Selection of Earth Rods (Electrodes) or Buried Earthing Conductors

Earth Rods

Although the material does not affect the earth resistance of an electrode, care should be taken to select a material that is resistant to corrosion in the type of soil in which it will be used.

There are two aspects which should be considered regarding the corrosion resistance of an earth electrode or an earthing conductor, compatability with the soil itself and possible galvanic effects when it is connected electrically to neighbouring metal work. The latter is most likely to come about when the earthing system is bonded to exposed metal structural components.

Corrosion and Type of Soil

The factors associated with the corrosion of metals in contact with soil are the chemical nature of the soil, in particular acidity and salt content, differential aeration, and the presence of anaerobic bacteria.

A general picture of the aggressiveness of soils is given by the following list, which places various types of soils in increasing order of aggressiveness:

  • Gravelly soils
  • Sandy soils
  • Silty soils (loam)
  • Clays
  • Peat and other organic coils
  • Made up soils containing cinders

Calcium carbonate in a soil will reduce the rate of corrosion. Non-cohesive soils, made from mixtures of the first three items above, are generally the least aggressive providing they are well drained and contain little or no dissolved salts.

Location of electrodes should be chosen to avoid the drainage of fertilizer and other materials into the area. Top soil should not be mixed with the backfill around an electrode. The least aggressive soils tend to be those having a high resistivity.

More detail can be obtained by measuring the electrical resistivity of the soil, which provides an indication of corrosivity under aerated conditions, and the redox potential, which indicates the risk of corrosion due to the presence of anaerobic bacteria.

The tests are described in BS 1377.

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Selection of Earth Rods (Electrodes) or Buried Earthing Conductors