The protection of the networks
By their nature and their function, the electrical grids are often the object of fortuitous defects. It is thus essential to install protection systems. This protection depends on the treatment of the neutral which can be done in three different ways:
Insulated neutral (fig.1) - This technique which induces important risks in the event of earthing of a phase is used only on networks of small size and weak tension . 
Earthing direct of the neutral (fig.2) - The neutral of the system is directly connected to the ground. In such a case, the current of short-circuit can reach 20 to 30 times the value of the rated current.
Earthing of the neutral with limitation of the fault current (fig.3) - This technique consists in inserting an impedance between the neutral and the ground. There are 3 types of impedances :- Résistance
- Réactance
- Wind of extinction of arc (winds of Petersen)
Of these three methods, only that using a resistance allows a detection simple and fast defect, while offering a potential of reference of which stability is suitable, the control of the current of short-circuit artificially created and a maximum safety for the environnement.
CALCULATION AND DIMENSIONING
The calculation of a resistance of earthing is done starting from the three criteria according to :
- t (en s.) duration of the defect
- U (en V) tension made up of the network
- I (en A) intensity of the current crossing resistance during the defect
One deduces the value from it from resistance by the formula:
The mass of resistance is then obtained by calculating its adiabatic heating lasting the defect thanks to the simplified equation :
All our resistances of earthing are conceived in respect of the standards lEEE32-1972 and lEC289 which defines an acceptable maximum temperature of resistance according to the duration of the defect. This temperature for resistant stainless elements is of 760°C for a defect of duration lower or equal to 600 seconds and of 385°C for a defect of higher duration.