Practical Intrinsic Safety for Engineers and Technicians

Transcription

1 Practical Intrinsic Safety for Engineers and Technicians

2 THIS BOOK WAS DEVELOPED BY IDC TECHNOLOGIES WHO ARE WE? IDC Technologies is internationally acknowledged as the premier provider of practical, technical training for engineers and technicians. We specialize in the fields of electrical systems, industrial data communications, telecommunications, automation and control, mechanical engineering, chemical and civil engineering, and are continually adding to our portfolio of over 60 different workshops. Our instructors are highly respected in their fields of expertise and in the last ten years have trained over 200,000 engineers, scientists and technicians. With offices conveniently located worldwide, IDC Technologies has an enthusiastic team of professional engineers, technicians and support staff who are committed to providing the highest level of training and consultancy. TECHNICAL WORKSHOPS TRAINING THAT WORKS We deliver engineering and technology training that will maximize your business goals. In today s competitive environment, you require training that will help you and your organization to achieve its goals and produce a large return on investment. With our training that works objective you and your organization will: Get job-related skills that you need to achieve your business goals Improve the operation and design of your equipment and plant Improve your troubleshooting abilities Sharpen your competitive edge Boost morale and retain valuable staff Save time and money EXPERT INSTRUCTORS We search the world for good quality instructors who have three outstanding attributes: 1. Expert knowledge and experience of the course topic 2. Superb training abilities to ensure the know-how is transferred effectively and quickly to you in a practical, hands-on way 3. Listening skills they listen carefully to the needs of the participants and want to ensure that you benefit from the experience. Each and every instructor is evaluated by the delegates and we assess the presentation after every class to ensure that the instructor stays on track in presenting outstanding courses. HANDS-ON APPROACH TO TRAINING All IDC Technologies workshops include practical, hands-on sessions where the delegates are given the opportunity to apply in practice the theory they have learnt. REFERENCE MATERIALS A fully illustrated workshop book with hundreds of pages of tables, charts, figures and handy hints, plus considerable reference material is provided FREE of charge to each delegate. ACCREDITATION AND CONTINUING EDUCATION Satisfactory completion of all IDC workshops satisfies the requirements of the International Association for Continuing Education and Training for the award of 1.4 Continuing Education Units. IDC workshops also satisfy criteria for Continuing Professional Development according to the requirements of the Institution of Electrical Engineers and Institution of Measurement and Control in the UK, Institution of Engineers in Australia, Institution of Engineers New Zealand, and others.

3 CERTIFICATE OF ATTENDANCE Each delegate receives a Certificate of Attendance documenting their experience. 100% MONEY BACK GUARANTEE IDC Technologies engineers have put considerable time and experience into ensuring that you gain maximum value from each workshop. If by lunchtime on the first day you decide that the workshop is not appropriate for your requirements, please let us know so that we can arrange a 100% refund of your fee. ONSITE WORKSHOPS All IDC Technologies Training Workshops are available on an on-site basis, presented at the venue of your choice, saving delegates travel time and expenses, thus providing your company with even greater savings. OFFICE LOCATIONS AUSTRALIA CANADA INDIA IRELAND MALAYSIA NEW ZEALAND POLAND SINGAPORE SOUTH AFRICA UNITED KINGDOM UNITED STATES idc@idc-online.com Visit our website for FREE Pocket Guides IDC Technologies produce a set of 6 Pocket Guides used by thousands of engineers and technicians worldwide. Vol. 1 ELECTRONICS Vol. 4 INSTRUMENTATION Vol. 2 ELECTRICAL Vol. 5 FORMULAE & CONVERSIONS Vol. 3 COMMUNICATIONS Vol. 6 INDUSTRIAL AUTOMATION To download a FREE copy of these internationally best selling pocket guides go to: On-Site Training SAVE MORE THAN 50% OFF the per person cost CUSTOMISE the training to YOUR WORKPLACE! Have the training delivered WHEN AND WHERE you need it! All IDC Technologies Training Workshops are available on an on-site basis, presented at the venue of your choice, saving delegates travel time and expenses, thus providing your company with even greater savings. For more information or a FREE detailed proposal contact Kevin Baker by ing: training@idc-online.com

4 IDC TECHNOLOGIES Worldwide Offices AUSTRALIA Telephone: Facsimile: West Coast Office 1031 Wellington Street, West Perth, WA 6005 PO Box 1093, West Perth, WA 6872 East Coast Office PO Box 1750, North Sydney, NSW 2059 CANADA Toll Free Telephone: Toll Free Facsimile: Suite 402, 814 Richards Street, Vancouver, NC V6B 3A7 INDIA Telephone : th Street, Kumaran Colony, Vadapalani, Chennai IRELAND Telephone : Facsimile: Caoran, Baile na habhann, Co. Galway MALAYSIA Telephone: Facsimile: Jalan Kota Raja E27/E, Hicom Town Center Seksyen 27, Shah Alam, Selangor NEW ZEALAND Telephone: Facsimile: Parkview Towers, 28 Davies Avenue, Manukau City PO Box , Manukau City POLAND Telephone: Facsimile: ul. Krakowska 50, Balice, Krakow SINGAPORE Telephone: Facsimile: Eu Tong Sen Street, #04-11 Pearl s Centre, Singapore SOUTH AFRICA Telephone: or Facsimile: Pretorius Street, President Park, Midrand PO Box 389, Halfway House 1685 UNITED KINGDOM Telephone: Facsimile: Suite 18, Fitzroy House, Lynwood Drive, Worcester Park, Surrey KT4 7AT UNITED STATES Toll Free Telephone: Toll Free Facsimile: Will Clayton #6175, Humble, TX 77338, USA Website: idc@idc-online.com

5 Presents Practical Intrinsic Safety for Engineers and Technicians By Geoff Bottrill Revision 4 Website: idc@idc-online.com

6 IDC Technologies Pty Ltd PO Box 1093, West Perth, Western Australia 6872 Offices in Australia, New Zealand, Singapore, United Kingdom, Ireland, Malaysia, Poland, United States of America, Canada, South Africa and India Copyright IDC Technologies All rights reserved. ISBN First published 2007 All rights to this publication, associated software and workshop are reserved. No part of this publication 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 permission of the publisher. All enquiries should be made to the publisher at the address above. Disclaimer Whilst all reasonable care has been taken to ensure that the descriptions, opinions, programs, listings, software and diagrams are accurate and workable, IDC Technologies do not accept any legal responsibility or liability to any person, organization or other entity for any direct loss, consequential loss or damage, however caused, that may be suffered as a result of the use of this publication or the associated workshop and software. In case of any uncertainty, we recommend that you contact IDC Technologies for clarification or assistance. Trademarks All logos and trademarks belong to, and are copyrighted to, their companies respectively. Acknowledgements IDC Technologies expresses its sincere thanks to all those engineers and technicians on our training workshops who freely made available their expertise in preparing this manual.

7 Contents 1 Introduction General introduction to explosion protection Historical background to explosion protection Scope of protection systems Responsibility Explosion of protection and certification Legality The role of standards and certification 4 2 Definitions and Terminology Definitions and terminology Protection concepts Principles of ignition Electricity as a source of ignition Properties of flammable materials Other relevant properties of flammable materials Other properties affecting general safety Summary Classification system Classification concepts Area classification locations Summary of area classification Methods of explosion protection Classification of equipment for use in hazardous areas Certification Dusts 23 3 Principles of Intrinsic Safety Origins of intrinsic safety Principles of IS Electrical theory to explain IS Implementation of IS The shunt diode safety barrier Associated apparatus Electrical apparatus in the hazardous area Simple apparatus or certified apparatus Enclosures Temperature 60 4 Systems Concepts The IS systems concept An IS system System documentation Assessment of safety Simple apparatus 66

8 4.6 Safety parameters Temperature classification of systems Systems concepts in other standards Conclusion to systems 68 5 Types of Explosion Protection Methods of explosion protection Separation Construction Containment Electrical design Special Multiple certification Selection of certification method Perceived advantages of IS 82 6 Earthing Systems Earthing Personal safety Hazardous area considerations Earthing and bonding Static electricity Clean and dirty earthing Electrical Interference Earthing terminology Connection of earthing systems Power supply systems Portable equipment using batteries Earthing arrangement standard solutions Earth loops Computer earthing Surge protection systems Standards and codes of practice Installations Introduction to installation requirements Installation requirements IEC79-14: Standard contents Other relevant installation standards and codes Safety documentation General requirements of the standard Practical aspects of IS installations Other considerations affecting installation Other installation issues Inspection and Maintenance Inspection and maintenance Integrity preserved Scope of IEC General requirements 139

9 8.5 Inspections The insulation test Maintenance Testing Unauthorised modification Earthing integrity verification BS5345 inspection requirements Standards and Certification Standards and certification Definition of a standard Standards for IS The European ATEX directive The international standards organisation Certification Safety documentation Marking of apparatus Example discussed Fault Finding Fault finding Fault-finding routine Safety assessment of testing Test equipment Use of uncertified test apparatus Interface testing Certified apparatus IS apparatus repair procedure Applications 185 Applications Loop device selection Terminology 186 Status inputs Switch conventions Circuit faults Relay objects IS switch status Switch transfer using galvanic isolators Switch input criticality Proximity switches Intrinsic safety aspects of switches Summary 198 Analogue applications High-level signals The 4/20mA transmitter Operational characteristics Transmitter operational characteristics Smart transmitters Safety parameters 201

10 11.18 Marking Safety assessment 202 Transmitter loops with interfaces Single channel barriers Two channel barrier solutions Referencing common connections with barrier systems Safety considerations 208 Active barriers Active vs passive barriers 209 Analogue galvanic isolation Principles of operation Functional aspects Non-energy storing input 211 Loop applications Safety aspects 213 Applications concepts Power supply connection to interfaces Earth return of faults 220 Temperature measurement Thermocouples Resistance thermometry Two-wire high resistance elements RTDs with galvanic isolating interfaces Temperature convertors Temperature multiplexing Strain gauge bridges Isolators in combination 240 Channel function concept Power supply Isolator supply techniques Higher power techniques Higher power IS channel interfaces 244 The solenoid valve Safety characteristics Solenoid valve operation Operation of barriers with SVs Application techniques Operation of isolating drivers with SVs 247 Analogue outputs I/P convertors Operational detail Controller analogue output modules 248 Vibration monitoring Pulse signal transmission Conductivity and ph 250 Techniques for operating through hazardous areas Safety considerations of transfer across hazardous areas Data communications Communication techniques Combined voltage and current Multi-dropping 255 I/O systems Redundancy 257

11 HART systems HART monitoring systems 261 Fieldbus concepts with IS 261 Selection of apparatus and interfaces Determination of important criteria Barrier vs isolator philosophy Option selection 265 Appendix A IEC Series Standard Titles 267 Appendix B List of IS Standards and Codes of Practice by Country 269 Appendix C IEC79 Ex i Inspection Schedule 271 Appendix D CENELEC Members 273 Appendix E IP Code 275 Appendix F Standards Reference 277 Appendix G Other Reference Documents 279 Appendix H European and International Standards 281 Appendix I Practical Exercises 287

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13 1 Introduction 1.1 General introduction to explosion protection Intrinsic Safety (IS) is one of a number of explosion protection techniques. Explosion protection (or explosion proofing ) may be described as the specific treatment of industrial electrical equipment, such that, in the presence of a potentially explosive gas atmosphere and under prescribed conditions, it cannot cause ignition. Equipment so treated is generally referred to as (electrical) Apparatus. There are considered to be nine different categories or techniques that can be employed by the suppliers of electrical apparatus to meet the industrial needs of users. The purpose of electrical apparatus designed to minimise the risk of ignition is to allow industry to handle situations where electrical power and signals can be used in hazardous atmospheres. The risk posed is never considered to be totally eliminated as this would be impossible to guarantee. The risk must be lowered to levels that are deemed acceptable by the Law, and all parties interested in safety. It is the consequences of an industrial explosion that are potentially catastrophic and therefore cannot be ignored. 1.2 Historical background to explosion protection Explosion Protection concepts were developed in parallel and as a consequence of the mining industry. Coal has been mined for more than 1000 years. Large-scale mining was practised as early as the 18th century in many countries. Mine safety procedures operated from this time when the risks of fire-damp were beginning to be understood. The initial way of dealing with the problem was to send fire-men, younger people covered with wet sacking, down mine roads, clutching a long lighted taper. The purpose of this was to burn off any residue of methane that had built up before the miners began work. This was an effective but somewhat sacrificial and barbaric practice! The renowned Davy Safety Lamp was first tested underground at Hebburn Colliery on 9 th January Its invention was as a result of the disaster on 25 th May 1815 at Felling Colliery, County Durham, in the UK, where 92 men and boys lost their lives. The scientist, Humphrey Davy, was approached on behalf of the Sunderland Society to try to provide a solution to the problem of firedamp in mines. The Sunderland Society was formed by concerned individuals which included

14 2 Practical Intrinsic Safety for Engineers and Technicians MPs and churchmen who realised that the danger was ever present and that solutions were required. Over the years, the Davy Safety Lamp must have saved countless lives. Electrical equipment came into use in mines well before the 1900s. It was not until 1905 that regulations in Britain were introduced to govern the use of electricity. The regulations suggested that equipment installed underground should be designed to minimise the risk of heat or sparks causing ignition. Even before this, equipment had been purposely over-engineered in order to withstand the adverse conditions underground. Enclosures and housings were made more substantial than would be otherwise required. The design criteria was that mechanical shock to the equipment, high levels of dust and moisture should not be able to penetrate the casing and render the equipment inoperable or unsafe. This heavy construction meant that it was also more likely to protect personnel from electrocution. An additional advantage was that the enclosures were found to withstand the effect of an internal explosion if a gas air mixture had seeped inside. It was Humphrey Davy who explained that the enclosure would prevent the internal ignition from reaching the external gas air mixture surrounding it, provided that certain constructional limits were observed. The origins of this method, as one of explosion protection, undoubtedly came from this realisation. The term Flameproof was not used until a British Standard was published in More explosions that were thought to be caused by the use of electricity in mines led to the development of British Standards in the early 1900s. On October 13 th 1913 at the Universal Colliery in Senghenydd, Glamorgan in the UK, 439 miners were killed. This was the largest explosion in a UK mine. The government of the day was pressured into appointing two scientists, Wheeler and Thornton, to investigate the cause of the explosion. Their findings, published in 1916, indicated that it was most likely to have been caused by sparks from the electric bare wire bell signalling system. This was a simple arrangement to aid communication between parties of miners at the coal face (filling coal trucks at one end of a roadway) and the winch operators (at the shaft end) preparing to receive the full trucks. The signalling system was operated from a low voltage and was exempt from the 1905 regulations. As a result of the two scientists work, the spark energy to ignite methane and firedamp was determined. This led to the formal development of explosion protection methods and standards. Crude forms of energy limiting were then used in low voltage circuits to prevent incendive sparks. This method of energy limiting was used for some time and comparatively recently received the name of Intrinsic Safety. Initial attempts to operate with IS systems at higher power levels than are accepted now were included into British Standard BS Scope of protection systems It is important to realise that modern explosion protection methods and standards are only in place to minimise the risk of electricity causing ignition of a flammable atmosphere. Use of these standards does not enable the equipment to continue to work when fire or explosions have been caused. This is a general misconception of the term. Systems which are designed to continue to operate even though some disaster has been caused are the subject of separate engineering consideration. Fail-Safety concepts are discussed in this manual because they form an important part of the safety strategy for instrumentation and control applications. There are many other safety issues that must be taken into account and must operate in parallel with Explosion Protection and IS. These techniques can and do operate in a complementary way and one technique does not necessarily alter the strategy of the other.

15 Introduction Responsibility The requirement placed upon industry to operate in a safe way is contained in the laws of most countries. A duty is placed on the owners of places of work, to provide conditions that minimise risks to such an extent as is reasonably practicable. This legal precedent is followed to encapsulate the level of responsibility needed by industrial management for the definition of this duty. It is clearly necessary to safeguard life and limb. It is also in the interest of the general public that property, investment, livelihood and social structures are similarly protected. Owners of industrial premises must be aware of all aspects of general safety and must take precautions to minimise risks to acceptable levels. The term, acceptable levels is not easy to define and is based on a matter of judgement. This may vary in definition from one country to another and so it is up to the laws of those countries to balance the probability according to their interpretation. An obvious question to ask is whether it is absolutely necessary to site a piece of electrical equipment in a position where it has an increased risk of causing ignition. If it is possible to design out this requirement then the risks are inevitably reduced. This is the responsibility of the plant design body. General electrical safety is just one of many aspects of safety that need to be considered by the plant owner in an industrial situation. The consequences of equipment and system failures causing accidents that include explosions must be considered by the factory owner. The location of a plant and the impact of an explosion on the surrounding location must be assessed. It is reasonable to take extra precautions if the factory is located in the middle of a city, but does not mean that safety can be relaxed if it is located in the middle of nowhere. Risk assessment is the art of judging the likely causes of disasters and balancing them against measures taken to prevent the occurrence of such a disaster. It must take the view that nothing is completely safe. It should be realised that explosion protection is just one tool which can be used in an industrial situation to minimise the risk of electricity causing ignition to an acceptable level. 1.5 Explosion of protection and certification The expense and inconvenience of safety testing each item of electrical equipment in each individual plant situation is impractical. A system has evolved to facilitate the selection of equipment for use in plants which utilise potentially flammable materials. The principle of the system is based on the matching of two sets of criteria that are defined under a common set of rules. These criteria are: The classification of a plant The classification of electrical equipment used in the plant. The common set of rules comes from national and international Standards and Codes of Practice. These have been developed over the years and describe acceptable ways of building equipment in such a way as to prevent electricity from causing ignition in certain hazards. The standards expect that the hazards into which the equipment is to be placed have been categorised and defined by these same common rules and so the hazard and equipment may be directly compared for suitability. The certification process merely states conformance with the standard to which equipment has been assessed. It does not imply that the equipment is safe.

16 4 Practical Intrinsic Safety for Engineers and Technicians The classification rules will be discussed in some detail in this manual. These have led to an almost internationally accepted set of markings that are used to identify Explosion Protection (Ex) equipment. The benefit of this system is that it helps both equipment manufacturers and users to design, select, install and maintain equipment in a more practical and cost effective fashion. Documentation of all criteria applied to safety is required by Law and by the Standards. The classification system makes documentation easier. In this way it may be said to contribute to the overall plant safety. A set of documents collected and collated by the management of any plant is fundamental in communicating the appropriate information on safety issues. It is referred to as Plant Safety Documentation. Its content will be discussed during the course. 1.6 Legality The present legal situation in the UK may be taken as an example of the way the Law relates to the responsibilities of factory owners. The law does not state how the owner must make his factory safe. It merely states that the owner must be able to demonstrate that he has taken all reasonably practicable steps to assure safety. The use of national or internationally agreed standards to which equipment has been tested/certified and which forms part of an overall scheme of safety, as recorded in the Plant Safety Documentation, is accepted as proof that adequate care has been taken. In the UK, as yet, it is not illegal, if certified equipment has not been used. In this case, the owner would be responsible for proving that the equipment on his plant was safe if an explosion were to have occurred. This burden of proof could only be satisfied if the owner maintained records demonstrating that the equipment used on the plant was selected from that which had a proven track record of safety. Note: The situation described above will remain true in Europe until the European ATEX Directive, known as Article 100a, came into force on 1 st July Thereafter, all equipment, of whatever origin, sold into and installed throughout Europe, must have been assessed for safe use and marked accordingly. This covers both electrical and mechanical equipment. It is discussed further in the section on Standards and Certification. In other countries, Standards may be cited by in the Law of the country as to how to proceed on these matters. 1.7 The role of standards and certification The local and national standards for the design and implementation of explosion protection systems are now converging into the International Electrotechnical Committee Standard series IEC79. Since the first publication of the IEC documents in the mid-1980s, there has been an escalation of effort to complete the series and maintain its technical content. Many countries have now adopted IEC79. It is therefore logical that where contracts to design and supply industrial processes requiring safety in operation are now global, safety standards are accepted across the world. This workshop therefore uses the international standards as the basis for training. Where localised experience can augment, explain or clarify the IEC Standards it is used to give examples of common practice methods.