General earthing system: earthing installation
The general earthing system connects the different parts of an electrical installation with the surface conductor of the Earth to guarantee security and functionality.
In an electrical installation, the soil itself provides equipotentialization for all of the metallic parts so that there are no differences in potential between each other. This is carried out in order to guarantee the safety of people and equipment from any possible defects in the actual facilities.
Depending on the purpose for which the earthing system is designed, there are two types of earthing:
- Protection or masses earthing: It is defined as all the metallic parts of the installation that normally are not under tension, but could be due to consequences such as damages, accidents, atmospheric discharges or overvoltages.
- Service or neutral grounding: It is defined as the return of the current in the installation and its mission is to set a reference potential.
How to install earthing in an electric system
Depending on the electric system that the general grounding installation requires, certain steps and guidelines must be followed. In all cases, earthing must be installed before the foundation, because if it is planned afterwards, more complex and expensive solutions would be required.
Grounding installation in low voltage systems
The aim of this earthing system is to guarantee that, if a person touches a metallic object that has a certain voltage due to an electric failure, it does not exceed the safety threshold, generally established as a maximum of 50 volts.
The elements that are used in an earthing system, will vary slightly depending of the resistivity of the land.
Below is a description of the parts of an earthing connection:
Electrodes for earthing
General earthing is usually done via a bare earth ring or horizontal conductor mesh buried in the ground, complemented by vertical electrodes to decrease resistance.
There are different types of electrodes depending on their characteristics and functionality:
- Earth rods, also known as spikes, are steel electrodes with an electrolytic copper coating which are used in the standard solutions for terrains with low resistivity. The rod is a copperbond steel rod which is recommended to have a copper coating thickness of more than 250 microns. Usually, the rods are usually 2 meters in length yet could be joined together for longer lengths. There are also solid cooper, galvanized steel and stainless steel rods.
- Dynamic electrodes are used in high resistivity grounds, for example, when the terrain is rock and a standard solution is not adequate.
In these cases, APLIROD© dynamic electrodes are used. These are composed of a hollow copper tube filled with an ionic compound mixture that improves the resistivity of the soil around the electrode between 30-80%.
- Earth plates are recommended for grounding in stony ground and are generally made of copper or galvanized steel.
- Graphite electrodes are characterized as being good conductors, inert to chemical agents, therefore they are ideal electrodes for high resistivity terrains. It is convenient to install them in deep wells, so that the electrode sits at least two meters below the surface.
Earth pits in earthing installations
Earth pits are usually situated towards the outside of buildings. Their function is to provide an accessible manhole for any tests and inspections.
The connection to the earth pit is made at the bottom of the excavation, through a device that allows the disconnection of the earth conductors and which must carry an earth symbol.
Earth bonds: permanent and mechanical
Connection components must ensure the continuity between the down conductor and the earthing. It is recommended that connections be made using the exothermic welding APLIWELD©, because unlike mechanical connections, welded joints do not degrade and are a permanent solution that is not going to deteriorated with the passage of time.
Conductivity enhancers are used in high resistivity terrains, where obtaining a low resistivity ground value could be impossible, even with the installation of multiple electrodes. Conductivity enhancers achieve a moisture retention in the soil and provide ions that significantly reduce the resistance in a long lasting manner and without any corrosion.
Ground connection to lightning rods
Lightning rods must have their own ground connection, independent from the general earthing, yet linked to it by means of spark gaps in order to avoid corrosion problems and to decrease any overvoltage.
In a lightning protection system, each down conductor must have a ground connection, comprised of the conductor elements in contact with the soil capable of dispersing the lightning current.
The ground connection of lightning rods must have a resistance value of less than 10 ohms. Due to a lightning bolt being an impulse current, it is important that the impedance of the earthing is low.
In an earthing system, it is not recommended to only use one long electrode. It is recommended to use deep electrodes if the surface resistivity is elevated and the underlying layers of the soil are moist.
Protective earth calculation
There are several methods to perform the earth calculation, depending on the installation characteristics one method or another is applied.
Depending on the earth calculation result, the characteristics of the ground installation are established, with the required number of rods. It is sought to establish a determined soil resistance that ensures the voltage supported by any person is within delimited values.
Aplicaciones Tecnológicas offers the engineering service for the calculation of earth connections based on international standards. For more information about this and another services, please contact us.