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#175511 - 03/03/08 09:51 AM Grounding Issues with Solutions
BPHgravity Offline
Member

Registered: 04/08/03
Posts: 141
Loc: Port Charlotte, Florida
Here is an interesting report recently released on a grounding study. Keep in mind this is for lightning protection and NOT NEC related installations:

 Quote:
Case Study: Grounding Issues with Solutions

This case study is a result of an NLSI client audit in southeastern United States. Lightning-caused damages to sensitive electronic equipment were traced to resistive soils. Architects and engineers had specified typical NEC 250 grounding designs. They were inadequate.

1. Soils Profile Report from C. Hoyle PE, dated 1/31/06:

Site B-1 near F Building. Sand to 15 ft. (NLSI measured 285 ohms resistance.)
Site B-2 near Six Building Complex. Sand to 15 ft. (NLSI sole available measurement was at Building A with 141 ohms resistance.)
Site A-2 near Transfer Switch. Sand to 6 ft. (NLSI test rod experiment measured 73 ohms resistance.)

2. NLSI Soils Measurement Analysis, dated 11/12/07:

Power poles with ground wires - north side of property.
Pole #14 = 260 ohms
Meter nearby = 170; same meter 1 hour after installing solution of Epsom salts = 70 ohms; 24 hours after applying salts = 52 ohms.
New pole = 73 ohms
Pole #15 = 142 ohms
Poles across Hillside Road.
Pole #090 = 196 ohms
Pole # 092 = 93 ohms
Pole to south of property = 124 ohms
Electrical ground at Building A = 141 ohms
At east side rod at new Building H = 285 ohms
NLSI 10 ft. test rod in grass near Transfer Switch = 73 ohms
CCTV enclosure = 19 ohms. Note this earth electrode is a ring ground.

3. Conclusion.

Native "grounds" are composed of highly resistive sandy soils. This poor earthing defeats a direct and preferential path for lightning. As a result, lightning follows other (many) pathways of lesser impedance through equipment.

4. Remedy.

Non-native engineered products (backfills) or salt-treatment of existing soils are the only options to reduce high impedance grounds. Driving additional ground rods will not increase volume of the earth electrode in any meaningful way. Conductive cement is preferred over "chemical ground rods" due to sand porosity which may dilute the effectiveness of soils conditioning.

5. Conductive cement, intended for creating artificial earth electrodes, is available from:

Sankosha Corp. tel 310-320-1661
ERICO Inc. 800-248-9353
Electric Motion Co. tel. 860-379-8515
Loresco Corp. tel 601-544-7490

Ask for MSDS and compare (lowest) sulphur content.

6. Installation of conductive cement.

Auger min. 10 in. dia. cylinder X min. 10 ft. deep.
Must not be installed dry. Install as a heavy slurry.
Install slurry to 12 in. below grade.
Install ¾ in. X 10 ft. copper-coated steel ground rod with 6 in. of rod above the top of the slurry and remaining 6 in. below grade.
Install access box approx. 18 X 18 X 18 with traffic-rated cover.
When concrete with rod is fully set-up, Cadweld and/or bolt connecting bonding straps to sources requiring grounding.
Inspect all bolts for tightness annually.
_________________________
Bryan P. Holland, ECO.
Secretary - IAEI Florida Chapter

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2014 / 2011 NEC & Related Books and Study Guides
#175516 - 03/03/08 10:49 AM Re: Grounding Issues with Solutions [Re: BPHgravity]
gfretwell Online   content

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
When the state was building lightning targets (radio towers and toll booths) they used 40' multipart rods to get down in the water table. All of the buildings in the complex were bonded together (more important than actually being "grounded").
As a similar experience, in my other life, I was presented with the problem of protecting point of sale terminals in dispersed buildings on a property and in some cases buildings that "grew" from several separate ones (separate services). Again our salvation was bonding the whole cludge together with fat wire bonding conductors (4ga or so).

I'm sure the whole mess went up and down with local lightning events but all going up and down together prevented blowing interface cards and that was our objective.
In that regard, think 680 equipotential bonding concepts.
You don't really care what the potential is, only that everything you can touch is the same. Birds on a wire.
_________________________
Greg Fretwell

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#175518 - 03/03/08 11:52 AM Re: Grounding Issues with Solutions [Re: gfretwell]
BPHgravity Offline
Member

Registered: 04/08/03
Posts: 141
Loc: Port Charlotte, Florida
I completely agree Greg as we have with this very discussion on other topics and forums.

Electrical and electronic equipment could care less if it has a high potential to a remote object. As long as nearby and interconnected equipment are bonded to it, no current can flow from a difference of potential.

This particular report is case specific. In general, grounding (earth) resistance is not of much concern when comes to the dynamics of lightning and other surging events.
_________________________
Bryan P. Holland, ECO.
Secretary - IAEI Florida Chapter

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#175524 - 03/03/08 02:46 PM Re: Grounding Issues with Solutions [Re: BPHgravity]
gfretwell Online   content

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Burt Lavin was the engineer on the I75 MM99 tool plaza project. He can give you all the details on the grounding and bonding there but it was substantial. It was so far beyond article 250 that my inspections were just an academic exercise.
_________________________
Greg Fretwell

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#175526 - 03/03/08 03:42 PM Re: Grounding Issues with Solutions [Re: gfretwell]
BPHgravity Offline
Member

Registered: 04/08/03
Posts: 141
Loc: Port Charlotte, Florida
I had the opportunity to visit the 911 call center in Winter Park about a year or so back. The grounding installation on the radio tower has been highly praised for it's design. I was pretty impressed but found much of it to be over-engineered to a degree.

I also got to visit the SW Florida International Airport early in its construction while they were installing the LPS on the parking garage. That too is an interesting and complex installation. The best part is that it is still very accessible for anyone to view and admire. Just go up to the third (top) level of the garage and take a look. Nearly all of the air terminals, down conductors, and various connectors can be viewed up close any even touched. (Not that I would suggest that on a summer afternoon)
_________________________
Bryan P. Holland, ECO.
Secretary - IAEI Florida Chapter

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