The Grounding resistance is the electrical resistance between the terminals of an earth electrode and the ground. The grounding resistance is an important characteristic of an earth electrode and should be as small as a rule.
Composition of the grounding resistance
The grounding resistance is primarily determined by the soil resistivity of the soil in the immediate vicinity of Erderelektrode and the geometry of Erderelektrode. The grounding resistance is also called spreading resistance.
Grounding systems, which include several kilometers in diameter in the spatial extent can be performed with very little grounding resistance in the area around and just below 1 Ω. These are in monopolar high voltage direct current transmissions, in which the operating ground currents in normal phase up to several kA, are used. An example of such a system is the Pacific DC Intertie in the USA.
The specific earth resistance ( spreading resistance ) is largely responsible for the level of grounding resistance. This specific earth resistance equal to the resistance of a cube with an edge length of one meter. Is a prerequisite for resistance determination that the cube is determined by flows from one edge to the other edge surface area. The amount of soil resistivity depends on several factors. In addition to the type of soil also assume the grain size and the density of the soil affect the soil resistivity. In addition, differences in moisture content affect the amount of soil resistivity. These moisture contents are subject to seasonal fluctuations. The seasonal variation of soil resistivity depends mainly on the depth of the soil. These variations extend approximately sinusoidal. Also, the temperature of the soil affects the amount of soil resistivity, frozen soil affects almost like an insulator.
By comparison, the resistivity ρ of good electrical conductors such as metals located in areas under 10-6 OMEGA.m.
Current distribution and potential profile
The current distribution and the potential profile of the earth electrode and thus the grounding resistance, depend on the dimensions and arrangement of the earth electrode. The larger the surface of the earth electrode, the large area is the area of contact with the ground. A larger contact surface with the ground is at the same time, a greater power output surface. Thus, this will reduce the grounding resistance. The shape and size of the Erderoberfläche essentially determined in Erdernähe the current distribution around the earth electrode. With increasing distance from the earth electrode decreases this influence.
In a Halbkugelerder the current from the center of the sphere spreads starting radialsymetrisch from the ground. However, this is only possible when the soil is homogeneous. The area, which is the power at the outlet of the grounding is available, is initially relatively small, but it is with increasing distance from the earth electrode getting bigger. In the drawing on two metallic hemispheres are each buried with the radius r at a distance d in the ground. It is assumed that the distance d is substantially greater than the spherical radius r. In this case, to be measured at the resistor Rk Pinch K is independent of the distance d, this is due to the cross-sectional sizes of the soil. Depending on the radius of the earth electrode and the soil resistivity ρ in the vicinity of the earth electrode resistance Rk is determined by the following equation:
Thus, the grounding resistance R of an earth electrode to give:
In practice hemispherical earth electrode will not be used.
The potential distribution at the surface is dependent on the design of the earth electrode. Earth Rods have a less favorable potential distribution as a surface earth at the surface. Generated, the flow through the earth electrode in the soil flow around the earth electrode a voltage gradient. To a Halbkugelerder arise concentric equipotential lines. When used in practice, earth electrodes differently shaped equipotential lines and thus, differently shaped voltage gradients arise. The potential distribution has a large influence on the step voltage.
Impulse earthing resistance
In high-frequency processes, such as when lightning currents can no longer be expected to ground resistance. This is expected due to the changed parameters with the impulse earthing resistance. The impulse earthing resistance (shock impedance) is due to eddy currents and radiation effects a higher value than the grounding resistance. The reason is that the earth resistance is measured at a frequency of 50 hertz. For practical Erderberechnung consideration of the determined at 50 Hertz Erderwiderstandes is usually sufficient.
Earthing resistance in practice
In practical grounding electrodes, additional resistance components as those of the connecting cable and the connecting elements are added (terminals ). These resistors are usually negligible, provided that the fasteners are tight, since the components are usually made of highly conductive metals. For other geometries of grounding conductors, differ significantly from the spherical shape, such as strip earth, there are other relations between the form of the earth electrode resistance and that is often not analytically, but are determined by practical measurements.
Measurement of earth resistance
For the measurement of earth resistance, there are several different methods. Important tools are ground stakes as auxiliary electrodes and probes and current probes for feeding and measuring the Erderströme.
Grounding resistance as a component
Alternatively, are referred to with the term grounding resistance also star earthing resistors are switched in energy supply systems between the star point of a power transformer or generator and earth, and which limit the fault current to acceptable values in case of error. In the field of low-voltage systems with solidly earthed neutral point of the non-operational current to earth to tripping of the protective device such as a fuse, circuit breaker or residual current circuit breaker leads. The size of the current is determined inter alia by the grounding resistance.