It can be difficult for an electrical contractor to determine whether a single electrode will be sufficient to be able to gain an adequate value of resistance prior to its installation and test. Here, Jake Green Head of Technical Engagement, Scolmore Group (owners of the Unicrimp brand) looks at how the earth electrode resistance might be improved as part of a planned approach to installation.
Earth electrode resistance is affected by several factors. These include:
• Climate conditions (wet/dry/frozen ground)
• Soil type (soil resistivity)
• Shape of the earth electrode (rod/plate/strip)
• Depth of burial
• Number of electrodes.
BS 7430: 2011+A1:2015 Code of practice for protective earthing of electrical installations provides recommendations and guidance on meeting the requirements for the earthing of electrical installations.
Soil conditions
To calculate the expected earth electrode resistance generally assumes a homogenous soil (of a common type) rather than the more likely multiple layers as an electrode is driven into the ground.
Table 1 of BS 7430 (pictured below) provides examples of soil resistivity for differing soil types. The table is no substitute for measuring the soil resistivity on-site.
The examples show that where the resistivity is naturally high (e.g. chalk/granite) it is difficult to create the conditions where the earth electrode resistance can be reduced significantly in value.
Earth electrode
BS 7430 recognises that the earth electrode must be robust, and that the earthing system remains safe.
The approximate resistance of a rod-type earth electrode may be calculated using:
Where:
ρ = soil resistivity (Ωm)
L = length of the electrode (m)
d = diameter of the rod (m)
Calculation allows the designer and installer to determine before work is carried out whether, or not, a single electrode driven to a specific depth will be sufficient.
Example:
Consider doubling the depth of an earth rod from 2 m to 4 m in a chalk soil!having a resistivity of 60 Ωm. The diameter of the rod is 38.5 mm.
2 m depth
4 m depth
Based on this calculation, doubling the depth of the rod will improve the earth electrode resistance in a uniform soil, but will not halve the value.
Increasing the diameter of the rod will have very little impact on the overall earth electrode resistance; depth is the key factor in reducing the earth electrode resistance. The limiting factors on increasing the depth of an earth rod are mechanical strength as the rod is driven into the ground, and the varying soil strata.
Paralleling earth rods
There will be times, for instance with the earlier example in chalk, where it isn’t possible to drive an earth rod deeper to reduce the earth electrode resistance. Where a reduction in earth electrode resistance is required, multiple earth electrodes (rods/plates etc.) can be installed in parallel with one another.
Where the soil is of a similar nature and where the electrodes are installed outside of the resistance area of each rod, the approximate resistance is the reciprocal of the number of rods employed.
Consider the previous example where the resistance was 24 Ω for an earth rod at a depth of 2 m. If a second rod were to be installed at the same depth and at, say, 6 m from the original rod, the resistance would fall to approximately 12 Ω (1/2). Were a third rod to be added the resistance would fall to approximately 8 Ω (1/3).
It is assumed that rods or pipes are outside each other’s resistance areas if the separation distance is not less than the driven depth. E.g. if the driven depth is 2 m then the separation distance should be at least 2 m. Little benefit exists once a distance of twice the driven depth exists.
There are a series of formulae in BS 7430 to aid the designer in determining the most effective configuration for parallel electrodes, plates, mesh or strip electrodes. Figure 15 in BS 7430 highlights five such configurations, including:
• Triangular (three rods)
• Two strips at right angles
• Three strips set at 120° meeting at the star point
• Four strips set in a cruciform, and
• Square (four rods at each corner).
Conclusion
There are other methods of reducing the resistance of the earth electrode including, for example, the use of structural steelwork and encasing electrodes in low resistivity material. However, these methods are beyond the scope of this article.
Prior assessment of the proposed installation method and selection of electrode will enable the designer and installer to avoid a ‘hit and miss’ approach to the earthing of installations using earth electrodes.
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