| 5. Cathodic Protection | |
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5.10 Anode-to-Earth Resistance |
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Anode Earth Resistance of a Single Vertical Anode
The anode earth resistance of a single vertical anode is calculated using Dwight equation
| where | Rv = resistance of one vertical anode to earth | |
| P = resistivity of backfill | ||
| L = length of the anode (or backfill) in ft | ||
| d = diameter of anode in ft |
The resistance of two or more vertical anodes in parallel can be calculated by Sunde equation:
| where | S = spacing between the anodes in ft | |
| N = number of anodes |
Example:
From the data provided below, calculate the resistance of one vertical anode to earth.
| Soil resistivity | 100 ohm-cm | |
| Backfill resistivity | 5 ohm-cm | |
| Backfill dimensions | 8.5" x 28" (0.7083' x 2.333') | |
| Anode dimensions | 5" x 5" x 20.5" (0.470' x 1.7081') | |
| Calculations: | ||
| First determine the resistance from the anode to the outer edge of backfill columns Rv(i) | ||
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| Determining the resistivity of the vertical anode to backfill: | ||
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| (use the dimensions of the backfill and the resistivity of the backfill as shown above) | ||
| Rv,1 = Rvi - Rv,backfill
= 0.3614 - 0.2538 = 0.1076 ohm |
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| Next, calculate the resistance of the anode to soil (use backfill dimensions and resistivity of the soil) | ||
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| To get the total resistance of one anode to
earth
Rv,total = Rv,1 + Rv,2 = 0.1076 + 5.074 = 5.181 ohm |
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