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[TowerTalk] conduit

Subject: [TowerTalk] conduit
From: Nick Pair <>
Date: Tue, 11 Jul 2006 14:31:47 -0700 (PDT)
List-post: <>
I have mentioned the conduit and its ability to choke current in grounds(which 
you don't want to happen ) and coax etc. (which you do want). Others have 
reminded that the outer braid/corrugated jacket of the coax should be grounded 
just above the point where the coax changes direction from down to horizontal. 
The conduit forms a one turn choke coil around the conductor. It does not lower 
the voltage and lessen the jumping power of the strike, it just limits how much 
damage it can do. Its still way, way above the lethal level.
  On telephones, they are the number one cause of injury by lightning. The 
spark gap protectors the phone co. uses are really only good for static bled 
off. Even the newest ones using the current name of customer interface box 
aren't much better and won't prevent death by phone.
  The two required rods at your service entrance are not sized for lightning 
protection just for fault current due to short circuits. The largest wire 
required is a AWG #6 because that's all the short circuit current a rod can 
dissipate in average soil. This current is not the 200 or 400 amps your service 
is rated at, but the short circuit current the utility transformer can deliver. 
This can be 10k amps or better fro a short duration. Utilities also require 
this ground in addition to the ground they use at pole on bare conductor 
neutral in triplex drop wire.
  The NEC and NFPA have separate sections for grounding and lightning 
  A copper water service pipe does not meet the NEC requirements for the tying 
together of your ground systems and would require a appropriately sized wire 
with no problem with the wire snaking its was around any obstacles. Common 
sense tells us that your straight run with UL approved connections would be 
more that adequate in this case of a existing installation.
  In the case of a lightning developed square wave there would be a lot of 
harmonically related RF generated. Think of that switching power supply in the 
computer you are using and how much engineering went into quieting it down to a 
level you can barely accept.
  Well my 2 cents are up, 73

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