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Fully charged Reducing earth resistance for rod-type electrodes A rod-type earth electrode is commonly used to provide a reliable electrical connection to the general mass of earth for a TT system, as required by Regulation 542.1.2.3 of BS 7671, or for a particular item of equipment such as a generator.
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he reliability of such a connection to the general mass of Earth can vary significantly from one location to the next, due to the resistivity of the soil at the particular location of installation. Because of this, clause 9.2.1 of BS 7430: 2011 recommends avoiding, wherever possible, areas of land made up of dry sand, gravel and chalk for the location of an earth electrode. Even where this is the case, the magnitude of the measured value of earth resistance may not be sufficiently low enough to confirm that the connection will function reliably. For example, regardless of the type of earth fault protection installed, a value greater than 200 1 may not be reliable (Note 2 of Table 41.5 of BS 7671: 2008 Amd 3 and Note 3 of clause 7.2.9 of BS 7430: 2011 Amd 2015 refer).
Therefore in this article, the following three general options that may be employed to achieve the desired reduction are reviewed: • Extending the length of the rod-type earth electrode • Installing additional rods • Treating the soil Extending the length of the rod earth electrode The resistance to Earth for most shapes of electrodes generally decreases with buried depth, but changes little with diameter, so an extensible rod type electrode which allows sections to be added to extend the rod deeper into the soil can provide a significant decrease in earth resistance (R). The effect of increasing the length of rod is illustrated using the following formula (clause 9.5.3 of BS 7430: 2011 refers).
where, is the length of the electrode in metres (m) fully driven in the soil is the diameter of the electrode in metres (m) is the resistivity of the soil (assumed uniform) in ohm metres ( ) ln is the natural logarithm (log to the base ‘e’)
Fig 1 Using a rod-type electrode to connect to the general mass of Earth
For example, using the formula the earth resistance (R) of a rod-type earth electrode having a length of 2 m and a diameter of 15 mm, installed in soil of resistivity( ), 100 m, is calculated as follows:
Repeating the calculation with the length of rod increased to 4 m gives R = 26.53 , and for a rod length of 8 m it gives R = 14.64 . Whilst these calculated results show the reductions that might be achieved by extending the rod length, for the purposes of the calculation it is assumed that the resistivity of the soil is uniform throughout the length of the electrode, which is unlikely in practice. For most practical situations such calculated values have an accuracy within 10 - 15% (clause 9.5.1 of BS 7430: 2011 refers). Although a rod-type electrode has a much lower earth resistance than a plate of equal Connections Winter 2015-2016
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