When an object is thunderstruck, principle of LV power system protection thanks to cascaded 3-stage protection
Protection system of LV power system composited of lightning current arresters and surge arresters SPD must be able to discharge lightning currents or their substantial parts without their damage.It is generally recommended to come out from the ohmic resistance of the building earthing, pipeline, power distribution system and so on for the purposes of establishing current distribution going through SPD in case of direct lightning stroke into a building protected by the outside lightning system. The following figure shows a typical example of lightning current distribution in an object hit by direct lightning stroke.
Where an individual evaluation is not possible, it can be assumed that:
– 50% of the total lightning current Iimp = 200kA (10/350)…. IS1 = 100kA (10/350) enters the earth termination system of the LPS (lightning protection system) of the structure considered
– 50% of Iimp = 200kA (10/350)…. IS2 = 100kA (10/350) is distributed among the services entering the structure (external conductive parts, el.power, communication lines, etc.) The value of the current flowing in each service Ii is given by IS/n, where n is the number of the above mentioned services (see the above figure).
For evaluating the current Iv in individual conductors in unscreened cables, the cable current Ii is divided by m, the number of conductors, i.e. Iv = Ii /m.
For shielded cables, the current will flow along the shield. Requirement on dimensioning of protective system SPD in the most usual connection of the building and LV power system (TNC – system 230/400V/50Hz) results from this reasoning:
For maximum lightning current size Iimp = 200kA (10/350) it is enough to dimension the protective cascade of each phase conductor entering the object on approx. 4% Iimp, that is on approx. 8kA (10/350) in most cases.