Lightning currents are load-independent currents, i.e. a lightning discharge can be considered an almost ideal current source. If a load-independent active electric current flows through conductive components, the amplitude of the current, and the impedance of the conductive component the current flows through, help to regulate the potential drop across the component flown through by the current.
In the simplest case, this relationship can be described using Ohm´s Law.
U = I · R
If a current is formed at a single point on a homo-geneously conducting surface, the well-known potential gradient area arises.
This effect also occurs when lightning strikes homogeneous ground (Figure 1).
If living beings (people or animals) are inside this potential gradient area, a step voltage is formed which can cause a shock current to flow through the body (Figure 2).
The higher the conductivity of the ground, the flatter the shape of the potential gradient area.
The risk of dangerous step voltages is thus also reduced. If lightning strikes a building which is already equipped with a lightning protection system, the lightning current flowing away via the earth-termination system of the building gives rise to a potential drop across the earthing resistance RE of the earth-termination system of the building (Figure 3).
As long as all conductive objects in the building, which persons can come into contact with, are raised to the same high potential, per-sons in the building cannot be exposed to danger. This is why it is necessary for all conductive parts in the building with which persons can come into contact, and all external conductive parts entering the building, to have equipotential bonding.
The rise in potential of the earth-termination system as a result of the lightning current also creates a hazard for electrical installations (Figure 4).
In the example shown, the operational earth of the low-voltage supply network is located outside the potential gradient area caused by the lightning current. If lightning strikes the building, the potential of the operational earth RB is therefore not identical to the earth potential of the consumer system within the building.
In the present example, there is a difference of 1000 kV. This endangers the insulation of the electrical system and the equipment connected to it.
Close Up Lightning Strike Compilation
Cant see this video? Click here to watch it on Youtube.
Reference: Lightning Protection Guide – Dehn