In this issue we introduce a highly effective method that guarantees low Dispersion resistance, high durability, minimum cost and, above all, ecological adequacy. It can be successfully applied to any high resistivity soil, especially for inner or domestic Ground Connections.

These findings, correlated with field measurements, allow us to establish away from cultivated areas, Equivalent Resistivities averaging 300 Ohm-m, a parameter that can be directly assumed taking into consideration that a 35% chance exists that it could be higher. It must also be pointed out the difficulty for the nailing of Ground Connection Electrodes, since excavations are required for their installation.

Within this context, the formal Design and Execution of Ground Connections with low Dispersion resistance is greatly hindered, resulting, on the one hand, in the proliferation of the use of chemicals, some of them of toxic nature (based on ferricyanides), and, on the other hand, in the indiscriminate and predatory use of Crop Lands for the preparation of the Landfills. A Ground Connection pit weakens or eliminates 5 m2 of cultivable soil.

Moreover, the unnecessary use of electrodes with great surface or development is suggested or also, contradictorily to the evacuation of current to the ground, the use of auxiliary spiral electrodes is recommended. All these hindrances have been overcome now in a rational way and the Simplified Ecological Method has gained strength.

Currents channeled to the ground have different origins and amplitudes; in all cases, their dispersion in the soil is achieved through the total Resistance of the grounding system (circuit, connections, components, and soil), which magnitude should be the minimum possible in order to guarantee human protection. This will depend on the efficiency achieved in the installation of the electrodes in the ground.

The behavior of the Ground as an endless load catch pit makes its Electrical Potential zero (V = 0); then, any device, which Mass is connected to the Ground, will be provided with such zero Referential Potential. This will foster the proper operation of itself and of the devices related to it.

Human protection against both dangerous voltage gradients and electrical shocks, as well as the supply of the Referential Potential are achieved through the connection of all the Masses of the electrical devices and the nearby metal infrastructure to the Grounding Electrode. This step must be taken regardless the kind of Electrical Supply System that feeds the inner circuit.

a. Permanent Operation

Dispersion of small Static Charge currents, Insulation Leaks, unbalance of Loads or erratic charges, such as Geomagnetic and Induced currents, preventing the accumulation of Load and the electrification of the Masses of the electrical devices and of nearby metal objects, which do not require low-resistance ground connections.

b. Occasional Operation

The need to evacuate great currents to the Ground results from insulation deficiencies in electrical devices or circuits. They can be "Free" or "Absorbed"; likewise, they may be originated by "direct or indirect" impacts of atmospheric discharges; in all these cases danger to human beings increases: Ground Connections resistance must be as low as possible.

In order to guarantee Grounding Protection, the Dispersion resistance (Rt) will be < 25 Ohm when the Neutral of the supply network is ground-insulated and < 2.0 Ohm when such Neutral is connected to the ground; in both cases, the Standards provide a limit of < 25 Ohm, while for atmospheric discharges (Rt), the limit must be < 2.0 Ohm.

4. WHAT ARE THE COMPONENTS OF A PUNCTUAL GROUND CONNECTION?

They have a simple configuration, based on a single electrode embedded into a conductor fill, inside an excavation, with one of its ends accessible for the connection of the grounding circuit of the Masses.

a. Ground Connections with a Vertical Electrode

These are the most commonly used ground connections due to the minimum space required. A single Copper-Rod (Javelin) electrode is used; the proposed model allows attaining Dispersion resistance values ranging from 6 to 12 Ohm.

b. Ground Connections with a Horizontal Electrode

These are seldom used -- only when the underground is rocky. A single Platen type Copper electrode or a bare thick Conductor is used; the proposed model allows achieving Dispersion resistance values ranging from 7 to 14 Ohm.

c.-  Parts of the Vertical or Horizontal Ground Connections

1. Manhole with cover (optional)
2. Main electrode
3. Removable clamp
4. Connection conductor
5. Vertical pit/Horizontal trench
6. Conductor Fill
7. Salt Reserve Beds
8. Impregnation levels

All the notions of a Safety Ground Connection Design are applied, except the control of the Touch and Passing voltages achieved with great coverage electrodes, while the electrodes used for Punctual Grounding are simple and concentrated, prevailing in both cases the achievement of the lowest Dispersion resistance, with the following evolution:

This is the assessment of the electrical behavior of the soil using a Stratified Model. It is carried out by measuring the Apparent Resistivity (Pai) using the WENNER method, recognized by the ASTM; each time with the allocation of four exploration electrodes (C1, C2, P1, P2) nailed to a solid ground at a depth of 0.1 mm with straight-line spacing with equal distance between them (ai)

With the four points (Pai, ai), the vertical electrical variation soil characteristics are determined and processed using the TAGG method. Soil parameters are obtained for at least two strata (P1, P2, h1); if (P1 = P2), then it is assumed that it is uniform resistivity soil.

The resulting Resistivities (P1, P2) are directly applied to the design of Horizontal Electrodes or after calculating the equivalent value (Pe), to the Vertical Electrodes that are to be placed in both strata.

The type and way to install the Grounding Electrode is determined taking into consideration the space available and the Design Resistivity. It must be made of Solid Copper due to its high resistance against the corrosion produced by the soil; likewise, its geometry and Dispersion capacity will be optimized with the use of a Neutral Fill with Low Resistivity (Pr) so that its radius (r) can be expanded to the maximum (r -> R).

a.- Vertical Electrodes in Pit

There are cut rods of Hard Temper Copper available in the market with standardized lengths (l) 2.0, 2.5, 3.0 m with diameters (d = 2r) ranging between 0.013 and 0.025 m. The most marketed size is chosen, i.e. 2.5 m long, and it is optimized with the Fill in the (D = 2R) diameter pit that according to the analytic model allows to achieve its own maximum Resistance.

There are copper platens available in the market from 3.0 m long (L) with different sections with radius equivalent to (r), being the most appropriate size 0.003 m x 0.04 m, with the emerging end bent at 0.5 m and a 0.013 hole drilled for the connection. Should this type be scarce in the market, a bare Copper conductor can be used with 100 mm2 Semi-Hard or Soft temper. They can also be optimized using fill in the trench, whose equivalent diameter is (D = 2R).

The dispersion of current in a Resistivity Soil (P) using expensive electrodes with a large surface (circular, quadrangular, polyedric, spherical, orthogonal, etc.) can be equaled using simple electrodes with optimized geometry by means of an installation that takes advantage of the excavations in order to include special fills, and the replenishment and reserve of natural salts, according to which the expressions of the Dispersion resistance of the Mathematical Model are found for both types of electrode.

For lower Dispersion Resistance values, near 3.0 Ohm, as from the first grounding made, the addition of others is tested. In the case of vertical electrodes, up to 4 can be installed in parallel 6 m apart from each other, in any type of arrangement. In the case of horizontal electrodes, the distance can be increased up to 18 m in open or closed arrangements. In these cases, and when combined electrodes are used, the existing analytic formula gives only approximate values.

The acquisition of materials, labor, and related services may be estimated using the following reference market prices as of November 1998

The difference in the grounding models with either vertical or horizontal electrodes is limited to the outer finishing, with or without manhole. Consequently, the costs, without including Technical Direction, Insurance, Profits or Taxes, are:

Table 2: Basic Costs of Completed Grounding:

6. HOW IS A PUNCTUAL GROUNDING CARRIED OUT?

The work must be started with the accurate location of the ground point or line to be removed, verifying beforehand that no channels or underground structures exist, whether for own or external services entering or passing through the area.

The personnel in charge of the work must be familiar with the electrical installations and must be skilled in reading and interpreting construction drawings in relation to the location of devices, channels and underground structures. Likewise, precautions must be taken in relation to cracks, accidental spills or leaks.

Being the installations rather simple, direct hand labor must be carried out by workers trained on masonry at helper level. Many times, excavations require the breaking of tiles that demand careful replacement tasks. Workers must be insured against accidents.

6.2 Excavation and Soluble Salt Replenishment

During the excavation, the fine soil will be segregated from the coarse conglomerate that cannot be reused for the fill. Likewise, if pipes, sewers or underground structures are found, actions will be taken to go alongside them without damaging them. If electrical channels are found, a relocation of the excavations must be made to ensure that they are made far from them.

For a 2.5 m (l) electrode, usually a 2.8-m deep pit is excavated with 1.0-m diameter at the top and 0.8-m diameter at the bottom. These dimensions allow the normal work of two workers during slightly over half a shift. In crumbly soils, the pit mouth is expanded with one or two 0.8-m high lateral steps in order to extract the material.

The preparation of the deep bed includes pouring into the pit a saline solution of 25 kg of NaCl in 150 liters of water (a drum), thereafter waiting until it is absorbed and then spreading the reserve of 15 kg of grain salt at the bottom.

b. Trench Excavation and Preparation

3.0 m (l) Platen-type Electrodes are located in 3.0-m long and 0.85-m deep trenches, with a mouth of up to 0.6-m wide for a 0.5-m bottom. This work will take half a shift of two workers. When the soil is crumbly, the excavation is made at the natural slope.

The soil preparation consists of pouring into the trench two doses of saline solution, each having 25 kg of NaCl in 150 liters of water, waiting for its filtration, and then spreading the reserve of 25 kg of salt at the bottom.

The fill is prepared by dry mixing fine earth from the site with Bentonite (natural clay), and, if required, with fine earth from external sources, excluding cultivated soil, since it is corrosive and attacks copper, and it would also result in a predatory use.

a. Pit Fill and Vertical Electrode Placement

The mixture of earth and bentonite is slowly spread with abundant water in order to form a mortar.

The electrode with straight-line ancillaries is placed at the center of the pit; if possible, it can be left to be nailed at the end.

At a height of 1.2 m from the bottom, a dose of saline solution is poured. After it has been absorbed, 10 kg of salt is spread on the pit walls (salt collar).

Next, at a height of 2.3 m from the bottom, a new dose of saline solution is poured. The filling will be finished after the saline solution is absorbed.

b. Trench Fill and Horizontal Electrode Placement

The mixture of earth and bentonite is slowly spread with abundant water in order to form a mortar.
At a height of 2.0 m from the bottom, the electrode (plate) is placed and the filling is continued.
At a height of 0.5 m from the bottom, a new dose of saline solution is poured. After it is absorbed, the filling is completed.

In both cases, the final cover consists of earth from the site in order to maintain the external aspect similar to the surroundings. It must be considered that after 24 hours the fill will get compacted and the surface level of the excavated area will sink (0.07 m).

It is the verification of the current evacuation and dispersion in the soil by the ground connection alone (disconnected). The measurement is made using a connection scheme that depends on the geometry of the buried electrode. A portable earth-resistance meter with 3 or 4 terminals is used.

a. Principle Scheme for Approximate Measuring [Rt]

Applying the principle of "Potential Drop", the current circuits (C1, C2) and the potential circuits (C1, P2) are established, defining three essential points that allow to measure (Rt) considering the respective distances in relation to the electrical image (ro) of the electrode.

C1: Ground Connection Electrode. Referential Point.
C2: Referential Electrode at distance (d > 20 ro).
P2: Potential Electrode at distance (p = 0.62 d)

b. Equivalent Hemispherical Radius (ro)

With the theoretical expression of the Dispersion Resistance (Re) of the electrode in natural soil, the electrical image of the electrode (ro) is determined by comparing it with the equivalent parameter (Rt), i.e. making (Re = Rt).

Due to their discrete location and operation, Ground Connections are forgotten as time passes. This damages their electrical features and reduces their performance.

It consists of removing the pressure clamp connecting the electrode in order to clean it with a cloth, a metal brush and fine sandpaper (120). Then the Dispersion Resistance is measured. If there are several Users, then this work must be carried out with the authorization of all of them; otherwise, a temporary ground connection will be used.

The optimum conservation of the proposed models installed in dry weather areas is obtained flooding every six months the manhole or the outer pit with about 30 liters of water. If the weather is weather or if the ground connection is located in a garden with irrigation, such conservation measures will not be required.

The domestic use of computers and high-fidelity or high-precision electronic equipment requires relatively low Dispersion Resistance. Consequently, any installation with over 10 Ohms should be renewed. In these cases, the proposed method will be applied, recycling the same earth from the pit fill, sanding thoroughly the electrodes and placing new connection accessories with pressure. The continuity of the connection conductor from the Masses must be reviewed and tested, too.

a. Due to the inconsistency and high cost of other methods for ground connections, an economical method is proposed that minimizes Dispersion Resistance, based on the optimization of simple electrodes with a natural conductor fill that is neutral and hygroscopic, as well as enough replenishment of the natural salt reserve of the soil.

b. The proposed method has been widely used since 1978 and assures a comparatively high efficiency due to a better and longer permanence of the low Resistance achieved in the installation, compared to the performance of similar Grounding connections made using other methods.

c. The proposed method provides optimum performance in soils with high resistivity and is clean and environment-friendly. This allows to eliminate the "justification" for the use of toxic chemicals (forbidden abroad), and of unnecessary electrodes, as well as the depreciation of crop lands.

This paper has been prepared by the creator of the method, Justo Yanque M., Mechanical-Electrical Engineer - UNI, M.Sc. App-FPMS-Belgium, who is a Specialist Consultant with wide technical experience in the execution of electromechanical projects.

If you are interested in obtaining more information regarding the subject, as well as the bibliographical references, please contact Procobre-Perú.