Guidance Note 6: Protection Against Overcurrent (IEE Guidence Notes) (No 6)

CONTENTS......Page 3
CO-OPERATING ORGANISATIONS......Page 5
ACKNOWLEDGEMENTS......Page 6
PREFACE......Page 7
INTRODUCTION......Page 8
1.1.2 Protective measures......Page 10
1.2.2 Overload and fault current......Page 11
1.4 Omission of protection......Page 13
1.6 Duration of overcurrent......Page 14
1.7.2 Discrimination between devices......Page 15
1.7.3 Co-ordination of devices......Page 16
1.7.4 Back-up co-ordination......Page 18
2.2 Load assessment......Page 20
2.2.3 Thermally equivalent current......Page 21
Fig 1:  Illustrating a varying load......Page 22
TABLE 1  Duration of load current for equivalent constant load (Duration in minutes)......Page 23
Fig 2:  Test for variable load......Page 24
2.3 Selection of protective device and conductor size......Page 25
Fig 3:  Overload protection......Page 26
2.3.1 Selection of conductor size, and value of Iz......Page 27
2.3.2 Nuisance tripping of overload protection......Page 28
TABLE 2  Selection of cb or rcbo type......Page 30
Fig 4:  Typical star-delta arrangement......Page 31
2.3.4 Cables in Parallel......Page 33
Fig 5:  Cable configurations for single-core cables in parallel......Page 35
Fig 6:  Typical ring final circuit arrangement......Page 36
2.3.6 Load assessment......Page 37
2.3.7 Mineral insulated cables and ring circuits......Page 38
2.4 Omission of protection against overload current......Page 39
3.2 Nature of damage and installation precautions......Page 42
3.2.2 Expansion forces......Page 43
3.2.3 Electromagnetic forces......Page 45
3.3 Fault impedance and breaking capacity of protective device......Page 46
3.4.2 Fault current and impedance at origin of installation......Page 47
3.4.3 Relocation of fault current protection......Page 49
Fig 7:  interconnections between switchgear items......Page 50
3.5.1 General......Page 51
3.6 The use of one device for both overload and fault current protection......Page 54
3.7 Harmonics......Page 55
4.1 Determination of fault current by enquiry......Page 57
Fig 8:  Reduction in fault level at the suppliers cut-outs......Page 59
4.2 Measurement of fault current......Page 60
4.3 Calculation of fault current......Page 61
4.3.1 Highest value of fault current......Page 62
4.3.2 Lowest value of fault current......Page 63
5.1 General equation for fault current......Page 64
Fig 9:  General arrangement of impedances......Page 65
5.2.2 Circuits for more than 100 A......Page 66
5.3 Conductor temperature and resistance......Page 67
5.3.1 Shock protection......Page 68
TABLE 3  Resistance coefficients......Page 69
5.3.3 Earth fault currents......Page 70
Fig 10:  Additional reactance......Page 72
5.5.3 Sources for values of resistance and reactance......Page 73
5.6.2 Circuits for more than 100 A......Page 74
5.6.4 Special considerations with single-core cables......Page 75
6.2 TN systems (TN-C, TN-S and TN-C-S)......Page 76
Fig 11:  Earth fault path, TN-C-S system......Page 77
6.2.3 Sources for values of resistance and reactance......Page 78
6.3.1 Steel-wire armoured multicore cables......Page 79
TABLE 4  Coefficients for steelwire armour......Page 81
6.3.2 Aluminium-wire armoured single-core cable......Page 82
TABLE 5  Values of Cr and Cx for aluminium-wire(1) armoured single-core copper conductor cables......Page 83
TABLE 6  Coefficients for aluminiumwire armour......Page 84
TABLE 7  Coefficients for copper sheaths......Page 85
6.3.5 Cables in steel conduit......Page 86
TABLE 8  Values of Fr and Fx for steel conduit......Page 87
6.4 TT system......Page 88
Fig 12:  Earth fault path, TT system......Page 89
7.2 Overload and shortcircuit protection by the same device......Page 90
7.4 Parallel cables......Page 91
7.7 Status of adiabatic equations......Page 92
7.8 Alternative values of k......Page 93
7.9 Cables in trenches......Page 94
8.1 General......Page 95
8.2 Influence of circuit design and length of flexible cord......Page 96
8.3.2 Acceptable length of flexible......Page 97
8.4.1 Ring circuits......Page 98
TABLE 9  Maximum lengths of cord for earth faults (circuit protection complying with 413-02-09)......Page 100
2 Line to neutral, single-phase, faults......Page 102
TABLE 1.1  Coefficient a......Page 103
3 Line to line, single-phase, short-circuit......Page 104
4 Three-phase short-circuit......Page 105
5 Earth faults......Page 107
TABLE 2.1  Coefficient B and constant k......Page 108
C......Page 109
F......Page 110
O......Page 111
R......Page 112
V......Page 113
Tables & figs......Page 0