Earth Conductors

Earth Conductors:

Having introduced the wide variety of grounding arrangements possible, it is now necessary to consider the grounding system itself. The most important functions of the earth conductors are explained and some definitions introduced. The different types of earth electrode available are described and surprisingly the same types are generally used whether the grounding system is for a house, factory or generating station.

Conductor sizing :

IEC 60364-5-54 Provides a guide on how to correctly dimension PE conductors:

Notes:

This simplified method can only be used if the PE/PEN conductor is of the same material as the line conductors.

c.s.a = cross sectional area

1. If the PE conductor is a separate cable from that of the phase conductors the minimum values below must be observed:

2.5 mm2 if the PE conductor is mechanically protected.

4 mm2 if the PE conductor is not mechanically protected.

2. For mechanical reasons the PEN conductor must have a c.s.a. not less than 10 mm2 in copper and 16 mm2 in aluminium.

Climate

Obviously, arid and good rainfall climates are at opposite extremes for conditions of soil resistivity.

Seasonal Conditions

The effects of heat, moisture, drought and frost can introduce wide variations in “normal” soil resistivity. Soil resistivity usually decreases with depth, and an increase of only a few percent of moisture content in a normally dry soil will markedly decrease soil resistivity. Conversely, soil temperatures below freezing greatly increase soil resistivity, requiring earth rods to be driven to even greater depths. for variations of soil resistivity with moisture content, and for variations of soil resistivity with temperature.

Other Factors

Other soil properties conducive to low resistivity are chemical composition, soil ionisation, homogeneous grain size and even grain distribution - all of which have much to do with retention of soil moisture, as well as providing good conditions for a closely packed soil in good contact with the earth rod.