Product reliability is one of the important aspects of product quality. It's dependent on the reliability of every component used in the system. In Reliability Basic Theories and Applications in Electrical Apparatus, authors Lu Jianguo, Wang Jingqin, Luo Yanyan, and Su Xiuping, colleagues at Hebei University of Technology, present a guidebook to help manufacturers ensure the soundness of their electrical products. Providing information about how to design, test, and interpret the results, Reliability Basic Theories and Applications in Electrical Apparatus; gives an introduction to the history of electrical apparatus development and the content of reliability work; expounds on the fundamental knowledge of reliability; discusses the basic theory of reliability sampling, inspection, and design; details the reliability indices, reliability test method, reliability verification test sampling plan, test procedure 'shows reliability tests for relays, contactors, miniature circuit breakers, residual current operated circuit breakers, low-voltage circuit breakers, and overload relays. Summarizing achievements from the last thirty years, Reliability Basic Theories and Applications in Electrical Apparatus offers a collection of the many standards used in the certification of electrical apparatus, such as relays, contactors, molded case circuit breakers, and miniature circuit breakers.

Reliability Basic Theories and Applications in Electrical Apparatus

By Lu Jianguo Wang Jingqin Luo Yanyan Su Xiuping

iUniverse, Inc.

Copyright © 2012 Lu Jianguo, Wang Jingqin, Luo Yanyan, Su Xiuping
All right reserved.
ISBN: 978-1-4759-5240-7

Contents

Preface..........................................................................................iiiChapter 1 Overview...............................................................................11 Survey of Electrical Apparatus Reliability.....................................................22 Definition of Reliability......................................................................183 Relation between Product Reliability and Quality...............................................194 Inherent Reliability and Use Reliability.......................................................195 Failure Laws...................................................................................206 Significance of Improving Electrical Apparatus Reliability.....................................217 Basic Content of Reliability Work..............................................................22Chapter 2 Basic Knowledge of Reliability.........................................................251 Basic Knowledge of Boolean Algebra.............................................................252 Failure Density Function and Cumulative Failure Distribution Function..........................263 Reliability Characteristic Parameters of Electrical Apparatus..................................294 Failure Distribution Types.....................................................................385 Estimation of Reliability Characteristic Parameters............................................71Chapter 3 Basic Theories for Electrical Apparatus Reliability....................................931 Theory and Methods of Sampling for Electrical Apparatus Reliability............................932 Reliability Design Theory of Electrical Apparatus..............................................111Chapter 4 Reliability of Relay...................................................................1731 Reliability Index..............................................................................1732 Test Requirements..............................................................................1743 Test Method....................................................................................1764 Reliability Test of Relay with Contact Load cc1................................................1805 Reliability Test of Relay with Contact Load cc2................................................1856 Reliability Test Device........................................................................186Chapter 5 Reliability of Contactor...............................................................1951 Reliability Index..............................................................................1952 Test Requirements..............................................................................1953 Test Method....................................................................................1974 Reliability Grade..............................................................................1985 Determined Method of Contactor Reliability in Practical Load Condition.........................2026 Reliability Test Device........................................................................203Chapter 6 Reliability of Miniature Circuit Breaker...............................................2091 Reliability Index..............................................................................2092 Test Requirements..............................................................................2133 Test Method....................................................................................2154 Sampling Plan and Test Procedures of Reliability Verification Test.............................2185 Reliability Test Device........................................................................223Chapter 7 Reliability of Miniature Residual Current Operated Circuit Breaker.....................2291 Reliability Index..............................................................................2292 Test Requirements..............................................................................2323 Test Method....................................................................................2364 Sampling Plan and Test Procedures of Reliability Verification Test.............................2385 Reliability Test Device........................................................................242Chapter 8 Reliability of Low-Voltage Circuit Breaker.............................................2531 Reliability Index..............................................................................2532 Test Requirements..............................................................................2543 Test Method....................................................................................2574 Sampling Plan and Test Procedures for the Reliability Verification Test........................2575 Reliability Test Device........................................................................260Chapter 9 Reliability of Overload Relay..........................................................2931 Reliability Index..............................................................................2932 Test Requirements..............................................................................2943 Test Method....................................................................................2954 Test Plan and Procedure of Reliability Verification Test.......................................2975 Reliability Test Device........................................................................298Reference........................................................................................303Appendix.........................................................................................307

Chapter One

OVERVIEW

The reliability of an electrical product refers to the ability of this product to perform its required functions under stated conditions for a specified period of time. In modern day reliability studies, the advancement of manufacturing must be accompanied by better understanding of the science and technology (S&T) of reliability to solve practical reliability problems.

Reliability technology refers to the engineering methods related to product reliability and has been in continuous development for decades. During the World War II, electronic devices were widely used in military equipment. However, it frequently occurred that various electronic devices were unreliable and could not be effectively put into service. In a battle in the early 1950s, this problem was so remarkable that the radar equipments of the U.S. were frequently incapable of normal operation and idle in repair-waiting state. The high repair and maintenance cost caused by low reliability of electronic devices lead the U.S. industry to start to attach importance to the reliability problems. This embarked the investigation, research and test works that raised the curtain of reliability research in the field of electronic devices. The early researches were focused on the electron tubes and their performance. This focus was not only on its electrical properties, but also attached importance to its resistance to shock and impact and other environmental adaptability.

The period from the 1950s to 1960s was the decades of rapid development of reliability technology during which the U.S. Defense Department established various reliability research organizations, such as: the AdHoC reliability group in 1950, the AGREE(Advisory Group on Reliability of Electronic Equipment) in 1952, and the ACGMR (AdHoC Group on Missile Reliability) in 1957, etc. Led by these organizations, the works of reliability management, analysis and test, etc. were carried out in an extensive scale. And the reliability researches were also developed extensively by various related technical associations, companies and manufacturers in the U.S. In the later period of 1960s, many standards on reliability management, design and test appraisal, etc. were published, and the reliability technology on electronic elements and devices became increasingly mature. In the 1970s, the reliability research in the U.S. has gradually penetrated into the fields of mechanics, electrical engineering, electrical power and chemical industry, etc.

Overall, the U.S. developed the earliest, most extensive and effective researches on reliability. In addition, Japan, Britain, France, Germany and the former Soviet Union, etc. have also developed the reliability researches and achieved tremendous result. The former Soviet Union has not only prepared many fundamental reliability standards, but also prepared the reliability standards and reliability requirements for many products. They also specified reliability test methods in the product standards, and published many books and manuals on reliability. For instance, the Technical Manual on System Reliability that was published in 1985, compiled by y .

1 SURVEY OF ELECTRICAL APPARATUS RELIABILITY

1.1 SURVEY OF ELECTRICAL APPARATUS RELIABILITY IN INDUSTRIALLY DEVELOPED COUNTRIES

Electrical apparatus manufacturers and research institutes in the industrial developed countries have identified the research and application of electrical apparatus reliability as a major task. Even in those developed countries, the stipulation of reliability indices is not uniformly applied. The result is that some electrical apparatus have and some do not have properly stipulated reliability indices. Product reliability has been controlled in most factories, and the level of product reliability has become the major competition among enterprises. The current research and application of electrical apparatus reliability in industrial developed countries are mainly focused on the following aspects:

1.1.1 FORMULATION OF RELIABILITY STANDARDS

The formulation of reliability standards has developed very fast, especially the preparation of fundamental reliability standards. Since established in 1965, the IEC TC56 Technical Committee for Reliability has published many standards on reliability and maintainability. In 1988, IEC TC56 held an annual conference in Tokyo and determined to constitute the TC56 standard system by adopting the "toolbox principle". The so called "toolbox principle" means dividing the standards and documents into four categories:

1. Top-level document (IEC300) - Management on Reliability and Maintainability is a fundamental standard on reliability management.

2. Application guide - mainly include reliability specification, reliability design analysis, reliability prediction of elements (components), reliability test, reliability growth, reliability screening, software reliability, maintainability technology and field evaluation.

3. Tool document - mainly include two system standards of IEC605 Equipment Reliability Test and IEC706 Maintenance Guidance.

4. Support document.

The evolution of reliability standards for electrical apparatus can be captured with the following milestones:

• The first reliability standard for electrical apparatus with reliability requirements was published in 1964 as the U.S. Military Standard MIL-R39016 General Specification on Electromagnetic Relay with Reliability Index.

• Japan has issued several standards afterwards: 1980 the Japanese industrial standard JIS C5440 General Reliability Rules of Small Relay used in Control Circuits, 1981 the test method of failure rate both in JIS C4530 Clapper Type Electromagnetic Relays and 1982 the JIS C4531 Contactor Type Relays.

• In 2002, 10 terms about reliability were listed in IEC60050-444 International Electrotechnical Vocabulary: Elementary Relays.

• In 2011, IEC issued the IEC 61810-2-1 Electromechanical Elementary RelayPart 2-1: Reliability.

• 1983, the former Soviet Union has listed the content of reliability requirement and test in many standards for electrical apparatus, such as the -83 General technical conditions for low voltage switch electrical appliance which stipulated the reliability requirement of product.

• Germany specified the ultimate on/off times to be achieved by the contactor with rated value of product mechanical life and electrical life based on the performance of 90% of all contactors in the VDE0660 Specification of Lowvoltage Switch Electrical apparatus. This became the means to the concept of 0.9 reliability in the mechanical life and electrical life of contactors.

• In France NFC 63–100 Industrial Low-voltage Control Equipment: Contactor, it's specified that for the batch produced electrical apparatus, especially those with agreed thermal current of less than or equal to 40A, the mechanical life is tested on representative prototype by repeat mode, and the manufacturer should provide the mechanical life value of the product after statistics of test results, which actually means to determine the mechanical life of contactors by the concept of reliability.

1.1.2 RELIABILITY TEST AND DEVELOPMENT OF RELIABILITY TEST DEVICE

Computers are commonly adopted for control and detection in the reliability life test of electrical apparatus in the U.S. and Japan.

For example, the Japanese Yaskawa Company adopts the auto test device that uses electronic computer for control and detection in the contact reliability test of relays; Panasonic Company adopts the microcomputer-controlled full auto test device in the reliability life test of relays; Fujitsu Company also adopts the computer-controlled test device in the reliability test of reed relays, which is capable for the life test of 200 reed pipes at the same time.

In the U.S., the reliability life tests of RT160 relays are with microprocessor-controlled systems that come with the functions of auto measurement of contact pressure drop at contact point and other parameters, display and print of test results, etc.

German electrical apparatus companies frequently conduct life tests of contactors. For example, the contactors produced by Siemens are sampled each week for reliability testing. About 20~30 sets for the low current contactors (I_N ≤ 32A) and 10~15 sets for the higher current contactors (I_N >32A) are sampled every two weeks as a group for mechanical life test until all test samples failed. The reliable life at reliability R=0.9 is determined by Weibull probability Paper according to the test result, which should not be less than the specified mechanical life value of the product sample. As for Siemens contactors, the number of sets of each group is the same for electrical life test and mechanical life test, but the tests are conducted once by sampling each month, and the products sampled each year constitute one group.

The contactors produced by French Telemecanique Company are sampled 10 sets each month for mechanical life test. The Japanese S-type contactors are also sampled 2~20 sets each month for mechanical life test.

1.1.3 RESEARCH ON ACCELERATED LIFE TEST

The mode and method of accelerated life test and data analysis method are researched in the U.S., Japan and many other countries. For example, in Japan, the accelerated life test of switches is conducted by using load voltage and current as accelerating variable; additionally, the accelerating factor at different load voltage and current values are calculated according to the test results.

1.1.4 RESEARCH ON RELIABILITY DESIGN

The U.S., Japan and many other countries attach great importance to the reliability design of products. For example, the leading electrical apparatus companies of the U.S., Japan and Germany designed and developed intelligent circuit breaker, which significantly improved the reliability of power supply. Advanced intelligent circuit breaker can not only transmit the signal to the computer in control room from remote distance, but also receive computer instructions, to realize system automation and two-way communication.

1.1.5 RESEARCH ON RELIABILITY PHYSICS

The reliability physics has been researched since 1960s in foreign countries. The U.S. Air Force ROME Air Development Center started the on-site failure analysis in 1960s of the failed element. J. Vaccro first proposed using the concept of "failure physics" for the research on element reliability. The conference on "failure physics" has been held every year since 1962 in the U.S and in 1967 renamed to conference on "reliability physics". The so called "reliability physics" is a special science on the product failure mechanism. It studies the specific physical and chemical process on the way and reason of product failure. It can be seen that the research on reliability physics is a fundamental research to improve product reliability.

Great importance was placed on the reliability physics in the U.S., Japan and other countries. In the area of electromagnetic relays and contactors reliability, in-depth research of the performances of electric contact, electromagnetic system and other parts of the contactor are conducted. The various forms and causes of the faults of operating relay are mastered through large amount of tests and investigation in actual application. Among others, special importance is placed on the contact reliability of the electric contact. They studied the influence of various application conditions on the contact reliability, conducted overall analysis on the material, shape, contact mode, contact pressure, etc. of the contact point, and proposed requirements for the product design and application. In electromagnetic contactor, a great deal of investigations have been conducted into the faults of operating contactors in Japan, and it's shown by the results that the reliability of contactor is not only determined by its design and manufacturing, but also significantly influenced by correct or wrong application.

1.2 SURVEY OF CHINESE ELECTRICAL APPARATUS RELIABILITY

The development history on reliability of Chinese electrical apparatus can be divided into the following several periods:

1.2.1 PERIOD OF POPULARIZING RELIABILITY KNOWLEDGE AND RESEARCH

The former Ministry of Machine-Building Industry paid great attention to the reliability of electrical products. As early as the late 1970s, it entrusted Hebei University of Technology to open classes to train the latest technology on electrical products and established that reliability technology be one of the main education contents. In October 1983, China Electrotechnical Society formed the Electrical Product Reliability Research Committee to further promote reliability research on electrical products. Since then, all academic communication activities were developed and organized under the leadership of the committee. Electrical product reliability training classes have since been organized many times to promote the importance of reliability development of Chinese electrical apparatus industry.

1.2.2 PERIOD OF PROMOTION BY THE GOVERNMENT

The former Ministry of Machine-Building Industry had convened several reliability working conferences. In 1986, it announced the "Improvement Of Electrical Product Reliability Notice" and assigned work to "Review The Reliability Index Of Mechanical And Electrical Products Within Limited Time" in mechanical and electrical industry. A total of 7 document batches (1,189 specifications in total) on mechanical and electrical products were issued with reliability index to be reviewed within limited time in 1986~1991. This industry wide review included manufactured products such as relays, contactors, transformers, measuring relays, motors, power electronic devices and many other electromechanical products. The result of the review played a significant role in promoting the development of reliability of Chinese mechanical and electrical products. More than twenty national standards on reliability have now been established in China.

1.2.3 PERIOD OF SETTING STANDARDS, TEST METHODS, TEST EQUIPMENTS AND IMPLEMENTATION IN INDUSTRY

1.2.3.1 FORMULATION OF STANDARD ON ELECTRICAL APPARATUS RELIABILITY

Hebei University of Technology and other institutions published five electrical apparatus reliability standards which were approved and issued by the National Development and Reform Commission and in 2007. The standards were adopted in 2008 by Standardization Administration of the People's Republic of China SAC as national standards 2008. By December 2008, eight national standards on the reliability of relays and representative low-voltage electrical apparatus had been issued in China:

• GB/T 15510-2008 "General Rules for Reliability Test of Electromagnetic Relay for Control Circuits"; • GB/Z 10962-2008 "General Rules of Reliability for Machine Tool Electrical Components";

• GB/Z 22200-2008 "Reliability Test Method for Lower Capacity Alternating Current Contactors";

• GB/Z 22201-2008 "Reliability Test Method for Contractor Relay";

• GB/Z 22202-2008 "Reliability Test Method of Residual Current Operated Circuit-breaker for Household and Similar Uses";

• GB/Z 22203-2008 "Reliability Test Method of Over-current Protection Circuit-breakers for Household and Similar Installation";

• GB/Z 22204-2008 "Reliability Test Method for Overload Relay";

• GB/Z 22074-2008 "Reliability Test Method for Molded Case Circuit-breakers".

These standards have regulated the reliability index, reliability test method, failure criteria and reliability verification test scheme of relay and representative low-voltage electrical apparatus.

1.2.3.2 RESEARCH ON RELIABILITY TEST TECHNOLOGY AND DEVELOPMENT OF DETECTION DEVICE

Hebei University of Technology established an electrical apparatus reliability test center and has passed the approval of CNACL in May 2001 to service the industry. It can certify the electrical apparatus reliability of products, and has developed reliability test equipment which conforms to the above national standards. Some of the equipment in the center are as shown: Figure 1-1, overview of relay reliability test device; Figure 1-2, overview of contactor reliability test device; Figure 1-3, overview of miniature circuit-breaker reliability test device; Figure 1-4, overview of residual current operated circuit-breaker reliability test device; Figure 1-5, overview of overload relay reliability test device; and Figure 1-6, overview of molded case circuit-breaker reliability test device.

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