Electricity + Control October 2017

EARTHING + LIGHTNING PROTECTION

Earthing and its application in electrical systems General Earthing / Bonding requirements Correct earthing system design is intended to achieve a required earth resistance (impedance) value as well as ensuring satisfactory surface po- tential distribution by selecting equipment that is both reliable and suitable and will ensure equip- ment and operator safety during varying operating conditions. The intention is for the earthing conductor ar- rangement to provide a connection to earth. Equipment and conductors being used for this purpose should therefore be: • Reliable • Suitable to carry earth fault currents without o Thermal o Thermo mechanical o Chemical and; o Electromechanical stresses arising from normal operational as well as fault currents. The construction and effectiveness of an earth electrode is dependent on factors such as: • The value of the expected fault current • Ground conditions on site where the earth electrode is planned Simple horizontal earth electrodes are metal rods, strips or pipes placed horizontally under the ground surface at a given depth t, which is usually in the range from 0.6m up to 1 m deep. In regions where the ground freezes these electrodes may be placed deeper than 1 m as horizontal electrodes should be mounted beneath the freezing line. Vertical Earthing Electrodes Simple vertical earth electrodes are metal rods or pipes placed vertically in the ground. The typical length of rods is in the range from 3 m up to 30 m and could; in extreme cases be even longer/deep- er. They are usually mounted as a set of rods with the length of about 1.5 m, placed successively one over other and driven into soil with a mechanical hammer. Vertical earth electrodes have considera- ble advantages as the rod passes through various layers of soil when it is embedded into the ground. Water or moisture content generally increases with depth of soil. A stable temperature can also be expected at increased depths below ground level. Thus, the resistance to earth of a vertical rod Different types of electrodes Horizontal Earthing Rods [1]

electrode is more stable than when compared to horizontal electrodes. This is due to sensitivities as a result of weather and ground conditions. Another advantage is that a smaller surface area is required to install the vertical rod electrode. Vertical rods are recommended in dense build- ing areas, or where the surface is covered with asphalt or concrete. Vertical earth electrodes are often also used in addition to horizontal rods in order to supplement and improve the resistance of existing horizontal electrodes. A disadvantage of vertical rod electrodes; is un-favourable surface potential distribution. Foundation earth electrodes Foundation earth electrodes are recommended as a very practical solution to building earthing and are constructed from conductive metal parts em- bedded in the concrete of the building foundation. Concrete buried directly in the ground has natural moisture content and can be consid- ered as conductive matter with conductivity similar to that of earth. Because of the large area of this type of electrode, low resistance can be achieved. Furthermore, concrete pro- tects metal parts against corrosion; and steel electrode elements embedded in the con- crete do not need any additional corrosive protection. In practice there are two basic foundation earth electrode constructions: • A foundation without concrete reinforce- ment • A foundation with concrete reinforcement In both cases the earth electrode is made from: • Steel strip, usually with rectangular cross sec- tion not less than 30 mm x 3,5 mm, or • Steel bar, usually with round cross section not less than 10 mm diameter. The steel elements can be galvanised (i.e. with a zinc coating), but this is not necessary if the layer of concrete covering the electrode is greater than 50 mm, because the concrete ensures sufficient protection against corrosion. In a foundation with- out concrete reinforcement the electrode usually follows the contour of building foundation, i.e. it is placed under the main walls. In buildings with extensive foundations, the electrode is usually in- stalled in the form of multiple loops covering the parts of foundation outlines, and connected to each other. In a foundation using concrete reinforcement the earth electrode is placed over the lowest layer

Designing and installing an earthing system that is in contact with the earth creates a path to earth.

Electricity + Control

OCTOBER 2017

33