KA3SGM
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West Chester, PA
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| reply to burner50
Re: The Science of Effective Grounding...
said by burner50  :When i put up an aerial mast I was planning on digging an 18 inch trench and sinking in Four 8' rods 18 inches apart and welding 8 gauge copper to them. I've been told its overkill, but i've lost 3 computers and 2 tv's to lightning this year along with other major appliances. Been trying to get the electric company to put in a new ground, but they say since my service is underground it doesnt matter very much... Except the service goes aerial to underground 1 pole away from my house  I believe the National Electrical Code specifies that individual ground rods be installed 6 feet apart, so a 6x6 square with 4 rods at the corners would be just the ticket.
I am also sold on #6 or larger copper for grounding, ideally a copper braid or strap, as it is far less likely to fuse 'open' under a high current strike.
Anyone else have a better idea??
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burner50
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EN22wm
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edit:
November 22nd, @12:53AM
| Guess i never consulted the NEC...
never heard of that either...
We put in a similar field at my last job except we used 24 rods, but this was for a freestanding structure not an aerial antenna.
Edit: A 15x15 structure with about 20 million worth of electronic equipment |
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KA3SGM
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| said by burner50 :Guess i never consulted the NEC... never heard of that either... We put in a similar field at my last job except we used 24 rods, but this was for a freestanding structure not an aerial antenna. Edit: A 15x15 structure with about 20 million worth of electronic equipment I think the NEC intent is that a 6 foot spacing between ground rods that are bonded together comprises two separate ground points.
Anything closer than 6 foot is obviously better than a single ground rod, but a cluster of closely spaced rods is considered a single ground point, the 6 foot spacing compensates for differential from one ground point to another.
Many commercial sites have several ground 'test wells' to determine the ground potential differential between different points at the same site.
Still, everything metal above ground is bonded to everything else, and all ground rods are bonded to each other.
This includes towers, buildings, antennas, feedlines, power and telco demarks, even the security fencing.
Everything metallic is connected to ground, and all ground points are bonded to each other.
Ideally, Lightning/Static/EMP flashes over the site uniformly, and finds an effective ground anywhere it touches.
At least that is the theory behind it, not that it always works that way.
Of course when you have 7 to 8 $ figures worth of electronics sitting on the ground, you do what you can to keep everything at an even ground potential, so nothing gets singled out to be 'THE' ground rod.
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Splitpair
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| said by KA3SGM :I think the NEC intent is that a 6 foot spacing between ground rods that are bonded together comprises two separate ground points. With widely spaced rods connected by the proper gauge bond wire an additional rod would halve the resistance under load of the first one a third rod would bring at all down to a third a fourth to a fourth and so on to the point of a diminishing return. However with close spacing such as 6 foot with average soil resistance/moisture the second rod will only lower the resistance under load by about 66% a third rod will make it 40% and the forth 33%.
To get a better understanding think of a ground rod as an antenna turned upside down and shoved into the earth. When passing power to earth the energy leaves the rod and spreads out in all directions from the rod producing a voltage gradient around the rod. If another bonded rod is too close its voltage gradient comes up against the other rods gradient reducing the effects of multiple rods.
The potential in volts per inch/foot varies by soil resistance and soil makeup. Generally speaking moist sandy soil will have the lowest resistance producing tight gradients and good protection.
Wayne
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