What is the difference between direct and indirect lightning strikes and how are their effects mitigated in industrial facilities?

The primary difference between direct and indirect lightning strikes lies in whether the lightning current physically enters a structure or system. A direct lightning strike occurs when the lightning channel makes direct contact with a building, equipment, or power line. This results in a large surge of current flowing through the structure, causing significant damage due to heat, electromagnetic forces, and overvoltage. An indirect lightning strike occurs when lightning strikes near a structure or power line. The electromagnetic fields generated by the lightning strike induce voltage surges in nearby conductors, even though the lightning current does not directly enter the structure. These induced surges can still damage sensitive electronic equipment. Mitigation strategies for direct lightning strikes include: Lightning rods: These are conductive rods installed on the highest points of a building to provide a preferred strike point for lightning. The rods are connected to a grounding system that safely conducts the lightning current to ground. Grounding systems: A well-designed grounding system provides a low-impedance path for lightning current to flow to ground, minimizing damage to the structure and equipment. Surge protection devices (SPDs): SPDs are installed at the service entrance and at sensitive equipment to divert surge currents to ground, protecting the equipment from overvoltage damage. Mitigation strategies for indirect lightning strikes include: Shielding: Shielding cables and equipment reduces the amount of induced voltage from nearby lightning strikes. Surge protection devices (SPDs): SPDs are installed at the service entrance and at sensitive equipment to divert surge currents to ground, protecting the equipment from overvoltage damage. For example, a tall distillation column in a chemical plant is at high risk of direct lightning strikes and would be protected by lightning rods connected to a robust grounding system. Electronic control systems within the plant are susceptible to damage from indirect lightning strikes and would be protected by SPDs installed at the power panels and at the equipment itself.