Grounding is one of the first concepts that we come across when studying electrical engineering. The need to introduce protective grounding is due to the fact that a necessary condition for safe operation of any electrical installation is the forced closure of all its conductive components to the ground.
The principle of grounding
It is known that the ingenious is in the overwhelming majority of cases simple. Likewise with grounding. It is based on several extremely simple and obvious provisions.
They boil down to the fact that:
- it is guaranteed to prevent both insulation breakdown on the conducting body of the electrical installation and a noticeable decrease in its resistance;
- potential hit on the body has no visible external signs;
- when touching such parts, a person is under the influence of a high potential;
- the striking factor is the current that appears at the moment of touching the conductive surface and flows through the human body to the ground;
- the current can be reduced to a safe level only by reducing the potential DIFFERENCE between the conductive element and ground, for which it is enough to forcibly connect the metal parts of the case to the ground with a wire with a small resistance.
As a result, during insulation breakdown, the current simply goes mainly into the ground, and does not pass through the human body. This is illustrated graphically in Figure 1.
Why is there little ground resistance
In accordance with Ohm's law, current always flows in a closed loop. In this case - from one pole of the power plant to the grounding electrode through the electrical installation with its grounding system. In this case, the low resistance of the grounding of the installation does not guarantee a small resistance of the reverse branch of the circuit under consideration. Indeed, the resistivity of the soil is quite large and, at first glance, the effect of shunting the human body by grounding elements does not occur.
For all the validity of this reasoning, it does not take into account one circumstance. In this case, the fact that due to the extremely large, almost infinite cross-section of the medium between the grounding electrodes of the power plant and electrical installation, the resistance of the reverse branch of our closed loop will be quite small.
This allows:
- the grounding system to qualitatively perform its protective functions;
- ensure high reliability of their implementation due to the absence of breaks
- significantly save on the laying of an additional cable connecting the earthing switches of the facility and the power plant.
Features of the grounding device
Protective grounding performs its functions qualitatively only if there is little transition resistance between the grounding electrode to which the unit is connected and the ground.
This is achieved:
- high contact area, which, in turn, is provided by several plates rigidly connected by welding, Figure 2;
- by deepening the electrodes below the freezing depth (the resistivity of the frozen soil increases sharply).
To fulfill the last condition, vertical earthing switches with a length of up to 3 m are well suited, Figure 3.
In addition, the grounding resistance must not exceed 4 ohms. The latter is determined by the fact that the resistance of the human body is at least several hundred ohms, i.e. two orders of magnitude higher.