Protection against lightning
Lightning consists of an electrical discharge involving an electrostatic phenomenon of breakdown of a dielectric. In this case, the dielectric being humid air, we understand that an enormous voltage is involved, which means a colossal amount of energy. This phenomenon can occur between clouds or between clouds and the earth. Lightning prioritizes low impedance paths; especially good conductors of electricity, such as metal. Usually, the journey is not unique. It occurs regularly in the form of multiple ramifications, of which a main branch can be distinguished. It is at this level that passes most of the energy that can cause damage.
Effects of lightning on buildings
Although the damage is unpredictable, as is the point of impact, an unprotected building poses a fire hazard and can suffer other significant harms, such as:
failure of electrical and electronic equipment
failure of the fire alarm system
irreplaceable loss of cultural heritage
the high cost for the replacement of the damaged structure.
On the other hand, a building with a poorly maintained lightning protection system can also present the same risks, and even worse, since the lightning is attracted, but the installation is inadequate to drain all the energy. contained, causing serious harm.
Principles of lightning protection
The basic principle of a lightning protection system is to provide a low impedance path allowing the electrostatic discharge to penetrate the ground without major damage. The metal circuit must ensure electrical continuity from the sensing device to the ground. The most vulnerable parts of structures are in particular those that are protruding (chimneys, towers, spires, bell towers, etc.). The lightning protection system consists of three basic elements providing a low impedance path:
aerial bollards and collection conductors on the roof and in other high places.
the earth connection(s) (generally more than one is required).
the down conductors which connect these two elements.
The cables will run away from the other conductors and the earthing will also be made as short as possible on the equipotential bonding bar or the ground of the cabinet.
We will thus seek to optimize the route of the conductors by ensuring that the incoming wires on the surge arrester are clearly distinct from the outgoing ones.
Outgoing feeders protected by a surge arrester will be taken from the very terminals of the surge arrester and cut-off device dedicated to end-of-life protection.
The total length of the connections, protection device and protection included, must not exceed 50cm.
No separate earth connection should exist.
If in a panel or cabinet, the general earth connection is too far, an intermediate earth terminal block will be used.
One earth connection per building or per protected installation is required.
To optimize the installation, the resistance of this earth connection must have the lowest possible HF impedance.
It should be checked that there are not coexisting within the same building or electrical cabinet connections on separate earthing distributions, where the equipotentiality is distant.
All these installation rules are valid for all protections.
The increasingly frequent presence of sensitive electronics makes electrical equipment more vulnerable to transient overvoltages. Of various other possible origins (industrial overvoltages, switching overvoltages on the networks, electrostatic discharges, etc.), transient overvoltages of atmospheric origin are the most harmful for electrical or electronic equipment and installations due to the enormous energies generated in very short times. They can be caused by a direct lightning strike on the electrical network (telephone line, etc.), induced by induction or by rising earth.
The insulation measurements made compulsory by the equipment manufacturing standards do not make it possible to cope with these effects (IEC 61000-4-5: equipment immunity). The equipotentiality of masses and earths must be achieved and the installation of lightning arresters (surge arresters or protection devices against overvoltages) is necessary to absorb the energies involved.
During a direct lightning strike on a power line or a pylon, the current can propagate and reach all the installations distributed by the line even if they are located several kilometers from the point of impact. These currents are all the more destructive as the majority of the energy of the lightning strike is “conducted” by the network.