A good grounding ensures safety and stability for the electrical installation, in addition to facilitating maintenance, as it provides the professional with greater security over the service he is performing. According to ABNT, electrical grounding means placing installations and equipment in the same electrical potential, so that the potential difference between the earth and the equipment is as close to zero as possible.
Among the main functions of electrical grounding, we can mention: Protect the user of the equipment from lightning strikes, by enabling an alternative path to earth, from lightning strikes; “Unload” static charges accumulated in the machine or equipment casings to the ground; Facilitate the operation of protection devices (fuses, circuit breakers, DPS, etc.), through the current diverted to earth.
When talking about grounding, there is one thing to keep in mind, safety! In order to provide the highest safety, the grounding must have the best possible quality, so we must observe the resistivity of the soil since it is through which the electric current will be drained through the ground wire. The types of grounding rods and their arrangement in space, taking into account that these factors change the resistivity and consequently the grounding resistance.
We need to understand what makes up a grounding resistance before talking about the grounding rods themselves. Grounding is understood as the electrical connection of equipment or component of an electrical system to earth, by means of suitable devices and conductors of electricity.
The resistance of the ground consists of: Resistance proper to the electrode and the electrical connections to the electrode, the contact resistance between the electrode and the earth adjacent to it, the resistance of the surrounding earth, which depends on the resistivity of the soil and the current distribution from the electrode.
The figure below illustrates the resistances mentioned above, where the grounded rod represents the electrode’s own resistance, the arrow where the current touches represent the contact resistance and the point that is “inside the earth” represents the resistance of the surrounding earth.
Grounding rods definition
The NBR-13571 sets the minimum requirements for steel-copper ground rods and their accessories (grounding connector and splice sleeve). According to the standard, the definition of steel-copper ground rods is “grounding electrode consisting of a rigid cylindrical steel-copper rod by electrodeposition”.
The steel-copper ground rods and their accessories must be manufactured with first quality materials that withstand the electrical, mechanical and chemical-corrosion resistance conditions, which are submitted when installed.
The steel-copper earthing rods and their accessories must look good, with regard to the overall finish, a steel-copper earthing rod consists of a drawn cylindrical bar made of carbon steel covered with a protective layer of copper against corrosion . The bimetallic steel / copper connection of the rods and their cylindrical steel core, must be through the electrodeposition process.
Grounding rod characteristics
The importance of earthing rods (electrodes) in the composition of the earthing resistance is evident, and obviously their dimensioning needs to be taken into account because this resistance must be as small as possible, close to zero, in order to guarantee a better efficiency of earthing . Ground rods are usually made of a copper-clad steel core.
The length of the grounding rod can vary between 1.5m and 4.0m. The 2.5 m ground rods are the most used, as they reduce the risk of reaching underground ducts in your installation. The geometry of the grounding rods (as they are available in space) has a direct influence on the resistivity of the grounding and its resistance. Below we can check some of the types of grounding arrangement: