Reducing earth resistance for rod-type electrodes

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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.

The 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 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).

Fig 1 Using a rod-type electrode to

connect to the general mass of Earth

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’)

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

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Fig 2 Separation

distance

area, the depth of the electrode should be of suffi cient depth so as not to be aff ected by seasonal variations in the soil resistivity (Regulation 542.2.4 of BS 7671 refers). The temperature of the soil drying out or freezing can rapidly increase the resistance of the upper part of the rod, causing a signifi cant increase in earth resistance. For these reasons, clause 9.3 of BS 7430: 2011, recommends that any part of an earth electrode system within 0.5 metres of the soil surface should be considered to be ineff ective under frost conditions.

In some locations, due to the composition of the subsoil, extending the earth electrode beyond a certain depth may not only be time consuming but also require specialist equipment and in some cases may achieve very little reduction in earth resistance, especially where the deeper soil layers are formed of materials having higher values of resistivity. For such reasons, clause 9.4 of BS 7430: 2011 recommends undertaking an on-site soil resistivity testing survey to determine whether it will be benefi cial to extend the length of the rod-type electrode or whether, subject to the space available, the installation of additional rods, connected in parallel is more appropriate.

Installing additional rods are connected in parallel, and the separation distance is equal and outside the resistance area of each rod (generally this is achieved by ensuring the separation distance is not less than the driven depth of the rod), then the combined resistance of all the rods can be considered to be connected in parallel and will have a resistance approximately proportional to the reciprocal of the number of rods employed (clause 9.5.3 of BS 7430: 2011 refers). For example, if one rod electrode has a measured earth resistance of 300 Ω, then 3 such rods connected in parallel will reduce the total earth resistance value to approximately : 300/3 = 100 Ω.

It should be noted that other methods, including formulae, the application of numerical methods or soft ware packages, may be used to provide results of greater accuracy (clause 9.5.4 of BS 7430: 2011 refers). Furthermore, rod-type electrodes may be installed in a range of confi gurations, including triangular or square formations, and details for calculating the total resistance for these respective arrangements are contained in clause 9.5.8 of BS 7430: 2011.

In accordance with Regulation 542.2.3, underground structural metalwork may be used as an earth electrode, and where this is intended clause 9.5.8.6 of BS 7430: 2011 contains further details.

Treating the soil

Another method of lowering the earth resistance is to treat the soil surrounding the earth electrode with a material of low specifi c resistivity, such as Bentonite.

Although soil treatments can produce earth resistance reductions of up to 20 %, usually it should only be used for temporary installations, as the additives migrate and leach away over time, requiring their constant monitoring and replacement (clause 9.2.2 of BS 7430: 2011 as amended refers). Harmful environmental eff ects may also result, and care must be taken to ensure that the chemicals used do not have an adverse eff ect upon the electrode material. In particular, coke breeze should not be used due to its corrosive nature.

Although for some temporary electrical installations in areas having high soil resistivity the application of soil treatment may prove to be the most economical solution, in many cases, especially in areas where the land is predominately rocky or chalky, it may prove more benefi cial to install an alternative electrode, such as plates or tapes.