Practical Guide Applying Low-Vltg Fuses

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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

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