Lightning strikes are responsible for tens of billions of dollars of damage to assets and related losses each year around the world, irrespective of the industry sector. Whilst some losses are due to the very obvious and visible โdirectโ lightning strikes, the majority are due to electrical transients from โindirectโ lightning strikes that couple inductively, capacitively or conductively into the conductive services entering or exiting a particular structure or facility.
The โcoupling mechanismsโ are important for understanding the need for surge and transient protection. Inductive coupling relates to magnetic field induction and capacitive coupling relates to electric field induction of surge currents into above and below ground conductors or circuits. Arguably, the most easily understood mechanism is conductive (or galvanic) coupling, where lightning currents or voltages enter electrical circuits through direct conduction on or through the soil or via earth potential rise (EPR).
Ironically, there is a poorly understood and often neglected subset of conductive coupling that potentially causes the most damage of all mechanisms, viz. โearth potential differenceโ (EPD). This damage mechanism arises when lightning hits the ground or a nearby structure, creates very large โearth potential riseโ at the injection point and sets up a large difference in earth potential between two points interconnected with a conductor.
This scenario is typical of most industrial sites and facilities. Many believe that a sitewide earthing system along with shielded routes, such as enclosed cable trays, obviates the need for surge protection. Obviously, proper shielding can help to mitigate inductive and capacitive coupling, and good earthing helps to limit the EPR. But these measures do not mitigate EPD due to the impulsive nature and broadband content of a lightning strike.
The aim of this paper is to explain the mechanism of EPD, present case studies that quantify the magnitude of the mechanism under lightning strike conditions, and finally to show how the damaging effects of EPD can be mitigated.