Cathodic Protection Training Course

Module 8

Groundbed siting

There is nothing mysterious about a cathodic protection impressed current groundbed. People add a mystery by using acronyms such as ICCPGB and TR in the UK, TRU in South Africa and of course other initials in other countries.

A ground bed is an electrical node or contact point between the positive terminal of a transformer/rectifier and the earth.

Mass earth is a resistanceless conductor because the charges pass through an infinite number of resistances in parallel. If you apply Kirchoff s law in which the total resistance is equal to the sum of the recipricals of all resistances in parallel then it is clear that there is no resistance in mass earth.

However, the charges supplied by the postive terminal of the transformer/rectifier pass into the cable to the anode that is buried in the ground making that at a higher potential that the surrounding earth.

The surface area of the anode metal determins the number of resistances, in parallel, that are immediately encountered by the charges (measured as the potential of that anode).

We construct a groundbed so that the backfill has little resistance and will not allow corrosion of the anode to create resistance in the form of a corrosion product.

The effect of the backfill is to increase the surface area of the conductive material and therefore increase the number of resistances in parallel as the potential equalises with remote earth.

The charges follow the line of least resistance and will disperse according to the inverse square law of radiation through a homogeonous ground condition.

The spread of charges cannot be truly calculated due to the natural electrical construct of the ground itself. There is always stratificaion and disturbances to the homogeneity caused by constant evolution of the earth itself and the influences of mankind and the organic community in the environment.

The Wenner four pin method of ground resistivity measurment is a good starting point and later methods using other energy discharge devices will give a reasonable indication of general ground resistivities in any area.

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