The IT earthing system (unearthed neutral) in LV

1 Introduction1.1 Protection of persons against electric shocksUse of Safety by Extra Low Voltage (< 25 V)-SELV- is the most drastic solution since iteliminates the electrical hazard. However it isapplicable only in low power distribution.Regarding everyday use of electricity, a number ofstudies have identified the causes of electric shocksand provided specific solutions for each one.Electric shocks have two causes, namely:c direct contact, i.e. a person or an animaltouching an exposed live conductor;c indirect contact, i.e. a person touching themetal frame of an electrical load on which aninsulation fault has occurred.Protection against direct contactTo provide protection against direct contact,insulation and/or distancing measures are taken.These measures can be reinforced in finaldistribution by additional protection in the form ofa high sensitivity Residual Current Device(RCD).Protection against indirect contactWith respect to protection against indirectcontact, between an accidentally energisedframe and the earth, the basic solution is to earthall the load frames via the protective conductors.However, this measure does not rule out theexistence of a contact voltage hazardous forpersons if it exceeds the conventional limit safetyvoltage U L defined by standard IEC 60479.This contact voltage depends on the earthingsystems defined in the international standardIEC 60364.1.2 The various standardised earthing systemsThe three earthing systems given official statusby international standards (IEC 60364) are alsostipulated by a large number of nationalstandards: in France by the LV installationstandard: NF C 15-100.A brief reminder of the protection principle ofthese systems will now be given beforedescribing the IT system in greater detail.The TN systemc Its principle:v the transformer neutral is earthed;v the electrical load frames are connected toneutral.This type of system has three possibilities:v the same conductor acts as a neutral and aprotective conductor: this is the TN-C system;v the neutral and the protective conductor areseparate: this is the TN-S system;v and the combination of these two systems,known as TN-C-S when the neutral and theprotective conductor are separated downstreamof part of the installation in the TN-C system.Note that the TN-S cannot be placed upstreamof the TN-C.c Its operation (see fig. 1 ):An insulation fault on a phase becomes a shortcircuitand the faulty part is disconnected by aShort-Circuit Protection Device (SCPD).The TT systemc Its principle:v the transformer neutral is earthed;v the electrical load frames are also earthed.c Its operation (see fig. 2 ):The current of an insulation fault is limited byearth connection impedance.Cahier Technique Schneider Electric no. 178 / p.4