A direct strike occurs when a structure or an electrical service conductor is hit directly by a cloud to ground lightning strike. A direct strike surge is the strongest and most destructive of surge sources. The average lightning strike represents 30,000A (30kA) of current flow up to a possible maximum of 200,000A (200kA).
A lightning strike does not have to hit equipment or cabling directly for it to cause damage. The majority of current surges are caused by the indirect effects of the lightning strike. For example, by lightning striking a building containing electrical and electronic equipment.
As the energy from a cloud to ground lightning strike is dissipated to earth, a radiating field of earth potential is created due to the resistance of the ground. Sites or equipment grounded near to the strike point will rise to a much higher potential than sites or equipment further away. Any power or signal cabling between these sites will be used as a current path due to the high potential difference. This is why surge protection is a requirement for cables running between equipment with discrete earthing. In the example below, even though lightning only strikes the building on the left, surge protection is required at both buildings.
Due to the high current and voltage present during a cloud to cloud or a cloud to ground strike, a ‘Lightning Electromagnetic Pulse’ (LEMP) is released into the atmosphere. This imposes a surge onto signal and power cabling through induction of the magnetic field and capacitive coupling of the electric field. The energy resulting from the LEMP effect is much lower than the previous sources, but still affects sensitive equipment and long signal interconnects. As before, surge protection is required at both ends of the cable, that is at each building.
Switching surges occur when reactive electrical equipment such as capacitors or inductors are switched into and out of the grid. Switching surges are by far the most prominent source of surges to which equipment may be exposed. Switching surges are generally quite low in energy but can still damage sensitive equipment and cause component degradation leading to operational failure.