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

Thomas Jones me at thomasjones.id.au
Mon Jan 25 10:23:31 EST 2021


There is definitely a relay internally, not sure what it's actually there for though - could be for applying power to the line when attempting to power the DPU, if a short is detected it can disconnect quickly.

Kind regards,
Thomas Jones 

-----Original Message-----
From: AusNOG <ausnog-bounces at lists.ausnog.net> On Behalf Of Matt Perkins
Sent: Monday, 25 January 2021 9:17 AM
To: Jrandombob <jrandombob at darkglade.com>
Cc: AusNOG at lists.ausnog.net
Subject: Re: [AusNOG] Lightning and FTTC - is it really this bad?

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



-- 
/* Matt Perkins
       Direct 1300 137 379     Spectrum Networks Ptd. Ltd.
       Office 1300 133 299     matt at spectrum.com.au
       Fax    1300 133 255     Level 6, 350 George Street Sydney 2000
      SIP 1300137379 at sip.spectrum.com.au
       Google Talk MattAPerkins at gmail.com
       PGP/GNUPG Public Key can be found at  http://pgp.mit.edu
*/

> 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.
> _______________________________________________
> AusNOG mailing list
> AusNOG at lists.ausnog.net
> http://lists.ausnog.net/mailman/listinfo/ausnog

_______________________________________________
AusNOG mailing list
AusNOG at lists.ausnog.net
http://lists.ausnog.net/mailman/listinfo/ausnog


More information about the AusNOG mailing list