Lightning is a high-frequency electrical phenomenon which causes overvoltages on all conductive items, especially on electrical cabling and equipment. Lightning strokes can affect the electrical (and/or electronic) systems of a building in two ways: by direct impact of the lightning stroke on the building (see Fig. J5 a); by indirect impact of the lightning stroke on the building: A lightning stroke can fall on an overhead electric power line supplying a building (see Fig. J5 b). The overcurrent and overvoltage can spread several kilometres from the point of impact. A lightning stroke can fall near an electric power line (see Fig. J5 c). It is the electromagnetic radiation of the lightning current that produces a high current and an overvoltage on the electric power supply network. In the latter two cases, the hazardous currents and voltages are transmitted by the power supply network. A lightning stroke can fall near a building (see Fig. J5 d). The earth potential around the point of impact rises dangerously.