I. Introduction Switchgear, especially low-voltage electrical appliances, is an indispensable component of electrical automation equipment, and its reliability is of paramount importance. Product reliability refers to the product's ability to perform its intended function under specified conditions and within a specified time. Reliability is an important quality characteristic of a product, distinct from its functional indicators and other quality indicators. It changes over time, representing characteristics such as durability, trouble-free operation, and effectiveness. With the increasing capacity and wider application range of low-voltage electrical products, the chances of product, component, and assembled equipment or system failures have increased, leading to greater economic losses. Therefore, improving the reliability level of switchgear is an urgent and challenging task for the electrical industry. In many applications, users are no longer satisfied with the vague statement of "product quality reliability" but demand clear quantitative indicators of product reliability. Therefore, scientific reliability testing methods based on reliability theory are particularly important. As an example, the methods described in this paper can also be used for reliability analysis of other electrical components and electrical systems. II. Reliability Testing Methods and Failure Criteria The reliability test of the auxiliary contacts of the 3TB40 AC contactor was conducted on a multi-functional relay and contactor reliability microcomputer testing system. Sixteen 3TB40 products with two normally open and two normally closed auxiliary contacts, manufactured by a certain factory, were randomly selected for online monitoring of contact voltage drop, coil current, and operating time. (I) Test Methods: Contact voltage drop monitoring uses a D/A converter to provide a threshold value for the contact voltage drop. This value is compared with the actual contact voltage drop value. If the actual contact voltage drop value is lower than the threshold value, the contact is considered to be in good contact, and the test sample's contact is deemed to have passed this monitoring. Otherwise, the contact is considered to be in poor contact, and the test sample's contact is deemed to have failed this monitoring. Coil current monitoring first converts the current signal into a voltage signal through a resistor, and then compares it with the threshold value for the contact voltage drop provided by the D/A converter to obtain the monitoring result of the coil current. The pull-in and reset time monitoring of the test sample is performed using an 8253 timer. Standard threshold values ​​for the pull-in and reset times of the test sample are provided. When the standard time threshold value is reached, the timer count pulse returns to zero and a signal is sent. The computer receives the signal and issues a command to monitor the contact voltage drop. If the contact voltage drop is qualified, the test sample's pull-in time or reset time is judged to be qualified; otherwise, the test sample's pull-in time or reset time is judged to be long. (II) Test Conditions: Apply a 24V DC voltage to each auxiliary contact circuit, with a 240Ω resistor connected in series, meaning the current in the auxiliary contact circuit is approximately 100mA. The continuous test frequency is 7200 times/hour. The test cutoff time is 300,000 times, which is a time-truncation life test. (III) Failure Criteria: The multi-functional relay and contactor reliability microcomputer test system monitors the contact voltage drop, coil current, and operating time online. Compare the monitoring results with the failure criteria (standard threshold value) to give a conclusion on whether the test sample is qualified this time. III. Test Results and Failure Analysis (I) Failure Analysis When the product life follows an exponential distribution, there are two methods to estimate its failure rate: Point estimation method λ=γ/T The failure rate is estimated by the following formula: Where, T—total test time (number of times) γ—number of test specimens that fail during the test In the time-cut-off life test without replacement, the total test time is: Where, n—total number of test specimens ti—failure time (number of times) of the i-th test specimen t0—test cutoff time (II) Interval estimation method Interval estimation has two forms: one-sided estimation and two-sided estimation. For one-sided estimation, the estimated interval of failure rate is: λL=00 where X20.6(2γ+2) — the lower quantile of the X2 distribution with 2γ+2 degrees of freedom and 0.6 (1-α=0.6) confidence level. For two-sided estimation, the estimated interval of failure rate is: where X20.2(2γ) — the upper quantile of the X2 distribution with 2γ degrees of freedom and 0.2 (α/2=0.2) confidence level. X20.8(2γ+2) — the lower quantile of the X2 distribution with 2γ+2 degrees of freedom and 0.8 (1-α/2=0.8) confidence level. For exponential distribution, after the failure rate is estimated, other characteristic quantities can be obtained: Reliability: R(t)=e-μλt Average life: Reliable life: Median life: Substituting the test results into equations (1) to (10), the reliability characteristic quantities of the 3TB40 AC contactor are obtained, as shown in Table 5. Meanwhile, based on the experimental data, the shape parameter m was estimated using Weibull probability, resulting in m = 0.95 ≈ 1.0. Therefore, it can be considered that its lifetime failure follows an exponential distribution, and estimating its reliability characteristic quantity using the exponential distribution is effective. (IV) Failure Mechanism The test results show that the test specimen has only one failure mode, namely, contact failure of the contacts. It can be seen that: 1. The electromagnetic mechanism of the 3TB40 AC contactor has a high reliability level, the operating time does not exceed the standard, and the mechanism does not malfunction. 2. The auxiliary contacts of the 3TB40 AC contactor experienced multiple contact failures. This is because during the test, the contact load voltage was DC 24V and the current was 100mA. The current is lower than the arc current of the silver contact, so no arc discharge will be formed during the energization process. Due to the oxidation of the auxiliary contacts in the air, the film resistance Rj of the auxiliary contacts increases, resulting in an increase in the contact voltage drop and contact failure of the contacts. 3. The contact failure of the auxiliary contacts may be caused by material defects. Table 2 shows that the second normally open contact (7.2K) of the seventh test sample exhibited a contact failure type of level three within the range of 44796 to 123215 cycles. The mechanism of this failure may be due to film resistance, or it may be caused by impurities or material inhomogeneity in the contact material. Similarly, the contact voltage drop of 15 2B is unstable, reflecting the highly unstable contact voltage drop Rj. 4. During the test, it was found that after 104 switching cycles, the faulty contact disappeared, or the frequency of the fault significantly decreased. This phenomenon may be due to the mechanical impact and spark discharge (glow discharge) during the switching process cleaning the inorganic film, dust film, and surface impurities on the contact surface. In particular, the mechanical force and electrothermal effect tempered the contact surface, eliminating or improving the surface stress formed during machining, improving thermal conductivity, improving surface contact, and reducing contact resistance. IV. Conclusion This paper introduces the reliability test method and failure criteria for the auxiliary contacts of the 3TB-40 AC contactor. Based on the test monitoring results, the reliability of the product was experimentally studied, and characteristic indicators of reliability were given. The main factors affecting the product's reliability indicators were identified. The failure mechanism of the product was discussed. This provides a reference for the reliability design of contactors. Regarding the auxiliary contacts of this batch of 3TB-40 AC contactors, the following conclusions were drawn: 1. The product's lifetime failure distribution follows a regular exponential distribution; 2. The electromagnetic mechanism has a high reliability level. The operating time did not exceed the standard, and no mechanism failure occurred; 3. Multiple contact failures occurred in the auxiliary contacts. This was mainly caused by the high film resistance Rj of the auxiliary contacts and defects in the auxiliary contact material; 4. Mechanical force and electrothermal effects can temper the contact surface, improving surface contact and reducing contact resistance.