On 1/2/2013 12:21 PM, Jim Lux wrote:
On 1/2/13 8:39 AM, K8RI wrote:
On 1/2/2013 10:12 AM, Jim Lux wrote:
I haven't found the one on the NWS event yet, but there is a comment
about half way down on
http://forums.mikeholt.com/showthread.php?t=131113
by dereckbc referring to a telecom site grounding to minimum spec.
Yeah, I saw that one.. it wasn't clear whether the 4AWG copper wire was
installed correctly
The NWS event I was referring to was part of a Sky Warn training session
a couple of years back, but I have little sense of time so all I can say
for sure is that it was probably some time in the last ten years. I've
not found the specific photos I was referring to, but all I did find
appear to be related to moisture getting into the metal/concrete
interface.
that seems to be a common thread in failures.
I suppose that if you had a failure like this (spalled concrete),
compared to, say, a rod that didn't make good contact with the soil,
you'd never see the "damage" buried with the rod, while the concrete
damage is obvious.
And, of course, neither says anything about whether the thing that was
being protected was damaged, or the relative cost to repair. You could
be in a situation where the rod grounding electrode survives, but the
equipment it's "protecting" is destroyed, or conversely, the concrete
spalls but the equipment survives.
But looking at the odds: I'd place a bet that if the grounding didn't do
its job, it's highly unlikely that the equipment survived unscathed.
Care to place a wager?
Third row down on (Depending on your screen resolution) shows a pier pin
tower base with a chunk blown out http://www.bing.com/images
/search?q=concrete+damage+by+lightning&qpvt=concrete+damage+by+lightning&FORM=IGRE
the one at ecm-web.. the guy anchor with the Ibeam sunk into the pier?
http://ecmweb.com/content/upgrades-tv-station-tower-ensure-247-operation
later on the bing page I found a better article with better pictures
http://www.copper.org/applications/electrical/pq/casestudy/a6137/a6137.html
ALong with the comment that the problem was that the rebar wasn't
adequately bonded, but that the quasi Ufer ground was so much better
that lightning took that path (causing damage) rather than the
conventional driven rod.
Yup!
Yah. The resistivity of the concrete may be high, but when you have a
big block, the overall resistance is pretty low (many thousands of cubic
centimeters) while the contact area with the soil is huge!
And I love this quote:
"Finally, we pulled out and inspected the grounding electrodes at the
anchors and found that several of the exothermic welds had failed.
Several conductors were also broken, possibly by earthmoving equipment
during construction. With all these problems, plus the poor connections
and high ground resistance, it’s no wonder lightning ignored this part
of the grounding system!”
Breaking a weld is quite the chore... what did they do, run a bulldozer
over it.
Or through it! Typically the weld is the strongest part.
Another interesting quote at the end of the article:
"Copper is cheap insurance compared with the equipment damage that could
occur if the system is inadequate."
This is where the difference between a ham installation and a broadcast
installation is most evident.
The magnitude is different, but a few hundred dollars to a thousand or
two more is still pretty cheap insurance compared to a good DX or
contest station. Think of the major DX stations or multi-op contest
stations. Just think of the thousands of top end rigs driving top end
amps. Those stations aren't cheap. They may not represent the average
station, but even then, when a ham has all he/she can afford into a
modest station, it really needs the best protection they can afford. IOW
They can't afford to cut corners here. Cutting corners on grounding is
really false economy!
At a multi million dollar installation
that has a 24/7 requirement, the decision to spend a few thousand
dollars on big copper and ring grounds etc is fairly easy. They spent
more on labor than on the copper most likely. But that's a different
cost and risk model than most hams would use.
With two complete stations (xceiver, amp, & tuner) plus relays, remote
antenna switches and close to several thousand feet of LMR-600 and over
a 100 coax connectors a $1000 in the ground system is still cheap insurance.
With over 600 feet of bare #2 and 32 or 33 8' ground rods with their
associated CadWelds(TM) I have a fair amount invested in the ground
system even if I did install it before the price of copper went up!<:-))
It boggles my mind that hams will spend many thousands of dollars, or
have a station that would cost thousands of dollars to replace and then
scrimp on the grounding system, tower(s), and/or connectors
73,
Roger (K8RI)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|