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3.3.3 Frequency of dangerous events

The following frequencies of dangerous events can be relevant for a structure:
ND        Caused by direct lightning strikes to a structure.
NM        Caused by nearby lightning strikes with magnetic effects.
NL        Caused by direct lightning strikes to incoming supply lines.
NI        Caused by lightning strikes near incoming supply lines.

A detailed calculation can be found in Annex A of IEC 62305-2.

To calculate the above frequencies, either the annual lightning flash to earth point of strike density NSG or the annual ground flash density NG is required. Both parameters are interrelated (Equation 3.3.3.1):

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