T.E. Charlton,
BEng, CEng, MIEE, MBA(Director, Strategy and Solutions Ltd) J.R. Wales,
DFH, CEng, MIEE(Consultant, Stemet Earthing Co. Ltd)
INTRODUCTION
Earthing of electrical installations is primarily concerned with ensuring safety. In power networks the earthing system helps to maintain the voltage of any part of the network within a predetermined range with respect to earth under steady state and fault conditions. If designed correctly, it should allow enough current to flow under fault conditions to operate the protective devices installed.The rise in potential expe- rienced during the fault combined with the speed of fault clearance, should be such as to minimise both the risk of electrocution to individuals near the site of fault and damage to equipment. The widespread use of electrical appliances, both in the factory and the home, also introduces many situations where efficient earthing is of paramount importance, especially to prevent electric shock under fault conditions.
The two main components of an earthing system are equipotential bonding and formal earth electrodes. Equipotential bonds seek to minimise the potential differ- ence experienced across exposed metallic conductive parts by connecting them together. The formal earth electrodes normally consist of metallic components in direct contact with the soil. They are required to disperse any fault current to ground in a safe, controlled and effective manner.
The installation of an earth electrode is an important factor in achieving a satis- factory earthing system. It involves burying conductive material which is in direct contact with soil or the general mass of the earth. Soil conditions can vary enor- mously from site to site and directly affect the resistance value of a given electrode.
It is thus necessary to consider the soil and other factors which affect the actual resistance of the earth electrode at the design stage.
The earthing system consists of conductive material above ground, and metal elec- trodes within the soil and the surrounding soil itself. Each of these will contribute towards the overall resistance value. There are contact resistances, for example at joints and at material interfaces. The contact resistance of joints must be kept to a minimum by using appropriate materials and installation practice. In a new instal- lation, the most significant contact resistance is likely to be at the interface between electrodes and soil. This arises mainly because the soil has not yet consolidated.
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To understand the influence of soil on the resistance of an earth electrode, con- sider a vertical electrode of one metre length inserted vertically into the ground.
The soil immediately adjacent to the rod will influence its contact resistance, which will form an important part of the overall resistance. Let us first assume that the soil is in perfect contact with the rod and is made up of an infinite number of cylin- drical shells of equal thickness surrounding it. In uniform soil the shells nearest the rod will have most influence on the rod’s resistance value. Each subsequent shell will affect the resistance but will have progressively less influence. In practice, the shells will not have the same resistivity value, so their effect will be a combination of their resistivity and distance away. An extreme case is if the natural soil is of low resistivity, but the rod is surrounded by soil which has dried out and has a high resis- tivity.The rod will have a high resistance dictated by the nearby high resistivity layer.
A rod installed in a cavity in rock may be surrounded by low resistivity soil. It will still have a high resistance. This is because the shells some distance away have a much higher resistivity value than those close to the rod. This resistivity factor over- whelms the effect of distance. As the size of the cavity increases, so the effect of the surrounding rock will fall, but it will always affect the resistance value. Possibly the best way to appreciate this effect is that the earth currents will flow many kilo- metres, so the effect of localised effects will be limited.
SOIL RESISTIVITY
It is important to know a little more about the electrical properties of soil because it is so critical to the eventual resistance value of the rod or electrode system. We are interested in its resistivity value, which is expressed in ohm metres (Wm). This is, in effect, the resistance between the two opposite faces of a one metre cube of material. Some typical resistivity values of soils are shown in Table 7.1.
The two main factors which influence soil resistivity are the porosity of the mate- rial and the water content. Porosity is a term which describes the size and number of voids within the material, which is related to its particle size and the pore
Table 7.1 Typical values of resistivity for different soils.
diameter. It varies between 80 and 90% in the silt of lakes, from 30 to 40% in sands and unconsolidated clay and by a few per cent in consolidated limestone.
An increase in water content causes a steep reduction in resistivity, until the 20%
level is reached, when the effect begins to level out. Dissolved minerals and salts in the water may help further reduce the resistance, particularly where these are nat- urally occurring and do not become diluted over time. The effect of water content is clearly shown in the resistivity values of concrete in Table 7.1. This has a high resistivity when dry, which falls dramatically when wet to a value close to that of the surrounding soil.
Moisture has so great an influence that it is sensible to make use of this property if possible in the earthing system design, for example by inserting earth electrodes into the water table to provide a consistently low resistivity. Site surveys seek to find such information.
RESISTIVITY SURVEYING
Some geophysical prospecting techniques based on electrical measurements of the subsoil to various depths, have been adapted to suit requirements of electrical earth- ing systems. The method devised by Dr F. Wenner in 1915 has proved to be partic- ularly suitable for the calculation of soil resistivity for use in earthing calculations.
Equipment necessary
The necessary equipment comprises:
(1) An earth resistance/resistivity tester, having four terminals and a built-in source of power (Fig. 7.1)
(2) Four metal test electrodes
(3) Four lengths of single insulated cable – used for connecting the test electrodes to the tester.
The leads should be as long as necessary, commensurate with the size of the earth grid to be installed, the length of ground available, and the accuracy of the instru- ment. For single vertical electrodes the test electrode separation (i.e. between each pair of test electrodes) would need to be several times the maximum depth antici- pated. For earth grids, the separation should be at least the same as the diagonal across the site. Lengths of 20 m for voltage leads and 60 m for current leads should be sufficient for most rods and small grids. Usually operators prefer to develop their own cable reel equipment for this operation. For example, the cables can be of different colours, wound on reels and mounted on a stand. Often the cables are multicore, marked or have tee connections when a practice of taking measurements at a set number of spacings is adopted. This can save much time on site.
Method of use
The tester is positioned at a convenient point, often coincident with the location for a vertical earth rod or the centre of a proposed earth grid. The four test probes are
positioned in as straight a line as practically possible and at equal distance from one another, as in Fig. 7.2. The probes do not have to be driven too deeply, normally no more than 0.25 m. The voltage probes need only be 5–15 cm deep. The probe depth should never exceed 1/20th of the spacing or a revised formula for the soil resistiv- ity calculation must be used.
Fig. 7.1 Megger DET2/2 auto earth tester. For accurate measurements of earth electrode resistance or soil resistivity.
Fig. 7.2 Diagram of connections for an earth tester when used for resistivity surveying.
The probes are then connected to the tester as shown in Fig. 7.2. Some of the modern instruments can be used to check that the test probe and lead resistances are acceptable prior to each measurement. To overcome polarisation and inter- ference effects, the polarity of the current source is normally reversed several times during the test. With older equipment this used to be a manual process, but in modern equipment the d.c. supply is converted into a switched wave- form whose frequency can be varied to improve accuracy if necessary.
The current produced by the instrument passes through the two end electrodes and the voltage between the two inner electrodes is measured. The relation voltage/current is read directly as ohms (R) from the tester. This reading represents the resistance presented by a hemispherical volume of soil beneath the tester.
As a practical approximation on site of the depth of soil being measured, Stemet assume that the Wenner method is examining the properties of the soil to a depth of 75% of the probe separation; for example, if the spacing is 4 m, the depth examined is 3 m.
For a more accurate estimate in uniform soil, a graph is available. This shows the percentage of the total current flowing through a vertical plane beneath the tester. Approximately 50% flows through a plane of depth equal to half the electrode separation, 70% through a plane equal to the electrode separation, and so on.
From the resistance figure obtained we are able to calculate the apparent resist- ance of the material contained within the hemisphere. This is via the following formula for resistivity (r):
where R =earth tester readings (W) A=spacing used (m).
The above equation assumes that the soil is of consistent structure, i.e. consistent grain size, type and water content throughout the whole volume. This is very rarely the case and the reason why the calculation provides the apparent resistivity and not the true resistivity. In fact the soil is likely to have several horizontal, sloping or vertical layers. Each layer will have a different real resistivity and the volume of each layer within the measured hemisphere will contribute towards the apparent resistivity.
This measurement technique can be used to examine how the real soil resistivity is changing with depth. To achieve this a series of resistance measurements are taken, each at a different probe spacing. It follows that the depth of material inves- tigated at each spacing will differ and will consist of different layers of soil. Typi- cally the probe spacing would start at about 0.5 m and increase in stages to 30 or 50 m. The results of a typical survey are set out in Table 7.2.
r=2pRAWm
INTERPRETING MEASUREMENTS
Normally the apparent resistivity readings from column 4 of Table 7.2 would be plotted against the Wenner probe spacing (column 1) on log/log graph paper. By examining the shape of the curve, much can be deduced about the type of soil structure. Normally each significant change in direction or gradient indicates a change in soil resistivity at a certain depth. Figure 7.3 is such a plot for a three- layer soil structure. For effective vertical earthing we are trying to identify layers where the soil resistivity is low, for example in layers below the water table.
Figure 7.3 illustrates that there is a low resistivity layer some distance beneath the surface. Software facilities are available which can derive a fairly accurate soil model from test data; for example, the soil model derived from the same data used in Fig. 7.3 is:
Surface layer: 105Wm, 1.1 m thick Central layer: 265Wm, 2.5 m thick Lower layer: 85Wm, at least 16 m thick.
This soil model would be used as the basis of accurate computer modelling of the proposed earthing system, if this is required. In multilayer soil the actual depth of the hemisphere of soil being tested can vary substantially, owing to the natural pref- erence of the current to seek a route through low resistivity layers. It is only really possible to account for this by using computer packages or experience to derive the soil model.
Stemet Earthing have developed a graphical method as a first approximation of computer based techniques. The readings from Table 7.2 are plotted as diagonals across logarithmic scaled graph paper, as shown in Fig. 7.4. From this an approxi- mation of the rod resistance can be read directly off the vertical scale. If the resis- tivity value is required, this can either be calculated (as shown in Table 7.2) or assessed graphically (Fig. 7.5). Diagonals have been superimposed on the grid to show the theoretical resistance of an earth rod driven into uniform soils of differ- ent resistivities.
Table 7.2 Results of a typical survey.
Rod resistance formula
The next stage is to convert the resistivity figures obtained into a resistance value for the earth electrodes. For example, it is useful to able to predict in advance the resistance value of a single earth electrode in order to optimise its length.
The simplified formula is:
where r =resistance of the rod (W)
r =resistivity measurement taken at certain spacings (Wm) D=depth of rod (m)
d =diameter of rod (m).
Example from Table 7.2
Taking a reading from Table 7.2, the approximate resistance of a rod driven to a depth of 3 m is calculated as follows:
r D
=0 366¥ 3d . rlog10 W
Fig. 7.3 Plot of apparent resistivity against Wenner probe spacing for a three layer soil.
In general, if the curve showing the calculated resistance drops off at least as sharply as the diagonals in Fig. 7.5, it is more advantageous to carry on driving a single rod rather than use a number of shorter rods connected in parallel. However, it must not be assumed that deep driven rods are the answer in every case. Some sub-soils are quite unsuitable and it is not worth continuing to drive into these. For example, the underlying soil may be rocky and have an increasing resistivity with depth. In
r= ¥ ¥ ¥
=
0 366 538 3
3 3 0 016 180
. log10
. W
W
Fig. 7.4 Traditional site chart: earth tester readings for different spacings enabling rod resistance values to be read directly off the vertical scale.
such cases, it is better to use a multiple number of rods connected in parallel, each of which is driven to about 4 m depth. Rods should always be at sufficient depth such that their resistance value is not significantly affected by seasonal changes which may change the soil resistivity. As discussed earlier, the variation in the water table is important in this respect and advice on this may be obtained via geo- technical studies or local knowledge. Generally, the deeper parts of the rod (i.e. beyond 1.5 m depth) are not prone to significant seasonal variation. Ideally, the separation of these earth electrodes should not be less than their depth; for example, if an electrode is driven 10 m deep, then the earth electrode nearest to it should be positioned at least 10 m away, otherwise the interaction between them will reduce the overall effectiveness.
Where multiple earth rods are used, they are normally connected together with bare copper tape or cable. A bare interconnection acts as an additional electrode and helps to reduce further the overall resistance value. The depth at which interconnecting tape or cable is installed should be sufficient to afford protection and mechanical strength, and maintain contact with damp soil. This is normally 0.6–1.0 m deep. In areas subject to extremely low winter temperatures, greater depth may be necessary. The top ends of the vertical earth electrodes are installed to the Fig. 7.5 Curves of specific resistivity and electrode resistance plotted on a logarithmic grid.
same depth (0.6–1.0 m) to reduce voltage gradients on the soil surface. Large elec- trode networks should have facilities for testing either individual rods or groups of rods. Contactless testers are used for the measurements so that the electrode remains intact and in service.
The improvement in the earth resistance that can be expected from the installa- tion of additional rods in terms of the percentage improvement each additional rod produces, is approximately as follows:
Second rod 60%
Third rod 45%
Fourth rod 35%.
Clearly, increasing the number of rods has a diminishing benefit, as illustrated in Fig. 7.6.
The above method of calculating the resistance is limited to straightforward struc- tures and conditions. For more complex earthing arrangements and soils which have several layers (particularly where one is rock), more detailed estimates or computer modelling is required. New standards require calculation of touch and step poten- tials across a substation site. The earthing installation directly affects these poten- tials and computer modelling enables them to be calculated accurately throughout the site. It is also possible to measure these potentials. To illustrate the power of the computational modelling tools available, Fig. 7.7 shows the potential on the soil surface above an earth grid, in three dimensions. The results obtained can also be presented in two dimensions, as in Fig. 7.7b. The two-dimensional facility is often Fig. 7.6 Total resistance of multiple earth rods expressed as a percentage of a single rod.
used to produce the 430 V or 650 V hot zone contours required by telecommunica- tion authorities.
Artificial method of reducing earth resistivity Low resistivity materials
It is widely thought that adding low resistivity materials to the soil in the immedi- ate vicinity of the earthing system or rod will have a dramatic effect on reducing its resistance. This is not normally true. There is always a contact resistance between the electrode and the soil. Where a new rod has been driven into the ground, the sideways movement will have increased the width of the hole in which the rod lies.
The gap between the rod surface and the compressed soil to its side will introduce a large contact resistance which will be apparent when testing the resistance of the rod. However, experience shows that this resistance falls over time as the soil becomes consolidated around the rod due to rainfall, etc. One way to accelerate this is to add a low resistivity material, such as bentonite slurry, as the rod is driven in.
Bentonite is a fine, naturally occurring clay powder. It is mixed with water to form a slurry. As the earth electrode is driven into the soil the bentonite is drawn down by the rod. By continuously pouring the mixture into the hole as driving proceeds, a sufficient quantity is dragged down to fill most of the voids around the rod and lower its resistance. Bentonite is thixotropic in character and therefore gels when in an inert state, so should not leach out. It has a low resistivity and does not dry out under normal conditions. However, its use in this manner does not necessarily produce a significantly lower earth resistance than that which would occur naturally over time. In some cases, installing the rods a little deeper can achieve a better result than using low resistivity material.
Adding bentonite and similar material, such as marconite, in a trench or larger drilled hole around the electrode has the effect of increasing the surface area of the earth conductor, assuming its resistance is lower than that of the surrounding soil. Figure 7.8 shows the effect on the resistance value of one rod when its radius (and hence surface area) is increased. Clearly there is an improvement, but this falls off rapidly with increasing radius, and the cost quickly becomes prohibitive. Addi- tional reasons for increasing the surface area may include a requirement to reduce the build up of electrochemical deposits on the electrodes which may reduce their efficiency, or for high frequency earthing (lightning protection, etc.).
Chemical treatment
Treating the soil surrounding the earth electrode with chemicals normally only pro- vides a temporary improvement as the chemicals will usually be dispersed over time by rainwater. The environmental effects may also mitigate against this undesirable practice. In addition, one must ensure that the chemicals used do not have any corrosive effect on the electrode material.
One interesting technique used in rocky sites in Hungary was to drill a hole several metres deep into the rock. Explosives were introduced and when detonated,
Fig. 7.7 (a) Potential rise on the soil surface above an earth grid and terminal tower; (b) touch voltage contours above a buried substation earth grid.