Peak Value of Deadly Lightning Current

Peak Value of Deadly Lightning Current (photo credit: William via Flickr)

Lightning Discharge

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.

Potential distribution of a lightning strike into homoge-nous soil

Figure 1 – Potential distribution of a lightning strike into homoge-nous soil


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).

Herders count the bodies of sheep in Hoboksar in the Xinjiang Uygur autonomous region on Tuesday. A lightning strike killed 173 sheep, a heavy loss for the herders.

Figure 2 – Herders count the bodies of sheep in Hoboksar in the Xinjiang Uygur autonomous region on Tuesday. A lightning strike killed 173 sheep, a heavy loss for the herders (source: peopledaily.com.cn)


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).

Potential rise of the earth-termination system of a build-ing compared to the remote earth due to the peak value of the lightning current

Figure 3 – Potential rise of the earth-termination system of a build-ing compared to the remote earth due to the peak value
of the lightning current


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.

If this is disregarded, there is a risk of dangerous shock hazard voltages if lightning strikes.

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).

Threat to electrical installations by potential rise at the earth-termination system

Figure 4 – Threat to electrical installations by potential rise at the
earth-termination system


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


author-pic

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears. Professional in AutoCAD programming and web-design. Present on Google+.



One Comment


  1. Dr Subhash Joshi
    Mar 08, 2014

    It is an excellent handout for practicing engineers. My greeting to Edward and EEP for dissemination

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