1、ELECTRICALINSTRUMENTS INHAZARDOUSLOCATIONS4th EditionBy Ernest C. MagisonMagison-front-2006update.fm Page i Friday, November 17, 2006 10:50 AMCopyright 1998 by International Society of Automation 67 Alexander DriveP.O. Box 12277 Research Triangle Park, NC 27709All rights reserved. Printed in the Uni
2、ted States of America. 10 9 8 7 6 5 4 3 2 ISBN 10: 0-9792343-1-X ISBN 13: 978-0-9792343-1-6No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permiss
3、ion of the publisher.Library of Congress Cataloging-in-Publication Data - In ProgressMagison-front-2006update.fm Page ii Friday, November 17, 2006 10:50 AMACKNOWLEDGMENTSThe first edition of this book was based largely on work done by ISA RP12.The author drew freely on concepts, attitudes, and concl
4、usions developedduring committee deliberations. Although specific references were citedwhenever possible, the most important contributions could not be soacknowledged because the patterns of thought, ideas, and concepts weredeveloped by the committee during its many years of activity.The committees
5、program of study, teaching, and standards writingdemanded substantial personal commitment from many individuals andcontinued support by their sponsoring organizations. Without that com-mitment there would have been no basis for this book.In recent years my sources of information and understanding ha
6、ve multi-plied. Some who have read an earlier edition have sent me copies of theirown works so that my knowledge base would be expanded. Most havebeen recognized by reference in the bibliographical references at the endof each chapter. It has now become almost impossible to identify tomyself, much l
7、ess credit in print, all those who in some way have contrib-uted to my understanding of the subjects covered in this book. I can onlyacknowledge the contributions of many dedicated colleagues in nationaland international committees on which I have been fortunate enough tohave served and learned.I ow
8、e thanks to Bill Lawrence for helping me make Chapter 3 more cur-rent and to Dave Bishop, Tom Dubaniewicz, and other members of ISASP12.21, who made many helpful suggestions about Chapter 12.As I get older, however, I am ever more aware of the contribution thatFred Maltby made to the work of ISA in
9、the field of safe use of electricalequipment in hazardous locations. Fred insisted that his committee followthese steps: Understand the technical background of the subject of the stan-dard. Agree on what the standard should say. If this disagrees with otherstandards, establish a plan to educate peop
10、le and change the erro-neous standard, if at all possible. Write the standard.Unfortunately, many committees have not been trained to act in this fash-ion and leap to the pen or word processor before they understand theirsubject.Thanks, Fred!Mag-frontmatter.fm Page iii Thursday, December 14, 2006 12
11、:10 PMMag-frontmatter.fm Page iv Thursday, December 14, 2006 12:10 PMTABLE OF CONTENTSPREFACE xiCHAPTER 1 HISTORICAL BACKGROUND AND PERSPECTIVE 1Growing Interest in the Safety of Electrical Instrument Installations, 1The Role of Underwriting and Standards Developing Organizations, 3Occupational Safe
12、ty and Health Administration, 12Progress in Electrical Safety Standards Development, 14References, 14CHAPTER 2 COMBUSTION AND EXPLOSION FUNDAMENTALS 17Some Underlying Theory, 17Ignition By a Point Source, 21Ignition Energy and Flame Velocity, 28Lower and Upper Explosive Limits, 29Most Easily Ignited
13、 Concentration, 31Minimum Ignition Energy, 32Effect of Test Conditions, 32Effect of Changing Inert Gases, 38Electrode Geometry, 39Time Scale of the Ignition Process, 40References, 41CHAPTER 3 CLASSIFICATION OF HAZARDOUS LOCATIONS AND COMBUSTIBLE MATERIALS 43Why Area Classification?, 43The North Amer
14、ican System, 43Classes and Groups of Materials, 44Determination of Area Classification Relative to Class I Hazards, 49Considerations in Classification of Class I Locations, 51Some Guides for Class I Area Classification, 55Special Cases of Area Classification in Instrument Systems, 58Area and Materia
15、l Classification Outside the United States, 60Material Classification in the United StatesTutorial Discussion and Opinion, 75References, 94Magison book.book Page v Thursday, December 14, 2006 12:03 PMvi Table of ContentsCHAPTER 4 PRACTICE AND PRINCIPLES OF HAZARD REDUCTION PRACTICE 99United States,
16、99Canada, 102Countries Following IEC or CENELEC Standards, 102The Types of Protection, 103Principles, 108References, 134CHAPTER 5 EXPLOSIONPROOF ENCLOSURES 137Why an Explosionproof Enclosure Works, 137Influence of Enclosure Design and Test Procedure, 139Pressure Piling, 153Correlation Between MESG a
17、nd Spark Ignition Energy, 157Design CriteriaIntroduction, 159References, 176CHAPTER 6 REDUCTION OF HAZARD BY PRESSURIZATION 183Recognition in the National Electrical Code, 183The Standardization of Pressurization, 184Classification of Pressurization Systems, 186NFPA 496 Requirements for Pressurizati
18、on Systems, 187Additional Requirements for Pressurized Enclosures in Class I Locations, 192Requirements for Pressurized Enclosures in Class II Locations, 193Requirements for Pressurized Control Rooms, 193Pressurization of Enclosures with Internal Source of ReleaseContinuous Dilution, 194CENELEC EN50
19、016, 204Installation of Pressurization Systems, 211References, 215CHAPTER 7 ENCAPSULATION, SEALING, AND IMMERSION 217Oil and Sand Immersion, 217Sealing, 219Defining Safety Requirements for Sealed Devices, 229Standards for Sealed Devices, 235Encapsulation or Potting, 244References, 247CHAPTER 8 INCRE
20、ASED SAFETY, TYPE OF PROTECTION e 251Construction Requirements, 253Creepage and Clearances, 254References, 263Magison book.book Page vi Thursday, December 14, 2006 12:03 PMTable of Contents viiCHAPTER 9 IGNITION OF GASES AND VAPORS BY ELECTRICAL MEANS 265Characteristics of Electric Arcs, 267Arcing a
21、t Closing Contacts and in Capacitive Circuits, 274Typical Test Equipment for Capacitive Circuits, 278Precautions in Testing, 281Typical Ignition Test Results, 284Effect of Resistance in Discharge Path, 286Effects of Electrode Material and Geometry, 294Opening Contacts in Inductive and Resistive Circ
22、uits, 294Test Equipment for Break-Spark Ignition Measurements, 296Influence of Contact Material, 299Effect of Contact Separation Speed, 306Ignition of Materials Other than Methane, 307Probability of IgnitionInfluence of Current and Voltage Level, 311Effect of Shunt Elements on Ignition Current, 315H
23、ot Wire Ignition, 324References, 335CHAPTER 10 INTRINSICALLY SAFE AND NONINCENDIVE SYSTEMS 341Historical Review, 341Interpreting the Definition, 345Specific Requirements for Intrinsically Safe Systems, 349Requirements for All Intrinsically Safe Apparatus, 354Requirements for Features and Components
24、on which Intrinsic Safety Depends, 359Requirements for Infallible (Protective) Components, Infallible Assemblies, and Infallible Connections, 367How Safe is Safe?, 374Installation of Intrinsically Safe Systems, 394Inspection of Intrinsically Safe Systems, 400Maintenance of Intrinsically Safe Systems
25、, 404Nonincendive Equipment and Wiring, 406References, 410Magison book.book Page vii Thursday, December 14, 2006 12:03 PMviii Table of ContentsCHAPTER 11 DESIGN AND EVALUATION OF INTRINSICALLY SAFE APPARATUS, INTRINSICALLY SAFE SYSTEMS, AND NONINCENDIVE SYSTEMS 415Design and Evaluation of Intrinsica
26、lly Safe Apparatus, 415Power Supplies, 428Inductors with Ferromagnetic Cores, 433Zener Diode Track Layout, 461Design of Intrinsically Safe Systems, 469References, 489CHAPTER 12 IGNITION BY OPTICAL SOURCES 493Overview, 493Ignition by Radiating Sources, 497The Nature of the Hazard, 507Standards Action
27、s to Address Hazards of Radiation - Opinions of the Author, 516Need for Further Study and Research, 517References, 518CHAPTER 13 DUST HAZARDS 521Nature of the Dust Hazard, 521Influence of Chemical Composition of the Dust, 523Influence of Shape, Size, and Concentration, 527Chemical Composition of the
28、 Suspending Medium, 529Distinguishing Features of Dust Hazards, 531Area Classification, 533Electrical Apparatus For Dusty Locations, 538Ways to Reduce Hazard, 541IEC and CENELEC, 541References, 544CHAPTER 14 HUMAN SAFETY 547Introduction, 547Effects of Electrical Shock, 547The Body as a Circuit Eleme
29、nt, 550What is a Safe Level of Circuit Voltage?, 553Conclusion, 554References, 554CHAPTER 15 DEGREE OF PROTECTION BY ENCLOSURES 555The NEMA System, 555IEC 529, 558Correlation of NEMA and IP Codes, 562References, 562Magison book.book Page viii Thursday, December 14, 2006 12:03 PMTable of Contents ixA
30、PPENDIX A DERIVATION OF INITIAL PURGE VOLUME REQUIREMENT 563APPENDIX B EXAMPLES OF DERIVATIONS 567Derivation of an Equation for Loss of Pressure in a Sealed System, 567Derivation of Expressions for Seal Breathing, 569Derivation of Expressions for Pressure Testing Sealed Enclosures, 572Derivation of
31、Flow Test Equation, 574INDEX 575Magison book.book Page ix Thursday, December 14, 2006 12:03 PMMagison book.book Page x Thursday, December 14, 2006 12:03 PMPREFACEThe first edition of this book went to press in 1966 just as ISA-RP12.2,“Intrinsically Safe and Non-Incendive Electrical Instruments,” was
32、 pub-lished. ISA-RP12.2 was the product of 15 years of intensive effort byInstrument Society of America (ISA) Committee RP12 (now SP12). Duringthat period, the committee, as individuals and as a whole, researchedinformation pertinent to the engineering of safe electrical systems; formu-lated, debate
33、d and reformulated the requirements of a safe system; andtook the lead to educate the American instrument industry about funda-mentals of electrical hazards and safe practices.The prominence of ISA RP12 in matters of electrical safety was evident inthe first edition of this book in the subjects cove
34、red, in the philosophy ofsafety that was presented, and in the many references to publications byRP12 members and to papers by other experts delivered at ISA-sponsoredsymposia.Most of the first edition was written as a text for the first ISA Short Coursein Electrical Safety, sponsored by the Wilming
35、ton Section in 1964. The firstedition came at the end of a period during which the American instrumentindustry and others concerned with safe electrical systems learned that thesafety problems of electrical installations in hazardous atmospheres yieldto the same analytical approaches that produce so
36、lutions to other engi-neering problems.When the first edition was published, intrinsic safety had been accepted asa powerful technique for safety. Many people concerned with safety hadaccepted the essential concept that safety can be engineered into a systemby applying a relatively few fundamental p
37、rinciplessafety by engineer-ing and design rather than by emotion and decree.Between the first and second editions there was a period of building onthe foundation prepared by SP12. This period saw activity in electricalsafety on a much broader scale. Both the National Fire Protection Associa-tion (N
38、FPA) and the U. S. Coast Guard published requirements forintrinsically safe systems that incorporated the principles of ISA RP12.2. Inaddition, Factory Mutual, Underwriters Laboratories, and Canadian Stan-dards Association (CSA) listed many intrinsically safe systems. Intrinsicsafety became a common
39、 tool of the American instrument industry.Adopting intrinsic safety as a working tool was only one highly visiblemanifestation of increased sophistication about electrical safety. Manyorganizations and individuals began to reevaluate past practices, trying toMagison book.book Page xi Thursday, Decem
40、ber 14, 2006 12:03 PMxii Prefaceascertain the facts and proposing new and different solutions to safetyproblems.With growing sophistication about electrical safety also came increasedawareness that work done in other countries, some of which predatedAmerican activity by decades, is pertinent to our
41、problems. Because thenature of the problem and the validity of the solution do not depend onthe language in which they are stated, parochial, chauvinistic attitudeswere supplanted by a real desire to learn from the work and experience ofthe rest of the world. An ever-growing number of individuals an
42、d organi-zations became committed to establishing standards of safe practice thatare acceptable worldwide.The second edition recognized the maturation processes of that period.There was less evangelical fervor, though no less enthusiasm for a subjectthat still had many unknowns and many problems to
43、challenge the techni-cally or politically inclined. Because the basic principles had been sold thetone of the second edition was, therefore, somewhat more matter of factand tutorial.During the period between the second and third editions important, andnot altogether welcome, changes occurred in the
44、field of electrical safety. Itwas a period when safety standards and regulations often became grist forbureaucratic mills. In all industrialized countries the achievement of safetybecame more a legislated, regulated endeavor and less an engineeringundertaking. The influence of the Occupational Safet
45、y and Health Act,and similar legislation, in the United States was matched by parallel regu-lations in other countries.The general statements of principle supplied by RP12.2 no longer sufficedin an environment where third-party certifications became the acceptednorm. Certifications, whether by gover
46、nmental or non-governmental bod-ies, demand ever-increasing detail in standards so that compliance ornoncompliance can easily be demonstrated by reference to the writtenword, unencumbered by the necessity for making professional judgments.Many design engineers and inspectors also want the solace of
47、unequivocalrules that need no interpretation.The trend toward more comprehensive and detailed standards was exac-erbated by movements toward harmonization of standards to make safetystandards the same in all countries and marketplaces. Harmonization ishealthy, as anyone employed by a manufacturer or
48、 user with plants ormarkets in many countries can attest. But, as one attempts to include in asingle document all the requirements of different jurisdictions, it becomesessential to add text to recognize historical national practices. More detailis needed also to enhance uniform interpretation among
49、 a larger, moreheterogeneous group of design and certifying engineers.Magison book.book Page xii Thursday, December 14, 2006 12:03 PMPreface xiiiIt was even more necessary to focus on fundamental principles in the thirdedition than it had been 15 years earlier. As standards give more attentionto detail it becomes much easier to forget fundamentals and to continuallyadd more restrictive requirements “for the sake of safety.”Although the third edition recognized these changes in the environmentin which issues of safety were addressed, like the first and second edi-tions, it was ded
