Mea Culpa. That makes perfect sense. I was considering it from an RF perspective wherein the mass of earth would theoretically shield the buried copper. I'd failed to consider that in the case of a ground strike the buried copper presents a low-resistance path through the lumped resistance of earth, so it will be the preferential path for the current to take.
In which case the best I can offer is that perhaps the apparent higher NTD mortality rate in high lightning areas with aerial lead-ins is maybe due to them being more susceptible to higher-frequency components which are induced RF-wise into the aerial cable? Though without solid data it's hard to say if there's actually a real correlation between the aerial lead-ins and failures. Since most aerial cables end up being underground somewhere along the line it could well be a remote ground strike that is to blame and it's just the human propensity for pattern matching telling us there is a correlation. On Fri, Jan 22, 2021 at 12:51 PM Ross Wheeler <aus...@rossw.net> wrote: > > > > On Fri, 22 Jan 2021, John Edwards wrote: > > > Underground copper is probably more vulnerable than aerial to lightning. > > Lightning strikes the ground, not the copper, but a voltage gets induced > > in the copper due to the nearby electromagnetic charge - something that > > doesn't happen in air because it's a fairly good insulator. > > My experience has shown a different path to lightning damage. > > When lightning strikes the ground, or a grounded object, that current > dissipates through the soil, which has a typical resistance of around 500 > ohms per metre. If you have tens of thousands of amps flowing, then ohms > law tells us we have potentially huge potential differences over even > fairly short distances. > > The copper cable has a very low resistance (by comparison). > If that cable happens to be radial (or oblique) to the current path from > the point of entry, the potential difference from one end of the cable to > the other will be hundreds to many thousands of volts. > > Even the insulation of the cable may not be enough to save it, and any > components connected to it which happen to be physically close to the > ground will certainly break down. > > This can happen at distances far further away than magnetic induction > alone would explain. It also explains (to me anyway) why I've seen burried > cables damaged part way along their length (where the greatest potential > difference has been). > > Just my take on it. > R. _______________________________________________ AusNOG mailing list AusNOG@lists.ausnog.net http://lists.ausnog.net/mailman/listinfo/ausnog