On 11/17/12 4:40 AM, K1TTT wrote:
Dale - WD4IFR
In any lightning strike, even a 2" solid piece of copper takes on the
characteristics of a light bulb filament.
It is simple statements like this that make the rest of what you say
questionable. The fact is that MOST lightning strokes hitting even small
conductors will not significantly heat them. To get any significant warmth
in 2" copper is probably even beyond even the largest measured mega strokes.
I agree... take a look at the Preece or Onderdonk equations for fusing
current. Onderdonk takes into account the pulse length.
BTW -- You do not have to use exothermic welds exclusively for your
ground connections. One of the companies that make crimp on grounding
connectors is "Burndy". These clamps are designed to get away from
Cadwelding but still require the use of a 12 ton crimper to bond the
I have never cadwelded a ground connection, most commercial electricians I
know would probably look at you with complete confusion if you asked them to
do that. The basic brass clamp sold in every hardware store is what
probably 99% of the ground connections in this country are connected with is
just fine. After all a discharge that has just jumped a mile to the ground
isn't going to be stopped by a few microns of oxide in a connection between
two conductors. Even lightning rod installers use easily hand crimped
splices and connectors.
the need for exothermic welding or good clamps isn't for lightning. As
'TTT says, a few thousandths of an inch gap or oxide isn't going to make
any difference.
The good quality bond is for more mundane electrical safety reasons,
when the 110V wire shorts to the metal case, you want the circuit
breaker to trip, or, at least, the voltage on the case relative to your
bare feet to be limited.
Oh, one more while I'm at it:
copper braid that are cadwelded (exothermic) to the tower legs. From
Braid?? Cadwelded?? I always thought this was a no-no. and who runs braid
for a lightning or safety ground? Don't all codes specify solid conductors?
You can use stranded in larger sizes. There's a lot of 2/0 stranded used
for this kind of application, for instance. (because handling solid 2/0
is hard work?)
I'm too lazy to go get my code book and check, but there IS some
threshold size for it.
I wouldn't use welding cable (zillions of tiny strands, so it's real
flexible). I had bad luck using welding cable for pulsed power
applications, although I never spent much time figuring out why.
I wouldn't trust braid for an outdoor application like that anywhere, unless
maybe you are talking about braid that uses something like 10ga strands.
AWG 2/0 stranded...19 strands of something like 16.
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