11:9 11 Earthing resistance Due to the high resistance of the soil (109 x resistivity metal) at current bleed in the soil a strong electrical field is formed in the earth connection, which diminishes in strength with distance from the earth connection. At a certain distance, this field can be neglected (removed earth). The earthing resistance of the earth connection is usually measured with the same type of instrument used to measure the resistance of the soil. However, this measurement requires only one voltage probe and a current electrode (auxiliary earth connection). The location of probes and electrodes varies between different measurement methods. The two methods that follow are a method of accurate technical measurement and a more practical, simplified method. Measuring earthing resistance of the earth connection - Method 1 Method 1 (acc. to the lightning protection standard SS 4870110). This method has a measurement error of +/ - 2%. The summary gives this method: - The probe and auxiliary electrode are placed in a straight line from the earth connection to be measured as illustrated. - If the ground is layered, measurement should be carried out in two directions. The largest value is used. - The reliability of the measurement result depends on the location of the probe/ auxiliary electrode. Note the distance table below. These provide normally acceptable measurement accuracy. earth connection - probe = 0.5a-0.6a earth connection - electrode = a a ≥ 40 m if l ≤ 4 m a ≥ 10 x l if l > 4 m Measuring earthing resistance of the earth connection - Method 2. Method 2 (acc. to EBR-standard U2:80) This method normally has a measurement error of more than 2%, but practically is easier to perform than Method 1. The summary gives this method: - Probes and electrodes are placed as illustrated, 90° from the main direction of the earth connection. - The position of the probe/electrode is equal when measuring both an individual earth connection as earth connection system, i.e. at least 80 m from the earth connection. - Measurement of an earth connection is carried out with an open earth conductor clamp. - Measurement of the resulting transition resistance on multiple earth connection systems is carried out with the clamp closed and with the measuring line connected on the top of the earth conductor clamp. With the help of the conductivity and the maximum earthing resistance, which is required by, among other things, the high current regulations, it is possible to estimate how much line may be needed according to the formula: l = p / R l = length in metres p = soil resistance in ohmmeter R = earthing resistance in ohms. In the discussion of the advantage of deep-earth connections compared to surface-earth connections, it should be mentioned here that the earthing resistance of a horizontal surface-earth connection is twice that of a similar line length in a deep- earth connection, i.e. R0 = 2 x p / l Parallel connection Parallel connection of several earth sockets is often necessary for practical reasons to achieve a sufficiently low value of earthing resistance during earthing. In order to limit reciprocal connection between individual earth connections, deep-earth connections shall be installed a distance apart of 1.5 times the depth of the earth connection l. Resulting earthing resistance: Rres = k x Rm where Rm is the mean of the resistance value of the earth connection and k a reduction factor, the value of which is obtained from the following table. No. of parallel earth connections k for a = 1.5l 2 0.60 3 0.40 5 0.25 10 0.13 Parallel connection. From economic aspects it can be pointed out that the diameter of the earth connection has a negligible role when calculating the earthing resistance in deep earthing. This means that when using Elpress deep-earthing systems with copper line, the cost will be lower than when using, for example, conventional systems. What is important in practice in terms of the cable diameter is what currents the system is dimensioned for and what rules and requirements apply. Examples of applicable requirements: the lightning protection standard states Culine min 25 mm², EBR prescribes min. Cu-line 35 mm² for earth connections in overhead line networks and min. 50 mm² for earth connections in ground cable networks. Corrosion The service life of an earth connection depends on its resistance to corrosion (rust). The prerequisite for all types of corrosion is an electrolyte fluid that allows the transport of positive metal ions from anode to cathode. At the anode, metal atoms are dissolved in the electrolyte, forming free positive ions - oxidation- and at the cathode these ions are neutralized and deposited on the metal surface - reduction. In galvanic corrosion caused by contact between two metals, the corrosion rate is proportional to the galvanic tension between the metals. A base metal has higher negative potential than a nobler metal and therefore forms the anode in a corrosion process. There is also a clear correlation between the corrosion rate and ground resistance. The rate of corrosion depends on the composition of the soil. Influencing factors are the pH of the soil, temperature, oxygen content, moisture content and resistivity. These factors affect the corrosion current lk, which is directly proportional to the rate of corrosion. lk can be determined by direct measurement with an Ammeter or calculated, if the transition resistance Rö between the two electrodes is known, according to formula: lk = Ug / Rö Ug = galvanic voltage In some cases, Rö can be measured with a resistance bridge of the same type used to measure the earthing resistance of an earth connection. The rate of corrosion is often expressed in μm/year where 1 μm represents 1/1000 of 1 mm and denotes the thickness of the corroded away outer metal layer for 1 year. The table below indicates some practical values as guide values for different soil resistivity. Resistivity Corrosion p < 1 Ωm 100 μm/year p < 1-10 Ωm 100-30 μm/year p < 10-100 Ωm 30-4 μm/year p > 100 Om negligible
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