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Chapter 1. BASIC AC CIRCUIT TERMS 1
Alternating current 1
AC system characteristics 2
Power and power factor 3
AC waveforms and phase relationships 4
Power factor and current/voltage waveforms 5
Chapter 2. SHORT-CIRCUIT EFFECTS AND BASIC INTERRUPTING REQUIREMENTS OF PROTECTIVE DEVICES 7
Analogy 7
Overloads and short circuits 7
How short circuits can occur 7
Short-circuit effects 8
Protective device ratings 8
Chapter 3. SIMPLIFIED ANALYSIS OF SHORT-CIRCUIT CURRENT BEHAVIOR 11
Fault current waveform study 11
Asymmetry due to DC component 15
Conversion of symmetrical amperes to withstand ratings 16
Chapter 4. STANDARDS ORGANIZATIONS AND FUSE APPLICATION TERMS 19
Standards organizations 19
Fuse application terms 20
Chapter 5. FUSE DEVELOPMENT AND CONSTRUCTION 25
The early years 25
Product development 25
Combination performance fuse 27
Chapter 6. FUSE STANDARDS, CLASSES, TYPES, RATINGS, AND INTERCHANGEABILITY 29
Low-voltage fuse categories 29
Low-voltage fuse standards 29
Important fuse characteristics 32
Fuse classes, types, and ratings 33
Supplementary protection 36
Interchangeability of fuses 39
Hazards of interchanging fuses 42
Chapter 7. INTERPRETING AVERAGE MELTING TIME/CURRENT CHARACTERISTIC CURVES 43
Scale designations 43
Average melting times 45
Fuse melting band 47
Melting time variations 47
Constructing the fuse band 47
Total clearing time 47
Probable clearing time curve 48
Value of time/current curves 49
Chapter 8. OVERLOAD, TEMPERATURE, AND DERATING CONSIDERATIONS 51
UL clearing time limits 51
Special UL clearing time requirements for Class L fuses 52
Long-time opening and time/current characteristic curves 52
UL testing procedures 54
Ambient temperature effects 54
Other sources of heat 56
Derating fuses 56
Chapter 9. AVERAGE MELTING TIME/CURRENT CHARACTERISTIC VARIATIONS 59
Fuse categories 59
Fuse class melting variations 61
Melting variations with voltage rating 62
Comparing melting times of fast fuses 65
Comparing melting times of slow fuses 65
Tying it all together 67
Chapter 10. INSTANTANEOUS PEAK LET-THROUGH CURRENTS OF CURRENT-LIMITING FUSES 69
Practical usage 69
Power factor and X/R ratio 69
Fuse interrupting ratings and X/R ratio 70
Short-circuit behavior 71
Current-limiting concept 71
Let-through curves and peak available current lines 73
Interpreting peak current let-through curves 76
Recap 80
Protecting against the worst 80
Follow these basic rules 80
Threshold current 81
Chapter 11. I2T LET-THROUGH ENERGY OF CURRENT-LIMITING FUSES 83
What I2t involves 84
Heating effects of I2t 85
Mechanical effects of I2t 85
Combination effects of I2t 85
Comparing I2t and Ip of various fuse classes 86
Published I2t data 87
Melting I2t 88
I2t and coordination 89
Coordination tables 90
I2t vs. fuse short-time and long-time operation 91
Let-through I2t and circuit X/R 92
Let-through I2t and applied voltage 94
Maximum I2t vs. maximum Ip s 94
Rules to observe in applying I2t data 96
Chapter 12. FUSE SELECTIVITY AND SYSTEM COORDINATION 99
Fuse selectivity tables 99
Series-combination system 100
Coordination problems with existing circuits 101
Fuse coordination in the current-limiting area 101
Computer programs and test-generated data 103
APPENDIX UL REQUIREMENTS 105
Chapter 6 105
Chapter 10 106
Chapter 11 108
