Thread: Bonding help
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Old November 10th 07, 03:30 AM posted to rec.radio.amateur.antenna
w_tom w_tom is offline
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First recorded activity by RadioBanter: Apr 2007
Posts: 3
Default Bonding help

On Nov 8, 5:49 pm, scooterspal wrote:
At the time I looked into my needs I was directed to read the NEC
codebook... articles 800, 810, 820 and 830 that deal with communications
wiring.


NEC is about wiring for human safety. However that ground wire is
also installed for lightning protection - from a direct strike. Those
who fail to first learn numbers will post only their feelings. That 6
AWG is sufficient to earth a direct lightning strike and still remain
intact.
... consider that a bare 18 AWG (1 mm diameter) copper wire, in
air, normally will conduct at least 10 amperes safely, with very low
self-heating temperature rise. If the current slowly rises, the
temperature will increase until the melting temperature of 1065 C
(1950 F) is achieved at about 83 A. This same temperature could
be reached "instantly" by an 8x20 s pulse at a current of 61 kA.


Above numbers are from the front page of a professional engineering
publication (Electrical Engineering Times) of 1 Oct 2007. That 6
AWG can be expected to conduct 200,000 amps which is ten times more
than current from the typically lightning bolt.

Meanwhile, every wire entering the building must connect to earth
ground before entering the building - also for lightning protection.
Even underground wires. Principles are summarized in a figure
entitled "The Need for Coordinated Protection":
http://www.erico.com/public/library/...es/tncr002.pdf

Unfortunately your WiFi antenna will act like a Ben Franklin
lightning rod. When (not if) lightning strikes it, then energy must
be dissipated somewhere. Either energy is dissipated harmlessly in
earth, or that energy is dissipated inside the house. How much energy
is dissipated in earth? Answer is determined by the quality of
earthing as the other end of that 6 AWG wire.

Often better is to mount the antenna so that lightning will strike a
higher mounting pole; to make a connection to earth that need not pass
through the antenna.

Better is to also route that WiFi wire to the building earth ground
before entering the building. Any transient that might try to enter
the transceiver via that antenna wire would first dump that energy
into earth via a ground block and short connection to the earthing
electrode. Dump energy into earth before wire enters the building.
Should you not do that, then better is to have that WiFi mounting pole
higher so that lightning will strike the pole; less likely to conduct
through antenna.