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Re: Lightning and neutral grounding



Yep, commercial transmitter sites get hit all the time.  We use a db
Products antenna for a lot of our stuff (i.e. SCADA, iDEN) and you'll see
them "splayed" out after a strike, but things still work.

Joel B. Black, W4JBB
w4jbb@charter.net


----- Original Message -----
From: "Jon Ogden" <na9d-2@speakeasy.net>
To: "Roy Welch" <rdwelch@swbell.net>
Cc: "Greg D." <ko6th_greg@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Tuesday, March 11, 2003 10:52 PM
Subject: Re: [amsat-bb] Lightning and neutral grounding


> Roy,
>
> You have it connected correctly.  You described it correctly.
>
> What destroys things in a lightning strike is voltage potential.  The
amount
> of actual current in a strike is not as great as one would think.  I
forget
> what Polyphaser says a typical strike current is, but it is less than you
> would think.
>
> You have a lot of electon charge energy though that needs to be dissipated
> in a very short period of time.  The earth is not an immediate ground
sink,
> but rather acts as a lossy capacitor.  This means that there is a finite
> amount of time involved to dissipate the strike energy.  The earth can
only
> handle so much charge at a given time.  This is why you connect your tower
> to ground over a ground network of typically 3 ground rods space 16 feet
> apart on each leg.  This effectively allows the earth to absorb the charge
> more quickly as you are spreading the energy over a larger area.  Perhaps
> that is not 100% technically correct, but it gets the point across.
>
> Now, when you tie all your grounds together (never mind the neutral here),
> you are ensuring that all your grounds rise and fall at the same
potential.
> If you don't do this, the strike energy at your tower, could cause the
> ground at the tower to possibly raise by several hundred volts potential
> above the ground at the service entrance.  The shortest path to that lower
> potential then may just be right through your house and radio.  But with
all
> grounds connected, then everything rises together when the strike occurs
and
> there is no potential difference between grounds.
>
> If you have a properly engineered system of protection with proper
grounding
> and surge suppression there is no need to disconnect anything.  Commercial
> radio sites on very large towers stay on the air all the time and get hit
> all the time.  I'm quite sure that our local TV antennas on the Sears
Tower
> in Chicago get hit all the time as it's the tallest thing around here for
> miles.  Yet, the stations (FM and TV) stay on the air through all the
> storms.  They have proper protection and that's on top of a 1200 foot
> building.
>
> You mention rotor cables.  That's another good point that people tend to
> forget.  The rotor lines can also carry strike energy and so it's
> recommended to have a surge suppressor in your rotor wires as well.
>
> If you chose to disconnect like you say, then that's your choice.  I've
> heard that it doesn't necessarily protect.  And what about the time you
> forget to disconnect everything and go away for the weekend and a big
storm
> comes by?
>
> 73,
>
> Jon
> NA9D
>
>
> on 3/11/03 10:18 PM, Roy Welch at rdwelch@swbell.net wrote:
>
> > If the Neutral line goes to ground at the Service entrance and the house
> > ground wires do also then they are common at the service entrance.  The
> > ground wire then comes back through the house wiring to the ground
terminal
> > on the 3 conductor power lead cord to the radio.  The ground lead goes
to
> > the chassis inside the radio as a safety ground.  The RF ground terminal
on
> > the radio as well as the coax shield (via the antenna) both go to the
> > outside ground rod by the station.  A number 4 wire runs from that
ground
> > rod to the TV antenna ground rod and on around to the service entrance
> > ground rod.  It seems to me that the service entrance grounded neutral
and
> > the safety ground in the house wiring are tied together and fan out from
> > the service entrance.  However there is another path from the house
ground
> > wire back to the service entrance, going via the radio chassis to the RF
> > ground rod and back via the three strapped ground rods to the service
> > entrance neutral.  This has always bothered me, especially now that I
hear
> > thunder in the west :-)  I disconnect all coax leads at my coax patch
> > panel, open the power at the desk fused box, unplug all rotor cables,
the
> > KCT to PC cable and power down all PCs when I hear thunder approaching.
I
> > have a whole house surge surpressor in the service entrance box on the
> > house side of the circuit breakers.  I respect that stuff.
>
> -------------------------------------
> Jon Ogden
> NA9D (ex: KE9NA)
>
> Citizen of the People's Democratic Republik of Illinois
>
> Life Member: ARRL, NRA
> Member:  AMSAT, DXCC
>
> http://www.qsl.net/na9d   <- Updated on 1/22/03!!!
>
> "A life lived in fear is a life half lived."
>
>
> ----
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