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Fig. 11.3. Fencing-off overhead lines on construction sites. meet the requirements of the relevant parts of the ESQCR to ensure compliance with the EAW Regulations and the HSW Act. The contractor is also under the obligation of section 6(1)(c) of the HSW Act to make available adequate information about any conditions necessary to ensure safety and should, therefore, advise the occupier, usually the main contractor, of the need for barriers to fence-off the overhead lines as shown in Fig. 11.3 and keep the civil engineering mobile plant at a safe distance from them. See also HSE Guidance Note GS6, Avoidance of danger from overhead electrical lines. ELECTRIC SHOCK Electrical Safety 281 The number of electrocutions (i.e. deaths from electric shock) per annum of people at work is, fortunately, small and has never yet attained three figures, but electric shock accidents are common and are frequently accompanied by burn injuries at the point(s) of contact.
282 Handbook of Electrical Installation Practice Shock parameters Electric shock data have been compiled by a number of researchers who have experimented mainly on animals. Their results are not entirely satisfactory because of the difficulty of relating animal responses to humans. Some work, up to ‘let go’ current severities, has been carried out on human volunteers and has been useful but there are still unexplored areas and the data is incomplete. What follows, therefore, is based on present knowledge and logical probability. The body is a non-linear resistor with an inverse characteristic with respect to the applied voltage.The resistance varies also with sex, state of health and between individuals. Most of the resistance is in the skin. Under dry conditions, point contacts and safety extra low voltages, the resistance may be thousands of ohms and there may be no shock sensation but a wet skin considerably reduces the resistance and a slight shock may be felt if sufficient current flows between the contact points. The shock sensation is frequency dependent and is at a maximum at about the mains frequency of 50Hz and diminishes at higher or lower frequencies. The shock sensation disappears at frequencies exceeding about 1.0MHz and also on a ripple-free d.c. However, contact with a d.c. conductor produces an initial shock as the current rises from zero to a steady-state value and another when contact is broken and the current again drops to zero. Any sudden change in current produces a shock sensation, so shocks are experienced from pulsating or oscillating currents but not from a pure d.c. and not anomalously from very high frequency a.c. There is a considerable difference in response to body currents between individuals but in general and for mains frequency, i.e. 50Hz in the UK, the responses to sustained currents are as follows: 0.5 to 2.0mA is the threshold of perception with no deleterious effect or unpleasant sensation. Over 2.0mA the shock sensation increases with increases in current from a mild to a painful shock and at about 9.0 mA for a man and 6.0mA for a woman, it begins to become difficult to break contact, due to partial muscular paralysis. At 20mA and over, muscular paralysis inhibits the ability to ‘let go’ and the victim is liable to become unconscious through paralysis of the motor nerves. He may stop breathing and/or his heart may stop beating. At currents in excess of about 50mA ventricular fibrillation, i.e. uncoordinated operation of the heart valves, may occur and the victim’s chances of survival are poor as this condition needs immediate specialised treatment. Time is the other factor in electric shock danger. It is known that considerable shock currents can be tolerated for a very short time and it is possible to produce safety curves of shock currents plotted against time. These will be found in BS PD 6519: 1988, Guide to the effects of current passing through the human body, and are used by the designers of rcds to determine the performance characteristics of their products. Harmless momentary shocks, however, can cause accidents because of the involuntary reaction of the victim; for example, a momentary shock can cause the victim to fall off a ladder and injure himself. Rescue To render assistance to a person undergoing an electric shock, a rescuer should first interrupt the circuit by switching off the supply or pulling out the plug. If he
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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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INTRODUCTION CHAPTER 7 Ground Earth
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INTRODUCTION CHAPTER 8 Cathodic Pro
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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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