Abstract: Life prediction based on diagnostic technology is a new stage in circuit breaker maintenance technology. This paper studies the electrical life of circuit breaker contacts and proposes a practical monitoring method for contact electrical life. Keywords: Circuit breaker, contact electrical life prediction, monitoring 0 Introduction With the development of science and technology, the diagnostic technology of power equipment has undergone a gradual development process from unscientific to relatively scientific. Life prediction based on diagnostic technology is based on the aging and deterioration laws of power equipment and the influencing and determining factors. By obtaining various information through various detection methods (offline or online), and through scientific and comprehensive analysis, the current condition and expected life of the equipment are grasped [1]. It is the foundation and prerequisite for realizing condition-based maintenance of power equipment. From 1988 to 1991, the International Conference on Large Electric Systems conducted a world survey on modern single-voltage SF6 circuit breakers of 72.5kV and above. The survey showed that mechanical faults accounted for 70% to 80%, and electrical faults were the second most common. Due to long-term planned maintenance, indiscriminate disassembly and repair has wasted a lot of manpower and resources, caused power outages and reduced equipment lifespan, making people more aware of the necessity of monitoring the electrical performance of circuit breakers, such as contact condition and electrical life. Previously, monitoring of electrical wear involved recording the cumulative breaking current or cumulative arc energy (I²t). In fact, even if the same circuit breaker breaks the same current twice under the same external conditions, the degree of burn-out will not be the same [2-3]. It is known that when the breaking currents differ greatly, the burn-out mechanisms of the circuit breaker contacts are different, resulting in significant differences in burn-out. Therefore, using the cumulative breaking current to judge the amount of contact burn-out is insufficient. Furthermore, although electrical wear depends on arc energy, it is also related to the contact breaking speed, etc., and there is no proportional relationship between electrical wear and arc energy. Therefore, further research is needed on the monitoring of electrical wear and electrical life of circuit breaker contacts. Monitoring the electrical life of circuit breaker contacts requires addressing the concepts of electrical life and engineering implementation methods. 1. Calibration Methods for Electrical Life of Different Circuit Breaker Contacts 1.1 Introduction to Relative Electrical Wear and Relative Electrical Life of Contacts Contact wear refers to the phenomenon of gradual damage (deformation, burning, material detachment, etc.) to the contact surface after repeated opening and closing. Contact wear is mainly electrical wear. Electrical wear is mainly manifested as net contact loss, metal transfer of contact material, and chemical corrosion. Net loss is mostly caused by the molten or vaporized contact surface under the high temperature of the electric arc being washed away or splashed by the fluid medium. The electrical wear of the contact depends on the arc energy, i.e., the breaking current and the arcing time. Numerous experimental results show that, considering the cumulative electrical wear of circuit breakers, although the arcing time is random for a single interruption, the average arcing time tends to converge for multiple interruptions (the average arcing time for 20 interruptions and 30 interruptions has almost the same standard deviation). This means that the influence of random factors on the dispersion of arcing time and whether it is the first phase interrupted can be ignored from a cumulative perspective. In other words, the electrical wear of circuit breakers can be measured using the interrupting current as a reference. Let M be the electrical wear of a single interruption under the rated short-circuit interrupting current, and N be the corresponding allowable number of interruptions. From the statistical average and cumulative effect, the total allowable wear of the circuit breaker can be considered to be NM. Alternatively, we can define the total allowable wear of the contacts of a brand-new circuit breaker as 100%, i.e., its relative electrical life as 100%. Then, the relative wear at each interruption under the rated short-circuit interrupting current is 1/N (defined as QM). Then, based on the N1-Ic curve of different circuit breakers, the corresponding allowable number of interruptions N1 for any interruption current Ic is obtained, and the relative electrical wear of a single interruption is 1/N1 (defined as Qm). Thus, the relative electrical wear amount at any interruption can be calculated: QM=1/N, Qm＝1/N1 (1) Since the total absolute electrical wear of the contacts is constant, we have: QMN=QmN1, MN=mN1 (2) So Qm/QM=N/N1=m/M (3) Qm=QM(m/M)=(1/N)m/M (4) The relative electrical life of the circuit breaker L=LN-∑Qm (5) Wherein, LN is the initial value of the electrical life of the circuit breaker contacts, which is a percentage ≯1. Its value is determined by the operation of the circuit breaker. The value of the circuit breaker that has just been put into operation or has undergone major repair can be 1. The following uses the widely used SF6 circuit breaker as an example to derive the relative electrical wear formula. 1.2 Derivation of the relative electrical wear formula for SF6 circuit breaker contacts For the electrical wear of SF6 circuit breaker contacts, the relationship curve between the allowable number of breaks and the breaking current in reference [2] and the corresponding formula are taken as representatives to derive its relative electrical wear formula. The equivalent allowable number of breaks for any breaking current Ic is N1 (assuming that the equivalent allowable number of breaks for a breaking current of 50% Ie is 1), then we have: The allowable number of breaks for the corresponding rated short-circuit breaking current Ie can be found from the type test or technical parameters of the circuit breaker as N. According to the relative relationship of the number of breaks given by formula (6), the actual allowable number of breaks is 3.247N1N. Taking the LW6-500 circuit breaker as an example, its rated short-circuit breaking current Ie is 50kA. The allowable number of breaking cycles for the rated short-circuit current is taken as 16. The relationship between the allowable number of breaking cycles and the breaking current can be obtained from equation (6) as shown in Table 1. It can be seen from the table that when Ic/Ie is very small, the allowable number of breaking cycles of the circuit breaker will be very large. When Ic/Ie is 10%, the allowable number of breaking cycles reaches 4079. According to the temporary maintenance regulations for SF6 circuit breakers, when the number of breaking cycles reaches more than 3000, the circuit breaker will need to be repaired due to mechanical failures or other reasons. Therefore, the allowable number of breaking cycles when Ic/Ie = 3% is taken as 3000 (as shown in Table 1). When Ic/Ie is between 15% and 3%, the relative electrical wear is approximated by Lagrange linear interpolation. If Ic/Ie>15%, the equivalent allowable number of interruptions N1 in equation (6) is the allowable number of interruptions of 1 when the interruption current is assumed to be 50%Ie, and then obtained according to the corresponding relationship. According to the above and the definition of relative electrical wear, the relative electrical wear Qm of the SF6 circuit breaker is: Qm＝1／（3.247×N1×N） (7) From equations (6) and (7), we have: Qm＝1/［5.942N（0.35Ie/Ic）3］0.15Ie≤Ic＜0.35Ie Qm＝1/［3.247N（0.5Ie/Ic）1.7］Ic≥0.35Ie (8) Thus, the relative electrical life can be calculated according to equation (5). 2. Conclusion This paper proposes a convenient and easy-to-implement method for online monitoring of the electrical life of circuit breaker contacts from an engineering practical perspective. Based on this method, an online monitoring method for predicting the electrical life of oil-less and vacuum circuit breakers can be derived. Applying this monitoring method, we have easily developed an online monitoring device for the expected electrical life of circuit breaker contacts. This device can simultaneously monitor the phase wear and electrical life of all circuit breakers in a substation in real time. When the circuit breaker opens, it can record the breaking current according to the current transformer ratio set by the user, and calculate the corresponding relative electrical wear according to the electrical wear model of different circuit breakers, thus obtaining the current relative electrical life of the circuit breaker; when the electrical life of the circuit breaker has expired, an alarm signal is issued. This provides a convenient means for operating departments to understand the condition and expected life of circuit breakers.