Those who cannot remember the past are condemned to repeat it.
The Life of Reason (Vol. 1), George Santayana
The electrical contact has always been an essential part of the electric circuit. The reliability of the electrical contact has also been an essential, but often ignored, factor—ignored, that is, until it fails. Thus, it sometimes seems as if problems that should have been solved many years ago keep recurring. This perception is, of course, only partially true. In their earliest forms, electric circuits usually carried only a limited range of currents: perhaps up to a few hundred amperes. Since then, this current range has increased considerably. Now electrical contacts of one type or another are found in very high power circuits passing currents in excess of 106 A (mega-amperes) [1] and in electronic circuits where currents can be as small as 10−6 A (microamperes) [2]. In past 60 years, there have been major advances in electrical contact science, including the recognition and the understanding of fretting corrosion; the development of flowing mixed gas laboratory systems for studying electrical contact corrosion; the study of the effects of dust on connector performance; the miniaturization of electro-magnetic relays; the introduction of MEMS (microelectromechanical systems) switches; the ease of use and the availability of a broad range of surface analysis systems; the advent of computers for recording and analyzing data, especially statistical data; the advent of user friendly computer software to design and develop switching systems; the continued development of silver-metal oxide contact materials; the development of contacts for use in new operating ambients such as SF6 and vacuum; a whole new class of electrical connectors, especially for the electronics and automobile industries; and new types of sliding contacts. The reader should, however, be aware of the considerable body of knowledge that has been accumulated since this subject was first studied. Published research on electrical contact phenomena and the switching of electric circuits can be found dating back to 1835. Reference to this early work is given in the bibliography and abstracts on electric contacts from 1835 to 1951 that were published in 1952 by the ASTM (American Society for Testing Materials) [3]. A large part of this work is still relevant today. In fact, one paper from the end of the ninteenth century that is still frequently referenced by electric contact researchers is that by Kohlrausch and Diesselhorst [4]. They first developed the relationship between the voltage drop across a conductor and its temperature (see Chapter 1). The ASTM continued to publish abstracts of electrical contact research until 1965 [5]. For those interested in the history of electrical contact studies as a subject in its own right, this series of books provides an invaluable source of reference. From 1965 to 1977, the recording of electric contact abstracts was continued by the Holm Conference Organization [6]. The complete papers from the various embodiments of the Holm Conference (now called the IEEE Holm Conference on Electrical Contacts) can be found on a DVD [7] edited by Schoepf.
In the United States, the study of electrical contacts as a distinct discipline can be traced back to the first Holm Seminar, which was held at the Pennsylvania State University in 1953. Here Dr. Ragnar Holm presented a series of lectures on the state of electrical contact research and knowledge up to that time. This seminar was organized by Dr. Erle Shobert (then at Stackpole Carbon, Inc.) and Professor Ralph Armington (then on the faculty of the Pennsylvania State University’s Electrical Engineering Department). The Holm Seminars developed into the IEEE Holm Conferences on Electrical Contacts [7]; 2013 being the 59th conference. The first International Conference on Electric Contacts (ICEC) was held in Orono, Maine, in 1961 and was followed by a second in Graz, Austria, in 1964. Since that time, the International Contact Conference has been held every two years in the Americas, Europe, and Asia [8]; 2012 being the 26th conference. The Albert Keil Kontacktseminar has also been held in Germany since 1972. The proceedings of these conferences contain all the important advances in the subject for the past half century and can also be found on the DVD edited by Schoepf [7,8]. There has been an annual Japanese conference (International Session on Electromechanical Devices) since 2001. There has also been and ongoing committee in Japan that discusses electrical contacts and electromechanical devices. They hold meetings about ten times a year. In China the International Conference on Reliability of Electrical Products and Electrical Contacts was begun in 2004. Further Conferences were held in 2007, 2009, and 2012.
Although a reliable electrical contact is essential to the successful operation of every electric circuit, remarkably few books have been written on the subject. In 1940, Windred [9] published the first comprehensive book, Electric Contacts, in English. This book treated the subject in great detail and must have been of great practical value in analyzing and designing current switching devices. In 1941, Ragnar Holm published his first book on the subject in German [10]. He continued to update and revise this work until the publication of his seminal work, Electric Contacts: Theory and Application, in 1967, which was reprinted in 2000 [11]. This book contained a comprehensive review of all aspects of electrical contact phenomena known up to 1966, and is still a frequently referenced source book for many active researchers in the field. In 1957, F. Llewellyn Jones published The Physics of Electrical Contacts [12], which had a more limited scope. It concentrated on arcing and the erosion of relay-type contacts. Unfortunately, this book has long been out of print. Two books, one in German [13] and one in French [14], have been published in 1983 and 1996, and the first edition of this book filled an important niche in 1999. Braunovic et al. published their book Electrical Contacts, Fundamentals, Applications and Technology, on static contacts in 2007 [15]. In this new edition of our book, Electrical Contact, Principles and Applications, we continue to present, in one volume, the basic background to this subject as well as a comprehensive review of the present state of electrical contact research, development, and application. Because the subject continues to grow it is impossible for one author to have detailed knowledge of its every aspect. We therefore present individual specialized chapters written by recognized experts in that particular field of electrical contact research and application, making it possible to cover the whole range of electrical contact phenomena. There is, of course, a strong link between the study of electrical contacts, the design of electrical connection systems, and the design of circuit interruption devices. We therefore show many examples of the use of electrical contacts and component design criteria. It would be impossible, however, to include a complete discussion of all the design intricacies for every component in which electrical contacts are used. The reader should therefore use this book in conjunction with other specialized books that cover the particular aspects of electrical component design [16,17,18,19,20,21,22,23,24,25,26,27,28]. This book is divided into six parts, as follows.
Part I: Contact Interface Conduction
The four chapters in Part I discuss the basic principles of making contact and the effects of the ambient atmosphere on the passage of current from one conductor to the other. Chapter 1 presents the theory of two metal surfaces coming into contact. It discusses the true area of contact and develops the concept of contact resistance, a subject that has been investigated since the earliest studies on electrical contacts [29,30]. The effect of an electric current passing through the true area of contact, its effect on contact heating, the effect of very high frequency currents and the formation of intermetallic compounds are also explained. Chapter 2 introduces contact corrosion and tarnishing, another subject that has a long history of study [31]. It discusses the variety of ambient atmospheres that contacts may be exposed to, gives a general outline of the important corrosion mechanisms, presents methods of measuring the effects of corrosion, and introduces Chapters 3 and 4. In Chapter 3, the work on developing laboratory test atmospheres is discussed in detail, indicating the importance of the flowing gas test chamber that has revolutionized laboratory corrosion studies. It also discusses how the mixture and concentration of the test gases play a critical role in the corrosion effects. The corosion of thin noble metal plating on electronic connections is covered in detail and the palliative effects of lubricants. Chapter 4 concludes this part with a discussion of the the deleterious effects of dust on electronic connectors.
Part II: Nonarcing Contacts
The four chapters in Part II present the practical requirements of making permanent contact. The discussion also includes the periodic breaking of such a contact under the condition that no current is passed through the contact interface. Chapter 5 discusses the important aspects of producing high-power contact joints such as may be found in power transmission and distribution circuits; this type of joint has been studied for many years [32]. Here, currents in excess of 105 A may be carried. The discussion is continued in Chapter 6, where connections that carry currents of less than 1 ampere up to tens of amperes are covered. Here also the effects of loose connections are reviewed. This topic has gained considerable interest in recent years. Chapter 7 discusses the effects of contact wear when plug-in connections are made and broken. It also extends the discussion begun in Chapters 5 and 6 on contact fretting, especially as applied to electronic connectors. The final chapter in this part reviews the subject of plating a thin layer of metal that has good connector contact characteristics onto a substrate metal.
Part III: The Electric Arc and Switching Device Technology
Part III begins the discussion of opening contacts that have a current passing through them. The electric circuits being switched have currents from about 10 mA to 2 × 105 A and, in this book, a few volts to about 1000 V. Chapter 9 introduces the formation of the electric arc from both the breakdown of an open gap and the opening of conductors in contact; the discussion of these phenomena dates back to the 19th century [33,34]. Here, a new interpretation is developed for the minimum current required for an arc to form. It also covers arcing in ac and dc circuits and how the arc is extinguished and the circuit is interrupted. In Chapter 10, the effects of the arc on the contacts—contact erosion [35], welding, and contamination—are addressed. Chapter 11, Chapter 12, Chapter 13, Chapter 13 and Chapter 15 present aspects of contact and arc chamber design for efficient operation of contacts interrupting electric circuits. The low current reed relay is discussed in Chapter 11 and Chapter 12 introduces contact requirements for the MEMS switch. Chapter 13 includes design aspects for other types of relay [36] and low current switches. In Chapter 14, the control of the high current arc in contactors [37], molded case circuit breakers used in circuits up to 1000 V, and vacuum interrupters are described together with the computer friendly software that is increasingly being used for switch design. Finally, Chapter 15 concludes this part with a discussion of the detection and elimination of low current arcing faults.
Part IV: Arcing Contact Materials
Part IV was developed by Gerry Witter, who authored or coauthored the four chapters. These chapters expand on the design of arcing contacts and the on how the electrical arc can affect their performance. This part also discusses the compromises that have to be made when choosing contacts that will be exposed to the electric arc [38]. These contacts not only have to conduct current reliably when closed, but also must resist severe surface damage when they are opening. The choice of materials is presented for a number of voltage and current ranges. The development of specialized contact compositions also has a long history [38,39,40,41,42]. Surface-arc-contamination reactions that involve changes in contact resistance during the life of the contact are also discussed. Chapter 16 describes arcing contact materials relating properties to performance. Chapter 17 provides information on arcing contact construction and attachment technology options. Chapter 18 gives theoretical guidelines to help in selecting contact materials based on contact properties and also provides guidelines for development of test methods to compare the performance of arcing contact materials. Chapter 19 discusses the effects on arcing contact performance from arc interaction with various contaminants on contact surfaces.
Part V: Sliding Electrical Contacts
Part V deals with sliding contacts, one of the earliest subjects of electrical contact research [43,44]. Chapter 20 presents the fundamentals of sliding contacts and the aspects of testing and evaluating the test results on sliding contacts. Although the principles addressed in this chapter are general, the authors are mostly concentrating on currents of a few amperes up to hundreds of amperes that can occur large in electric motors. Chapter 21 gives examples of the application of such sliding contacts. In Chapter 22, low current sliding contacts are discussed, in particular the low current contacts that would be used in slip rings and would be employed in harsh environments, such as outer space. Chapter 23 presents the work on high current density brushes.
Part VI: Contact Data
The last part of this book presents updated tables of physical data for a wide range of contact materials. It also gives some common conversion factors as well as a table of physical constants used frequently by those performing research on electrical contacts.
Future Developments
Based on my own experience of electrical contact research over five decades, I would anticipate that continuing research and the use of computer-aided recording and analysis equipment will continue to develop the science of electrical contacts. I would also expect to see advances in material selection and application specifications for power connections. Electronics connector technology will continue to be driven by cost and reliability, two requirements that are not always compatible. For circuit interrupter applications, the control of the electric arc and contact material development will lead to more efficient and reliable component design. The marriage of miniaturized electronics for detection and control of switches will expand the use of switching systems that use electrical contacts and electric arcs for making and interrupting electric circuits. Also, the increasingly user-friendly computer software will greatly facilitate the future design and development of these switching systems. For sliding contacts, the continued work on brush contacts promises a new generation of contact that will complement the existing styles manufactured from solid materials. Finally the three environmental initiatives begun by the European Union will continue to have a significant effect in the use of materials that are considered risky, these initiatives are: (1) RoHS (Restriction of Hazard Materials), (2) REACH (Regulation, Evaluation, Authorization and Restriction of Chemicals) and (3) SVHC (Substances of very high Concern) [45].
REFERENCES
1. HA Calvin JJ Anderson, PG. Slade. The development of mega-ampere contact structures for an electromagnetic launcher. IEEE Trans Comp Hybrids Manuf Technol 16(2):203–210, March 1993.
2. HC Slade. A study of electroplated and evaporated low resistance ohmic contacts to n-type GaAs. Southeaster, 1994 IEEE Students Paper Book, part of MS Thesis, Department Electrical Engineering, University of Virginia, Charlottesville, VA, 1994.
3. Bibliography and Abstracts on Electrical Contacts 1835–1951. Philadelphia, PA: American Society for Testing Materials, 1952.
4. F Kohlrausch, H Diesselhorst. Über den Stationaren Temperaturzustand Eines. Elektrisch Geheizten Leiters. Ann Phys 1:132–138, 1900.
5. Bibliography and Abstracts on Electrical Contacts. Philadelphia, PA: American Society for Testing Materials, in 11 volumes, one for each year, 1954–1964.
6. (a) Bibliography and Abstracts on Electrical Contacts. Circuit Breakers and Arc Phenomena 1965–1969. (IEEE No. 70M65 PMP), Holm Conference, 1970. (b) Bibliography and Abstracts on Electrical Contacts. Circuit Breakers and Arc Phenomena 1970–1971; Proceedings of the 1971 Holm Seminar on Electric Contact Phenomena, IIT. (c) Bibliography and Abstracts on Electrical Contacts. Circuit Breakers and Arc Phenomena, 1972; 1972 Holm Seminar on Electric Contact Phenomena, IIT. (d) Bibliography and Abstracts on Electrical Contacts. Circuit Breakers and Arc Phenomena 1975; Proceedings of the 1975 Holm Seminar on Electric Contact Phenomena, IIT. (e) Bibliography and Abstracts on Electrical Contacts, Circuit Breakers and Arc Phenomena 1976–1977; Proceedings of the 1977 Holm Conference on Electric Contact Phenomena, IIT. (f) Title Word Index for the 1st 24 Holm Conference and the 1st, 3rd, 6th and 9th International Contact Conferences; Proceedings of the 25th Holm Conference on Electric Contacts, IIT, 1979. (g) Title Word Index for the 1st 29 Holm Conferences and the 1st 11 International Contact Conferences; Proceedings of the 29th Holm Confernce on Electric Contacts, IIT, 1983. (h) Cumulative Index of Holm Conferences and International Contact Conferences 1953–1988 (IEEE Catalog No. JH 9412–8), 1989.
7. Proceedings of the Holm Seminars on Electrical Contact Phenomena 1953–1976, of the Holm Conference on Electrical Contacts 1977–1984, and of the IEEE Holm Conference on Electrical Contacts 1985–2012 are now available on the DVD (ISBN 978-3-8007-3522-4) from VDE Verlag GMBH. An updated version of this compendium is given each year at the IEEE Holm Conference on Electrical Contacts. Selections of papers from these conferences have been published in IEEE Transactions Parts Materials and Packaging; Components, Hybrids and Manufacturing Technology; and Components Packaging and Manufacturing Technology since 1968.
8 Proceedings of the International Conference on Electrical Contacts: 1st, Orono, Maine, 1961 to the 26th, Beijing, China, and the complete proceedings of the Albert Keil Kontaktseminars are also available on the DVD (ISBN 978-3-8007-3522-4) from VDE Verlag GMBH.
9. G Windred. Electric Contacts. London: Macmillan and Co., and New York: D Van Nostrand Co. Inc., 1940.
10. R Holm. Technical Physics of Electrical Contacts. Berlin: Julius Springer, 1941.
11. R Holm. Electric Contacts: Theory and Application. New York: Springer, 2000.
12. F. Llewellyn Jones. The Physics of Electrical Contacts. Oxford: Clarendon Press, 1957.
13. A Keil, WA Merl, E Vinaricky. Elektrische Kontakte und ihre Werkstoffe. Berlin: Springer Verlag, 1984.
14. L Féchant (ed.). Le Contact Electrique, Vol. 1 and 2. Paris: Hermes, 1996.
15. M Braunovic, V Konchits, N Myshkin. Electrical Contacts: Fundamentals, Applications and Technology. Boca Raton, FL: CRC Press, 2007.
16. TE Browne (ed.). Circuit Interruption: Theory and Techniques. New York: Marcel Dekker, 1984.
17. RD Garzon. High Voltage Circuit Breakers: Design and Application. New York: Marcel Dekker, 1997.
18. GL Ginsberg (ed.). Connections and Interconnections Handbook, Vol. 2, Connector Types. Philadelphia, PA: The Electronic Connector Study Group, Inc. 1979.
19. RS Mroczkowski. Electronic Connectors. New York: McGraw Hill, Inc., 1998.
20. CH Flurscheim. Power Circuit Breaker Theory and Design. London: IEE, 1985.
21. A Greenwood. Vacuum Switchgear. London: IEE, 1994.
22. RT Lythall. JSP Switchgear Book. London: Newnes-Butterworths, 1972.
23. RL Peck, HN Wagar. Switching Relay Design. Princeton, NJ: Van Nostrand, 1955
24. S Dzierbicki, E Walcuk. Wylaczniki Orgraniczajace pradu Przemiennego. Warsaw: Wydawnictwa Naukowo-Techniczne, 1976.
25. TH Lee. Physics and Engineering of High Power Switching Devices. Cambridge, MA: MIT Press, 1975.
26. WT Shugg. Handbook of Electrical and Electronic Insulating Materials, 2nd Edition. New York: IEEE Press, 1998.
27. PG Slade. The Vacuum Interrupter: Theory, Design and Application. Boca Raton, FL: CRC Press, 2007.
28. RS Mroczkowski. Electronic Handbook. New York: McGraw-Hill Press, 1998.
29 L Binder. Contact resistance. E und M 30:781–788, 1912.
30. R Holm. Contact resistance especially at carbon contact. Zeit fur Tech Phys 3(9):290–294; 3(1):320–327; 3(11):349–357, 1922.
31. GE Luke. Resistance of connections. Elec J 21:66–69, 1924.
32. SW Melsom, HC Booth. Efficiency of overlapping joints in copper and aluminium bus-bars. IIEE 60:889–899, 1922.
33. OE Gunther. Energy and resistance of sparks caused by closing and breaking electric circuits. Ann der Phys 42(11):94–132, 1913.
34. W. Höpp. Arc formation in switchgear. ETZ 34:33–38; 55–58, 1913.
35. R Holm. Vaporization of electrodes in butt contacts. Zeits fur Tech Phys 15(11):483–487, 1934.
36. S Keilien. Testing snap switches. Elec J 33:521, 1936.
37. EW Seeger. Electromagnets and contacts for magnetic contactors. Prod Eng 5(5):181–182, 1934.
38. G Windred. Electrical contacts. Engineer 160:222, 1935.
39. H. von Fleischbein. Improved contact design and material. ETZ 39:445–446, 1918.
40. EF Kingsbury. The Use of Noble Metals for Electrical Contacts. Reprint-298. Murray Hill, NJ: Bell Telephone Laboratory, April 1928.
41. Sintered silver alloys for electrical contacts. Met Ind (Lond) 51:461, 1937.
42. G Windred. Compound metals for electrical contacts. Elec Eng 56:1045, 1937.
43. FG Baily, WSH Cleghorne. Communication. J IEE 38:150–182, 1907.
44. HFT Erben, AH Freeman. Brush friction and contact losses. J AIEE 32:559, 1913.
45. Full details of these initiatives can be found at: http://en.wikipedia.org