(I'm also lucky to live in southern California, where lightning is rare. 
  Those of you in Orlando probably have a different take on things 
<grin>.  )


And as Jim points out, there's a lot of lore out there that might do OK 
at protecting against disaster (e.g. it's no worse than doing it 
"right"), but makes day to day operation MUCH worse, so over time, the 
protection gets removed, because you're a lot more sensitive to daily 
annoyances than speculative disasters every 20 years.