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

John Edwards jaedwards at gmail.com
Fri Jan 22 12:05:27 EST 2021

All of the DSLAMs that Australian carriers are throwing in the bin right
now have $20 gas-discharge lightning arrestors on every port to comply with
TEBA rules around LSS connections.

I imagine that FTTC has no such requirement because there is no expensive
voice exchange to protect.

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.

If your disconnected FTTC copper line is still part of a bundle that it
connected back in the exchange, then you could be affected by a lightning
strike that is kilometers away.

Even better, since your line is shorter than the long lines back to the
exchange, you are now potentially on the high-voltage side of an un-coiled

Consider also that VDSL2 FTTC CPE needs a more sensitive receiver than
legacy ADSL2+ to achieve the high-order modulations, so it's probably more
susceptible to overvoltage conditions.

Annual reminder: use hierarchical cable sizes when constructing an earthing
system to protect against lighting. Electrical contractors will use a
"bigger is better" rationale to upsell you to larger cables which can
inadvertently redirect lightning to where you don't want it. There are not
many professionals around that really understand this stuff.

I hope the E-waste guys know what's in those DSLAMs and recycle them
appropriately, but that's probably wishful thinking!


On Fri, 22 Jan 2021 at 11:14, Jrandombob <jrandombob at darkglade.com> wrote:

> Yeah, I'd say that's a good bet.
> Aerial lead-ins are always going to be more susceptible to induced
> spikes from nearby lightning than buried cable.
> On Fri, Jan 22, 2021 at 11:30 AM <mike at ozonline.com.au> wrote:
> >
> > On Friday 15th we had 30 FTTC NCDs "fried"
> > in a single 1km2 area due to an electrical storm.
> > No other devices were impacted in the affected households and
> > damage occurred irrespective of whether NCDs were plugged to
> > surge protectors or not.
> >
> > It seems unlikely that lightning hit lead-ins for the
> > affected services.
> >
> > The area has mainly aerial lead-in delivery.
> >
> > Induced power spike?
> >
> > - Michael Bethune
> > Australia On Line
> >
> >
> >
> >
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