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

Mon Jan 25 10:45:16 EST 2021

This matches with what I've seen/heard/read - the clicking is the NTD turning off/on power to the DPU, while it tries to check if it's coming online. 

We had a DOA DPU and have had one since after a storm, in ... under three months.

James

On 2021-01-25 09:30 Matt Perkins wrote:
> There’s nothing quick about a disconnecting relay. But I do have the 
> reports of them clicking so perhaps power is applied and they are 
> looking for some condition that does not appear so power is removed a d 
> re-applied. Whatever condition they are looking for can not be sensed 
> due to the fault perhaps there’s some type of op-amp in that sense 
> circuit that’s sensitive to potential difference and the problem lies 
> there. Wherever the problem lies given the massive volumes involved 
> there appears to be a design fault / fit for purpose issue at play 
> here. 
> 
> 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 25 Jan 2021, at 10:24 am, Thomas Jones <me at thomasjones.id.au> wrote:
> > 
> > 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.
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