Lightning

Since 2002, the French wiring regulations, NF C15-100, has defined the situations in which it is obligatory or strongly recommended to have a parafoudre in the installation. The requirement for a parafoudre is determined by the niveau kéraunique, or Nk level, for a particular department. This is the thunderstorm severity and it is calculated by the number of days that lightning is observed in a given department per year. If the resultant Nk figure is greater than 25 a parafoudre is obligatory on all domestic electrical installations unless the supply cabling is entirely underground, i.e. right back to the sub-station. In our region, the Dordogne (24) is the only department where the Nk figure is greater 25, whereas the Creuse (23) and the Haute Vienne (87) have a value of 23 and the Charente (16) has 21. This is not to say that a parafoudre is unnecessary in those areas, but it is not obligatory.

A parafoudre may be installed into the household wiring system either in series or parallel. However, the crucial factor is that the connection to the earth is as a short as possible and never in excess of 50 cm and that this length is much shorter than the earth wires of any of the other circuits because the purpose of the device is to direct any excessive tension to earth before the sensitive appliances receive the surcharge. However, it is also important that the distance between the parafoudre and the equipment to be protected is not greater than 30 metres. If this is the case, then an additional parafoudre should be placed in the proximity of the equipment.

This is where the multiblock extension leads with incorporated surge protectors have a role to play. As a stand alone device, they cannot be relied upon to give a satisfactory level of protection, but their function down the line in front of the equipment will ameliorate the risk created by the voltage drop and residual tension from the parafoudre at the head of the installation. A parafoudre for use in the consumer unit should cost in the order of 140 € plus the cost of fitting.

Lightning Protection

After the long cold winter and all of the difficulties that caused, i.e. burst pipes, split radiators and frozen boilers, we are moving into a different period of climatic problem. That is the time of year when the thunderstorms come in from the Bay of Biscay and move across our part of France in a north-eastward direction towards the mountains. Each year, the territory of France is struck by lightning around 2 million times. The majority of these strikes are harmless and cause no damage to either people or property, but it is a different story for those that find their way into our electrical and telecommunications equipment. The lightning starts between two highly charged storm clouds, or between a cloud and the ground.

The clap of thunder is the sound of the lightning striking and discharging its energy. At this point, a current of several million amps flows between the cloud and the earth via the ionised channel that has been created. The effects of this are well known and for electrical, electronic and telecommunications equipment the outcome is usually disastrous. There are two forms of protection against lightning damage. The first is by using a lightning conductor, or paratonnerre, to prevent the direct effects and these are used on exposed sites and high buildings. The other method is with a surge protection device and these are known as parafoudres. The indirect effects of the lightning are the consequences of a direct strike on or close to the electrical or telecommunication distribution system and then carried by the network of lines to our apparatus. The effects of the voltage surge can also be created by lightning strikes in the vicinity of buildings, or by the discharge of energy through a lightning conductor. The strength of the surge will depend on the intensity of the strike and the distance from the point where the electrical equipment is installed.

The parafoudres are categorised in three groups dependent on the level of risk, the expected strength of the surge and method of installation. A type 1 parafoudre is used where there is a high risk, such as where the building has a lightning conductor and where the strength of the strike would be consistent with a direct impact. They are fitted to buildings on exposed sites, or high buildings such as apartment blocks and churches. Type 2 parafoudres are installed at the head of the electrical panels where the risk is only likely to be through an indirect effect. Type 3 parafoudres are used as a supplementary protection adjacent to a specific piece of equipment.