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

[Matt Perkins]

They had a few hundred to replace in the eastern suburbs in the first week of Jan the cable there is almost all underground. If anyone has one and can post a detailed photo of the PCB we can get to the bottom of it but suspect the HV protection is non existent. 

I have heard mention from customers that there is some sort of clicking sound on a dead NTD not sure what that would be why there would be a relay in there. Might be just false info 
Matt



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> On 24 Jan 2021, at 7:00 pm, Jrandombob <jrandombob at darkglade.com> wrote:
> 
> 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 <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.
>> R.
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