Conclusion
Method :
In earthing system TN or IT, the fault current is a short circuit (Phase-Neutral in TN, Phase-Phase in IT)
In both cases, the value of the short circuit current must be enough to act the over-current protective devices (Fuses, Magnetic relay)
For every circuit, the calculation of the short circuit current must be done and the protective equipment selected according to this calculation.
Using only one earthing system is not always the best choice.
In many cases it is thus advisable to install several earthing systems in the same installation. As a rule, a "rake shaped" installation, with clear identification of priorities, using emergency sources or UPS systems, is preferable to a monolithic tree-shaped installation.
For more details, extracts from the "Cahiers Techniques" (collection of Technical Books) published by Schneider Electric
CT no. 172EN - earthing systems in LV.
CT no. 173EN - Earthing systems worldwide and evolutions.
CT no. 177EN - Disturbances in electronic systems and earthing systems.
