[AusNOG] Lightning and FTTC - is it really this bad?
jrandombob at darkglade.com
Sun Jan 24 18:59:03 EST 2021
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
On Fri, Jan 22, 2021 at 12:51 PM Ross Wheeler <ausnog at 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.
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