Arc Flash Hazard Analysis and Mitigation
Description
An arc flash, an electrical breakdown of the resistance of air resulting in an electric arc, can cause substantial damage, fire, injury, or loss of life. Professionals involved in the design, operation, or maintenance of electric power systems require thorough and up-to-date knowledge of arc flash safety and prevention methods. Arc Flash Hazard Analysis and Mitigation is the most comprehensive reference guide available on all aspects of arc flash hazard calculations, protective current technologies, and worker safety in electrical environments. Detailed chapters cover protective relaying, unit protection systems, arc-resistant equipment, arc flash analyses in DC systems, and many more critical topics.
Now in its second edition, this industry-standard resource contains fully revised material throughout, including a new chapter on calculation procedures conforming to the latest IEEE Guide 1584. Updated methodology and equations are complemented by new practical examples and case studies. Expanded topics include risk assessment, electrode configuration, the impact of system grounding, electrical safety in workplaces, and short-circuit currents. Written by a leading authority with more than three decades' experience conducting power system analyses, this invaluable guide:
* Provides the latest methodologies for flash arc hazard analysis as well practical mitigation techniques, fully aligned with the updated IEEE Guide for Performing Arc-Flash Hazard Calculations
* Explores an inclusive range of current technologies and strategies for arc flash mitigation
* Covers calculations of short-circuits, protective relaying, and varied electrical system configurations in industrial power systems
* Addresses differential relays, arc flash sensing relays, protective relaying coordination, current transformer operation and saturation, and more
* Includes review questions and references at the end of each chapter
Part of the market-leading IEEE Series on Power Engineering, the second edition of Arc Flash Hazard Analysis and Mitigation remains essential reading for all electrical engineers and consulting engineers.
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J.C. DAS, PHD, is President and Principal of Power System Studies, Inc. He is the former Head of Power System Analysis at Amec Foster Wheeler, where he served for thirty years. He is specialist in conducting power system studies, including short-circuit, load flow, harmonics, stability, arc-flash hazard, grounding, switching transients, and protective relaying. He is the author 70 technical publications, hundreds of study reports for real-world power systems, and several books, including Power System Harmonics and Passive Filter Designs and Understanding Symmetrical Components for Power System Modeling. Mr. Das is a member of the IEEE Industry Applications and IEEE Power Engineering societies, a Fellow of Institution of Engineering Technology, and recipient of the IEEE Meritorious Award in Engineering.
Content
Foreword xix
Preface to Second Edition xxi
Preface to First Edition xxiii
Acknowledgement xxv
About the Author xxvii
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 5
1.5 Arc Blast 6
1.6 Electrical Shock Hazard 9
1.6.1 Resistance of Human Body 11
1.7 Fire Hazard 13
1.8 Arc Flash Hazard Analysis 15
1.8.1 Ralph Lee's and NFPA Equations 17
1.8.2 IEEE 1584 Guide Equations 17
1.9 Personal Protective Equipment 21
1.10 Hazard Boundaries 23
1.10.1 Working Distance 24
1.10.2 Arc Flash Labels 24
1.11 Maximum Duration of an Arc Flash Event and Arc Flash Boundary 25
1.11.1 Arc Flash Hazard with Equipment Doors Closed 25
1.12 Reasons for Internal Arcing Faults 27
1.13 Arc Flash Hazard Calculation Steps 28
1.13.1 NFPA Table 130.7(C)(15)(a) 29
1.14 Examples of Calculations 30
1.15 Reducing Arc Flash Hazard 33
1.15.1 Reduction 34
1.15.2 Arc Flash Labels 37
Review Questions 38
References 38
2 Safety and Prevention Through Design: A New Frontier 41
2.1 Electrical Standards and Codes 42
2.2 Prevention through Design 44
2.3 Limitations of Existing Codes, Regulations, and Standards 45
2.4 Electrical Hazards 46
2.5 Changing the Safety Culture 49
2.6 Risk Analysis for Critical Operation Power Systems 49
2.6.1 Existing Systems 50
2.6.2 New Facilities 50
2.7 Reliability Analysis 51
2.7.1 Data for Reliability Evaluations 52
2.7.2 Methods of Evaluation 53
2.7.3 Reliability and Safety 53
2.8 Maintenance and Operation 54
2.8.1 Maintenance Strategies 55
2.8.2 Reliability-Centered Maintenance (RCM) 56
2.9 Safety Integrity Level and Safety Instrumented System 56
2.10 Electrical Safety in the Workplaces 58
2.10.1 Risk Assessment 58
2.10.2 Responsibility 58
2.10.3 Risk Parameters 58
2.11 Risk Reduction 61
2.12 Risk Evaluation 62
2.13 Risk Reduction Verification 63
2.14 Risk Control 63
Review Questions 64
References 64
3 Calculations According To IEEE Guide 1584, 2018 68
3.1 Model for Incident Energy Calculations 68
3.2 Electrode Configuration 69
3.3 Impact of System Grounding 69
3.4 Intermediate Average Arcing Current 70
3.5 Arcing Current Variation Factor 71
3.6 Calculation of Intermediate Incident Energy 73
3.7 Intermediate Arc Flash Boundary (AFB) 75
3.8 Enclosure Size Correction Factor 77
3.8.1 Shallow and Typical Enclosures 77
3.9 Determine Equivalent Height and Width 77
3.10 Determine Enclosure Size Correction Factor 77
3.11 Determination of Iarc, E, and AFB (600 V < Voc = 15,000 V) 78
3.11.1 Arcing Current 78
3.11.2 Incident Energy (E) 78
3.11.3 Arc Flash Boundary (AFB) 79
3.12 Determination of Iarc, E, and AFB (Voc = 600 V) 80
3.12.1 Arcing Current 80
3.12.2 Incident Energy 80
3.12.3 Arc Flash Boundary (AFB) 80
3.13 A Flow Chart for the Calculations 80
3.14 Examples of Calculations 81
References 82
4 Arc Flash Hazard and System Grounding 84
4.1 System and Equipment Grounding 84
4.1.1 Solidly Grounded Systems 85
4.2 Low Resistance Grounding 89
4.3 High Resistance Grounded Systems 89
4.3.1 Fault Detection, Alarms, and Isolation 92
4.4 Ungrounded Systems 96
4.5 Reactance Grounding 97
4.6 Resonant Grounding 97
4.7 Corner of Delta-Grounded Systems 97
4.8 Surge Arresters 98
4.9 Artificially Derived Neutrals 99
4.10 Multiple Grounded Systems 102
4.10.1 Comparison of Grounding Systems 102
4.11 Arc Flash Hazard in Solidly Grounded Systems 102
4.12 Protection and Coordination in Solidly Grounded Systems 107
4.12.1 Self-Extinguishing Ground Faults 110
4.12.2 Improving Coordination in Solidly Grounded Low Voltage Systems 113
4.13 Ground Fault Coordination in Low Resistance Grounded Medium Voltage Systems 116
4.13.1 Remote Tripping 119
4.13.2 Ground Fault Protection of Industrial Bus-Connected Generators 119
4.13.3 Directional Ground Fault Relays 124
4.14 Monitoring of Grounding Resistors 125
4.15 Selection of Grounding Systems 126
Review Questions 127
References 128
5 Short-Circuit Calculations According To ANSI/IEEE Standards For Arc Flash Analysis 130
5.1 Types of Calculations 131
5.1.1 Assumptions: Short-Circuit Calculations 131
5.1.2 Short-Circuit Currents for Arc Flash Calculations 132
5.2 Rating Structure of HV Circuit Breakers 132
5.3 Low-Voltage Motors 135
5.4 Rotating Machine Model 136
5.5 Calculation Methods 136
5.5.1 Simplified Method X/R = 17 136
> 17 137
5.5.3 E/Z Method for AC and DC Decrement Adjustments 137
5.6 Network Reduction 140
5.7 Calculation Procedure 140
5.7.1 Analytical Calculation Procedure 141
5.8 Capacitor and Static Converter Contributions to Short-Circuit Currents 143
5.9 Typical Computer-Based Calculation Results 143
5.9.1 First-Cycle or Momentary Duty Calculations 143
5.9.2 Interrupting Duty Calculations 146
5.9.3 Low Voltage Circuit Breaker Duty Calculations 146
5.10 Examples of Calculations 146
5.10.1 Calculation of Short-Circuit Duties 152
5.10.2 K-Rated 15 kV Circuit Breakers 152
5.10.3 4.16-kV Circuit Breakers and Motor Starters 157
5.10.4 Transformer Primary Switches and Fused Switches 157
5.10.5 Low Voltage Circuit Breakers 161
5.11 Thirty-Cycle Short-Circuit Currents 161
5.12 Unsymmetrical Short-Circuit Currents 162
5.12.1 Single Line-to-Ground Fault 163
5.12.2 Double Line-to-Ground Fault 165
5.12.3 Line-to-Line Fault 168
5.13 Computer Methods 171
5.13.1 Line-to-Ground Fault 172
5.13.2 Line-to-Line Fault 173
5.13.3 Double Line-to-Ground Fault 173
5.14 Short-Circuit Currents for Arc Flash Calculations 175
Review Questions 176
References 176
6 Accounting For Decaying Short-Circuit Currents In Arc Flash Calculations 178
6.1 Short Circuit of a Passive Element 178
6.2 Systems with No AC Decay 181
6.3 Reactances of a Synchronous Machine 182
6.3.1 Leakage Reactance 182
6.3.2 Subtransient Reactance 183
6.3.3 Transient Reactance 183
6.3.4 Synchronous Reactance 183
6.3.5 Quadrature-Axis Reactances 183
6.3.6 Negative Sequence Reactance 184
6.3.7 Zero Sequence Reactance 184
6.4 Saturation of Reactances 184
6.5 Time Constants of Synchronous Machines 184
6.5.1 Open-Circuit Time Constant 184
6.5.2 Subtransient Short-Circuit Time Constant 184
6.5.3 Transient Short-Circuit Time Constant 185
6.5.4 Armature Time Constant 185
6.6 Synchronous Machine Behavior on Terminal Short Circuit 185
6.6.1 Equivalent Circuits during Fault 186
6.6.2 Fault Decrement Curve 190
6.7 Short Circuit of Synchronous Motors and Condensers 194
6.8 Short Circuit of Induction Motors 194
6.9 A New Algorithm for Arc Flash Calculations with Decaying Short-Circuit Currents 197
6.9.1 Available Computer-Based Calculations 198
6.9.2 Accumulation of Energy from Multiple Sources 198
6.9.3 Comparative Calculations 200
6.10 Crowbar Methods 203
Review Questions 204
References 205
7 Protective Relaying 206
7.1 Protection and Coordination from Arc Flash Considerations 206
7.2 Classification of Relay Types 210
7.3 Design Criteria of Protective Systems 210
7.3.1 Selectivity 211
7.3.2 Speed 211
7.3.3 Reliability 211
7.3.4 Backup Protection 212
7.4 Overcurrent Protection 212
7.4.1 Overcurrent Relays 213
7.4.2 Multifunction Overcurrent Relays 215
7.4.3 IEC Curves 217
7.5 Low Voltage Circuit Breakers 219
7.5.1 Molded Case Circuit Breakers (MCCBs) 219
7.5.2 Current-Limiting MCCBs 225
7.5.3 Insulated Case Circuit Breakers (ICCBs) 227
7.5.4 Low Voltage Power Circuit Breakers (LVPCBs) 228
7.5.5 Short-Time Bands of LVPCBs Trip Programmers 230
7.6 Short-Circuit Ratings of Low Voltage Circuit Breakers 231
7.6.1 Single-Pole Interrupting Capability 235
7.6.2 Short-Time Ratings 235
7.7 Series-Connected Ratings 236
7.8 Fuses 237
7.8.1 Current-Limiting Fuses 238
7.8.2 Low Voltage Fuses 240
7.8.3 High Voltage Fuses 240
7.8.4 Electronic Fuses 241
7.8.5 Interrupting Ratings 242
7.9 Application of Fuses for Arc Flash Reduction 243
7.9.1 Low Voltage Motor Starters 243
7.9.2 Medium Voltage Motor Starters 243
7.9.3 Low Voltage Switchgear 244
7.10 Conductor Protection 247
7.10.1 Load Current Carrying Capabilities of Conductors 248
7.10.2 Conductor Terminations 249
7.10.3 Considerations of Voltage Drops 249
7.10.4 Short-Circuit Considerations 249
7.10.5 Overcurrent Protection of Conductors 251
7.11 Motor Protection 252
7.11.1 Coordination with Motor Thermal Damage Curve 253
7.12 Generator 51-V Protection 261
7.12.1 Arc Flash Considerations 262
Review Questions 265
References 265
8 Unit Protection Systems 267
8.1 Overlapping the Zones of Protection 269
8.2 Importance of Differential Systems for Arc Flash Reduction 271
8.3 Bus Differential Schemes 272
8.3.1 Overcurrent Differential Protection 272
8.3.2 Partial Differential Schemes 273
8.3.3 Percent Differential Relays 273
8.4 High Impedance Differential Relays 274
8.4.1 Sensitivity for Internal Faults 277
8.4.2 High Impedance Microprocessor-Based Multifunction Relays 278
8.5 Low Impedance Current Differential Relays 278
8.5.1 CT Saturation 282
8.5.2 Comparison with High Impedance Relays 282
8.6 Electromechanical Transformer Differential Relays 283
8.6.1 Harmonic Restraint 285
8.7 Microprocessor-Based Transformer Differential Relays 286
8.7.1 CT Connections and Phase Angle Compensation 287
8.7.2 Dynamic CT Ratio Corrections 290
8.7.3 Security under Transformer Magnetizing Currents 293
8.8 Pilot Wire Protection 294
8.9 Modern Line Current Differential Protection 296
8.9.1 The Alpha Plane 297
8.9.2 Enhanced Current Differential Characteristics 299
8.10 Examples of Arc Flash Reduction with Differential Relays 300
Review Questions 303
References 303
9 Arc Fault Detection Relays 305
9.1 Principle of Operation 306
9.2 Light Intensity 306
9.3 Light Sensor Types 307
9.4 Other Hardware 312
9.5 Selective Tripping 313
9.6 Supervision with Current Elements 315
9.7 Applications 315
9.7.1 Medium Voltage Systems 315
9.7.2 Low Voltage Circuit Breakers 317
9.7.3 Self-Testing of Sensors 317
9.8 Examples of Calculation 317
9.9 Arc Vault(TM) Protection for Low Voltage Systems 317
9.9.1 Detection System 321
Review Questions 323
References 323
10 Overcurrent Coordination 325
10.1 Standards and Requirements 326
10.2 Data for the Coordination Study 326
10.3 Computer-Based Coordination 328
10.4 Initial Analysis 328
10.5 Coordinating Time Interval 329
10.5.1 Relay Overtravel 329
10.6 Fundamental Considerations for Coordination 329
10.6.1 Settings on Bends of Time-Current Coordination Curves 331
10.7 Coordination on Instantaneous Basis 331
10.7.1 Selectivity between Two Series-Connected Current-Limiting Fuses 333
10.7.2 Selectivity of a Current-Limiting Fuse Downstream of Noncurrent-Limiting Circuit Breaker 333
10.7.3 Selectivity of Current-Limiting Devices in Series 337
10.8 NEC Requirements of Selectivity 340
10.8.1 Fully Selective Systems 342
10.8.2 Selection of Equipment Ratings and Trip Devices 343
10.9 The Art of Compromise 346
Review Questions 356
References 357
11 Transformer Protection 358
11.1 NEC Requirements 358
11.2 Arc Flash Considerations 360
11.3 System Configurations of Transformer Connections 361
11.3.1 Auto-Transfer of Bus Loads 366
11.4 Through Fault Current Withstand Capability 366
11.4.1 Category I 367
11.4.2 Category II 367
11.4.3 Category III and IV 367
11.4.4 Observation on Faults during Life Expectancy of a Transformer 369
11.4.5 Dry-Type Transformers 370
11.5 Constructing the through Fault Curve Analytically 374
11.5.1 Protection with Respect to Through Fault Curves 374
11.6 Transformer Primary Fuse Protection 375
11.6.1 Variations in the Fuse Characteristics 375
11.6.2 Single Phasing and Ferroresonance 377
11.6.3 Other Considerations of Fuse Protection 377
11.7 Overcurrent Relays for Transformer Primary Protection 377
11.8 Listing Requirements 379
11.9 Effect of Transformer Winding Connections 383
11.10 Requirements of Ground Fault Protection 385
11.11 Through Fault Protection 385
11.11.1 Primary Fuse Protection 385
11.11.2 Primary Relay Protection 387
11.12 Overall Transformer Protection 387
11.13 A Practical Study for Arc Flash Reduction 388
11.13.1 System Configuration 388
11.13.2 Coordination Study and Observations 388
11.13.3 Arc Flash Calculations: High Hazard Risk Category (HRC) Levels 393
11.13.4 Reducing HRC Levels with Main Secondary Circuit Breakers 395
11.13.5 Maintenance Mode Switches on Low Voltage Trip Programmers 395
11.13.6 Addition of Secondary Relay 401
Review Questions 404
References 405
12 Current Transformers 406
12.1 Accuracy Classification of CTs 407
12.1.1 Metering Accuracies 407
12.1.2 Relaying Accuracies 407
12.1.3 Relaying Accuracy Classification X 408
12.1.4 Accuracy Classification T 409
12.2 Constructional Features of CTs 409
12.3 Secondary Terminal Voltage Rating 411
12.3.1 Saturation Voltage 412
12.3.2 Saturation Factor 412
12.4 CT Ratio and Phase Angle Errors 412
12.5 Interrelation of CT Ratio and C Class Accuracy 415
12.6 Polarity of Instrument Transformers 417
12.7 Application Considerations 418
12.7.1 Select CT Ratio 418
12.7.2 Make a Single-Line Diagram of the CT Connections 420
12.7.3 CT Burden 420
12.7.4 Short-Circuit Currents and Asymmetry 420
12.7.5 Calculate Steady-State Performance 420
12.7.6 Calculate Steady-State Errors 421
12.8 Series and Parallel Connections of CTs 425
12.9 Transient Performance of the CTs 425
12.9.1 CT Saturation Calculations 426
12.9.2 Effect of Remanence 427
12.10 Practicality of Application 428
12.11 CTs for Low Resistance-Grounded Medium Voltage Systems 430
12.12 Future Directions 430
Review Questions 433
References 433
13 Arc-Resistant Equipment 435
13.1 Calculations of Arc Flash Hazard in Arc-Resistant Equipment 436
13.1.1 Probability of Arcing Fault 436
13.2 Qualifications in IEEE Guide 437
13.3 Accessibility Types 438
13.3.1 Type 1 438
13.3.2 Type 2 438
13.3.3 Suffix B 438
13.3.4 Suffix C 438
13.3.5 Suffix D 439
13.4 IEC Accessibility Types 439
13.5 Arc-Resistant Ratings 440
13.5.1 Duration Ratings 440
13.5.2 Device-Limited Ratings 441
13.5.3 Effect of Cable Connections 444
13.6 Testing According to IEEE Guide 444
13.6.1 Criterion 1 444
13.6.2 Criterion 2 445
13.6.3 Criterion 3 445
13.6.4 Criterion 4 445
13.6.5 Criterion 5 445
13.6.6 Maintenance 446
13.7 Pressure Relief 446
13.8 Venting and Plenums 448
13.8.1 Venting into Surrounding Area 448
13.8.2 Plenums 450
13.9 Cable Entries 450
Review Questions 452
References 452
14 Recent Trends and Innovations 454
14.1 Statistical Data of Arc Flash Hazards 454
14.2 Zone-Selective Interlocking 456
14.2.1 Low Voltage ZSI Systems 456
14.2.2 Zone Interlocking in Medium Voltage Systems 463
14.3 Microprocessor-Based Low Voltage Switchgear 466
14.3.1 Microprocessor-Based Switchgear Concept 466
14.3.2 Accounting for Motor Contributions 467
14.3.3 Faults on the Source Side 469
14.3.4 Arc Flash Hazard Reduction 470
14.4 Low Voltage Motor Control Centers 470
14.4.1 Desirable MCC Design Features 471
14.4.2 Recent Design Improvements 471
14.4.3 Higher Short-Circuit Withstand MCCs 478
14.5 Maintenance Mode Switch 478
14.6 Infrared Windows and Sight Glasses 480
14.7 Fault Current Limiters 483
14.8 Partial Discharge Measurements 487
14.8.1 Online versus Offline Measurements 488
14.8.2 Test Methods 489
14.8.3 Current Signature Analysis: Rotating Machines 491
14.8.4 Dissipation Factor Tip-Up 491
Review Questions 493
References 494
15 Arc Flash Hazard Calculations In Dc Systems 496
15.1 Calculations of the Short-Circuit Currents in DC Systems 497
15.2 Sources of DC Short-Circuit Currents 497
15.3 IEC Calculation Procedures 498
15.4 Short Circuit of a Lead Acid Battery 501
15.5 Short Circuit of DC Motors and Generators 505
15.6 Short-Circuit Current of a Rectifier 510
15.7 Short Circuit of a Charged Capacitor 515
15.8 Total Short-Circuit Current 516
15.9 DC Circuit Breakers and Fuses 517
15.9.1 DC Circuit Breakers 517
15.9.2 DC Rated Fuses 520
15.10 Arcing in DC Systems 520
15.11 Equations for Calculation of Incident Energy in DC Systems 525
15.12 Protection of the Semiconductor Devices 527
15.12.1 Controlled Converters 529
Review Questions 530
References 531
16 Application of Ethernet and IEC 61850 Communications 533
16.1 IEC 61850 Protocol 534
16.2 Modern IEDs 535
16.3 Substation Architecture 536
16.4 IEC 61850 Communication Structure 537
16.5 Logical Nodes 539
16.6 Ethernet Connection 539
16.7 Networking Media 543
16.7.1 Copper Twisted Shielded and Unshielded 543
16.7.2 Fiber Optic Cable 544
16.8 Network Topologies 545
16.8.1 Prioritizing GOOSE Messages 547
16.8.2 Technoeconomical Justifications 547
16.9 Application to Arc Flash Relaying and Communications 549
Review Questions 549
References 549
Appendix A Statistics and Probability Applied to Electrical Engineering 551
A.1 Mean Mode and Median 551
A.2 Mean and Standard Deviation 552
A.3 Skewness and Kurtosis 553
A.4 Normal or Gaussian Distribution 554
A.5 Curve Fitting: Least Square Line 556
References 559
Appendix B Tables for Quick Estimation of Incident Energy and PPE in Electrical Systems 560
Index 588
1
ARC FLASH HAZARDS AND THEIR ANALYSES
In the past, industrial electrical systems in the United States have been designed considering prevalent standards, that is, ANSI/IEEE, NEC, OSHA, UL, NESC, and the like, and arc ?ash hazard was not a direct consideration for the electrical system designs. This environment is changing fast, and the industry is heading toward innovations in the electrical systems designs, equipment, and protection to limit the arc ?ash hazard, as it is detrimental to the worker safety. This opens another chapter of the power system design, analysis, and calculations hitherto not required. There is a spate of technical literature and papers on arc ?ash hazard, its calculation and mitigation. References [1-8] describe arcing phenomena and arc ?ash calculations, sometimes commenting on the methodology of arc ?ash hazard calculations in IEEE Guide 1584 [9] (see Chapter 3).
These issues have become of great importance in the power system planning, designs and protective relay applications. "Safety by Design" is the new frontier (see Chapter 2).
Awareness of the various hazards caused by arc ?ash has increased signi?cantly over the past decade. Arc ?ash is a dangerous condition associated with the unexpected release of tremendous amount of energy caused by an electric arc within electrical equipment [10]. This release is in the form of intense light, heat, sound, and blast of arc products that may consist of vaporized components of enclosure material-copper, steel, or aluminum. Intense sound and pressure waves also emanate from the arc ?ash, which resembles a con?ned explosion. Arcing occurs when the insulation between the live conductors breaks down, due to aging, surface tracking, treeing phenomena, and due to human error when maintaining electrical equipment in the energized state. The insulation systems are not perfectly homogeneous and voids form due to thermal cycling. In nonself restoring insulations, treeing phenomena starts with a discharge in a cavity, which enlarges over a period of time, and the discharge patterns resemble tree branches, hence the name "treeing" (Figure 1.1). As the treeing progresses, discharge activity increases, and, ultimately the insulation resistance may be suf?ciently weakened and breakdown occurs under electrical stress. Treeing phenomena is of particular importance in XLPE (cross-linked polyethylene) and nonself restoring insulations. Surface tracking occurs due to abrasion, irregularities, contamination, and moisture, which may lead to an arc formation between the line and ground. An example will be a contaminated insulator under humid conditions. Though online monitoring and partial discharge measurements are being applied as diagnostic tools, the randomness associated with a fault and insulation breakdown are well recognized, and a breakdown can occur at any time, jeopardizing the safety of a worker, who may be in close proximity of the energized equipment. Arc temperatures are of the order of 35,000°F, about four times the temperature on the surface of the sun. An arc ?ash can therefore cause serious fatal burns.
Figure 1.1. Treeing phenomena in nonself-restoring insulation, leading to ultimate breakdown of insulation.
1.1 ELECTRICAL ARCS
Electrical arcing signi?es the passage of current through what has previously been air. It is initiated by ?ashover or introduction of some conductive material. The current passage is through ionized air and the vapor of the arc terminal material, which has substantially higher resistance than the solid material. This creates a voltage drop in the arc depending upon the arc length and system voltage. The current path is resistive in nature, yielding unity power factor. Voltage drop in a large solid or stranded conductor is of the order of 0.016-0.033 V/cm, very much lower than the voltage drop in an arc, which can be of the order of the order of 5-10 V/cm of arc length for virtually all arcs in open air (Chapter 3). For low voltage circuits, the arc length consumes a substantial portion of the available voltage. For high voltages, the arc lengths can be considerably greater, before the system impedance tries to regulate or limit the fault current. The arc voltage drop and the source voltage drop are in quadrature. The length of arc in high voltage systems can be greater and readily bridge the gap from energized parts to ground.
Under some circumstances, it is possible to generate a higher energy arc from a low voltage system, as compared with a high voltage system.
In a bolted three-phase short circuit, the arcing resistance is zero, and there is no arcing, and no arc ?ash hazard. Sometimes, when short circuit occurs, it can be converted into a three-phase bolted short circuit by closing a making switch or circuit breaker, which solidly connects the three-phases. The fault current is then interrupted by appropriate relaying. This method, however, will subject the system to much greater short-circuit stresses and equipment damage, and, is, therefore, not recommended.
1.1.1 Arc as a Heat Source
The electrical arc is recognized as high-level heat source. The temperatures at the metal terminals are high, reliably reported to be 20,000 K (35,000°F). The special types of arcs can reach 50,000 K (about 90,000°F). The only higher temperature source known on earth is the laser, which can produce 100,000 K. The intermediate (plasma) part of the arc, that is, the portion away from the terminals, is reported as having a temperature of 13,000 K.
In a bolted three-phase fault, there is no arc, so little heat will be generated. If there is some resistance at the fault point, temperature could rise to the melting and boiling point of the metal, and an arc could be started. The longer the arc becomes, the more of the system voltage it consumes. Consequently, less voltage is available to overcome supply impedance and the total current decreases.
Human body can exist only in a narrow temperature range that is close to normal blood temperature, around 97.7°F. Studies show that at skin temperature as low as 44°C (110°F), the body temperature equilibrium starts breaking down in about 6 hours. Cell damage can occur beyond 6 hours. At 158°F, only a 1-second duration is required to cause total cell destruction.
1.1.2 Arcing Phenomena in a Cubicle
The arc formation in a cubicle may be described in four phases:
- Phase 1: Compression. The volume of air is overheated due to release of energy, and the remaining volume of air inside the cubicle heats up due to convection and radiation.
- Phase 2: Expansion. A piece of equipment may blow apart to create an opening through which superheated air begins to escape. The pressure reaches its maximum value and then decreases with the release of hot air and arc products.
- Phase 3: Emission. The arcing continues and the superheated air is forced out with almost constant overpressure.
Figure 1.2. The various stages of pressure buildup and its release for an arc in a cubicle. A: Compression, pressure rises; B: Expansion, relief of pressure; C: Emission, gases exhausted; D: Thermal, pressure equalizes (not to scale).
- Phase 4: Thermal. After the release of air, the temperature inside the switchgear nears that of an electrical arc. This lasts till the arc is quenched. All metals and insulating materials undergo erosion, may melt and expand many times, produce toxic fumes, and spray of molten metal.
Figure 1.2 shows these four phases.
1.2 ARC FLASH HAZARD AND PERSONAL SAFETY
The phenomenal progress made by the electrical and electronic industry since Thomas Edison propounded the principle of incandescent lighting in 1897 has sometimes been achieved at the cost of loss of human lives and disabilities. Although reference to electrical safety can be found as early as about 1888, it was only in 1982 that Ralph Lee [11] correlated arc ?ash and body burns with short-circuit currents. This article is considered by many as pioneering work on arcing phenomena in the open air. It quanti?ed the potential burn hazards. Lee established the curable burn threshold for the human body as 1.2 cal/cm2, which is currently used to de?ne the arc ?ash boundary. Lee published a second article in 1987, "Pressure Developed from Arcs" [12].
Doughty et al., published two articles [13, 14], and Jones et al. published an article in 2000 [15]. The IEEE 1584 Guide can be considered a breakthrough for arc ?ash analyses. The previous methods in NFPA 70E were based upon theoretical concepts or drawn from limited testing. The new testing concentrated on arcing faults in a variety of electrical equipment enclosures, arcs in boxes, which is more typical of actual work locations. Yet some researchers are critical of the methodology of the IEEE 1584 Guide; for example, Stokes and Sweeting in "Electrical Arc Burn Hazards" [5], critique Lee's models and IEEE 1584 Guide equations and testing setup for arc ?ash burns. Yet the statistics collected on the prevention of arc ?ash hazard injuries shows that such injuries were prevented when the workers used the required personal protective equipment (PPE) calculated according to the IEEE Guide; see Chapter 3. Wilkins et...
