On 11/17/2012 10:51 AM, Jim Lux wrote:
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.
I've never seen a conductor melt that has take a lightning strike here.
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'm not sure I'd go quite that far as I've seen holes blown right
through the wings of large aircraft from mega strokes.
I agree... take a look at the Preece or Onderdonk equations for fusing
current. Onderdonk takes into account the pulse length.
Many lightning strikes are a repetition of strokes that add up to
substantial time (relative to the length of a single stroke). Yet I have
never lost a coax tied to an antenna that has taken a hit and that
includes 8X, RG-6 CNT-240, LMR-400, Davis BuryFlexTM) and LMR-600. I
have had insulators fail in N-Type connectors from over voltage.
I have to admit I do not like CN-240 and would never recommend it over
8X. It has a tiny, copper plated, steel center conductor that is easily
broken with cable flexing. The shield is foil with braid, but the braid
consists of very find wires that are easily broken while making connections.
The current is a summation of a rather complex wave form with relatively
steep rise and fall times. Most strikes with thousands of peak amps
probably have less than a hundred amps average for less than one second.
(I'd guess at less than a 1/4 second for the event) If you are
calculating fuse time, how can you not take into account duration which
would be a summation of the current wave form for the duration of the
event. IE you need to know how many jouls the fuse can stand for a
specific interval.
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
Big difference between those crimp connections and clamps.
I have never cadwelded a ground connection, most commercial
electricians I
All of the ground rod connections in the antenna systems here are
cadwelded. The idea is to keep the "voltage" that eventually gets to
the equipment relatively low while attempting to keep all of the
connections going into the equipment at close to the same potential.
There can be thousands of volts difference from one end of the house to
the other, but hopefully everything in any one room including computers
and rigs are very close together. The rigs can be bonded to each other,
but the network, cable (broad band) telephone and USB/serial connections
from computers to rigs needs to be kept within just a few volts which is
a bit more challenging.
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.
I've found those to be dangerous. Electricians and the power company use
them at the entrance/meter. I've never seen one of those connections
last a year at this location. The soil is quite acetic and usually very
moist about 6 months out of the year. After a year you can dig the dirt
away and simply lift the clamp off the ground rod. Not what I'd call
good for safety. OTOH they did require 3 rods and they only used #6
stranded (the green wire from the breaker panel to the meter and ground)
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.
I'm far more interested in the thousand volt voltage drop across that
tiny gap.
The antenna systems that are above the top of a 100' 45G have taken many
hits. (17 verified in the first 6 years, none in the last 5 or 6) On
several occasions the strike has removed all the water proofing from the
connections along with all the silver plating leaving the connectors
with a "sand blasted" appearance and no trace of what they were plated
with. OTOH those 5 runs were all still connected. I lost one
polyphaser with no damage to any equipment.
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.
If memory serves, The GFIs depend on very little current flowing through
the green wire and they are quick. I don't remember which manufacturer
put on the demo, but the guy used his body for grounding. When he took
hold of the second wire the GFI tripped. He said he didn't feel
anything. I don't have that much faith in man made objects.
In the shop the electrician used a GFI duplex outlet as the first in
each string, so it serves as the GFI for the entire string without
requiring an expensive GFI breaker in the panel.
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?
We use all stranded in this area. The electrical inspector really didn't
care what I used, but I went with #2 stranded as it was all I could get.
Nothing was available larger than #6 or #8 solid and that was at Lowe's.
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?)
That too was unavailable so we had to use 3/0 Al from the meter to the
panel. That has a second problem. The Al cold flows so you have to
retighten the bolts. Stretching my memory, after 1 year it took over 1
full turn to get the bolts tight. After another year it took about a
half turn and the 3rd year added another 1/8th to 1/4 turn. 10 years
later I've not been able to do any additional tightening.
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.
Probably all those zillions of tiny connections/diodes in there<:-))
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.
The strands in the #2 I use appear to be about #14. The stuff is about
as flexible as a rod. It's available in various configurations.
73
Roger (K8RI)
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