Design and Calculation of an Earth Electrode

People using electrical installations have to be protected against electrical shock. For purposes of protection a distinction is made between direct and indirect contact. Direct contact is contact with a live conductor. Protection is provided by the insulation of cables or the screening of live parts. An indirect contact happens when someone touches exposed metal parts which are not intended to carry current but have become live as a result of a fault. In this case metallic parts raise the metal to a dangerous potential (contact voltage). Here protection is provided by connecting the exposed metal part (i.e. the case of the electrical machine) to the earthing point of the installation. A protective device will disconnect the circuit as soon as a fault current flows to earth. The earth fault value will depend on the impedance of the path taken by the fault current, which is known as the earth fault loop. The resistance of the earth electrode plays an important role in the final impedance of the earth fault loop, especially when the neutral of the transformer is earthed without any impedance. A good earth electrode should have the lowest possible value compared with the rest of the earth loop (voltage divisor). If this can be achieved, the contact voltage will be limited and the current that trips the protections will be of a higher value. The maximum duration of a contact voltage is established according to IEC 60364-4 413.1.1.1. An important constraint in obtaining a good resistance value for the earth electrode is the resistivity of the soil.