6.4.2 Necessity of internal down conductors

Extensive structures (e.g. production halls or logistics centres) have a large roof area. When lightning strikes the air-termination systems in the centre of the roof, the lightning current must travel a long distance to reach the ground via the down conductors and the earthing system. This results in a large separation distance being required between the air-termination systems on the roof surface and the steel roof structures located beneath the roof surface. If the required separation distance cannot be maintained, a conductive connection must be established between the air-termination system and the supporting structure. In such cases, the ducts through the surface of the roof should be installed by a roof tiler since he is responsible for the rain safety of the roof.

In large structures, the roof structure is supported not only by the walls around the perimeter of the building, but also by internal columns.