6.4.4.2 Installation of down conductors

The down conductor is the electrically conductive connection between the air-termination system and the earth-termination system. The function of a down conductor is to conduct the intercepted lightning current to the earth-termination system without damaging the building e.g. due to intolerable temperature rises.
To reduce damage caused by the discharge of lightning current to the earthing system, the down conductors must be installed in such a way that there are several parallel current paths from the point of strike to earth, the length of the current paths is kept as short as possible (straight, vertical, no loop formation) and connections to conductive parts of the structure are made wherever necessary.

The down conductors must be arranged in such a way that they are the direct continuation of the air-termination conductors. They must be laid straight and vertically so that they provide the shortest possible direct connection to the earth.
Loops, e.g. on projecting eaves or structures, must be avoided. If this is not possible, the distance measured where two points of a down conductor are closest and the length I of the down conductor between these points must fulfil the requirements on the separation distance s (Figure 6.4.4.2.1). The separation distance s is calculated using the total length l = l1 + l2 + l3.