3.3.10 Summary

Lightning protection risk analyses aim to objectify and quantify the risk posed by direct and indirect lightning strikes to structures and their contents. There have been some important changes compared to the first and second editions of IEC 62305-2.

Significant changes in the third edition include:

1. Only one risk value is calculated; i.e. no distinction is made between different types of damage. This risk calculation applies to structures and their systems that are of 'public importance'. This conveys that such systems, or their failure, also affect people who are not involved in the risk management decision-making process (the public). A risk assessment can be carried out for private facilities; this is then the responsibility of the owner or the appointed management team.
2. In addition, a failure rate (damage frequency) is calculated and evaluated for electrical and electronic systems. This only covers damage caused by power surges. The consequences and extent of such damage are not taken into account here. It is exclusively about the number of possible failures within a defined period of time. A tolerable failure rate can again be defined in general terms for systems of 'public importance' (proposals from the IEC, national committees, legislators), while for 'private' systems, such a value is the responsibility of the owner or the appointed management.
3. The ground flash density N_g is replaced by the lightning flash to earth point of strike density N_sg for some sources of damage. This takes into account the fact that many lightning discharges have more than one lightning base point, i.e. a subsequent strike usually deviates from the partially ionised lightning channel of the previous partial strike and has a different point of strike. 
4. The reduction of a few risk components can be achieved by using preventive temporary measures activated by a thunderstorm warning system (TWS) in accordance with IEC 62793. The risk of a direct lightning strike on people in open areas was introduced because this risk can be reduced by a TWS.
5. IEC 62305-1 specifies in more detail the impulse currents (parts of lightning currents or induced currents) to be expected or assumed for surge protective devices at different installation locations for the various sources of damage. In addition to this, IEC 62305-2 now specifies tiered, highly detailed damage probabilities (P_SPD) for surge protective devices, depending on the lightning protection level and source of damage for low-voltage systems and telecommunications systems.