6.4.1 Determination of the number of down conductors

The following points should be observed when installing the conductors so as to prevent damage caused by lightning currents that are discharged to earth via the external lightning protection system (LPS):

• Provide multiple down conductors: There should always be several parallel down conductors from the point of strike to the ground. This distributes the current across multiple paths and reduces the load on individual conductors.
• Keep cable routes short and direct: The shorter the path to earth, the lower the voltage drops and thus the risk of uncontrolled flashovers. Conductors should therefore be laid as straight as possible and without avoidable circuitous routes.
• Ensure equipotential bonding: All conductive parts of the structure (e.g. metal parts, pipes, electrical systems) must be connected to each other. This prevents dangerous voltage differences from occurring. 

The minimum number of down conductors required depends on the circumference of the external edges of the roof (circumference of the projection onto the ground area) of the structure. The down conductors must be arranged to ensure that, starting at the corners of the structure, they are distributed as uniformly as possible over the circumference. Depending on the structural conditions (e.g. gates, precast components), the mutual vertical spacing between the down-conductors may vary. In each case, there must be at least the total number of down conductors required for the respective class of LPS. IEC 62305-3 specifies typical vertical distances between down conductors depending on the protection class (Table 6.4.1.1).