burner50
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| reply to KA3SGM
Re: The Science of Effective Grounding...
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  |
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KA3SGM
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| 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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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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Cow Towne
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edit:
November 22nd, @04:11AM
| reply to burner50
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. Generally speaking your ground rods should not be closer to each other than the length of the rod. So with 8 foot rods 10 foot spacing would be fine.
If you are a really anal such as I am you will place a new ground rod at each service (power Telco cable satellite and antennas) in addition to a ground rod at each corner of the building. Those rods are bonded by a bare buried copper wire to form as close to a ring as possible to avoid dead-end-points.
Your minimum wire gauge for grounding and bonding should be #6 bare copper with all connections exothermically welded.
ALL building grounds shall be bonded together to prevent any possibility of a ground potential difference between grounds. Failing to do so is like begging ole mother nature to destroy your electronics.
A common grounding point should be established in the hut/shack and all cables as well as equipment shall be grounded to this point before connection to equipment.
When building your common grounding point don’t forget to insulate it from the wall to prevent any secondary paths to ground.
Where not placed by the service provider place primary surge protectors as close as possible to where the service enters the building but if possible always external to the building to minimize the chance of fire should a hard cross with power cause a catastrophic failure of the surge protector.
Don’t fail to include in the above an entire-home surge protector for the power entering the home. Be sure that protector provides hot to hot, hot to neutral and all lines to ground protection.
Contrary to common belief underground service is not immune to surges and lighting hits. The reality is at least around here that equipment connected to underground residential distribution (URD) is more susceptible to being damaged as FP&L’s engineers have decided that URD transformers don’t require primary surge protection as the primary is buried which is true. However they install streetlights which do get hit and carry that hit to the secondary which is in parallel to the service feeding the homes connected to the URD system.
At least in aerial residential distribution (ARD) the primary acts as a static line taking the hit and passing it to ground via. the primary surge protectors on the ARD transformers. Us folks on URD do not have that luxury.
I am currently (pardon the pun) in the process of re-writing this FAQ »AT&T Southeast Forum FAQ »How can I protect my DSL/dialup equipment from surges? to make it more relevant to Ham radio and will submit for inclusion to the FAQ’s here once completed. In the mean time I have included a couple PDF attachments from my employer on grounding and bonding. Just a little light reading to keep ya busy for now. 
Wayne
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KA3SGM
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| said by Splitpair : 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. Generally speaking your ground rods should not be closer to each other than the length of the rod. So with 8 foot rods 10 foot spacing would be fine. If you are a really anal such as I am you will place a new ground rod at each service (power Telco cable satellite and antennas) in addition to a ground rod at each corner of the building. Those rods are bonded by a bare buried copper wire to form as close to a ring as possible to avoid dead-end-points. Your minimum wire gauge for grounding and bonding should be #6 bare copper with all connections exothermically welded. ALL building grounds shall be bonded together to prevent any possibility of a ground potential difference between grounds. Failing to do so is like begging ole mother nature to destroy your electronics. A common grounding point should be established in the hut/shack and all cables as well as equipment shall be grounded to this point before connection to equipment. When building your common grounding point don’t forget to insulate it from the wall to prevent any secondary paths to ground. Where not placed by the service provider place primary surge protectors as close as possible to where the service enters the building but if possible always external to the building to minimize the chance of fire should a hard cross with power cause a catastrophic failure of the surge protector. Don’t fail to include in the above an entire-home surge protector for the power entering the home. Be sure that protector provides hot to hot, hot to neutral and all lines to ground protection. Contrary to common belief underground service is not immune to surges and lighting hits. The reality is at least around here that equipment connected to underground residential distribution (URD) is more susceptible to being damaged as FP&L’s engineers have decided that URD transformers don’t require primary surge protection as the primary is buried which is true. However they install streetlights which do get hit and carry that hit to the secondary which is in parallel to the service feeding the homes connected to the URD system. At least in aerial residential distribution (ARD) the primary acts as a static line taking the hit and passing it to ground via. the primary surge protectors on the ARD transformers. Us folks on URD do not have that luxury. I am currently (pardon the pun) in the process of re-writing this FAQ » AT&T Southeast Forum FAQ » How can I protect my DSL/dialup equipment from surges? to make it more relevant to Ham radio and will submit for inclusion to the FAQ’s here once completed. In the mean time I have included a couple PDF attachments from my employer on grounding and bonding. Just a little light reading to keep ya busy for now. Wayne AMEN Brother...Thanks !!
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Splitpair
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Cow Towne
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| reply to KA3SGM
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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Splitpair
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Cow Towne
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Re: The Science of Effective Grounding... A few more links
»www.eeel.nist.gov/817/pubs/spd-a···uide.pdf
»www.eeel.nist.gov/817/pubs/spd-a···ning.pdf
»www.nist.gov/public_affairs/prac···sfnl.pdf
»www.eeel.nist.gov/817/pubs/spd-a···link.pdf
Wayne
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If you cannot fix it with a buttset and some beanies you ain't a technician. |
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dandeman
join:2001-12-05
Chapel Hill, NC
·AT&T Southeast
edit:
December 2nd, @12:17PM
| Good reading.. especially links 2 and 4 above..
Both point out a concept I feel strongly about..
That is, even with whole house protection at the service entrance and applicable surge suppressors on phone, cable, etc there tied to common ground... as distance to electronic devices throughout the house increases, there is an increasing need for "surge reference equalizers" (to use their terminology) to be installed AT THE LOCATION of any equipment having multiple paths (e.g. power and telephone line/cable, etc).
As the distance to the equipment grows and the enclosed area in the loop formed by the multiple paths to a piece of equipment grow larger (borrowing from Ampere's Circuital Law for any so interested) otherwise in simple terms, the loop works like a 1 turn transformer winding... the magnetic field from any nearby 18kA statistcal average lightning hit makes use of this one turn transformer winding quite effectively and to the detriment of the equipment at the end of the loop. Without an SRE, the connected equipment closing the loop, becomes the fuse in series with the loop. With the SRE, it closes the loop ahead of the equipment leaving the equipment on a short protected stub.
The distributed SREs in turn pretty much mandates a perimeter or some other pretty solid bonding system to ground the distributed systems together.
Not a new idea, see it fairly often in commercial installations. This concept was a major consideration in designing my system pictured in the previous post..
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