Re: Bonding help

w_tom wrote:
On Nov 8, 5:49 pm, scooterspal <t...@xxxxxxxxxxxxxx> 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.

Sort of true..
One can find the equations at http://home.earthlink.net/~jimlux/hv/fuses.htm

Onderdonk's equation takes into account the time course of the pulse, and is probably more relevant here. AWG16 for a 10 microsecond pulse melts at 12.6 kA. AWG6 is 100 times the size, so one might expect 1.26 MA fusing current.

HOWEVER.. in a fast pulse, the current is carried only in the surface of the conductor, which Onderdonk's and Preece's equations don't deal with, nor does the simple approximation in EETimes.

Also, for fast high current pulses, the magnetic fields are important: a) because of the forces on the conductor which tend to deform it and b) because of the effects of inductance.


Note also that the code also allows oddball stuff like AWG17 copperclad steel for the grounding conductor. I surmise that the sizes are chosen more for mechanical ruggedness than for current carrying capacity.


It is true, however, that a short length of AWG6 can carry a typical stroke current without fusing or failing.

.

Relevant Pages

  • Re: Bonding help
    ... NEC is about wiring for human safety. ... also installed for lightning protection - from a direct strike. ... AWG is sufficient to earth a direct lightning strike and still remain ... temperature will increase until the melting temperature of 1065 C ...
    (rec.radio.amateur.antenna)
  • Re: Lightning protection
    ... Pulses are sums of numerous frequencies - numerous sine ... A pulse is not just a pulse. ... That is also true of lightning. ... I never said the earthing wire for a lightning rod must not ...
    (sci.electronics.repair)
  • Re: Lightning protection
    ... sine waves go forever in time. ... exist only during the period of that pulse. ... Wavelet as a better description would be nice. ... even know how to begin to describe a lightning pulse in terms ...
    (sci.electronics.repair)