Planning with SIVACON 8 PS - Siemens

Further information about planning7.1 Dimensioning and selectionTT systemIn TT systems, one point is directly earthed; the exposed parts of the electrical installationare connected to earth electrodes which are isolated from the system earth electrode.Today's TT systems feature protective measures in the form of protective earthing, RCCBsand voltage-operated e.l.c.b. systems.Figure 7-4TT systemIT systemThe IT system has no direct link between active conductors and earthed parts; the exposedparts of the electrical installation are earthed.Today's IT systems feature protective measures in the form of a protective-conductorsystem.Figure 7-5IT system1 Exposed part 2 ImpedanceFirst letter: Earthing condition of the incoming current sourceT =I =Direct earthing of a pointEither insulation of all active parts from earth or connection between a point and earth via animpedanceSecond letter: Earthing condition of the exposed parts of the electrical installationT =N =Exposed part directly earthed regardless of whether or not a point in the power supply isearthedExposed part linked directly to system earth, in alternating voltage systems, the earthed pointis usually the star point.Additional letters = Arrangement of the neutral conductor and the protective conductorS =C =Neutral conductor and protective conductor functions via separate conductorsNeutral conductor and protective conductor functions combined in a single conductor (PEN)Planning with SIVACON 8 PS242 Planning Manual, 10/2007, A5E01541101-01