Direct or nearby lightning strikes can cause lightning to enter buildings, adjacent facilities, or even penetrate buildings through power distribution cables, communication lines, and data cables.

Lightning currents from a direct strike that enter a building can cause a significant potential rise relative to the grounding equipment. The voltage drop across the grounding resistance and the potential rise in the surrounding environment can create surges, which are among the greatest threats to the building's electrical system.

Surges can also be induced in the building’s electrical installations, interconnected systems, and equipment due to the electromagnetic field generated by lightning.