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# 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 resistivities 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:

im = current output in mA

P = soil resistivity in ohn-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.

Anode Weight (pounds) | Standard Anodes | Factor (f) |
---|---|---|

3D3 | (packaged) | 0.53 |

5D3 | (packaged) | 0.60 |

9D3 | (packaged) | 0.71 |

17D3 | (packaged) | 1.00 |

20D2 | (packaged) | 1.60 |

32D3 | (packaged) | 1.06 |

48D5 | (packaged) | 1.09 |

**Step 3:** Select potential correction factor from Table 2.

P / S | Magnesium |
---|---|

-0.70 | 1.14 |

-0.80 | 1.07 |

-0.85 | 1.00 |

-0.90 | 0.93 |

-1.00 | 0.79 |

-1.10 | 0.64 |

-1.20 | 0.50 |

**Step 4:** Calculate anode current output with formula.

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

No. of Anodes in Parallel | ADJUSTING FACTORS Anode Spacing in Feet | |||
---|---|---|---|---|

5′ | 10′ | 15′ | 20′ | |

2 | 1.839 | 1.920 | 1.946 | 1.964 |

3 | 2.455 | 2.705 | 2.795 | 2.848 |

4 | 3.036 | 3.455 | 3.625 | 3.714 |

5 | 3.589 | 4.188 | 4.429 | 4.563 |

6 | 4.125 | 4.902 | 5.223 | 5.411 |

7 | 4.652 | 5.598 | 6.000 | 6.232 |

8 | 5.152 | 6.277 | 6.768 | 7.036 |

9 | 5.670 | 6.964 | 7.536 | 7.875 |

10 | 6.161 | 7.643 | 8.304 | 8.679 |

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