It is important to understand a little the lightning phenomenon with regards to the different types of protection against lightning strikes: The main lightning discharge is preceded by several "stepped" leaders building up ionized channels and proceeding in a number of quick steps downwards. When a stepped leader approaches the ground, conductive discharges (positive streamers) can develop from pointed and tall objects (trees, masts, towers...). Once a streamer hits a leader, a conductive channel is established between the cloud and the ground, allowing the main lightning discharge.

A lightning protection system comprises an external and an internal lightining protection:

Direct lightning strike (External protection)

At a direct impact, the lightning current can penetrate a roof or a structure and cause a fire due to intense heating. A system with one or more metallic spikes (lightning rods), spanned wires and intermeshed conductors -the air-termination system- can "attract" the lightning and conduct the discharge through cables with adequate cross-section to the ground where it is distributed in the earth. The external protection thus consists of 3 parts: Air-termination system, down conductor and earth termination system.

A side effect of the lightning occurs at the point of contact with the earth through galvanic coupling: There comes up a short, very high electric field which can amount several hundred kV. Because most electrical equipment is grounded (equipotential bonding), another protection needs to be installed to prevent the high potential difference to reach the equipment: The surge or overvoltage arresters.

Indirect lightning strike (internal protection)

Indirect effects are caused when lightning strikes close to an object. Short-time voltage impulses, so-called transients, spread along supply lines and can access various electrical lines in the object through electromagnetic induction. These additional voltage peaks of several thounds volt can damage connected devices (televisors, satellite receivers, PC's, house appliances, alarm and monitoring systems, photovoltaic systems,...). Even phone lines and computer networks are affected. Same consequences can also occur without thunderstorms during switching operations in electrical power systems!

A protection of electrical equipment against this kind of electromagnetic coupling is done with surge arresters; protection devices having a very high resistance under a certain threshold voltage. Once a surge voltage appears, their resistance becomes very small, thus reducing abruptly the surge voltage. Surge arrestors have to show a very small time delay; transients are very short-term (10 to 500 μs during lightning stroke).

Source: Pröpster

A lightning protection is recommended especially when the object to be protected is situated in an isolated location, on a hill or near a tall structure. Furthermore, resulting damages may be very expensive and cause time consuming repairs to the internal installations.

Source: Dehn

Further information can be found on the websites of our partners:

Dehn

Proepster

and an interesting introduction to lightning protection under:

Dehn-online training

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