Effect of lightning strike

Different effects

The effects of lightning are those of a high-strength impulse current that propagates initially in a gaseous environment (the atmosphere), and then in a solid, more or less conductive medium (the ground):

  • visual effects (flash): caused by the Townsend avalanche mechanism
  • acoustic effects: caused by the propagation of a shock wave (rise in pressure) originating in the discharge path; this effect is perceptible up to a range of around 10 km
  • thermal effect: heat generated by the Joule effect in the ionised channel
  • electrodynamic effects: these are the mechanical forces applied to the conductors placed in a magnetic field created by the high voltage circulation. They may result in deformations
  • electrochemical effects: these relatively minor effects are consist in the form of electrolytic decomposition in accordance with Faraday’s law –– induction effects: in a varying electromagnetic field, each conductor becomes the seat of an induced current
  • effects on a living being (human or animal): the passage of a transient current of a certain r.m.s value is sufficient to incur risks of electrocution by heart attack or respiratory failure, together with the risk of burns.

Lightning causes two major types of accidents:

  • accidents caused by a direct stroke when the lightning strikes a building or a specific zone. This can cause considerable damage, usually by fire. In order to prevent any risk of accident, lightning air terminals should be used
  • accidents caused indirectly, as when the lightning strikes or causes power surges in power cables or transmission links.
    Hence the need to protect with SPD the equipment at risk against the surge voltage and indirect currents generated.

Direct effects

Thermal effects

Thermal effects: These effects are linked to the amount of charge associated with lightning strikes. They result in fusion points melting holes of varying sizes at the point of impact of materials with high resistivity. For material which is a poor conductor, a large amount of energy is released in the form of heat. The heating of water vapour contained in the material results in very high abrupt localized pressure which may cause it to explode.

Effects due to the initiation: In the event of a lightning strike a substantial increase in the ground potential of the installation will occur depending on the grounding network and soil resistivity. Potential differences will also be created between various metal elements. Hence the need to pay particular attention when installing earth rods and inter-connection of metal structures adjoining the conductors.

Acoustic effects

Acoustic effects - thunder: Thunder is due to the sudden increase in pressure (2 to 3 atmospheres) of the discharge channel developed by the electrodynamic forces during the lightning flash. The duration of a thunder clap depends on the length of the ionized channel. For high frequencies, propagation of the spectral components released by the shock wave is perpendicular to the channel. For low frequencies, propagation is omnidirectional; hence the different forms of rumbling or claps heard by an observer according to the distance and orientation of the successive channels used by the lightning flash.

Luminous effects: A lightning strike nearby violently sensitizes the retina of an observer. The eye is dazzled and vision is lost for several long seconds.

Electrodynamic effects: Electrodynamic effects between conductors and other parts occur due to large magnetic field of the lightning current.. This results in substantial mechanical forces, both attractive and repulsive, that are all the stronger when the conductors are close together or the current is high.

Electrodynamic effects

Electrochemical effects: The fleeting nature of lightning impacts (compared to stray ground currents) mean that these effects are highly negligible and without influence on earth rods

Indirect effects

The ever increasing use of sensitive electronics means that electrical equipment is becoming more and more vulnerable to transient overvoltages caused by lightning.The overvoltages are either of atmospheric origin or industrial origin. The most harmful are however atmospheric overvoltages which are the result of three main effects:

Conduction

Conduction: An overvoltage that propagates along a conductor which has been in direct contact with the lightning strike. This effect is all the more destructive as the majority of the lightning energy is propagated through the entire network. This problem is resolved by fitting the installation with suitable device able to support high currents.

Induction

Induction: caused by the electromagnetic field radiated by the lightning strike. It generates an overvoltage on conductors within a range that is proportional to the power and the rate of speed variation of the lightning strike. Consequently, under the influence of abrupt variations in current, the cables, and even the ducts which act as aerials, may be subjected to destructive overvoltages. This is the reason that placing the network underground does not guarantee lightning protection.

Rising up from the ground

Rising up from the ground: When a lightning strike hits, an overvoltage can rise up from the ground attempting to find a more favourable path to ground. This can, in part, be dealt with through a) equipotential bonding between the metal structures and ground of the entire installation of a structure. b) overvoltage protection installed on services.