Grounding Issues with Solutions

[bphgravity]

Here is an interesting report recently released on a grounding study. Keep in mind this is for lightning protection and NOT NEC related installations:

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 ? 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.

 

[dereckbc]

Brian like you, I have been a student of lightning and grounding for over 30 years. In fact I have specialized in the field for the last 15 years with the Telecom industry and worked with IEEE as a Co-Author of the Emerald Book, and many Telecom industry standards.

While I appreciate you publishing these results, I cannot see any direct correlation or conclusion with the findings. While earth impedance has some importance with respect to lightning grounding system performance, it is not the most important factor in system design. Would really like meeting you someday to discuss SHOP and War Stories.

 

[Pierre C Belarge]

Bryan
With lightning being the type of event it is (it does not necessarily behave the same as man made current and voltage), will it even notice the resistance of 1 & 2 in the information you have provided in your post? Does that resistance really stop the lightning from passing into the earth?

 

[dbchamblee]

Lightning behaves more like a current source than the voltage source most of us are familar with. With a voltage source the applied voltage creates a specified current based on the loads impedance. With a current source, however, the supply (lightning, in this case) delivers a given current in to the load (the earth). The loads impedance determines the resulting voltage drop. A lower impedance will create a lower voltage drop. The reactive components (capacitance and inductance) of a grounding system are more critical during the initial rising edge of the wavefront. The resistance comes more into play during the decaying tail of the waveform (where the bulk of the energy resides). While simply acheiving a 5 ohm (or whatever value) earth resistance will not insure adequate protection, it is also untrue that the earth resistance has little or no effect on lighting protection. See this article from the NEMA Surge Protection Institute:

http://www.nemasurge.com/news/grounding.pdf

 

[bphgravity]

I am glad to see these type of responses.

I too am surprised that the NLSI would publish this finding when the organization has frequnetly commented on the importance, actually, lack of importance of earth impedance when it comes to protection of the structure and its components when it comes to suring events.

This is even supported in the NFPA 780 which is closely reviewed by NLSI members.