w_tom wrote:
> On Jan 23, 11:07 am, m...@sushi.com wrote:
>> Isn't there an issue with the potential difference between the grounds
>> of the two buildings?
>

Excellent information on surges and surge protection (including
lightning produced) is in an IEEE guide at:
http://omegaps.com/Lightning%20Guide...ion_May051.pdf
And one from the NIST at:
http://www.nist.gov/public_affairs/p.../surgesfnl.pdf

>
> The discussion is about an ethernet cable between two buildings.
> Why is this a problem? Lightning striking one building can find earth
> ground, destructively, via equipment in the other. IOW one building
> becomes a lightning rod connected directly to electronics in the other
> building. Damage is created only when current has both in incoming
> and outgoing path through that electronics.
>
But lightning doesn’t have to strike one of the buildings to be a problem.

>
> If every wire connects directly to earth, then no service. If
> every wire inside that ethernet cable connects directly to earth, then
> no ethernet. So we make that earthing connection via a protector.
> The protector is not protection. Earthing is the protection.
> Protector is simply a connecting device. Better earthing and a
> shorter connection to earth make protection better.

If you have a 1000A surge current to earth and a very good resistance to
earth of 10 Ohms the ground reference system will rise to 10,000V above
absolute earth potential. Within a building the protection is largely by
having the ground reference of all systems rise together. That means
having a short connection from the phone and other entrance protectors
to the earthing electrode conductor at the power service. The author of
NIST guide has written “the impedance of the grounding system to ‘true
earth’ is far less important than the integrity of the bonding of the
various parts of the grounding system.”

I agree with w_ that ethernet between buildings should connect to an
entrance protector at each building with a short ground connection to
the earthing wire at the power service. That may not be possible. An
example of a long connection is in the IEEE guide starting pdf page 40.

As Bill and Jeff wrote, fiber is a safer option. Depends on risk, value
of what you are protecting, cost of outage.

> But when one assumed the protector is some kind of
> 'magic box' protection, then damage becomes acceptable.
>
w_ refers to plug-in surge suppressors as a ‘magic box’ because he can’t
figure out how they work. It is explained in the IEEE guide starting pdf
page 40. Both guides say plug-in suppressors are effective.

When using plug-in suppressors all interconnected equipment needs to be
connected to the same plug-in suppressor, or interconnecting wires need
to go through the suppressor. External connections, like phone, also
need to go through the suppressor. Connecting all wiring through the
suppressor prevents damaging voltages between power and signal wires.
These multiport suppressors are described in both guides.

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