Following data provided by Sirf (the Italian system for the detection of lightening), about 750.000 lightning strikes hit our country every year.Everyone knows how dangerous they are, as they may cause major accidents both when they hit people directly and when they hit buildings. Among the indirect effects of lightning strikes are fires and the trees falling next to buildings. This is why it is paramount putting in place specific types of intervention that can be effective also for prevention. In the case where, inside a building there are business activities, a further risk may be the damage to the machinery that may cause a financially detrimental machine downtime. Also a partial or total damage to the electrical system could happen. In particular, with regards to the protection of machinery, the technologies used are usually able to guarantee a certain level of immunity to overvoltage, although within a well-defined limit. If this is passed, then malfunctions will set in; if instead the values are especially high and the lightning event happens repeatedly the risk is incurring in permanent faults. Also, sometimes a damaged machine can cause serious events such as fires. In order to offer a higher level of reliability of the home electric systems and their more effective protection from lightning strikes, the CEI 64/8 regulation with the Variation 3, in effect since 1st September 2011, has provided for the installation of specific dischargers of overvoltage, also within the main grid of residential units.
The SPD
SPDs have the undeniable advantage of being easy to install and if compared to other systems, they are cheaper. They can be added to a pre-existing system guaranteeing a perfect performance if installed correctly. On a practical level, SPDs appear like switches, positioned in parallel to the electrical line they are to protect. In case of overvoltage their elevated impedance is able to decrease to very low values, draining the overvoltage itself to the ground, thus protecting the line. Once the overvoltage will end, they will revolve to be an open switch. In any case, those who choose to rely on SPDs (with or without fuse) have to use models that can provide alerts on their status. Should an SPD have a fault that is not shown, the system would continue to work without a problem but it will no longer be protected. In the event of the first serious discharge the system would therefore be exposed to risks.
For buildings equipped with photovoltaic systems
With regards to buildings equipped with photovoltaic systems, these also provide for maximum tension; up until this limit is reached the system works without irregularities but, once the limit is passed, the risk of damage is high. In the case of photovoltaic systems, many believe that the risk of those being hit by lightning is even higher; this is because these systems are usually located at the top of buildings. It has to be taken into account that, when these systems adhere optimally to the surface of the building, not altering significantly its shape, they do not present a higher risk if compared with other buildings.
The Lightning rod
On the other hand, even when a lightning strike hits the neighbourhood of the photovoltaic system it can cause damages. Examining the direct fulmination of a photovoltaic system, in the case where the latter alters the original shape of a building the adoption of a prevention system such as the LPS (that is the lightning rod) is fundamental. This can be used as long as the photovoltaic system is placed within an area protected from the lightning rod. Also, it is necessary that the system and the LPS are not placed in a small space in order to avoid dangerous discharges from one to the other. If the space available does not allow complying with the minimum distance it is necessary to resort to a Surge Protective Device. With regards to the indirect fulmination, this can generate currents that can cause a magnetic field able to induce tension within the spires it passes through. In order to prevent this risk the advice is to make the links between the modules of the voltaic systems in such a way that the spires are reduced to the minimum and, at the same time, make it so that the spires split; this will reduce the effect produced from a spire on the others. Although this is a good solution often it is ignored making the systems far too sensitive to lightning. In the case where, in spite of the protection methods adopted, a building (of any type) is damaged by lightning it will help to use the experience to adopt the appropriate counter-measures.