Overvoltages are short-term voltage surges with values above the maximums established between two points of an electrical installation or circuit.
Currently the need for protection is much greater, as technology has evolved making electronic components smaller and more sensitive to electromagnetic disturbances.
Lightning surges can have two possible causes: direct / near discharge or distant discharge.
Direct or close lightning strikes are those that hit in a building's external protection system, in its immediate vicinity or in any of the conducting electrical systems that penetrate it (for example, low-voltage power supplies, control cables ...)
The shock currents and voltages that are produced by direct lightning strike represent a very serious threat to the system to be protected, in terms of its amplitude and energy content.
In the event of a direct or close lightning strike, overvoltages (case 1a) originate due to the voltage drop in the resistance of the shock earth connection, and the resulting increase in potential in the building compared to the distant environment. This constitutes the maximum effort that electrical installations in buildings are subjected to.
They are those that take place at a great distance from the installation under protection: lightning strikes in the medium voltage air network or in its proximity or also lightning strikes from cloud to cloud (cases 5, 6 and 7)
Analogously to induced surges, the repercussions of distant lightning strikes on the electrical installation of a building are neutralized by components dimensioned according to the 8/20 µs shock current wave.
The most common transient overvoltages are due to machinery commutations. However, the most destructive are those due to atmospheric discharges. The effects of these surges range from the simple momentary interruption of work to the total destruction of a piece of equipment or installation:
DISRUPTION: Interruption of system operations, data loss and corruption, unexplained computer failures ...
DEGRADATION: Exposure to transient surges will degrade, without the user being aware of it, the electronic components and circuits, reducing the effective life of the equipment and increasing the chances of failure.
DAMAGE: Large transient overvoltages can damage components, circuit boards, etc. Even burning them, possibly causing the destruction of the equipment and the electrical installation, as well as most likely the focus of a fire. They affect to a greater extent electronic, computer and telecommunications equipment. All these effects entail economic losses due to the replacement of the damaged elements, as well as the indirect cost of the breakdown of the production processes. In addition, these effects may entail a risk to people that must be avoided according to the Occupational Risk Prevention Law: Minimum health and safety provisions for the use of work facilities and equipment by workers. R.D. 1215/97. Annex II, point 12: "Any installation or machinery used for work, and that can be struck by lightning, must be protected against its effects by appropriate devices or measures."