Many years ago, I visited a museum of science and technology in Munich. This museum had extensive exhibits of high-voltage engineering technology and offered an "arcs and sparks" demonstration. During this demonstration, a person was seated inside a metal cage shaped like a sphere, which was then hoisted by a crane high into the air adjacent to the business end of a Van de Graaff generator. The generator, charged to several hundred thousand volts, was then discharged to the metal sphere with no ill effect whatsoever on its occupant. The same principle, which safeguarded the occupant of this "Faraday Cage," is critical to using grounding and bonding to protect against voltage surges associated with lightning or power system transients.
Lightning strikes to overhead power lines, or to the ground in the vicinity of underground lines, can introduce surges directly on the power system in the absence of a strike to the building itself. Electrical faults and switching operations on high-voltage transmission systems can also produce surges with respect to ground on the power system. Transient voltage surge suppressor (TVSS) devices are often used throughout a building's electrical system to divert these surges and they also require an effective connection to ground.