Electrical Installation Guide 2009 - the global specialist in energy ...

B - Connection to the MV publicdistribution network1 Supply of power at mediumvoltageBDiagramRs valueA - TN-aMVLV12B - IT-aMVLV12Cases A and BNo particular resistance value for Rs is imposedin these cases33NNR SR SC - TT-aMVLV1D - IT-bMVLV1Cases C and DUw - UoRs yIm23N23NWhereUw = the rated normal-frequency withstandvoltage for low-voltage equipment atconsumer installationsUo = phase to neutral voltage at consumer'sinstallationsIm = maximum value of MV earth-fault currentR SR SE - TT-bMVR SLVR N123NF - IT-cIn cases E and F the LV protective conductors (bonding exposed conductive parts) in the substationare earthed via the substation earth electrode, and it is therefore the substation LV equipment (only)that could be subjected to overvoltage.MVR SLVR N123NCases E and FRs y Uws - UImWhereUws = the normal-frequency withstand voltagefor low-voltage equipments in thesubstation (since the exposed conductiveparts of these equipments are earthedvia Rs)U = phase to neutral voltage at the substationfor the TT(s) system, but the phase-tophasevoltage for the IT(s) systemIm = maximum value of MV earth-fault currentNotes:b For TN-a and IT-a, the MV and LV exposed conductive parts at the substation and those at the consumer’s installations, together with theLV neutral point of the transformer, are all earthed via the substation electrode system.b For TT-a and IT-b, the MV and LV exposed conductive parts at the substation, together with the LV neutral point of the transformer are earthed viathe substation electrode system.b For TT-b and IT-c, the LV neutral point of the transformer is separately earthed outside of the area of influence of the substation earth electrode.Uw and Uws are commonly given the (IEC 60364-4-44) value Uo + 1200 V, where Uo is the nominal phase-to-neutral voltage of the LV systemconcerned.Fig. B10 : Maximum earthing resistance Rs at a MV/LV substation to ensure safety during a short-circuit to earth fault on the medium-voltage equipment for differentearthing systemsThe combination of restricted earth-fault currents, equipotential installations andlow resistance substation earthing, results in greatly reduced levels of overvoltageand limited stressing of phase-to-earth insulation during the type of MV earth-faultsituation described above.Limitation of the MV earth-fault current and earth resistance of the substationAnother widely-used earthing system is shown in diagram C of Figure B10. It will beseen that in the TT system, the consumer’s earthing installation (being isolated fromthat of the substation) constitutes a remote earth.This means that, although the transferred potential will not stress the phase-to-phaseinsulation of the consumer’s equipment, the phase-to-earth insulation of all threephases will be subjected to overvoltage.© Schneider Electric - all rights reservedSchneider Electric - Electrical installation guide 2009