Lightning Surges Where the lightning surges come from □ From power lines : When lightning strikes nearby a surge can be induced to the low-voltage distribution lines. The operation of a high-voltage arrester could happen at the lead-in entrance, and it results in a rise of the potential of the earth. By forcing its way into the low-voltage distribution lines from a class 2 ground wires, or some other route, the lightning surges enter into the low-voltage circuits through the power lines. □ From signal lines or load lines : When a lightning discharge occurs in the vicinity of the signal lines drawn in overhead from the outside, the lightning surge is induced to them. Also, when outside-mounted sensors or control equipment receives a lightning strike, extremely large lightning surges are passed along the signal lines and jump into the control circuits. □ From ground lines : Lightning that strikes a building causes a large lightning current to flow through the building’s lightning rod. This results in the potential of the earth rising extraordinarily and the lightning surge forces its way in as it is conveyed along the ground line of the equipment. Lightning surges force an entry via various routes and there are records of more than 12,000V developing between lines and earth, and lightning surge currents reaching 1,000A. In contrast to this attacking threats, the lightning surge withstand voltages of the low-voltage equipment (for the mostly operating at 230V or less) are on the order of 1,000V to 1,500V. Semiconductor circuits themselves have a withstand voltage of only several volts and so lightning surges pose a great threat to computerized low-voltage equipment and to so called digital convergence equipment. What are the forms of transient surges ? Surges by the direct lightning strikes : When lightning strikes a lightning rods or other grounded metal structures of a building, the lightning current is dissipated into the ground. The impedance of the ground and the current flowing through it create huge potential as the surge. This transient voltage, surge, then propagates the LV system through the cables, damaging equipment on the way. Surges by the indirect effects of lightning strikes : The transient overvoltages, surges, of above mentioned routes are also found when lightning strikes in the vicinity of a facility, due to the increase of ground potential at the point of impact. The electromagnetic fields of the lightning create indirect surges by inductive, capacitive and resistive coupling. Within a number of kM distance, the electromagnetic field caused by lightning in clouds can also create sudden increases in voltage.