Electrical Safety of Low-Voltage Systems Book

Preface xiii

1 Basic Definitions and Nomenclature 1

1.1 Introduction 1

1.2 Basic Definitions and Nomenclature 1

1.2.1 Basic Insulation 1

1.2.2 Class 0 Equipment 1

1.2.3 Class I Equipment 2

1.2.4 Class II Equipment 2

1.2.5 Class III Equipment 2

1.2.6 Direct Contact 2

1.2.7 Indirect Contact 2

1.2.8 Disconnection of Supply 2

1.2.9 Exposed-Conductive-Part (ECP) 2

1.2.10 Extra-Low Voltage 2

1.2.11 Extraneous-Conductive-Part (EXCP) 3

1.2.12 Functional Insulation 3

1.2.13 Ground 3

1.2.14 IT Grounding System 3

1.2.15 Neutral-Protective Conductor (PEN) 4

1.2.16 Protective Bonding Conductor 4

1.2.17 Protective Conductor (PE) 4

1.2.18 Remote or Zero Potential 4

1.2.19 TN Grounding System 5

1.2.20 TN-C Grounding System 5

1.2.21 TN-C-S Grounding System 5

1.2.22 TN-S Grounding System 5

1.2.23 TT Grounding System 5

1.2.24 Prospective Touch Voltage 6

1.2.25 Touch Voltage 7

1.2.26 Prospective Step Voltage 7

1.2.27 Step Voltage 7

2 Fundamentals of Electrical Safety 9

2.1 Introduction 9

2.2 Protection Against Direct Contact 10

2.2.1 Insulation of Live Parts 10

2.2.2 Enclosures and Barriers 11

2.2.3 Protection by Obstacles 14

2.2.4 Additional Protection by Residual Current Devices 15

2.3 Protection Against Indirect Contact 18

2.3.1 Protection by Automatic Disconnection of Supply 19

2.3.2 Protection Without Automatic Disconnection of Supply 20

FAQs 26

Endnotes 26

3 Mathematical Principles of Electrical Safety 29

3.1 Introduction 29

3.2 Mathematical Definition of Safety 29

3.3 Risk of Indirect and Direct Contact 32

3.4 The Acceptable Residual Risk 33

3.5 Safety and Risk of Basic Insulation 36

3.6 Safety and Riskof Class 0 Equipment 37

3.7 Safety and Risk of Class I Equipment 38

3.8 Safety and Risk of Class II Equipment 39

3.9 Safety and Risk of Electrical Separation 41

3.10 A Qualitative Comparison Between Safety and Risk of Protective Measures 42

FAQs 43

Endnotes 44

4 The Earth 45

4.1 Introduction 45

4.2 The Earth Resistance 45

4.3 The Earth Potential 49

4.4 Independent and Interacting Earth Electrodes 51

4.5 Spherical Electrodes 55

4.6 Voltage Exposure Upon Ground Faults 59

4.6.1 Touch Voltage 59

4.6.2 Extraneous-Conductive-Part (EXCP) 63

4.7 Voltage or Current? 65

FAQs 67

Endnotes 68

5 Effects of Electric Currents Passing Through the Human Body, and Safety Requirements 71

5.1 Introduction 71

5.2 The Human Body as an Electrical System 72

5.2.1 On the Electrical Nature of the Cells 72

5.2.2 Action Potential 73

5.3 Influence of Frequency on the Effects of Current 76

5.4 Physiological Response to Electrical Currents 77

5.4.1 Tetanization 77

5.4.2 Ventricular Fibrillation 78

5.4.3 Thermal Shock 79

5.5 Permissible Body Current and Person's Body Mass 81

5.6 Permissible Body Current Independent of Human Size 82

5.6.1 Heart Current Factor 83

5.7 Human Body Impedance 84

5.8 Current Paths 86

5.9 Permissible Prospective Touch Voltage VSTp 87

5.10 Effects of Direct Currents 90

FAQs 93

Endnotes 93

6 TT Grounding System 95

6.1 Introduction 95

6.2 Voltage Exposure in TT Systems 96

6.3 Protection Against Indirect Contact in TT Systems by Using Overcurrent Devices 99

6.4 Protection Against Indirect Contact by Using Residual Current Devices 102

6.5 Neutral-to-Ground Fault in TT Systems 103

6.6 Independently Grounded ECPs in TT Systems 104

6.7 Leaking-to-Ground ECPs in Three-Phase TT Systems 106

6.8 Electrical Interferences in TT Systems 106

6.9 The Neutral Conductor in TT Systems 108

6.9.1 Resistance of the Utility Neutral in TT Systems 109

6.10 Main Equipotential Bonding 110

6.10.1 Should We Bond Incoming Pipes Made of Plastic? 111

6.10.2 Should We Bond Incoming Pipes Electrically Separated by a Dielectric Joint? 112

6.11 Supplementary Equipotential Bonding 113

6.12 Potential Differences Among Metal Parts in Fault Conditions in TT Systems 113

FAQs 115

Endnotes 115

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 Systems 122

7.4 Protection Against Indirect Contact in TN-S Systems by Using Overcurrent Devices 123

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

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

9 IT Grounding System 155

9.1 Introduction 155

9.1.1 Insulation Monitoring Device 158

9.1.2 Equipotential Bonding 159

9.2 Overvoltages Due to Faults in IT Systems 159

9.3 Resonant Faults in IT Systems 161

9.4 Protection Against Direct and Indirect Contact by Using RCDs in IT Systems 163

9.5 Protection Against Indirect Contact in the Event of a Second Fault to Ground 164

9.5.1 ECPs Earthed Individually or in Groups 164

9.5.2 ECPs Earthed Collectively to a Single Grounding System 165

9.6 Role of the Fault Resistance in TT and IT Systems 167

FAQ 170

Endnote 170

10 Extra-Low-Voltage Systems 171

10.1 Introduction 171

10.2 Separated Extra-Low-Voltage (SELV) Systems 172

10.2.1 Protection Against Indirect Contact 172

10.2.2 Protection Against Direct Contact 174

10.3 Protective Extra-Low-Voltage (PELV) Systems 175

10.3.1 Application of PELV Systems to Control Circuits 176

10.4 Functional Extra-Low-Voltage (FELV) Systems 178

Endnotes 180

11 Earth Electrodes, Protective Conductors, and Equipotential Bonding Conductors 181

11.1 Introduction 181

11.2 Earth Electrodes 182

11.2.1 Corrosion Phenomena 183

11.3 Protective Conductors 185

11.3.1 Analytical Calculation of the Minimum Cross-Sectional Area of PEs 186

11.3.2 Metallic Layers of Cables as Protective Conductor 192

11.4 Equipotential Bonding Conductors 193

11.4.1 Where Should We Use Equipotential Bonding Conductors? 195

11.5 Earthing Conductors and Main Earthing Terminal 196

11.6 The PEN Conductor 197

FAQs 198

Endnotes 199

12 Safety Against Overvoltages 201

12.1 Introduction 201

12.2 Temporary Overvoltages and Safety 202

12.2.1 High-Voltage Ground Faults in TN Systems 202

12.2.2 High-Voltage Ground Faults in TT Systems 204

12.2.3 High-Voltage Ground Faults in IT Systems 206

12.3 External Overvoltages 208

12.3.1 Characterization of Earthing Systems Under Impulse Conditions 209

12.3.2 Induced Overvoltages 210

Endnotes 212

13 Safety Against Static Electricity 213

13.1 Introduction 213

13.2 Generation of Static Electricity 215

13.3 Static Charge Energy 216

13.4 Mitigation Strategies 218

13.5 Residual Voltages 219

Endnotes 222

14 Testing the Electrical Safety 223

14.1 Introduction 223

14.2 Soil Resistivity Measurement 223

14.3 Earth Resistance Measurement 226

14.4 Earth Resistance Measurements in Industrial Facilities 228

14.5 Earth Resistance Measurement in TT Systems 230

14.6 Measurement of the Fault-Loop Impedance in TN Systems 232

14.7 Touch Voltage Measurement in TN Systems (Low-Voltage Earth Faults) 234

14.8 Step and Touch Voltage Measurements in TN Systems 235

14.9 Fundamental Measurements in IT Systems 238

14.10 Protective Conductor Continuity Test 239

14.11 Insulation Resistance Test 240

Endnotes 242

15 Applications of Electrical Safety in Special Locations and Installations 243

15.1 Introduction 243

15.2 Electrical Safety in Marinas 244

15.3 Electrical Safety Requirements for Equipment Having High Protective Conductor Currents 246

15.4 Electrical Safety in Train Stations 248

15.5 Electrical Safety in Swimming Pools 250

15.6 Electrical Safety in Restrictive Conductive Locations 252

15.7 Electrical Safety in External Lighting Installations 253

15.8 Electrical Safety in Medical Locations 254

15.8.1 Microshock 254

15.8.2 Leakage Currents 255

15.8.3 Local Equipotential Earthing Connection 256

15.8.4 Electrical Separation 257

Endnotes 262

A Sinusoids and Phasors 263

A.1 Sinusoids 263

A.2 Phasors 267

Endnotes 272

B Fundamental Conventions and Electric Circuit Theorems 273

B.1 Introduction 273

B.2 Fundamental Electrical Conventions 273

B.3 Kirchhoff's Laws 274

B.4 Voltage and Current Dividers 275

B.5 Superposition Principle 276

B.6 Thevenin's Theorem 277

B.7 Millman's Theorem 278

B.8 Impedance Bridge 278

Endnote 279

C Fundamental Units, Symbols, and Correct Spellings 281

C.1 Synoptic Table 281

Index 283