Page 11 - Electrical Safety of Low Voltage Systems

viii
Dr. Massimo A. G. Mitolo

7 TN Grounding System 117
7.1 Introduction 117

7.1.1 Why Earthing the Transformer? 118
7.2 Voltage Exposure in TN-S Systems 120

7.2.1 Ground Fault in the Vicinity of the User’s Transformer 121
7.3 Potential Differences Between ECPs, and Between ECPs and EXCPs in TN 122
Systems
7.4 Protection Against Indirect Contact in TN-S Systems by Using Overcurrent 123
Devices

7.4.1 Calculation of the Approximate Minimum Value of the Phase-to-Protective 126
Conductor Fault Current

7.5 Protection Against Indirect Contact in TN-S System by Using RCDs 127
7.6 Transferred Potentials Between Distribution and Final Circuits in TN Systems 128

7.6.1 Supplementary Equipotential Bonding 129
7.7 Local Earth Connection of ECPs in TN Systems 130

7.8 TN-C Systems and the PEN Conductor 132
7.9 The Neutral Conductor in TN Systems 134

7.10 The Touch Voltage in TN Systems 135
7.11 Step Voltage 139

7.11.1 A Comparison Between the Dangerousness of Touch and Step Voltages 140
FAQs 141

Endnotes 141
8 Protective Multiple Earthing (TN-C-S Grounding System) 143

8.1 Introduction 143
8.1.1 Fault-Loop Impedance in PME Systems 146

8.2 Energization of the PEN Conductor in PME Systems 146
8.2.1 Ground Fault on the Low-Voltage Utility Distribution System 146

8.2.2 Ground Fault on the Medium-Voltage Utility Distribution System 148
8.2.3 Faults Phase-to-PEN in Low-Voltage PME Networks 149

8.3 Interruption of the PEN Conductor in PME 150
8.4 Stray Currents 151

8.5 Stray Voltages 152
Endnotes 153