LIGHTNING CONSULTING FOR WIRELESS LOCATIONS WHICH REQUIRE ENGINEERING DESIGN BECAUSE OF GROUND POTENTIAL RISE (GPR) Part 2 of a two part series

FOR IMMEDIATE RELEASE
Castle Rock, Colorado, United States of America (Free-Press-Release.com) May 2, 2011 -- Lightning consulting recommendations will use both regular Portland as well as Sankosha lo-ohm conductive cement to augment almost any kind of electrode, and the results are easy to calculate and predict and are permanent. Cement is well known to contractors to protect buried metal from corrosion. Not just any kind will do for grounding, however. Conductive cements have over 200 times lower resistivity than ordinary cement, low enough that standard formulas can be used for calculating the resistance of electrodes made with them, just as if the electrodes were made of metal.

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LIGHTNING CONSULTING FOR WIRELESS LOCATIONS WHICH REQUIRE ENGINEERING DESIGN BECAUSE OF GROUND POTENTIAL RISE (GPR) Part 2 of a two part series

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Conductive cement effectively enlarges the wire, creating a conductive plate. A horizontal-strip configuration, or groundbed, and the formula for calculating its resistance. The most common installation procedure follows:

1. Dig a trench, 30 in. deep, 20 in. wide, and as long as required to obtain the desired resistance. (The length is a design calculation, discussed later.) Center a 4/0 stranded wire in the bottom of the trench.
2. Pour in the cement as a dry powder (it will later absorb moisture and harden) by dragging an open bag of it down the trench. Use one 50-lb bag every 10 ft. Heap the cement up as shown.
3.`Lift the wire slightly so it is completely covered by the cement for corrosion protection. Tamp the cement with feet or a shovel toward the tapered edges.

4. Carefully shovel in a 4-in. layer of soil and tamp it down.
5. Push in the rest of the removed soil using construction equipment.

A lightn ing consulting design procedure may be as follows:

1. Decide upon the desired resistance of the electrode.
2. Measure the soil resistivity with an earth tester.
3. Determine the required length from the table, based on the desired resistance (5 or 10 ohms) and the soil resistivity.

A typical pad-mounted wireless site has a buried ground ring around the pad, about 2 ft out from the pad, and another ring around the antenna. The formula given in Figure 4 applies; however, the resistance thus obtained must be multiplied by 1.12 to account for the reduced grounding efficiency of a square ring compared to a straight strip.

Meanwhile, back at the substation, the source of the GPR from power faults, the GPR can be reduced by lowering the resistance of the grounding grid. If conductive cement is used to surround grid wires on a 10-by-10-ft spacing, the grid area can be reduced by 10 or 20 percent, with a concomitant money saving and reduction in the extent of the critical 300-V GPR contour. Use IEEE Std. 80 data or EPRI Substation Grounding Workstation software and assume strip conductors of 2-in.-by-18-in. cross section. For further information, refer to manufacturers' application notes.

Existing ground grids also can be improved by extending the grid area by 10 or 15 percent and using conductive cement. In one application in high-resistivity soil, grid resistance was reduced from 10 ohms to 2 ohms. In another, resistance was reduced from 0.96 to 0.2 ohm. Consolidated Edison and Boston Edison have used conductive cement to ground transmission towers and substations.

About 50% of the resistance between a ground rod and remote earth is in a shell within the first 6 in. from the rod. If this shell is shorted out by encasing the rod in 6 in. of conductive cement, as shown in Figure 5, the resistance is halved. This is a good example of how the resistance of any electrode can be decreased without making the electrode longer. This is important wherever bedrock limits the length of ground rods or when property lines limit the length of a horizontal electrode.

If you want to study this more then go to the www.gpr-expert.com web site and view the figures and tables associated with this press release.

More information can be found online at http://www.lightning-protection-institute.com

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