TN-S Type of System Earthing :
If a type of system earthing TN is used, it shall be type TN-S (with separate neutral N and protective conductor PE) in the hazardous area, i.e. the neutral and the protective conductor shall not be connected together, or combined in a single conductor, in the hazardous area. At any point of transition from TN-C to TN-S, the protective conductor shall be connected to the equipotential bonding system in the non-hazardous area.
TT Type of System Earthing :
If a type of system earthing TT (separate earths for power system and exposed conductive parts) is used, then it shall be protected by a residual current device. NOTE : Where the earth resistivity is high, such a system may not be acceptable.
IT Type of System Earthing :
If a type of system earthing IT (neutral isolated from earth or earthed through an sufficiently high impedance) is used, an insulation monitoring device shall be provided to indicate the first earth fault.
NOTE 1 If the first fault is not removed, a subsequent fault on the same phase will not be detected, possibly leading to a dangerous situation.
NOTE 2 Local bonding, known as supplementary equipotential bonding, can be necessary (see IEC 60364-4-41).