A well-designed earthing system is the basis for any electrical installation in order to ensure the danger associated with fault currents is avoided, protecting both equipment and individuals.
In a lightning protection system, a low earth resistance is essential as dispersion of the lightning current takes place within it. Each down conductor must have a ground connection, consisting of conductive elements in contact with the ground capable of dispersing the lightning current.
According to national low voltage electrotechnical regulations: “Earthing that is mainly established in order to limit the tension that, with respect to earth, metallic masses may present at any given moment, ensure the protection and eliminate or reduce the risk of a breakdown in the electrical materials used.”
The main objectives of a low voltage earth termination system are:
- Provide safety for persons by limiting the touch voltage.
- Protect installations and equipment by providing a low impedance path.
- Improve the quality of the signal by minimizing the electromagnetic noise.
- Set a reference voltage for the system equipotentialization.
How to obtain a perfect earth termination
In order to obtain an effective grounding, it is essential to achieve a low ground resistance, using conductors with a high resistance to corrosion and a suitable section to transport the expected current. It is also advisable to bond the connections of said conductors by exothermic or aluminothermic welding.
Other determining factors when designing an earthing system include:
- The resistance should be measured regularly making it necessary to install an earth inspection pit.
- The soil humidity will reduce the earthing resistance.
- Earthing enhancing compounds reduce the soil resistivity.
- Buried electricity and gas installations should be identified in order to respect the security distances for each.
- Buried pipes and water tanks should be identified in order to equipotentially bond them with the earth system.
To obtain a suitable earth resistance in earthing systems, special electrodes should be used for low-conductivity soils – enhanced electrodes, deep electrodes or ring conductors should be used in order to obtain a suitable earth resistance.
Earthing for lightning protection systems
In a Lightning Protection System, both with Early Streamer Emission (ESE) or with Franklin rods and meshed conductors, the Earth Termination System is essential for lightning protection, since the lightning current should disperse to ground through iy. Each downconductor must have an Earth Termination System.
The regulations recommend that the earthing resistance for the lightning protection system, separated from any other element, be less than 10Ω. Also, it should be noticed that lightning current is an impulse, so it is important that the impedance of the grounding is not high. Therefore, it is not advisable to use a single, very long electrode.
The use of deep electrodes is interesting if the resistivity of the surface is particularly high and there are lower layers of the ground that are more humid. For lightning bolt dispersion, the radial type configurations in triangle or goosefoot are suitable.
In general, it is recommended to connect the grounding of the lightning rods to other existing ground connections of the installation, preferably with an isolation spark gap to avoid noise interference and galvanic pair problems.