6.5.10 Meshed earth-termination systems

An earth-termination system has a variety of functions. This has already been described in chapter 6.5.1.
The purpose of protective earthing is to safely connect electrical installations and equipment to the earth potential and to protect human life and property in the event of an electrical fault.
Lightning protection earthing ensures that the current from the down conductors is safely discharged to the ground.
The function of functional earthing is to ensure safe and faultless operation of electrical and electronic systems.
The earthing system of a structure must be used for all earthing tasks, i.e. one earthing system performs all earthing tasks. If this is not the case, potential differences can occur between the installations earthed on different earth-termination systems. In the past, a so-called 'clean earth' separate from the lightning protection earthing and protective earthing was sometimes used in practice for the functional earthing of electronics. This is extremely unfavourable and can even be dangerous. In the event of lightning effects, very high potential differences up to a few 100 kV occur in the earth-termination system, which can lead to the destruction of electronic equipment and harm to people. For this reason, IEC 62305-3 and IEC 62305-4 require complete equipotential bonding within a structure.
Electronic equipment within a structure can be earthed radially, centrally or by meshes. An intermeshed structure should preferably be used. This depends both on the electromagnetic environment and on the characteristics of the electronic equipment. If a large structure comprises more than one building and these buildings are connected by electrical and electronic connecting cables, the (total) earth resistance can be reduced by combining the individual earth-termination systems (Figure 6.5.10.1). In addition, the potential differences between the buildings are also considerably reduced. This significantly reduces the voltage load on the electrical and electronic connecting cables. The individual earth-termination systems of the buildings should be interconnected to form a meshed network. This meshed earthing network should be designed such that it contacts the earth-termination systems at the point where the vertical down conductors are connected. The smaller the mesh size of the earthing network, the lower is the potential differences between the buildings in the event of a lightning strike. This depends on the total area of the structure. Mesh sizes of 20 m x 20 m up to 40 m x 40 m have proven to be cost-effective. If, for example, industrial chimneys (preferred points of strike) are installed, the connection components around the relevant part of the installation should be closer, and, if possible, arranged radially with circular cross-connections (potential control). Corrosion resistance and compatibility of materials must be observed when choosing the material for the conductors of the meshed earthing network.