When somebody asks us ¿how does a lightning rod work? We explain that the lightning rod is the external terminal protection of a building or other structures against direct lightning strikes.
Before explaining how the lightning rod works, we would like to context it through the history and introduce you the possible effects of lightning strikes.
On June the 15th , 1752, during a stormy day in Philadelphia, an inventor scientist named Mr. Benjamin Franklin blew up a metal frame kite tied with a silk cord to which he had previously inserted a metal key, placing it near his hand. Thanks to that experiment he was able to observe that through the silk thread the electricity reached the key and electric sparks flew.
He could confirm that the metal key was electrostatic charged, and he demostred that the clouds were electrically charged and that the lightning strikes were large electrostatic discharges.
Franklin discovered that if the lightning strike or the electric fire as he called it, when it came out of the clouds and it found a metal conduit on his way to earth to get into, it would stay there and dissipate. As a result of that crazy experiment, a year later in 1753, he discovered the lightning rod called Franklin type and that kite became the most famous in history.
Lightning strikes effects
Among some different effects that can produce lightning strikes, we can mention some as the thermal, physiological, electrodynamic, electrochemical effects, etc. Due to its importance we will emphasize the thermal and physiological ones.
Thermal effects occur due to the high temperature reached by the channel through which the lightning current flows, it may reach up to 20,000 ° C, which causes great damage when the electric shock reaches, for example, a tree or impacts on a structure.
On the other hand, the physiological effects, they mainly affect living beings and occur due to the step and contact voltages that appear when the lightning is discharged to earth. To combat and mitigate those effects, the regulations for protection against lightning establish safety measures for people and animals, such as those drafted in Annex D of the UNE 21186: 2011 standard.
There are also international regulations that deal with the effects of lightning current on the human body and livestock (IEC TR 60479-4: 2011). And other regulations, which establish safety procedures for risk reduction when we are outside a structure or building (IEC / TR 62713).
The thunderbolt also has two very characteristic associated effects, the lightning that is its luminous effect due to the high current circulation (up to 200 kA) and the thunder that is the sound effect due to the expansive wave of the air that it is heated in few micro seconds to a very high temperatures.
When people ask us How does a lightning rod work? We indicate that it’s an air terminal that gives external protection to a building or structure from direct lightning strike impacts. On that way, the lightning rod needs to be installed always over the highest point of the building or structure we need to protect, it will be in charge of capturing and conduct safely the lightning discharge to ground.
In order to capture this discharge, the lightning rod has a tip and a metallic body, which are connected by a conducting network to a low impedance earthing system (less than 10 Ω) where the lightning discharge dissipates.
In storm conditions, high voltage occurs between the cloud - earth system due to the large number of electrical charges that are present both at the base of the cloud and on the ground. This high voltage is the trigger to start the leader descended from the beam, which will drill the dielectric air between the cloud and the ground. The very high electric field E (kV / m), that it appears in that zone, produces a circulation of ascending electric charges through the body of the lightning conductor of opposite sign initiating an ascending tracer, which will meet and recombine with the descendant leader, capturing it and unloading it to the ground.
External lightning protection systems
At present, there are 4 external protection systems that are endorsed by regulations:
Due to its advantages with respect to other external protection systems, the ESE lightning rod (Early Streamer Emission) is currently the most used, it provides a greater protection radius than other systems (up to 80m radius in level I protection). ) and its installation is very simple, because in some cases, it only requires a down conductor to drive the lightning current and a ground connection to dissipate all its energy. As a consequence of all these factors, the installation of an ESE lightning rod system is simple, easy, fast and has a very low cost when compared to other systems.
Design and installation
For a correct design of a lightning protection system in a structure, we must first carry out an analysis of the risk of the same, to determine if its protection is necessary. In the affirmative case that lightning protection is necessary, we must calculate what is the level of protection or security factor to apply at this structure (I, II, III or IV). On the website of INGESCO there is a free online software to get this risk calculation and evaluation.
Once the protection level of the structure is calculated, we will choose the external lightning protection system that best fit your needs at each project between those 4 protection systems.
If the selected external lightning protection system is the ESE lightning rod we will follow up all the guidelines set by the international standards (UNE 21186:2011, NFC 17.102:2011, NP 4426:2013)
At the article installation of lightning rods you will find more information about how to install an ESE lightning rod in according to the mentioned regulations.
How to know if you need to install an external lightning protection system
You can consult directly with the INGESCO engineering department and that they will carry out a free study, in accordance to the international regulations and detail it in a technical report if it is necessary to install an external protection system against lightning, or if you prefer, you can carry out the study with the free ingesco software.