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

Johnathon Brandis jcbrandis at gmail.com
Thu Jan 21 22:43:32 EST 2021

Dundas NSW - Have had three jobs at the one place since xmas including a
fire as the result of lightning  - I recall there is another fellow on list
from the area maybe has similar stories - The process is annoying for the
FTTC (can be 5 days waiting, for a quick hardware replacement from the good
friends at Optus)

Have a great night all - JB2

On Thu, Jan 21, 2021 at 1:54 PM Brent Paddon <brent.paddon at gmail.com> wrote:

> Agree, and as per the text below (cut 'n pasted from:
> https://www.cablinginstall.com/cable/article/16465312/ground-potentials-and-damage-to-lan-equipment),
> maybe part of the problem is that each house has a different earth
> potential?
> I'm not an electrical engineer - so I'm assuming the below is correct.
> "Consider the case where a workstation in Building A is sending data to
> another network device in Building B. The ground potential of each building
> will be a function of the impedance of its ground system and the current
> flowing through the ground. The data line, in addition to carrying data, is
> also connecting together the ground systems of the two buildings. If the
> ground potentials of Building A and Building B are different, a ground
> current flows in the data line. This is known as a ground potential
> difference. The voltage level of the data signals is increased or decreased
> by the ground potential difference, causing data transmission errors.
> It is not unusual for a nominal, steady-state ground potential difference
> to exist between two buildings. There are cases where the potential
> difference has burned open data cables because of the current flowing from
> one building ground to another. This usually indicates an electrical
> equipment fault or incorrect building wiring. Weather conditions such as
> rain can affect ground potential differences. The water-saturated soil is
> better able to carry current to earth ground. Note that the improved ground
> conductivity can either improve or worsen the potential difference problem.
> Under normal conditions there should be very little current flowing in the
> ground conductor.
> Transient events are a much greater source of ground potential
> differences. Lightning strikes are the most obvious source and often
> involve the building ground system. During a strike, instantaneous currents
> of 100,000 A are possible. If the strike occurs near Building A, as in the
> example above, some of this current flows through its ground system on its
> way to earth ground. Besides the damage done in Building A, the high
> current impulse will cause an instantaneous rise in the Building A ground
> potential. For example, a 10,000-A lightning current flowing through the
> building`s (ideally) 0.1-ohm ground impedance creates a 1000-V transient
> rise in the ground potential of Building A. The potential difference of the
> two buildings` grounds causes current to flow through any electrical path
> between the two buildings. In this case, a transient surge appears on the
> network cable connecting Building A to Building B. This transient surge can
> last for several microseconds. Any unprotected LAN equipment connected to
> the network cable in Building B will be damaged."
> On Thu, Jan 21, 2021 at 11:30 AM Paul Jones <paul at pauljones.id.au> wrote:
>> > -----Original Message-----
>> > From: AusNOG <ausnog-bounces at lists.ausnog.net> On Behalf Of Karl Auer
>> > Sent: Thursday, 21 January 2021 11:24 AM
>> > To: ausnog at ausnog.net <ausnog at lists.ausnog.net>
>> > Subject: Re: [AusNOG] Lightning and FTTC - is it really this bad?
>> >
>> > There are two ways in to the CPE - the FTTC connection and the power
>> > supply to the CPE.
>> >
>> > The FTTC connections are themselves powered at the curb, and so may be a
>> > conduit for spikes into CPE.
>> >
>> > The likelihood of the cable run from the curb to the CPE getting hit
>> directly is
>> > probably very low, but the likelihood of the power grid getting hit and
>> > sending a spike down the line to either the curb equipment and thence to
>> > the CPE or to the CPE directly is unchanged.
>> > Actually it's probably higher, given the greater number of powered
>> devices in
>> > the network.
>> I would think a direct hit would generally let the smoke out, and then
>> some. Just the EM fields from a nearby strike is enough to damage poorly
>> designed equipment connected to long wires. You should see how much extra
>> protection is provided in something as simple as an alarm system used in
>> tropical storm locations (like Taiwan). Things like spark gaps and spike
>> adsorbers.
>> I'm guessing the designers of the CPE made the same assumption we all did
>> - the copper is not as long so the problem won't be as bad (i.e. save money
>> on protection that is normally used when connecting to phone lines).
>> Paul.
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