11:8 System structure and function - deep earthing Earthing An earth connection is a conductor placed in the soil, with the aim of diverting electrical current from an installation connected to the earth connection and into the surrounding soil. A customer who buys power takes earthing for granted. This is despite the fact that the use of power without, or with poor, earthing incurs great risks. All power suppliers must have approved earth connections at their installations. This means that voltage surges that can occur for various reasons are led into the ground so that they do not cause damage. Earthing thus functions as, among other things, personal protection, property protection, signal transfer protection, lightning protection and the like. An approved earthing should have: (1) low electrical resistance, (2) ability to conduct voltage stably (despite weather changes) and (3) long service life, i.e. good resistance to corrosion. Soil conditions or external conditions? The importance of the soil as a conductor of electric current is great. The technical specifications and requirements for earthing demonstrate the advantages of deep-earth connections, both as a technical and economic solution, in relation to surface-earth connections. Current conduction occurs in the soil through electrolytic processes, known as ionic conduction. Solid particles such as gravel are not usually conductive. The electrical conductivity of the soil therefore mainly depends on the proportion of saline water bound by capillary forces and osmotic pressure in the pores between grains of sand and in hygroscopic humus particles (e.g.clay). The water in deeper lying ground layers usually has a higher salinity than the water in the surface layer. The higher the moisture content of the soil, the better the conductivity. Soil humidity normally varies between 5-40%. At variations below 14-18%, conductivity deteriorates significantly. Cold (frost) significantly impairs the ground’s conductivity. It is of great importance that all this is taken into account for earth connections or earth connection systems. Weather conditions - cold, heat, rain and wind - mainly affect the upper layer of the soil (0 - 1.5 m), which therefore exhibits the most powerful variations. The most efficient earthing is thus reached when the electrode is placed deep enough so as not to be affected by changes in soil humidity and temperature. Soil resistance in relation to temperature. Soil resistance in relation to humidity. Resistivity in different soil conditions. Resistivity The electrical properties of the soil are quality declared by means of its resistivity, which is measured in Ωm (former unit Ωcm, 1 Ωm =100 Ωcm). Soil with good electrical conductivity thus has low resistivity: 10 - 100 Ωm. For each case of different soil type, soil resistance must be measured and preferably during several seasons and in different weather conditions. In measurement today almost exclusively voltage compensated electronic resistance bridges are used (measurement method according to Wenner) with 4 connection contacts, 2 of which are for current electrodes and 2 for voltage probes. The connectors are connected to 4 vertical metal tips that are driven down in a row about 0.3-0.5 m deep a metre apart. (See image) If the instrument reading is R, the resistivity of the soil is calculated according to the following equation: p = 2 x a x R Ωm In unlayered soil, resistivity is independent of the electrode distance a. By increasing distance a, the current penetrates deeper into the ground and the measured resistivity can fall or increase depending on the resistivity of the ground layer at 1 metre’s depth. When calculating approximate earthing resistance of the earth connection when the depth is l, the resistivity of the soil must be measured with electrode distance a ≈ 0.75 x l. Measurement of ground resistance. Measuring earthing resistance of the earth connection
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