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Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

1 National Electrical Safety Code Overview

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Rule 230. Clearances, General. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Rule 231. Clearances of Supporting Structures From Other Objects. . . . . . . . . . . . . . . . . . . . . 5

Figure 1-3. Clearances from railroad tracks. Rule 231C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Rule 232. Vertical Clearances of Wires, Conductors, and Equipment Above Ground,

Roadway, Rail, or Water Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Rule 233. Clearances Between Wires, Conductors, and Cables Carried on

Different Supporting Structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Rule 234. Clearance of Wires, Conductors, Cables, and Equipment from Buildings,

Bridges, Rail Cars, Swimming Pools, and Other Installations . . . . . . . . . . . . . . . . . . . . . . . . . 23

Rule 235. Clearance for Wires, Conductors, or Cables Carried on the

Same Supporting Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Rule 236. Climbing Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Rule 237. Working Space. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Rule 238. Vertical Clearance Between Certain Communications and

Supply Facilities Located on the Same Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Rule 239. Clearance of Vertical and Lateral Facilities From Other Facilities and

Surfaces On the Same Supporting Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Application of the NESC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

2 Pole Line Design

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Route Selection and Control Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

NESC Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Conductor Loading Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Conductor Tensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Ruling Spans. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Conductor Sag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Weight Span (Load Span) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Negative Load Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Wind Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Horizontal Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Total Bending Moment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Vertical Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

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Extreme Wind Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Anchors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Slack Spans (Reduced Tension Spans) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Pole Foundations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Selection of Pole Top Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Pole Line Design Parting Thoughts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

3 Wood, Steel, and Concrete Poles

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Wood Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Wood Pole Specifications, Codes, Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Steel Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

Concrete Poles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

4 Fiber Optic Cable

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

Basics of Optical Fiber Communications Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138

Fiber Optic Transmission Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

Fiber Optic Cable Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

Overall Design and Construction Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

5 Street Lighting

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

Factors Contributing to Roadway Design Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

Roadway Lighting Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

Roadway Lighting Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

State Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

Example of Roadway Lighting System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

Power Supply and Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

Street Lighting Voltage Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Voltage Drop Example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

Lighting System Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186Closing Thoughts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

6 Line Protection

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

Distribution Transformer Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

Fuse-Fuse Coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

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Reclosers and Breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

Recloser Recloser Coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

Sectionalizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

Recloser Sectionalizer Fuse Coordination. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

Switches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221

Surge Arresters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222

7 System Grounding

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232

Types of Grounding Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232

Earth as a Grounding Medium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

The Grounding Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

Pole Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

Grounding of System Neutral. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Pole Grounding for Line Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246

Improving System Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247

Substation Grounding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

Electric Shock and the Human Being . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249

8 Capacitors

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

Power Factor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

Capacitor Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260Capacitor Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262

Distribution Line Capacitor Banks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

Design of Line Capacitor Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

Substation Capacitor Banks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

9 Protective Relaying

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

Fault Current Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298

Transformer Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301Relay Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

Instrument Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

Overcurrent Relaying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

Zone/Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343

Differential Relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349

Miscellaneous Relaying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

Other Transformer Protection Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 360

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368

Basics of Distribution Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368

SCADA Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369

Fault Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373

Power Quality. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

Distribution Sensing, Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378

Distribution Automation Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381

Communication Systems for Distribution Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387

Automation Software. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391

Benefits of Distribution Automation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393

Environmental Conditions to Consider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394

11 Underground DistributionIntroduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402

Overhead vs. Underground. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402

Engineering URD Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405

Code Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

Design of URD Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

Installation of URD Facilities Some Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 442

Operating the URD System Some Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445

12 Transformer ConnectionsIntroduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452

Transformer Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452

Paralleling Single-Phase Distribution Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455

Phase Rotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

Ferroresonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

Single-Phase Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470

Considerations for Polyphase Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

Common Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

Banks With Two Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485

13 Metering

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490

Basic Construction and Operation of an Induction Watthour Meter . . . . . . . . . . . . . . . . . . 490

Demand Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

Reactive Power Metering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502


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Standard Metering Installations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 510

Special Metering Installations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 513

Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 514

Automated Meter Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 516

Safety Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 519

14 Dispersed Generation

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524

Distribution Generation Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 524

Commonly Applied Distributed Resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 525

New and Emerging Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533

Switching and Protection Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534

Overview of the DG Utility Interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 536

Requirements of IEEE 1547 Standard for Interconnecting Distributed Resourceswith Electric Power Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539

Glossary of Interconnection Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549

DG Interconnection Standards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 551

15 Engineering Economics

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554

Cost Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554

Cost Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 556

Introduction to Engineering Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557Time Diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561

Inflation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568

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12

18. For uncontrolled water flow areas, the surface area shall be that enclosed by its annual high-water mark.

Clearances shall be based on the normal flood level; if available, the 10-year flood level may be assumed as the

normal flood level.

19. The clearance over rivers, streams, and canals shall be based upon the largest surface area of any 1-mile-long

segment that includes the crossing. The clearance over a canal, river, or stream normally used to provide access for

sailboats to a larger body of water shall be the same as that required for the larger body of water.20. Where an overwater obstruction restricts vessel height to less than the applicable reference height given in

Table 232-3 in the NESC, the required clearance may be reduced by the difference between the reference height

and the overwater obstruction height, except that the reduced clearance shall not be less than that required for the

surface area on the line-crossing side of the obstruction.

21. Where the US Army Corps of Engineers, or the state, or surrogate thereof has issued a crossing permit, clearances

of that permit shall govern.

22. See Rule 234I for the required horizontal and diagonal clearances to rail cars.

23. For the purpose of this Rule, trucks are defined as any vehicle exceeding 8 feet in height. Areas not subject to truck

traffic are areas where truck traffic is not normally encountered nor reasonably anticipated.

24. Communication cables and conductors may have a clearance of 15 feet where poles are back of curbs or other

deterrents to vehicular traffic.

25. The clearance values shown in this table are computed by adding the applicable Mechanical and Electrical (M&E)

value of Table A-1 to the applicable Reference Component of Table A-2a of Appendix A in the NESC.

26. When designing a line to accommodate oversized vehicles, these clearance values shall be increased by the

difference between the known height of the oversized vehicle and 14 feet.

Vertical Clearances of Service Conductors

Shown below is a summary diagram of the most commonly required vertical clearances for

service conductors, limited to 300 V to ground.

Vertical clearances shall not be less than those shown in Figure 1-4, and shall be applied asdescribed earlier in Rule 232A. Application. The clearances shown here are as designed and are

not clearances under ambient conditions.

Figure 2.4. Vertical clearances of service conductors.

Street

Residentialdriveway

Not less than16.5 ft (open wire)16.0 ft (multiplex)

See Table 1-2.Not less than

16.5 ft (open wire)16.0 ft (multiplex)

See Table 1-2.Not less than

12.5 ft (open wire)12.0 ft (multiplex)

Accessible areapedestrians only

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70

Figure 2-3. Point of maximum sag and low point can be different.

Horizontal distance:

and

Vertical distance:

Where:

X = horizontal distance to low point in sag from the lower support, ft

Y = vertical distance to low point in sag from the lower support, ft

L = span length, ft

H = difference in attachment elevations, ft (if higher, the value is positive

and if lower, it is negative)S = conductor sag in question (any sag), ft

Note: If the value of (1 H/4S) is negative, the low point in sag (theoretical) occurs beyond the

lower support and thus not in the given span. In this case, the low point in sag is at the

attachment point of the conductor at the lower support.

Anatomy of a Span

Figure 2-4 depicts the important terminology associated with components of conductor sag.

These components are used constantly when designing overhead utility lines.

Y = S 1 H

4S

2

X = 1 L2

H4S

Low point on sag

S

H

Y

L

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76

Wind Loading on the Conductors

The horizontal loading due to the wind force on a conductor is defined by this equation:

FWC = WH LW OFT

Where:FWC = horizontal load due to wind force on each conductor, lbs

WH = horizontal wind factor, lbs/ft

LW = wind span, ft

OFT = overload factor for transverse wind as required by NESC Table 253-2.

Wind Loading on the Structure (Pole)

For manual calculations, the wind loading on the structure (pole) is to be applied to that

portion of the pole above the support level, see Figure 2-8. For an unguyed pole the support

level is the ground line and for a guyed pole the support level is defined as the top guy level.

The wind loading on that defined pole section is found by using the following formula:

FWS = WPH DA L OFT

Where:

FWS = wind load on structure (pole), lbs

WPH = horizontal wind pressure for given loading conditions, lbs/ft2

DA = average diameter of pole (above pole support level) ft

L = length of pole section above pole support level, ft

OFT = overload factor for transverse wind as required by NESC Table 253-2.

Figure 2-8. Wind loading on a structure.

Guyed pole support level

Unguyed pole support leve

L (guyed) (guyed)

L (unguyed

(unguyed

WS (unguyed

FW (guyed)

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122

Figure 3-8. Prestressed spun concrete pole.

Here are brief descriptions of the major components that make up a prestressed concrete pole:

Prestressed Strands

Prestressed strands are the main strength members of a concrete pole. The number of

prestressed strands, their size and location depends on the manufacturer and the class and/or

strength requirement of the pole. For ease of drilling, the location of these prestressed strands

can be specified. The prestressed strands are loaded to a specific tension to insure the concrete

stays in compression. Strands are generally located symmetrically, which makes it fairly easy to

avoid them when drilling. Prestressed strands are made of high strength carbon steel and can

only be cut with a torch. Due to their importance in the overall strength of the pole, they

should never be drilled, cut, or left exposed. Poles with this type of damage should be removed

from service.

Spiral Wrap

Spiral wrap wires are smaller than the prestressed strands and are made of mild steel. They arewrapped helically around the prestressed strands for the full length of the concrete pole.

Although they provide some strength to the concrete pole, cutting the spiral wrap wires for

boltholes does not generally have a significant effect on the strength of the pole.

Non-Tensioned Reinforcement

Non-tensioned reinforcement is nothing more than mild steel rebar. It is often used at the

ground line to provide extra strength in this area. It is placed beside the prestressed strand and

normally will not be located where it might be cut.

Concrete cover

Concrete cover

Spiral wire isc oser toget er

at each en

Longitudinal

Longitudinal wire(if required)

Prestressed str

Prestressed s

Spiral wire

Spiral wire

Non-tensionedreinforcement(if required

on-tens one ren orcem(if required) Seam line

s requ re

5 8 in. minimum

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150

Figure 4-8. Typical hardware for optical ground wire (OPGW). Courtesy: Preformed Line Products.

Figure 4-9. Typical hardware for ADSS cable. Courtesy: Preformed Line Products.

Air flow spoiler Dielectricdamper

ADSScoronaTM

coil

Downleadcushion &mountingaccessories

Splice casewithin steelballistic shield

Dielectricdeadends

or litetension

deadend

Cable abrasionprotector

In spanstorage system

Verticalcablestorage

Closure orsplice case

Dielectricsupport,aluminum

support, orlite support

Dielectricsuspension or

aluminumsuspension

Air flow spoiler

Spiral vibration damper

Support

Downleadcushion & lattice

tower clamp

Formed wire deadend

OR

Suspension

Cushion clamp

OR

U-Bolt deadend

Defender& vertical

cable storage

Splicecase/

closure

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180

Figure 5-6. Street lighting example.

From Table 5-5, we determine the roadway surface classification as R1. The residential area in

question is classified as a low pedestrian conflict area as listed in Table 5-3. From Table 5-6 we

see that the average maintained illuminance value (footcandles) for a major roadway in a low

pedestrian conflict residential area is 0.6 footcandles.

To calculate the average maintained footcandles, we assume the Lamp Depreciation Factor to

be 90% and the Luminaire Dirt Depreciation to be 80%. Using Figure 5-3 (American Electric

Roadway Luminaire) photometric data, the Coefficient of Utilization graph shows the CU to

be 43%, based on this calculation:

Street sideHouse side

CL of luminaire

Sidewalk

30'

Sidewalk

45'

39'6'

P2

P2

Points P1 and P2avg. maint. 0.6 FC

P1

P1

160.52'

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LINE PROTECTION

203

Figure 6-7. Inrush and damage curves.

Fuse-Fuse CoordinationFuse link coordination can be achieved by the use of TCC curves, coordination tables, or

industry established rules of thumb. When fuses are installed in series on a power system, the

down-line fuse is referred to as the protecting link and the up-line fuse is referred to as the

protected link. To achieve coordination, the protecting link should operate for faults in its zone

of protection without causing damage to the protected link.

0.01 0.6

0.02 1.2

0.03 1.8

0.04 2.4

0.05 3

0.06 3.60.07 4.20.08 4.80.09 5.40.1 6

0.2 12

0.3 18

0.4 24

0.5 30

0.6 360.7 420.8 480.9 54

1 60

2 120

3 180

4 240

5 300

6 3607 4208 4809 540

10 600

20 1200

30 1800

40 2400

50 3000

60 360070 420080 480090 5400

100 6000

0.5

0.6

0.7

0.80

.9 1 2 3 4 5 6 7 8 910

20

30

40

50

60

70

80

90

100

200

300

400

500

Current in Amperes: x 1 at 12.5 kV.

TimeinSeconds

TimeInCycles(60-Hz

Basis)

Damage CurveInrush Curve

Transformer Fuse

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248

Deep Driven Ground Rods

Using longer rods or sectional ground rods to drive the rod deeper into the earth usually

decreases the electrode resistance. In addition, with the ground electrode down further into the

earth it is more likely to exhibit lower levels of resistance due to water tables and constant soil

temperatures. It may also encounter soil with a lower resistivity. A rule of thumb is thatdoubling the length of the ground rod will lower the resistance by 40%.

Multiple Ground Rods

Paralleling multiple ground rods can achieve lower resistances by providing multiple paths for

the current to divide. However, paralleling two rods will not reduce the resistance by one-half

as is usually the case with parallel conductors. The reason for this is that the rods are not

widely separated from one another and the rods will mutually interact on each other through

their common resistances to remote earth. Separation of the rod grounds will greatly influence

the reduction in resistance. It is recommended that the rods be spaced from one another at a

distance of two times the length of the rods and tied together with a #6 or #4 copperconductor. If they are spaced in this manner, the reduction in resistance using two ground rods

is about 60%, three reduces it to 40%, and four reduces it to 33%.

Install Ground Away From the Pole

Ground rods installed adjacent to poles are typically in soil which was disturbed when the pole

was installed. This can result in a relatively high ground resistance. Installing the rod 2 to 3 feet

from the pole may reduce the resistance considerably. This is especially true when installing

ground rods for padmounted equipment.

Chemical TreatmentsIn some extreme cases, the soil surrounding the ground electrode must be chemically treated to

reduce the ground resistance. This method also helps to reduce the change in soil resistivity

during wet and dry seasons. The major disadvantages of this method are that it is expensive

and is not generally permanent (chemicals are washed away by rain). However, it may be the

only solution for areas with underlying rock layers preventing the use of deeper or even

multiple rods. The materials used for this purpose are generally referred to as Grounding

Enhancement Material or GEM, and this abbreviation is used in some trade names. Buried

electrodes are sometimes placed in Bentonite clay which has a favorable resistivity.

Substation GroundingThe function of a substation ground is to provide proper operation of electric equipment and

to provide personnel safety. These functions can be met by using the lowest practical resistance

between the circuit neutrals and the earth. Typically, a ground grid that is designed to meet the

safety standards for personnel will also be satisfactory for equipment operations.

The substation ground grid generally consists of driven ground rods tied together with buried

cables and equipment ground mats, which are all tied to the system neutral. Current flows into

the ground from lightning surges, ground faults. Switching surges can cause potential

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PROTECTIVE RELAYING

333

Figure 9-17. Line breaker with instantaneous relay.

As can be seen, the two are properly coordinated. However, instantaneous relaying can be

applied at breaker A to increase its sensitivity without mis-coordinating with breaker B. This is

accomplished by setting the instantaneous relay to see 80-85% of the distance to B. This can

be easily calculated as a ratio of fault current decrease to line length since this can be approxi-

mated as a linear relationship.

IInst = 8000 - [(8000 6000) 0.8] = 6,400 amps

The instantaneous relay can, for all practical purposes, be drawn as a vertical line at the 6,400

amps location. This time-overcurrent curve can be indicated with a dashed line for all currents

greater than the instantaneous setting.

Figure 9-17 has been redrawn in Figure 9-18 with a tap at point C that is protected by a fuse.

Source20 miles

80-85

Current

ime

6000 A 6400 A

6,400=

IF @ B IF @ A

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Figure 9-18. Radial line with fused tap.

As can be seen, for a fault with a magnitude of 7,200 amps on the tap at C just beyond the fuse,

the instantaneous trip setting on breaker A will operate faster than the fuse. The only way to

alleviate this problem is to set the instantaneous relay above 7,200 amps or to remove the

instantaneous relay. However, this greatly reduces the sensitivity of the breaker relay scheme to

high-magnitude, close-in faults.

Application of instantaneous relaying can be accomplished by utilizing a reclosing relay with

an instantaneous trip lockout feature. After a preset number of reclosers (usually one or two),

this device locks out the instantaneous relay from operation. Thus, breaker A can be set tooperate twice on instantaneous and then revert to time delay for two additional operations. As

a result, an intermittent (transient) fault on the tap at C can be cleared without the fuse

blowing. It is estimated that two-thirds of all faults are transient in nature, this results in a

considerable reduction in outage time for the consumer. If the fault is persistent, the instanta-

neous relay will be locked out and, under time delay at breaker A, the fuse will clear the fault

providing correct coordination.

For a persistent fault on the main feeder, the time delay of the final two operations can be

undesirable. However, due to the small percentage of persistent faults and, by liberal

application of fuses on all taps off the main feeder, this problem can be reduced to an

acceptable level.

Ground Overcurrent Relaying

Though all of the principles above are applicable to relaying for ground faults, ground fault

protection requires special consideration. The magnitude and detection of ground faults

depends upon transformer connection (delta or wye) and the presence of a ground impedance

in the case of a wye system. The following discussion will concentrate on the solidly grounded

wye system prevalent in most distribution systems.

C

6400 A

Tim

e

A

7200 A

ource

8000 A

IF @ B IF @ AIF @ C

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