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.
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.
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:
Ask for MSDS and compare (lowest) sulphur content.
- Sankosha Corp. tel 310-320-1661
- ERICO Inc. 800-248-9353
- Electric Motion Co. tel. 860-379-8515
- Loresco Corp. tel 601-544-7490
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.