There are many mechanisms by which lightning can cause personnel injuries and deaths. Most of the mechanisms are electrical, including: (i) “Direct strike” to the person. (ii) “Side flash” from an adjacent structure. (iii) Touching a conductor that has risen to a dangerous voltage (“touch voltage”). (iv) “Step voltage” from a lightning strike to a structure or ground.
The above mechanisms account for 3-5%, 20-30%, 15-25%, and 40-50% of human fatalities from lightning events (Cooper & Holle 2019), noting that serious injuries from lightning are ten times more likely than fatalities (Holle 2005, Mills et al 2006). These statistics show that earth potential rise (EPR) accounts for over 95% of all lightning fatalities and injuries.
Lightning protection standards provide varying degrees of guidance on EPR hazards. For example:
• AS 1768 (2021): Section 6.3.2 provides only generic, qualitative measures to reduce, and in some cases mitigate, exposure to electrical injury mechanisms, such as avoiding being outdoors during a thunderstorm, having sufficient separation from conductors carrying lightning currents, avoiding touching conductors carrying lightning currents, and managing earth potential rise by insulation or voltage equalisation.
• IEC 62305-3 (2024): Sections 8.1 and 8.2 address protection against touch and step voltages. o Touch Voltages: Mitigation for people around dedicated downconductors is suggested to be a clearance of 3 metres or a contact resistance1 with the soil within 3 m of the down conductor of ≥ 100 kΩ over an area of a typical footprint (0.02 m2). When natural components are used as downconductors, there must be at least 10 columns of structural steel or 10 pillars of interconnected steel, “with the electrical continuity ensured”. If these conditions are not met, exposed downconductors shall be insulated to an impulse withstand voltage of 100 kV when wet and physical restrictions or warning notices installed. o Step Voltages: Same mitigation conditions as for touch voltages, with equipotentialisation installed and physical restrictions / signage. This standard nominates a tolerable impulse step voltage of 25 kV.
The problem with these standardised guidelines is that they are either too generic, difficult to verify, or impractical to implement in some cases. Also, questions arise, such as “is 100 kV of insulation sufficient or too conservative?”.
The present paper attempts to address these issues by presenting calculations of step and touch voltage at typical lightning frequencies for a range of common scenarios, such as around a HV power system (substation), around a tall building in the city, a technician working in the field, personnel on an industrial site, etc. A separate study is also conducted of step voltage as a function of the distance and intensity of a lightning strike in open areas.
Conclusions are drawn regarding the recommendations in standards as well as any new guidelines that may be appropriate. Where possible, novel mitigation measures are suggested.