5. Cathodic Protection 5.10 Anode-to-Earth Resistance

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) Determining the resistivity of the vertical anode to backfill: (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 Next, calculate the resistance of the anode to soil (use backfill dimensions and resistivity of the soil) To get the total resistance of one anode to earth Rv,total = Rv,1 + Rv,2             = 0.1076 + 5.074             = 5.181 ohm