The IT earthing system (unearthed neutral) in LV

a) b)I dI dNNPENSCPDPESCPDR BU dR dR BU dR dc)I d12N3NPENPEPESCPDR BU dR dFig. 1 : insulation fault on a network operated in TN-C [a], TN-S [b] and TN-C-S [c].NRCDI dProtection is provided by the Residual CurrentDevices (RCD): the faulty part is disconnectedas soon as the threshold I∆n, of the RCD placedupstream, is overshot by the fault current, so thatI∆n R B iU L .R B R AU dR dFig. 2 : insulation fault on a network operated in TT.The IT systemc Its principle:v the transformer neutral is not earthed, but istheoretically unearthed. In actual fact, it isnaturally earthed by the stray capacities of thenetwork cables and/or voluntarily by a highimpedance of around 1,500 Ω (impedanceearthedneutral);v the electrical load frames are earthed.Cahier Technique Schneider Electric no. 178 / p.5