Under what condition can the structural steel framework of a structure serve as a main lightning conductor?

The structural steel framework of a building can serve as a main lightning conductor when it is made electrically continuous. This means that all parts of the steel framework are interconnected properly to ensure that electrical current can flow freely throughout the structure without encountering resistance or breaks in the circuit.

When the steel is electrically continuous, it can effectively direct lightning strikes into the ground, helping to protect the structure and its occupants from lightning-induced damage. Achieving electrical continuity often involves ensuring that all connections are secure and that there are no gaps in the conductive path.

The other options might seem relevant but do not secure the functionality of the structural framework as a lightning conductor in the same way. For instance, even if the steel contains no insulation, is entirely exposed, or is painted, these factors do not guarantee that the steel is electrically continuous. In fact, insulation may enhance safety and exposure might increase the risk of damage, but without the necessary continuity, the steel cannot effectively serve its purpose as a lightning conductor.