Page 12 - Handbook of Electrical Engineering

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12.8.3 Negative phase sequence 344
12.8.4 Core balance earth fault 345
12.8.5 Stalling current 345
12.8.6 Limitation to the number of successive starts 345
12.9 Low Voltage Static Load Protection 345
12.9.1 Time-delayed overcurrent 346
12.9.2 Instantaneous or high-set overcurrent 346
12.9.3 Core balance earth fault 346
12.10 Mathematical Equations for Representing Standard, Very and Extremely
Inverse Relays 346
References 349

13 Earthing and Screening 351
13.1 Purpose of Earthing 351
13.1.1 Electric shock 351
13.1.2 Damage to equipment 353
13.1.3 Zero reference potential 353
13.2 Site Locations 353
13.2.1 Steel structures 354
13.2.2 Land-based plants 354
13.2.3 Concrete and brick-built structures 356
13.3 Design of Earthing Systems 356
13.3.1 High voltage systems 356
13.3.2 Low voltage three-phase systems 357
13.3.3 IEC types of earthing systems 360
13.3.4 Earth loop impedance 365
13.3.5 Earthing rods and grids 367
13.4 Construction Details Relating to Earthing 371
13.4.1 Frames, casings and cubicle steelwork 371
13.4.2 Screwed and clearance hole entries 371
13.4.3 Earthing only one end of a cable 372
13.5 Screening and Earthing of Cables used in Electronic Circuits 373
13.5.1 Capacitance and inductance mechanisms 373
13.5.2 Screening against external interference 374
13.5.3 Earthing of screens 379
13.5.4 Screening of high frequencies 380
13.5.5 Power earths, cubicle and clean earths 381
References 383

14 Variable Speed Electrical Drivers 385
14.1 Introduction 385
14.1.1 Environment 386
14.1.2 Power supply 386
14.1.3 Economics 387
14.2 Group 1 Methods 388
14.2.1 Simple variable voltage supplies 388
14.2.2 Pole-changing of the stator winding 389