[AusNOG] Lightning and FTTC - is it really this bad?

Ross Wheeler ausnog at rossw.net
Fri Jan 22 12:50:57 EST 2021

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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