Current Output

Current output is controlled by three factors:

• Soil resistivity – Current output increases as soil resistivity decreases. Generally magnesium anodes are installed in relatively low resistivity soils. Economic application decreases significantly in soil resistivity exceeding 5,000 ohm-cm. Practically, magnesium anodes are not effective above 10,000 ohm-cm.
• Anode surface area – Current output is proportional to surface area. As the surface area increases, current output increases. Increased surface area is usually achieved by increasing the length of the anode.
• Alloy potential – High potential anodes have open circuit potentials approximately 20-25% higher than H-1 alloy. (-1.75 volts versus -1.40 volts). This creates a higher anode current and results in a lower total anode requirement.

Anode current output is calculated using Ohm’s Law:

V = IR
I = current in amperes
V = potential differential in volts
R = anode resistance in ohms

The voltage difference is the protected potential of -0.85 volts subtracted from the open circuit potential of the anode. For high potential anodes, the difference is -0.9 volts. For H-1 alloy, the difference is -0.55 volts.

Resistance to earth can be calculated by Dwight’s Equation:

R = resistance in ohms
  = soil resistivity in ohm-cm
L  = anode length in feet
d  = anode diameter in feet

Tefankjian, in his article “Application and Maintenance of Control Facilities” offers a simple process for determining anode current output with consideration of coating, potential, and multiple anode installation. The process involves calculation of anode current for a single high-potential anode installed 10′ away from a bare pipeline using the formula:

  i_m = current output in mA
P = soil resistivity in ohm-cm
f = size correction factor – Table 1
Y = potential correction factor – Table 2

Step 1: For a well-coated pipeline, the constant of 150,000 should be reduced 20% to 120,000.

Step 2: Select anode size correction factor from Table 1.

Step 3: Select potential correction factor from Table 2.

Step 4: Calculate anode current output with formula.

Step 5: Adjust for multiple anode installation by application of adjustment factors in Table 3.

As Tefankjian states in his article, the above process should be used only as a guide to estimate current output.