Electrical Safety: a guide to causes and prevention of hazards
This book explains the hazards associated with electricity in its many forms, including electromagnetic radiation. It describes methods of reducing risks to health and to the environment, giving rules and codes of practice to be followed. Guidelines are also given for the use of electrical equipment in specialised environments (such as locations subject to explosive gases and flammable dusts), the guarding of machine tools and the control of earth currents. Human safety and care for the environment are of increasing concern and the broad scope of the book makes it essential reading for those involved in engineering and technology at all levels; no specialised knowledge of electro-technology is presumed. It is a reference book for personnel responsible for their company's safety policy and for municipal authorities (particularly in Commonwealth countries) and other bodies concerned with technical training, industrial development and planning.
Inspec keywords: electromagnetic waves; electrical safety; installation; explosives; flames; hazardous areas; health and safety
Other keywords: safety policy; specialised environments; electrical safety; human safety; earth current control; electrical hazard prevention; electromagnetic radiation; electrical equipment; industrial development; health risks reduction; technical training; industrial planning; explosive gas; flammable dusts; machine tools; municipal authority
Subjects: Plant engineering, maintenance and safety; Manufacturing facilities; Health Physics; Electrical contracting and installation; Health and safety aspects; Electromagnetic compatibility and interference
- Book DOI: 10.1049/PBPO019E
- Chapter DOI: 10.1049/PBPO019E
- ISBN: 9780852968062
- e-ISBN: 9781849194341
- Page count: 204
- Format: PDF
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Front Matter
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1 Physiological effects of electric current
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This chapter discusses the physiological effects of electric current and shock hazards.
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2 Electric power systems
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This chapter discusses low voltage overload and short-circuit protection.
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3 The philosophies of earthing
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This chapter discusses earthing and electrical apparatus protection.
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4 Cables and fires
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The title of this chapter is not meant to imply that cables are a major cause of fires, but rather that the organic materials used to insulate and protect the conductors contribute to the severity of fires in buildings, tunnels, ships etc. In the United Kingdom the classic cable types have been paper-insulated lead-covered single-wire armoured (PILCSWA) cables for underground power distribution and vulcanised India rubber (VIR -or vulcanised rubber insulated, VRI) wiring for internal distribution. The sulphur used in vulcanising the rubber attacks copper so that it is necessary for rubber-insulated copper conductors to be tinned.
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5 Electrical equipment for use in explosive atmospheres
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With the wider use of natural gas and of petroleum accidental ignitions of accumulations of flammable gases and vapours are a major industrial hazard. The mode of combustion is usually explosive and often results in injury, death and widespread destruction. In today's complex society the only wholly safe procedure when gases and vapours are released to the atmosphere is to ensure that they are immediately ignited. This may sound anomalous but the safest way to vent a flammable gas is to burn it off at the point of release. A flare stack works on this principle. A processing furnace using an oxygen-free atmosphere and gas such as hydrogen for instance can be protected from dangerous release of gas by a so-called flame curtain consisting of small flame jets round the furnace portals.
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6 Protection by flameproof enclosure
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The principle of flameproof protection is to place electrical equipment in an enclosure which does not need to be sealed but which will not ignite a surrounding explosive gas if the same explosive mixture is ignited within the enclosure. A flameproof enclosure is therefore in effect a type of pressure vessel in which all openings and running clearances have been shown by test to be reliable flame traps.
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7 Protection by intrinsic safety
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In conclusion, it will be seen that protection by the method of intrinsic safety has grown rapidly due to its eminent suitability for very low voltage, low power process control and information transfer installations. It differs from other types of protection for electrical apparatus in potentially explosive atmospheres because the flow of energy from the safe area must be carefully controlled. Its growth has been greatly accelerated by the development of barrier devices which protect the hazardous area from the possible intrusion of dangerous sources of energy from the safe area parts of the system.
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8 Electrical apparatus in areas subject to flammable dusts
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This chapter discusses electrical apparatus in areas subject to flammable dusts.
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9 Design, workmanship and maintenance
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The failure or breakdown of electrical equipment can sometimes be blamed on faulty design or poor workmanship. Often, but not necessarily more often, it is the result of inadequate maintenance. This chapter discusses the effects of faulty design.
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10 Stored energy
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Stored energy is not hazardous until it is released or transformed. This applies in a mechanical context, as when falling out of a window, in an electrical context, as when being struck by lightning and in a chemical context, as when stepping on a land-mine. In each of these examples the potential energy is discharged very rapidly. Batteries are devices for storing chemical energy which can be released at a controlled rate in the form of an electric current. The maximum rate of release - when the terminals are short-circuited - will be limited by the internal resistance of the battery. The internal resistance of a battery is thus one of its three most important characteristics, the other two being its maximum stored energy/unit volume and maximum stored energy/unit weight. These parameters apply to both primary (non-rechargeable or throw-away) devices and secondary (rechargeable) devices.
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11 Electric welding
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There are two ways in which electricity is used for welding; by an arc and by resistance. Both depend on heat generated by an electric current. The welder must always protect him or herself with suitable personal protective equipment PPE as a defence against some of the above hazards. This equipment should include safety boots, apron, gauntlets, head, face and eye protection. It is also very advisable to keep ones overalls buttoned at the neck.
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12 Lightning phenomena and protection
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This title can be summarised by the letters ESD, for lightning with all its power and complex behaviour is no more than an electrostatic discharge. The awesome crack of a lightning stroke close by is no different in principle from the faint tick which accompanies a spark from the finger to the filing cabinet. Both are the shock waves from the sudden thermal expansion of air produced by a static spark.
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13 Coping with static
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Electrostatics is the study of electrical phenomena that take place when there are no moving charges. Static, the commonly used term for both charges and discharges, has many uses such as in printing and copying, dust precipitation, micro-electronics and in sprayed paint and powder coating. Electrostatic discharges (ESD) present many problems particularly with regard to ignition hazards, ranging from macro-scale disasters in grain silos and supertankers, to the explosion of the lungs of hospital patients while under anaesthetic. In addition, ESD costs the micro electronics industry untold millions.
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14 Electromagnetic radiation
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The more one thinks about this subject the more puzzling it becomes. Work at a distance, with nothing between the transmitter and the receiver is as mind boggling as the square root of minus one. Electromagnetic radiation concerns not only electrical apparatus. The warmth from a coal fire as much as that from an electric fire and the light from a candle as much as that from a torch are all electromagnetic radiation. It is only the frequencies which are different from radio transmission, but whatever the frequency or distance, the speed of propagation through space is the same. One of the most fundamental constants of the universe is thus the speed of light. An electric charge per se produces an electrostatic field but no magnetic field. When the charge moves, as for example in an electric current, a magnetic field is produced round the current vector.
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15 Earth currents and their effects
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As is well known, the direct current motor was invented many years before the alternating current induction motor. The d.c. motor goes back to the discovery by Faraday in 1821 of electromagnetic induction and rotation. His work was rapidly followed during the next few decades by a great variety of designs and patents for both motors and dynamos. Many of these incorporated novel and complex ideas. One of the early machines designed by Professors Elihu Thomson and Edwin J. Houston of Philadelphia included a brush-rocking device controlled by the load current. The commutator had only three segments and, to prevent damage to the surface, the sparks were blown out by synchronised blasts of air from an integral rotary vane pump as the gaps between the segments passed under the brushes.
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Back Matter
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