The life cycle of most products will go through three phases: the early failure phase, the accidental failure phase, and the loss-loss phase. The relationship between the failure rate and time corresponding to these three stages is a bathtub, which is commonly referred to as the bathtub curve (see). The failure rate in the early failure phase is higher and decreases significantly over time. The main cause of early failure is due to defects in design and manufacturing, such as poor structural design, unreasonable tolerances, defective raw materials, poor production process, damage to parts during assembly adjustment, and unclean production environment. , production equipment failure and operator negligence. The products that fail early can be removed by aging, adjustment, and, if necessary, by screening tests, to enter the optimal working period and an accidental failure phase. The reliability screening test is for the first author Luo Yanyan, female, born in 1971, Ph.D., lecturer. Tests that select products with certain characteristics or reject early failure products. Generally speaking, screening refers to the elimination and elimination of products that are not in compliance with the requirements. The screening test is generally carried out after the production process is completed and before the settlement test. It is an effective measure to improve the reliability of the entire batch. The main failure modes of miniature circuit breakers include operation failure, misoperation failure and failure failure. The reliability of the circuit breaker directly affects whether the equipment on the distribution line can operate safely. Because of its large output and wide application range, If its reliability is not high, the economic loss caused to the user due to its failure will be very serious. Therefore, research on the reliability screening test of miniature circuit breakers has far-reaching significance for ensuring and improving the quality and reliability of products. For the research of control electrical products, such as relays and contactors, the reliability screening test has been matured at home and abroad, and has been specified in relevant standards. For protective appliances, it is more profound to be able to eliminate early failure products early. In recent years, the early failure of circuit breakers has received increasing attention. Some researchers in China have done some testing on circuit breakers for this purpose, and some circuit breaker manufacturers have also developed their own reliability based on experience. Pre-factory tens of operations on the circuit breaker to filter the specification 丨(t-Iu2)2/2(t)dt Otherwise, as shown, smaller, even if you can choose a T, pi is small, but P2 is quite Big, the good products are also screened out, thus losing the meaning of screening. Batch product composition diagrams that are not suitable for screening From the point of view of failure rate, the failure rate of many products has a tendency to decrease with time at the beginning, as shown. Therefore, under certain stress, a time T product failure rate curve can be selected regardless of the requirement. The aging screening time "~ is often related to the distribution function of life T. The following is only for the early failure life distribution when the Weibull distribution is used. The method of T ~. The early failure life distribution of a typical product is a Weibull distribution with m < 1. In this case, in order to make the product failure rate after screening is lower than a certain constant X', the failure rate data of the bottom test can be used to determine the required screening time T' Weibull distribution failure rate, where A is and m, The constant associated with n is obtained by taking the logarithm of the two sides. Therefore, the data obtained by the bottom test ((/, X(t,)), i=1, 2, n can be used to obtain the regression line by regression analysis, and then use this regression. The out-of-line out-of-line failure rate is the screening time required for the predetermined requirement X. T' 3 Reliability screening test example 3.1 Reliability screening test data A batch of miniature circuit breaker samples are tested for operational reliability, and the failure rate X(t) is obtained. The data of t is listed in Table 1. Table 1 product failure rate t / time) 丨 3.2 description, drawing line 5), can be found that the failure rate curve is basically a straight line. Use the linear regression method to find this regression line. To do this, first compare the t and the input (make the transformation x=lgt, y=lg into (t), and substitute the equation (3), and obtain the regression line equation as the bilateral pair of failure rates. The number curve is: a = lgA; b = m - 1, both are constants. Table 2 shows. Table 2: Logarithmic calculation of product failure rate T. According to the least squares estimation method, the relevant formula for fitting the regression line can be obtained: 2 a=y-bx can be calculated by using the data in Table 2; The regression linear equation is / as we require that the product's failure rate is less than A~, then the regression time lg can be calculated to calculate the required screening time T%, that is, the lg input is 0.90, and the F distribution is found by the F distribution table. 0i(1,4)=4.54. Substituting the above data into equation (5) yields a 3.4T confidence upper bound estimate of 0.12). The screening time is approximately 90 times. It must be pointed out that the T obtained here is only a point estimate. If the screening time is T, the failure rate of the product cannot be completely guaranteed to be lower than A (=0.12). A more reasonable method should find the confidence upper limit T of T with a confidence level of 1 to ensure that the product has a failure rate below A (= 0.12) after passing T screening. To find the upper confidence limit of T, first find the confidence upper limit of x=lnT. Xu. The interval estimation formula of some parameters in the regression line is fitted by the least squares estimation method. The confidence of x is the confidence upper limit of 1a. This equation can be solved by simplification and the following equation is solved: 2) is the upper-side quantile of the F distribution with a degree of freedom of 1, n - 2, that is, the confidence of the screening time is 0.90. For the second solution, it is obviously not the meaning of the question. The first one is the required confidence limit, that is, T is taken as 96. Based on the analysis of the working characteristics and failure mode of the miniature circuit breaker, the reliability screening test is studied. The theory and preliminary discussion of the specific method of reliability screening test for miniature circuit breakers provides a method for related manufacturers to improve product batch reliability. P=1-a for a given confidence level 1 a can check the F distribution table for the value of Fa(1,n); for the residual standard deviation of the regression equation (4), the calculation formula is based on the data of Table 2 and the front Calculated values ​​of a and b Most blow dryers intended for home use have a DC or an AC motor. AC motors reduce noise output, making them a quiet option for early morning or late night hair-styling. If you live with someone else in cramped quarters, look for these quiet-running hair dryers. DC motors, on the other hand, are lighter in weight than AC motors and make it easier to blow dry longer hair without arm fatigue. 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