Jim Lux wrote in
:

Owen Duffy wrote:
"Rick (W-A-one-R-K-T)" wrote in
news

On Sun, 15 Jul 2007 17:23:13 +0000, Walter Maxwell wrote:


This topic has aroused my curiosity. As a grounding device, why
would a solid rod be better than a hollow pipe, except for the
current carrying capability?

Good afternoon, Walt.

The thing is, the current carrying capability for transient events
like lightning strikes should be about the same for the same diameter
pipe or rod, since most of the current is carried in skin effect
anyway.


I think that is flawed thinking.

A lighning down conductor needs to carry something like 20kA for
100ms, so it needs to be substantial enough that it doesn't melt and
remains in place to protect against the next strike.

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.

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).

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.)

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. 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.

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.

Owen