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Posted by bud-- on January 14, 2008, 12:00 pm
w_tom wrote:
>> I didn't say a strike, I said a surge.
>> I've lost 3 tv's at a different location fromlightningsurges
>> (nearby but not direct strike) coming into the tuner section via
>> CATV.
>
> Third, protection is defined in another post on 11 January 2008 in
> the newsgroup newsguy.general entitled "Lightning Strikes" at:
> http://tinyurl.com/22race
> Everything in that post defines what provides TV protection.
Direct strikes to a house are very uncommon and lightning rods are
seldom cost-effective. The post has minimal information relevant to
Red's cable TV.
Excellent information on surges and surge protection is in an IEEE guide at:
http://omegaps.com/Lightning%20Guide_FINALpublishedversion_May051.pdf
And one from the NIST at:
http://www.nist.gov/public_affairs/practiceguides/surgesfnl.pdf
The IEEE guide is aimed at those with some technical background. The
NIST guide is aimed at the unwashed masses.
>
> Fourth, you have assumed lightning surge entered on cable. Then what
> is the outgoing path to earth? Any properly installed cable first
> connects to earth ground before rising up to enter the building.
> Connected to earth means a surge will not seek earth ground,
> destructively, via the TV. What is the incoming surge path? What
> good is a 'sacrificial' amp when cable should already dump the
> incoming surge to earth before entering the building?
As Red points out, a cable entry ground block only grounds the shield,
leaving the center conductor unprotected. The IEEE guide notes that the
voltage between cable center conductor and sheath is then limited by the
breakdown of F-connectors which is typically 2-4,000V. The guide notes
that connected equipment can be damaged at those voltages
A plug-in suppressor, with the cable going through the suppressor, will
clamp the voltage.
Or a ground block that clamps the voltage could be used. These must be
grounded to the common ground point at entry (as below) to be effective.
>
> Fifth, surges typically enter from wires located highest on poles -
> AC electric. Incoming on AC electric, into TV, and out to earth
> ground via tuner and cable. Protection means the incoming wire should
> be earthed before entering the building. That is what one properly
> earthed 'whole house' protector does. Earthed to the same electrode
> that cable TV wire connects.
Not just to the same electrode. The IEEE guide has an example of too
long a ‘ground’ wire from a cable entry ground block to the earthing
wire at the power service starting pdf page 40. The distance to the
common bonding point for power, cable, phone is critical, not the
distance to the grounding electrode. The author of the 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.” If the entry protector for phone or cable is
distant from the power service a short connection is not possible. In
that case, the IEEE guide says "the only effective way of protecting the
equipment is to use” a plug-in suppressors with power plus cable and/or
phone going through the suppressor.
According to NIST guide, US insurance information indicates equipment
most frequently damaged by lightning is
computers with a modem connection
TVs, VCRs and similar equipment (presumably with cable TV
connections).
All can be damaged by high voltages between power and signal wires.
The 2 examples of surge protection in the IEEE guide are for TV/related
equipment with cable connection, and a computer with phone connection.
A cable amp should eliminate the hazard from high voltage on the center
conductor, but would not necessarily eliminate problems with power and
cable ground references being at high voltage with respect to each other.
A power service surge suppressor is a real good idea, but will provide
no protection from the 2 problems above.
--
bud--
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