Methodology for choosing an earthing system
First do not forget that all three earthing systems can exist side by side in the same electrical installation: this guarantees the best possible response to safety and availability needs.
Second, ensure that choice of earthing system is not recommended or stipulated by standards or legislation (decrees, ministerial orders).
Then dialogue with the user to identify his requirements and resources:
need for continuity of supply,
whether or not he has a maintenance service,
fire hazard.
Method : As a rule:
• Continuity of supply and maintenance service: the solution is IT,
• Continuity of supply but no maintenance service: there is no fully satisfactory solution; prefer TT for which discrimination on tripping is easier to implement and that minimises damage with respect to TN.
Advice : Extensions are easy to make (no calculation).
Continuity of supply not essential and competent maintenance service: prefer the TN-S (rapid repairs and extensions according to proper practices),
Continuity of supply not essential and no maintenance service: prefer the TT,
Fire hazard: IT if maintenance service and use of 0.5 A RCD or TT.
Advice : Allow for the specificity of the network and loads:
Very extensive network or network with high leakage current: prefer the TN-S,
Use of replacement or emergency power supplies: prefer the TT.
Loads sensitive to strong fault currents (motors): prefer the TT or IT,
Loads with low natural insulation (furnaces) or with large HF filter (large computers): prefer the TN-S,
Supply of control and monitoring systems: prefer IT (continuity of supply) or TT (better equipotentiality of communicating devices).
