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[TowerTalk] Antenna Discharge Unit and NEC Re: Coax for ground wire

To: Mike Short <mike.short@mchsi.com>
Subject: [TowerTalk] Antenna Discharge Unit and NEC Re: Coax for ground wire
From: jimlux <jimlux@earthlink.net>
Date: Thu, 23 Oct 2008 07:41:58 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
jimlux wrote:

> 
> Other stuff not often brought up.. NEC requires an "antenna discharge 
> unit" for each cable coming into the structure, outside or inside, as 
> near as practicable to the entrance of the conductors, not near 
> combustible material, grounded with that AWG10 or larger wire, etc. 
> It's not clear (yet.. i'm researching it) whether an "antenna discharge 
> unit" can be a bulkhead coax feed through in a grounded plate.
> 


OK.. done some more research.  the 2005 code says that a *listed* ADU is 
needed where the wires enter, except that for coax cable, if the shield 
is permanently bonded to the ground, that is acceptable.  Typically 
that's done with a "grounding block".

An interesting question is whether a bulkhead feedthrough would meet the 
need.  If there's a removable connector on the outside, probably not, 
but if you were to "stake" the connector after connecting it (epoxy or 
solder, for instance), would that work?  (the intent is that the 
grounding can't be accidentally disconnected).

If your cable terminated in a chassis mount connector, which is bolted 
to the panel, that would probably meet the intent of the code.  However, 
that's a fairly unusual setup.

Interestingly, it's not clear that if you have the shield permanently 
bonded that you need anything else, from a code standpoint. (the code is 
a "minimum".. you might want something more..)


Interestingly, Polyphaser doesn't have many "listed" products (just a 
network protector, I think), so their products, while useful, aren't 
going help you "meet code" if you're interested in doing so.

Neither would products from ICE, like their open wire arrestor
(http://www.iceradioproducts.com/impulse1.html#2)..

aside from not being listed (so knocked out of NEC right there).. the 
high power unit is in a plastic box, which doesn't meet the NEC 
requirements for non-combustible materials. (the low power unit's in an 
aluminum box, which would.. how's that for an odd irony)

If anyone knows of "listed" units, it would be interesting to take a 
look.  They must exist for commercial installations (I'll take a look at 
the TESSCO catalog when I get to work.)

Jim, W6RMK
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