Re: http://h71000.www7.hp.com/?
- From: "w_tom" <w_tom1@xxxxxxx>
- Date: 17 Jul 2006 21:50:54 -0700
Little difference whether a cable is overhead or underground. An
example. Lightning strikes the other building. Lightning seeks earth
that is some four miles distant from that building. Your building is
in that path. What is the shortest path, electrically, from that
building to earthborne charges four miles away? Via that buried cable
and through electronics inside your building, then to earth, and onward
for four miles. Your electronics are now in a destructive path that
included buried cable.
EMP is so trivial that even an NE-2 neon glow lamp (a bulb rated in
milliamps) will make such transients irrelevant. If a transient
overwhelmed protection already inside appliances, this it was not EMP.
It was a direct strike.
Every incoming wire in every cable that enters a building must
connect to a common earthing electrode, either by direct (hardwire
connection) or via a protector. In most buildings, only one AC
electric wire is grounded. Other AC electric wires are ungrounded (due
to no 'whole house' protector) and therefore connect nearby lightning
directly into electronics. With only one AC electric wire grounded
means AC electric is not sufficiently earthed. Most all buildings do
not have sufficent earthing ('whole house' protectors) on AC electric.
No 'whole house' protector on AC electric means AC electric wires are
not properly earthed.
This figure from an industry professional demonstrates two structures
with single point ground. Even a buried wire must connect to that
earthing before entering a building.
http://www.erico.com/public/library/fep/technotes/tncr002.pdf
Electronics already contain internal protection. Protection that can
be overwhelmed if a destructive surge is not earthed before entering
the building. My apathy for adjacent protectors is because anything at
the electronics that will protect those electronics is already inside
electronics. Experience has demonstrated how adjacent (and not
earthed) protectors even contributed to damage of adjacent (and powered
off) electronics.
Many reasons why telco lines are not 'most often' struck by
lightning. One is that AC electric lines are above telephone lines and
therefore protect those phone lines. A most common source of damage to
telephone appliances are transients that enter on AC electric. There
are exceptions. But this typical example also demonstrates how to
identify those exceptions. Bottom line - the analysis must answer how
lightning traveled to get to earth. The solution must change that path
so that the transient finds earth without entering the building.
Earthing is THE most critical protection component in a building wide
solution.
Dave Froble wrote:
I seem to remember your apathy for the battery backup and surge
protection units. :-)
Still, one does what one can. I have the units supplying power to most
electronics. I use the RJ45 ports in the units for network lines.
The problem is that I'm connecting 4 buildings via underground conduit.
Not very far underground. I think one is about 6 inches deep, another
maybe 12 inches.
Electrical entrances are all properly grounded. The phone lines have
the protection you mentioned. Unless there is a direct contact, I
should be protected. I'm thinking that the several hundred feet in each
conduit just might be picking up an EMP. I never would have thought
this would happen, but I can not find anywhere in the system that isn't
protected.
One note, this last time the telco device must have taken a hit, I
needed to call for repair. I've noted your thought that the transient
may have hit this from inside, not from outside.
.
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