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Re: Lightning and neutral grounding



Thanks Jon.  This brings up another question.  In a 240v circuit to an 
electrical range or clothes dryer, The romex type cable through the house 
is a three conductor two phase with neutral/ground circuit.  Here, the 240 
volts appears across the two phases with no current in the third wire which 
goes back to the service entrance neutral and ground.  The third wire is 
the safety ground and normally carries no current.  If this 240v is split 
into two 120v phases at the radio desk, why can't the 120v safety grounds 
from the radio desk be tied to the third wire from the 240v service since 
it is used only as a safety ground?  Does this violate anything?  Perhaps 
in this case the third wire is carrying current because the split load is 
not equal on both sides of the phases.

Jon Ogden wrote:
> 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."
> 
> 


-- 
73, Roy -- W0SL

E-Mail: rdwelch@swbell.net
Home Page: http://home.swbell.net/rdwelch

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