On 2/3/15 8:37 AM, N3AE wrote:
I just passed the local electrical inspection for my tower a few weeks ago. Around
here, it's rare that anyone pulls permits for amateur radio towers, so all of this
was a bit new for the inspector. Our county follows the 2011 NEC and Article 810 of
that version is what he was using. Apparently, he just wanted to see two things. He
wanted to see the tower ground rod (yes, currently just one) bonded to the nearby
(10ft away) ground rod from my detached garage sub-panel, and he wanted to see the
coax shield grounded at the base of the tower (used one of KF7P's very nice coax
grounding plates). The tower is grounded its nearby rod using 2" copper strap
and KF7P strap clamps, stainless at the tower and copper at the rod. I used solid #4
to bond the two ground rods together. The tower foundation is a UFER, having the
rebar cage bonded to the tower anchor bolts using listed direct bury clamps and #4
copper. The electrical inspector never asked about the foundation/UFER.
Search "Challenging Grounding/Bonding Situation" in the archives for a
description of my installation. That thread unfortunately got hijacked to discuss
requirements for grounding subpanels.
So the inspector was happy, but I'm not. I still plan on additional ground rods in a fan
around the tower plus a run of bare #4 copper from the tower to the ground rod at the
service entrance of the house almost 200 feet away. Using #4 more for mechanical
durability reasons than electrical. Plus the usual "SPG" entrance panel at the
house bonded to the overall system ground. I also plan on a transient suppressor at the
base of the tower for the rotor cable and coax transient suppressors at the entrance
panel.
For what it's worth, one of the better articles I've found on antenna grounding
and bonding per the NEC can be found at:
http://www.reeve.com/Documents/Articles%20Papers/AntennaSystemGroundingRequirements_Reeve.pdf
Interesting side note: 2011 NEC Article 810 says you have to have a "listed antenna discharge
unit" unless the coax shield is grounded. As far as I can tell, none of the common 50 Ohm coax
transient suppressors (Polyphaser, Alfa/Delta, I.C.E) are "listed." I've not seen a U.L.
sticker on any of them. Guess if you had a really finicky inspector he might get you on that. But
the Code gives you a way out by grounding the coax shield, which is good practice anyway.
yeah that whole "listed antenna discharge unit" thing...They're sort of
an exotic beast, I've decided. Maybe they made them back in the 1920s
or something.
I'll bet hams are the only people who still use balanced lines where the
"ground the shield" technique won't work.
(In a big Shortwave broadcast setup, like VOA in Delano, where they used
balanced lines, the entire thing is engineered, and they probably make
use of some other exception, or they're not subject to the NEC)
Some inspectors will make you have the grounding of the coax shield
permanent. That is, you can't have a barrel feedthrough in a grounded
plate, and then connect the coax to that.
This is why you have those coax grounding kits from Tessco, Andrew,
etc., where you pull the outer jacket off, wrap bolts, a clamp, or a
flexible plate around the coax shield, and then that's what's grounded.
They have a similar thing for cable TV.
What's kind of inconsistent is that they might make you do the
"permanent ground" thing, but there's no rule saying you can't have any
connectors in the run from the antenna, so you could conceivably have a
connector 2 feet from the ground connection.....
Fortunately, most inspectors are reasonable people..
What you can't do is run the uninterrupted coax in through a hole the
wall and have your grounding on the inside of the wall. (well, you
might.. but the clear intent is that the shield be connected to earth
ground before it goes inside)
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|