ARC FLASH HAZARD
ANALYSIS AND
MITIGATION
J.C. Das
IEEEPRESSSERIES0N
POWERENGINEERING
Mohamed E. El-Hawary, Series Editor
IEEEIEEE PRESS
©WILEYA JOHN WILEY & SONS, INC., PUBLICATION
CONTENTS
Foreword xix
Preface xxi
About the Author xxiri
1 ARC FLASH HAZARDS AND THEIR ANALYSES 1
1.1 Electrical Arcs 2
1.1.1 Arc as a Heat Source 3
1.1.2 Arcing Phenomena in a Cubicle 3
1.2 Arc Flash Hazard and Personal Safety 4
1.3 Time Motion Studies 5
1.4 Arc Flash Hazards 6
1.5 Arc Blast 6
1.6 Electrical Shock Hazard 9
1.6.1 Resistance of Human Body 11
1.7 Fire Hazard 14
1.8 Arc Flash Hazard Analysis 15
1.8.1 Ralph Lee's and NFPA Equations 17
1.8.2 IEEE 1584 Guide Equations 18
1.9 Personal Protective Equipment 23
1.10 Hazard Boundaries 24
1.10.1 Working Distance 25
1.10.2 Arc Flash Labels 25
1.11 Maximum Duration of an Arc Flash Event and Arc
Flash Boundary 26
1.11.1 Arc Flash Hazard with Equipment Doors Closed 28
1.12 Reasons for Internal Arcing Faults 29
1.13 Arc Flash Hazard Calculation Steps 30
1.13.1 NFPA Table 130.7(C)(15)(a) 32
v
Vi CONTENTS
1.14 Examples of Calculations 32
1.15 Reducing Arc Flash Hazard 36
Review Questions 37
References 37
2 SAFETY AND PREVENTION THROUGH DESIGN: A NEW FRONTIER 40
2.1 Electrical Standards and Codes 41
2.2 Prevention through Design 43
2.3 Limitations of Existing Codes, Regulations, and Standards 44
2.4 Electrical Hazards 45
2.5 Changing the Safety Culture 48
2.6 Risk Analysis for Critical Operation Power Systems 48
2.6.1 Existing Systems 49
2.6.2 New Facilities 49
2.7 Reliability Analysis 50
2.7.1 Data for Reliability Evaluations 51
2.7.2 Methods of Evaluation 52
2.7.3 Reliability and Safety 52
2.8 Maintenance and Operation 53
2.8.1 Maintenance Strategies 54
2.8.2 Reliability-Centered Maintenance (RCM) 55
2.9 Safety Integrity Level and Safety Instrumented System 55
Review Questions 57
References 57
3 CRITIQUE OF IEEE GUIDE 1584 ARC FLASH CALCULATIONS 60
3.1 Variations of Arcing Currents 60
3.2 Gap between Electrodes 62
3.3 Variations of Incident Energy 64
3.4 Some Anomalies in IEEE Equations 64
3.5 Lee's Arc Model 66
3.6 IEEE Experimental Model Setup 68
3.7 Electrical Arc Burn Hazard 70
3.8 Effect of Insulating Barriers 72
3.8.1 Without Barrier 72
3.8.2 With Barriers 75
3.9 Arc Flash Test Models 76
3.10 Alternate Equations 77
3.11 Further Testing and Research 78
CONTENTS Vii
3.12 Effectiveness of PPE Calculated Based on IEEE 1584 Guide 79
Review Question 80
References 80
4 ARC FLASH HAZARD AND SYSTEM GROUNDING 82
4.1 System and Equipment Grounding 82
4.1.1 Solidly Grounded Systems 83
4.2 Low Resistance Grounding 87
4.3 High Resistance Grounded Systems 87
4.3.1 Fault Detection, Alarms, and Isolation 90
4.4 Ungrounded Systems 94
4.5 Reactance Grounding 95
4.6 Resonant Grounding 95
4.7 Corner of Delta-Grounded Systems 95
4.8 Surge Arresters 96
4.9 Artificially Derived Neutrals 97
4.10 Multiple Grounded Systems 100
4.10.1 Comparison of Grounding Systems 100
4.11 Arc Flash Hazard in Solidly Grounded Systems 100
4.12 Protection and Coordination in Solidly Grounded Systems 105
4.12.1 Self-Extinguishing Ground Faults 108
4.12.2 Improving Coordination in Solidly Grounded Low
Voltage Systems 111
4.13 Ground Fault Coordination in Low Resistance Grounded
Medium Voltage Systems 114
4.13.1 Remote Tripping 117
4.13.2 Ground Fault Protection of Industrial Bus-Connected
Generators 117
4.13.3 Directional Ground Fault Relays 122
4.14 Monitoring of Grounding Resistors 123
4.15 Selection of Grounding Systems 124
Review Questions 125
References 126
5 SHORT-CIRCUIT CALCULATIONS ACCORDING TO ANSI/IEEE
STANDARDS FOR ARC FLASH ANALYSIS 128
5.1 Types of Calculations 129
5.1.1 Assumptions: Short-Circuit Calculations 129
5.1.2 Short-Circuit Currents for Arc Flash Calculations 130
viii CONTENTS
5.2 Rating Structure of HV Circuit Breakers 130
5.3 Low-Voltage Motors 133
5.4 Rotating Machine Model 134
5.5 Calculation Methods 134
5.5.1 Simplified Method X/R<\7 134
5.5.2 Simplified Method X/R > 17 135
5.5.3 E/Z Method for AC and DC Decrement Adjustments 135
5.6 Network Reduction 138
5.7 Calculation Procedure 138
5.7.1 Analytical Calculation Procedure 139
5.8 Capacitor and Static Converter Contributions to
Short-Circuit Currents 141
5.9 Typical Computer-Based Calculation Results 141
5.9.1 First-Cycle or Momentary Duty Calculations 141
5.9.2 Interrupting Duty Calculations 144
5.9.3 Low Voltage Circuit Breaker Duty Calculations 144
5.10 Examples of Calculations 144
5.10.1 Calculation of Short-Circuit Duties 150
5.10.2 tf-Rated 15 kV Circuit Breakers 150
5.10.3 4.16-kV Circuit Breakers and Motor Starters 155
5.10.4 Transformer Primary Switches and Fused Switches 155
5.10.5 Low Voltage Circuit Breakers 159
5.11 Thirty-Cycle Short-Circuit Currents 159
5.12 Unsymmetrical Short-Circuit Currents 160
5.12.1 S ingle Line-to-Ground Fault 161
5.12.2 Double Line-to-Ground Fault 163
5.12.3 Line-to-Line Fault 166
5.13 Computer Methods 169
5.13.1 Line-to-Ground Fault 170
5.13.2 Line-to-Line Fault 171
5.13.3 Double Line-to-Ground Fault 171
Review Questions 173
References 174
6 ACCOUNTING FOR DECAYING SHORT-CIRCUIT CURRENTS
IN ARC FLASH CALCULATIONS 176
6.1 Short Circuit of a Passive Element 176
6.2 Systems with No AC Decay 179
CONTENTS IX
6.3 Reactances of a Synchronous Machine 180
6.3.1 Leakage Reactance 180
6.3.2 Sub-transient Reactance 181
6.3.3 Transient Reactance 181
6.3.4 Synchronous Reactance 181
6.3.5 Quadrature-Axis Reactances 181
6.3.6 Negative Sequence Reactance 182
6.3.7 Zero Sequence Reactance 182
6.4 Saturation of Reactances 182
6.5 Time Constants of Synchronous Machines 182
6.5.1 Open-Circuit Time Constant 182
6.5.2 Subtransient Short-Circuit Time Constant 182
6.5.3 Transient Short-Circuit Time Constant 183
6.5.4 Armature Time Constant 183
6.6 Synchronous Machine Behavior on Terminal Short Circuit 183
6.6.1 Equivalent Circuits during Fault 184
6.6.2 Fault Decrement Curve 188
6.7 Short Circuit of Synchronous Motors and Condensers 192
6.8 Short Circuit of Induction Motors 192
6.9 A New Algorithm for Arc Flash Calculations with
Decaying Short-Circuit Currents 195
6.9.1 Available Computer-Based Calculations 196
6.9.2 Accumulation of Energy from Multiple Sources 196
6.9.3 Comparative Calculations 198
Review Questions 201
References 202
7 PROTECTIVE RELAYING 203
7.1 Protection and Coordination from Arc Flash Considerations 203
7.2 Classification of Relay Types 207
7.3 Design Criteria of Protective Systems 207
7.3.1 Selectivity 208
7.3.2 Speed 208
7.3.3 Reliability 208
7.3.4 Backup Protection 209
7.4 Overcurrent Protection 209
7.4.1 Overcurrent Relays 210
7.4.2 Multifunction Overcurrent Relays 212
7.4.3 IEC Curves 214
X CONTENTS
7.5 Low Voltage Circuit Breakers 216
7.5.1 Molded Case Circuit Breakers (MCCBs) 216
7.5.2 Current-Limiting MCCBs 222
7.5.3 Insulated Case Circuit Breakers (ICCBs) 223
7.5.4 Low Voltage Power Circuit Breakers (LVPCBs) 225
7.5.5 Short-Time Bands of LVPCBs Trip Programmers 227
7.6 Short-Circuit Ratings of Low Voltage Circuit Breakers 228
7.6.1 Single-Pole Interrupting Capability 232
7.6.2 Short-Time Ratings 232
7.7 Series-Connected Ratings 233
7.8 Fuses 234
7.8.1 Current-Limiting Fuses 235
7.8.2 Low Voltage Fuses 237
7.8.3 High Voltage Fuses 238
7.8.4 Electronic Fuses 238
7.8.5 Interrupting Ratings 239
7.9 Application of Fuses for Arc Flash Reduction 239
7.9.1 Low Voltage Motor Starters 239
7.9.2 Medium Voltage Motor Starters 241
7.9.3 Low Voltage Switchgear 241
7.10 Conductor Protection 244
7.10.1 Load Current Carrying Capabilities of Conductors 246
7.10.2 Conductor Terminations 247
7.10.3 Considerations of Voltage Drops 247
7.10.4 Short-Circuit Considerations 247
7.10.5 Overcurrent Protection of Conductors 249
7.11 Motor Protection 250
7.11.1 Coordination with Motor Thermal Damage Curve 252
7.12 Generator 51-V Protection 259
7.12.1 Arc Flash Considerations 261
Review Questions 263
References 264
8 UNIT PROTECTION SYSTEMS 266
8.1 Overlapping the Zones of Protection 268
8.2 Importance of Differential Systems for Arc Flash Reduction 270
8.3 Bus Differential Schemes 272
8.3.1 Overcurrent Differential Protection 272
STENTS Xi
8.3.2 Partial Differential Schemes 273
8.3.3 Percent Differential Relays 273
8.4 High Impedance Differential Relays 276
8.4.1 Sensitivity for Internal Faults 278
8.4.2 High impedance Microprocessor-BasedMultifunction Relays 280
8.5 Low Impedance Current Differential Relays 280
8.5.1 CT Saturation 282
8.5.2 Comparison with High Impedance Relays 283
8.6 Electromechanical Transformer Differential Relays 285
8.6.1 Harmonic Restraint 288
8.7 Microprocessor-Based Transformer Differential Relays 288
8.7.1 CT Connections and Phase Angle Compensation 288
8.7.2 Dynamic CT Ratio Corrections 293
8.7.3 Security under Transformer Magnetizing Currents 293
8.8 Piiot Wire Protection 295
8.9 Modern Line Current Differential Protection 296
8.9.1 The Alpha Plane 298
8.9.2 Enhanced Current Differential Characteristics 299
8.10 Examples of Arc Flash Reduction with Differential Relays 301
Review Questions 304
References 304
9 ARC FAULT DETECTION RELAYS 306
9.1 Principle of Operation 307
9.2 Light Intensity 307
9.3 Light Sensor Types 308
9.4 Other Hardware 313
9.5 Selective Tripping 314
9.6 Supervision with Current Elements 316
9.7 Applications 316
9.7.1 Medium Voltage Systems 316
9.7.2 Low Voltage Circuit Breakers 318
9.7.3 Self-Testing of Sensors 318
9.8 Examples of Calculation 318
9.9 Arc Vault Protection for Low Voltage Systems 31?
9.9.1 Detection System 32
Review Questions 3'
References -
xii CONTENTS
10 OVERCURRENT COORDINATION 326
10.1 Standards and Requirements 327
10.2 Data for the Coordination Study 327
10.3 Computer-Based Coordination 329
10.4 Initial Analysis 329
10.5 Coordinating Time Interval 330
10.5.1 Relay Overtravel 330
10.6 Fundamental Considerations for Coordination 330
10.6.1 Settings on Bends of Time-Current
Coordination Curves 332
10.7 Coordination on Instantaneous Basis 332
10.7.1 Selectivity between Two Series-Connected
Current-Limiting Fuses 334
10.7.2 Selectivity of a Current-Limiting Fuse Downstream
of Noncurrent-Limiting Circuit Breaker 334
10.7.3 Selectivity of Current-Limiting Devices in Series 339
10.8 NEC Requirements of Selectivity 343
10.8.1 Fully Selective Systems 343
10.8.2 Selection of Equipment Ratings and Trip Devices 346
10.9 Energy Boundary Curves 346
10.10 The Art of Compromise 353
Review Questions 363
References 363
11 TRANSFORMER PROTECTION 365
11.1 NEC Requirements 365
11.2 Arc Flash Considerations 367
11.3 System Configurations of Transformer Connections 368
11.3.1 Auto-Transfer of Bus Loads 373
11.4 Through Fault Current Withstand Capability 373
11.4.1 Category I 374
11.4.2 Category II 374
11.4.3 Category III and IV 374
11.4.4 Observation on Faults during Life Expectancy
of a Transformer 376
11.4.5 Dry-Type Transformers 377
11.5 Constructing the Through Fault Curve Analytically 381
11.5.1 Protection with Respect to Through Fault Curves 381
CONTENTS Xiii
11.6 Transformer Primary Fuse Protection 382
11.6.1 Valuations in the Fuse Characteristics 382
11.6.2 Single Phasing and Ferroresonance 384
11.6.3 Other Considerations of Fuse Protection 384
11.7 Overcurrent Relays for Transformer Primary Protection 384
11.8 Listing Requirements 386
11.9 Effect of Transformer Winding Connections 390
11.10 Requirements of Ground Fault Protection 392
11.11 Through Fault Protection 392
11.11.1 Primary Fuse Protection 392
11.11.2 Primary Relay Protection 394
11.12 Overall Transformer Protection 394
11.13 A Practical Study for Arc Flash Reduction 395
11.13.1 System Configuration 395
11.13.2 Coordination Study and Observations 395
11.13.3 Arc Flash Calculations: High Hazard Risk
Category (HRC) Levels 400
11.13.4 Reducing HRC Levels with Main SecondaryCircuit Breakers 402
11.13.5 Maintenance Mode Switches on Low Voltage TripProgrammers 402
11.13.6 Addition of Secondary Relay 408
Review Questions 411
References 412
12 CURRENT TRANSFORMERS 413
12.1 Accuracy Classification of CTs 414
12.1.1 Metering Accuracies 414
12.1.2 Relaying Accuracies 414
12.1.3 Relaying Accuracy Classification X 415
12.1.4 Accuracy Classification T 416
12.2 Constructional Features of CTs 416
12.3 Secondary Terminal Voltage Rating 418
12.3.1 Saturation Voltage 419
12.3.2 Saturation Factor 419
12.4 CT Ratio and Phase Angle Errors 419
12.5 Interrelation of CT Ratio and C Class Accuracy 422
12.6 Polarity of Instrument Transformers 424
xiv CONTENTS
12.7 Application Considerations 425
12.7.1 Select CT Ratio 425
12.7.2 Make a Single-Line Diagram of the CT Connections 427
12.7.3 CT Burden 427
12.7.4 Short-Circuit Currents and Asymmetry 427
12.7.5 Calculate Steady-State Performance 427
12.7.6 Calculate Steady-State Errors 428
12.8 Series and Parallel Connections of CTs 432
12.9 Transient Performance of the CTs 432
12.9.1 CT Saturation Calculations 433
12.9.2 Effect of Remanence 434
12.10 Practicality of Application 435
12.11 CTs for Low Resistance-Grounded Medium Voltage Systems 437
12.12 Future Directions 437
Review Questions 440
References 440
13 ARC-RESISTANT EQUIPMENT 442
13.1 Calculations of Arc Flash Hazard in Arc-Resistant
Equipment 443
13.1.1 Probability of Arcing Fault 443
13.2 Qualifications in IEEE Guide 444
13.3 Accessibility Types 445
13.3.1 Type 1 445
13.3.2 Type 2 445
13.3.3 Suffix B 445
13.3.4 Suffix C 445
13.3.5 Suffix D 446
13.4 IEC Accessibility Types 446
13.5 Arc-Resistant Ratings 447
13.5.1 Duration Ratings 447
13.5.2 Device-Limited Ratings 448
J 3.5.3 Effect of Cable Connections 451
13.6 Testing According to IEEE Guide 451
13.6.1 Criterion 1 451
13.6.2 Criterion 2 452
13.6.3 Criterion 3 452
13.6.4 Criterion 4 452
CONTENTS XV
13.6J Criterion 5 452
13.6.6 Maintenance 453
13.7 Pressure Relief 453
13.8 Venting and Plenums 455
13.8.1 Venting into Surrounding Area 455
13.8.2 Plenums 457
13.9 Cable Entries 457
Review Questions 459
References 459
14 RECENT TRENDS AND INNOVATIONS 461
14.1 Statistical Data of Arc Flash Hazards 461
14.2 Zone-Selective Interlocking 463
14.2.1 Low Voltage ZSI Systems 463
14.2.2 Zone Interlocking in Medium Voltage Systems 470
14.3 Microprocessor-Based Low Voltage Switchgear 473
14.3.1 Microprocessor-Based Switchgear Concept 473
14.3.2 Accounting for Motor Contributions 474
14.3.3 Faults on the Source Side 476
14.3.4 Arc Flash Hazard Reduction 477
14.4 Low Voltage Motor Control Centers 477
14.4.1 Desirable MCC Design Features 478
14.4.2 Recent Design Improvements 478
14.4.3 Higher Short-Circuit Withstand MCCs 485
14.5 Maintenance Mode Switch 485
14.6 Infrared Windows and Sight Glasses 487
14.7 Fault Current Limiters 490
14.8 Partial Discharge Measurements 494
14.8.1 Online versus Offline Measurements 495
14.8.2 Test Methods 496
14.8.3 Current Signature Analysis: Rotating Machines 498
14.8.4 Dissipation Factor Tip-Up 498
Review Questions 500
References 501
15 ARC FLASH HAZARD CALCULATIONS IN DC SYSTEMS 503
15.1 Calculations of the Short-Circuit Currents in DC Systems 504
15.2 Sources of DC Short-Circuit Currents 504
xvi CONTENTS
15.3 IEC Calculation Procedures 505
] 5.4 Short Circuit of a Lead Acid Battery 508
15.5 Short Circuit of DC Motors and Generators 512
15.6 Short-Circuit Current of a Rectifier 517
15.7 Short Circuit of a Charged Capacitor 522
15.8 Total Short-Circuit Current 523
15.9 DC Circuit Breakers and Fuses 524
15.9.1 DC Circuit Breakers 524
15.9.2 DC Rated Fuses 527
15.10 Arcing in DC Systems 527
15.11 Equations for Calculation of Incident Energy in DC Systems 532
15.12 Protection of the Semiconductor Devices 534
15.12.1 Controlled Converters 536
Review Questions 537
References 538
16 APPLICATION OF ETHERNET AND IEC 61850
COMMUNICATIONS 540
16.1 IEC 61850 Protocol 541
16.2 Modern lEDs 542
16.3 Substation Architecture 543
16.4 IEC 61850 Communication Structure 544
16.5 Logical Nodes 546
16.6 Ethernet Connection 546
16.7 Networking Media 550
16.7.1 Copper Twisted Shielded and Unshielded 550
16.7.2 Fiber Optic Cable 551
16.8 Network Topologies 552
16.8.1 Prioritizing GOOSE Messages 554
16.8.2 Technoeconomical Justifications 554
16.9 Application to Arc Flash Relaying and Communications 556
Review Questions 556
References 556
Appendix A Statistics and Probability Appliedto Electrical Engineering 558
A. 1 Mean Mode and Median 558
A.2 Mean and Standard Deviation 559
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