Lightning currents are impressed currents, in other words a lightning discharge can be considered to be an almost ideal current source. If an impressed electric current flows through conductive parts, a voltage drop across the part carrying the current occurs due to the amplitude of the current and the impedance of the conductive part carrying the current. In the simplest case, this relationship can be described using Ohm's Law.

U = I \: .\: R

I Peak value of the lightning current 
R Earth resistance
Figure 2.2.1 Potential distribution in case of a lightning strike to homogenous ground
Figure 2.2.1 Potential distribution in case of a lightning strike to homogenous ground

If a current is formed at a single point on a homogeneously conducting surface, a potential gradient area arises. This ef­fect also occurs when lightning strikes homogeneous ground (Figure 2.2.1).If living beings (persons or animals) are inside this potential gradient area, step voltage is formed which can cause electric shock.The higher the conductivity of the ground, the flatter is 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 via the earth-termination system of the building causes a voltage drop across the earth resistance RE of the earth-termination system of the building (Figure 2.2.3).

Figure 2.2.3 Potential rise of the building’s earth-termination system with respect to the remote earth caused by the peak value of the lightning current
Figure 2.2.3 Potential rise of the building’s earth-termination system with respect to the remote earth caused by the peak value of the lightning current
Figure 2.2.4 Risk for electrical installations resulting from a potential rise of the earth-termination system
Figure 2.2.4 Risk for electrical installations resulting from a potential rise of the earth-termination system

As long as all exposed conductive parts in the building are raised to the same high potential, persons inside the building are not in danger. There-fore, it is necessary to establish equipotential bonding for all exposed conductive parts in the building and all extraneous conductive parts entering the building. If this is disregarded, dangerous touch voltages may occur in case of a lightning strike. The rise in potential of the earth-termination system as a re­sult of the lightning current also creates a hazard for electrical installations (Figure 2.2.4). In the example shown, the opera­tional earth of the low-voltage supply system 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 with the earth potential of the consumer's installation inside the building. In the example, the difference is 1000 kV. This endangers the insulation of the electrical installation and the equipment connected to it.