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

Brent Paddon brent.paddon at gmail.com
Thu Jan 21 13:53:23 EST 2021

Agree, and as per the text below (cut 'n pasted from:
maybe part of the problem is that each house has a different earth

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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