Owen

The fusing/melting current for 1/2" copper pipe is probably well above
20kA, even for 100ms pulses. A more interesting potential failure mode
might be from the mechanical forces due to the magnetic field. (see,
e.g., quarter shrinking or can-crushing)



Hi Jim,

I note the "probably" in your comment, and the "dunno" in N5MK's
response.
That's because I was lazy and didn't want to actually compute it. I've
put multi tens of kA pulses through 1/4" copper pipe, but they're not
100ms long.



The uncertainty in my statement is over the exact lightning scenario,
they vary, and the circuit response (ie current waveshape, amplitude,
duration, ringing etc) depend on the specific excitation and circuit
elements (parameters of the down conductor, nature of the earth system,
ground, environment etc).

One could certainly use the standard double exponential approximations..
either a 2/50 waveform for a strike or the longer surge impulse (I can't
remember the exact rise/fall times for the surge..)



As far as supposition as to the fusing current for conductors, that is
determinable for a given scenario. I have at hand the Protective Earthing
Code of Practice published by the Electricity Authority of NSW June 1975
and it shows that a 35mm^2 copper conductor has a fault current withstand
of 20kA for 100ms. (I have considered implementing the underlying
formulas in an online calculator.)

Preece or Onderdonk?
(http://home.earthlink.net/~jimlux/hv/fuses.htm

N5MK stated "A #10 wire can handle that job". If he is talking copper, I
understand that #10 means 2.5mm diameter, or ~5mm^2, or less than 15% of
the recommended conductor csa for the stated scenario.


Preece equation gives fusing current for AWG10 (2.5mm diameter, as you
say) as 316 amps, but that's sort of for a steady state.

Onderdonk's equation, plugging in 100 ms for the melt time, gives 4.7kA,
which I can believe. I've blown up a lot of AWG10 wire with those sorts
of currents in a quarter shrinker. Partly melting, partly mechanical
stresses in that application.

The purpose of the National Electrical Code (National, here, referring
chauvinistically to the U.S.) required AWG 6 (diam 0.15 inches, 3.8 mm)
bonding wire for grounds is NOT to carry the lightning current (which it
wouldn't, in most cases) but to carry fault currents from things like
shorts from line to grounding conductor, which are usually in the
hundreds of amps range. Say an energized power line falls down and hits
the antenna. You want the antenna's grounding conductor to carry the
likely fault current and not go open, and carry enough current to trip
any overcurrent protective devices.

Lightning protection is usually things like 2/0 (0.364 inch diameter,
9.25 mm), which has a fusing current (viz Onderdonk) of 65kA.

I am not familiar
with your water pipe sizes. If it were, say, a half inch diameter #19, it
has a CSA of around 35mm^2, so the #10 wire should melt before the pipe
electrode, thus protecting the pipe electrode from failure. Yes,
mechanical forces are also relevant to lightning conductors, but my
comment was about the fusing current.

35 mm^2 would have a fusing current of around 30-35 kA.

1/2" Copper pipe is 0.625" od and 0.545" id (very close to 1mm wall)
so, has about 47 mm^2 area.


In this part of the world there is an Australian Standard (AS1768)
relating to lightning protection, there may be a similar standard or
"code" in other jurisdictions, and they would not be a bad place to start
in understanding lightning protection and designing a protection scheme.

Another source of information is to walk around a mobile phone base
station and look at the earthing system from the outside. It is even more
enlightning (no pun) to look inside. These things withstand lightning
events quite well. Are they over engineered? Probably not, they do suffer
damage from time to time.

It is my view that there is a significant risk that an inadequate
lightning protection scheme may be much worse than doing nothing.

I would agree..



Owen