At 02:52 PM 1/29/2006, Dick Green WC1M wrote:
> > Might be too late, but perhaps your ground radials shouldn't
> > be in the
> > plans to be approved? You're going to install them later,
> > after all the
> > structural work is done.
>
>Not necessarily. I had planned to do it while the concrete cures, which
>takes close to a month.
> > You could point out to the electrical guy that those radials
> > are purely for
> > RF grounding, and don't form part of the electrical or
> > lightning protection
> > grounding system.
>
>Not so. These three radials are designed strictly for lightning protection,
>according to recommendations from various sources, incuding Polyphaser.
>That's why I'm using 1/0 conductor and placing 8' ground rods every 12 feet.
>In addition to being tied to the tower and rebar, they'll be tied to the
>utility box next to the tower. Inside will be a grounding panel to which
>lightning suppressors will be mounted. Of course, I'll tie this ground
>system into any radial system I put down for vertical antennas hanging off
>the tower.
Ahh.. now I understand... these aren't RF radials, they're part of your
electrical grounding system. Well, you need to get the relevant codes and
make sure your design complies with the *local* codes, as interpreted by
the "authority having jurisdiction", i.e. the plan check folks. If it's in
some other book, it might well be a good idea, but you'll have to convince
the local folks of that fact. It is NOT unusual to have local requirements
actually require something that is less safe than some alternate
implementation, but unless you have a lot of time and money, you'll
probably have to lump it. Rules about grounding of temporary generators
spring to mind here.
(Of course, nothing prevents you from "improving" the already constructed
system later, as long as the improvements don't conflict with the code).
> > However, bear in mind
> > that you'll
> > probably have some number of feet of steel guy attached to the anchor
> > before going to the synthetic.
>
>Yes, but I'll be using a fairly short length of steel wire as I'm not
>concerned about vandalism or brush fires at the tower location.
But the building officials might, legitimately, be concerned about things
like a power line falling and contacting the steel guy, or lightning
striking the steel guy, etc. Your job is to convince them that this isn't
an issue, or that your design is actually safer.
> > Or, you can bury it less deep and cover it with concrete, if
> > that's an issue.
>
>200+ feet of concrete?
It's been done... it's a tradeoff between the cost of digging a 4 foot deep
trench a foot wide and a 18" deep trench 4" wide. The concrete doesn't
have to go to the surface. I forget the numbers, but it's something like a
4" or 6" thick slab over the conduit, so, for a 200 ft long, 6" wide by 6"
high ribbon, you're looking at 50 cubic feet, or a bit less than 2 yards of
concrete. 2 yards of concrete might cost less than excavating 15 yards of
dirt, especially if it involves blasting or jackhammer work.
> > yes, the electrical code DOES address low voltage cabling AND
> > also coax and
> > antennas. Go to the Mike Holt website (http://www.mikeholt.com/) and
> > download his lowvoltage handbook (it's free). It covers most
> > of the code
> > issues around low voltage control, coax, phone, etc. along
> > with grounding.
>
>Thanks! This is very helpful.
>
>73, Dick WC1M
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|