6.5.5 Earthing systems for lightning protection systems as per IEC 62305-3

The earth-termination system is the continuation of the air-termination systems and down conductors to discharge the lightning current to the earth. Other functions of the earth-termination system are to establish equipotential bonding between the down conductors and to control the potential in the vicinity of the building walls. 
It should be noted that a common earth-termination system is to be preferred for the different electrical systems (lightning protection systems, low-voltage systems and telecommunications systems). This earth-termination system must be connected to the equipotential bonding system via the main earthing busbar (MEB).
Since IEC 62305-3 requires consistent lightning equipotential bonding, no particular value is specified for the dissipation resistance. In general, a low earth resistance (≤ 10 Ω, measured with a low frequency) is recommended.

The standard distinguishes between type A (single earth electrode) and type B (ring earth electrode) earth electrode arrangements. Both earth electrode arrangements have a minimum earth electrode length I1 of the earthing conductors depending on the protection class (Figure 6.5.5.1).

The soil resistivity can be determined using the methods described in chapters 6.5.2.2 and 6.5.2.3. If this cannot be done in densely built-up areas, a specific earth resistivity of 1,000 Ωm can be assumed to determine the minimum length of the earth electrode.