Guidance Note 6 Protection Against Overcurrent BS 7671:2018+A2:2022, 9th Edition 2022 (PDF)

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About the book:

Publisher: IET
Place of Publication: United Kingdom
Pages: 126
Edition: 2022
Language: English
Size: 55 Mb

Guidance Note 6 Protection Against Overcurrent + A2:2022 has now been updated to BS7671:2018+A2:2022.

Content:

Cooperating organizations 7
Acknowledgements 8
Preface 9
Introduction 11
Chapter 1 The regulations concerning protection against overcurrent 13
1.1 Scope 13
1.1.1 Fundamental principles 13
1.1.2 Protective measures 13
1.1.3 Load equipment and flexible cables 13
1.2 Nature of overcurrent and protection 14
1.2.1 General 14
1.2.2 Overload current and fault current 14
1.2.3 General protection characteristic 14
1.3 Statutory requirements 15
1.4 Omission of protection 15
1.5 Overcurrent protective devices 15
1.6 Duration of overcurrent 18
1.7 Coordination and selectivity 18
1.7.1 Operation of protective devices in series 18
1.7.2 Selectivity between devices 18
1.7.3 Coordination of devices 19
1.7.4 Back-up protection and conditional short-circuit rating, lcc 2 2
1.7.5 Coordination of low voltage (LV) assemblies to the BSEN 61439 series 23
Chapter 2 Protection against overload 25
2.1 Introduction 25
2.2 Load assessment 25
2.2.1 Diversity 26
2.2.2 Varying loads 26
2.2.3 Thermally equivalent current 26
2.2.4 Small overloads of long duration 26
2.3 Selection of protective device 27
2.4 Selection of conductor cross-sectional area (csa) with overload protection 29
2.4.1 Tabulated current rating, lt 29
2.4.2 What is the relationship between lt and lz? 30
2.4.3 Rating factor, Cf 30
2.4.4 Rating factor, Cc 31
2.4.5 Rating factors, Cg, Ca, Cs and Cd 31
2.5 Omission of protection against overload 31
2.5.1 Selection of conductor csa for circuits not requiring overload protection 31
2.5.2 Omission of overload protection (Regulation 433.3) 32
2.6 Grouping of cables 3 5
2.6.1 Simultaneous overload 3 5
2.6.2 Non-simultaneous overload 35
2.6.3 Circuit loads of not more than 30 7o grouped rating 35
2.6.4 UK overcurrent devices 35
2.7 Avoidance of unintentional operation of circuit-breakers 3 6
2.8 Current ratings of low voltage (LV) assemblies (switchboards,panel boards, distribution boards,etc.) 38
2.8.1 General 38
2.8.2 Prosumer's electrical installations (embedded generation) 39
2.8.3 Conclusion 40
2.9 Cables for star-delta starters 40
2.10 Cables in parallel 42
2.10.1 Appendix 10ofBS7671 42
2.10.2 General 43
2.10.3 Single-core cables 43
2.10.4 Multicore cables 43
2.1 1 Ring final circuits 44
2.12 Load assessment 47
2.12.1 What is a small overload? 47
2.12.2 What is a long period? 47
2.13 Mineral-insulated cables and ring final circuits 48
Chapter 3 Protection against fault current 49
3.1 Types of fault to be considered 49
3.2 Nature of damage and installation precautions 49
3.2.1 Installation precautions 50
3.2.2 Expansion forces 50
3.2.3 Electromagnetic forces 51
3.3 Fault impedance and breaking capacity of protective device 52
3.4 Short-circuit rating of low voltage (LV) assemblies (switchboards,panel boards, distribution boards, etc.) 54
3.5 Position of fault current protection and assessment of prospective current 5 4
3.5.1 Assessment of fault current 54
3.5.2 Fault current and impedance at the origin of the installation 55
3.5.3 Relocation of fault current protection 5 6
3.6 Omission of fault current protection 5 8
3.7 The use of one device for both overload and fault current protection 60
3.8 Harmonics 60
3.8.1 Introduction 6 0
3.8.2 The neutral current 61
3.8.3 The line current 61
3.8.4 Harmonic rating factors 62
3.9 Prosumer’s electrical installations (embedded generation) 65
Chapter 4 Determination of fault current 67
4.1 Determination of fault current by enquiry 67
4.2 Measurement of fault current 69
4.3 Calculation of fault current 70
4.3.1 Highest value of fault current 70
4.3.2 Lowest value of fault current 70
4.4 Prosumer's electrical installations (embedded generation) 71
Chapter 5 Equations for the calculation of short-circuit current
5.1 General equation for fault current 73
5.2 Single-phase, line-to-neutral fault 74
5.2.1 Circuits up to about 100 A 75
5.2.2 Circuits for more than 100 A 75
5.2.3 Sources for values of resistance and reactance 75
5.3 Conductor temperature and resistance 76
5.3.1 Shock protection 76
5.3.2 Protection against short circuit 78
5.3.3 Earth fault currents 78
5.4 Single-phase circuits 79
5.4.1 Single-phase circuits fed from three-phase board 79
5.4.2 Effect of single-core cable spacing 79
5.5 Line-to-line short circuit 8 0
5.5.1 Circuits up to about 100 A 8 0
5.5.2 Circuits for more than 100 A 81
5.5.3 Sources for values of resistance and reactance 81
5.6 Three-phase short circuit 81
5.6.1 Circuits up to about 100 A 81
5.6.2 Circuits for more than 100 A 82
5.6.3 Sources for values of resistance and reactance 82
5.6.4 Special considerations with single-core cables 82
Chapter 6 Equations for the calculation of earth fault current
6.1 General 83
6.2 TN-S and TN-C-S systems 83
6.2.1 Circuits up to about 100 A 8 4
6.2.2 Circuits for more than 100 A 84
6.2.3 Sources for values of resistance and reactance 85
6.2.4 Value of touch voltage 85
6.3 Cable enclosure used as a protective conductor 85
6.3.1 Steel wire armoured multicore cables 85
6.3.2 Aluminium-wire armoured single-core cables 87
6.3.3 Copper sheathed cables 88
6.3.4 Auxiliary conductors 89
6.3.5 Cables in steel conduit 89
6.3.6 Cables in steel ducts and trunking 90
6.4 TT system 91
6.4.1 Introduction 91
6.4.2 Lowest prospective earth fault current If for automatic disconnection of supply 92
6.4.3 Highest prospective earth fault current 92
6.4.4 Touch voltage 93
Chapter 7 Selection of conductor size 95
7.1 General 95
7.2 Overload and short-circuit protection by a single device 95
7.2.1 General 95
7.2.2 Cables connecting generators downstream of a final circuit protective device 95
7.3 Earth fault current 96
7.4 Parallel cables 97
7.5 Energy let-through characteristics 97
7.6 Duration of short-circuit current 99
7.7 Status of adiabatic equations 100
7.8 Alternative values of k 101
7.9 Appendix 4 of BS 7671 101
Chapter 8 Prosumer’s electrical installations (PEIs) 103
8.1 Introduction to prosumer's electrical installations (PEIs) 103
8.2 Magnitude of overcurrents and short-circuit currents 104
8.3 Direction of current flow 105
8.4 Overcurrent protection and connection of generation 107
8.5 Location of overcurrent protective devices 107
8.6 Combined short-circuit protection 108
Appendix A Calculation of reactance 109
A1 General 109
A2 Line-to-neutral, single-phase faults 109
A3 Line-to-line, single-phase, short-circuit 111
A4 Three-phase short-circuit 111
A5 Earth faults 113
Appendix B Calculation of k for other temperatures 115
B1 Adiabatic equation 115
Index 117

Tags: Guidance Note 6-2022, Guidance Note 6-2022 pdf, Protection Against Overcurrent+ A2:2022