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INTRODUCTION CHAPTER 9 Lightning Protection J. Sherlock (Technical Manager, Furse/Thomas & Betts) P. Woods (Marketing Manager, Furse/Thomas & Betts) Traditionally, the subject of lightning protection has developed in two directions: (1) Protecting structures (2) Protecting electronic systems. It will be appreciated that, although the topics are related, the sensitivity of the two elements to lightning strikes differs considerably. It is essential that there should be no confusion as to the precautions that need to be taken for each condition. For this reason this chapter has been divided into two stand-alone parts, which in some instances duplicate important data. PART 1. PROTECTION OF STRUCTURES Characteristics of lightning Lightning strokes are the visible discharge of static electricity accumulated in storm clouds created by meteorological conditions. Strokes may occur within the cloud, between clouds, or from the cloud base to earth or earthed structures. Lightning is formed as a result of a natural build-up of electrical charge separation in storm clouds. The base of a storm cloud is commonly 5–10km above the earth’s surface, with the cloud 12km high. The charge at the base of the cloud is usually negative and induces an equal and opposite charge on the earth’s surface and earthed objects beneath the cloud. Buildings of masonry, concrete and timber are sufficiently conductive to reach the same potential as the earth’s surface. The earth’s atmosphere contains drifting pockets of ionised air, which also take on a charge of opposite polarity to the cloud base, and the presence of such a pocket near the cloud can create a potential difference sufficient to cause electrical breakdown of the air, and a downward leader stroke develops from the cloud, having a current of a few hundred amperes.A step progression can then develop from pocket to pocket of the leader stroke towards the earth in a few microseconds until the tip of the leader is within a distance of 50–100m from some point on the earth. At this 197
198 Handbook of Electrical Installation Practice Fig. 9.1 Development of the downward stepped leader and upward streamer. stage the potential difference is sufficient to initiate an upward discharge or streamer from the earth or some object on it (Fig. 9.1). The distance at which this occurs is the striking distance of the stroke. The downward stepped leader and upward streamer rapidly advance until they meet, forming a continuous ionised path from cloud to earth, initiating the full visible discharge. The fast rise time and large peak amplitude of the stroke can produce severe mechanical effects. Long-duration currents can cause fire, while short-duration highcurrent peaks tend to tear or bend metal parts when they make contact. A typical current amplitude against time waveform is illustrated in Fig. 9.2. Current magnitudes can range from several kA up to 200kA. Fortunately super bolts of over 100kA are rare, the statistical average being 20kA. The core of the discharge may reach a temperature of 30000K causing explosive expansion of the air and the typical sound of thunder. The resulting shock wave close to this stroke readily dislodges tiles from roofs. The ‘rolling sphere’ concept Given the lightning process already described, it is logical to assume that a lightning strike terminates on the ground (or on structures) at the point where the upward streamer was originally launched.
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HANDBOOK OF ELECTRICAL INSTALLATION
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© 2003 by Blackwell Science Ltd, a
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vi Contents System operation 79 Ide
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viii Contents Training and systems
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x Contents 18 Lighting 475 h.r. kin
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Preface My first task as editor is
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INTRODUCTION CHAPTER 4 Cable Manage
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CHAPTER 5 Electricity on Constructi
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CHAPTER 6 Standby Power Supplies D.
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CHAPTER 10 Special Installations or
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CHAPTER 12 Standards, Specification
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CHAPTER 13 Distribution Transformer
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DEFINITIONS CHAPTER 14 Switchgear A
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Table 14.1 (contd) 374 Handbook of
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CHAPTER 15 Rotating Machines D.B. M
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CHAPTER 19 Mains Cables G.A. Bowie,
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APPENDIX I Main Authoritative Docum
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670 Index oil, 364, 366 oil filling
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672 Index earthing, 234 earth termi
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674 Index swimming pools, 252 switc
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