## 2 Circuit Breaker Overload Protection Working Principle and Its Service Status
### Overload Protection Service State Considering Safety Margins for Action Characteristics
State 2 (mild degradation state): 60% \(\geq\) safety margin > 30%, the overload protection action time can meet the standard requirements, the overall performance has been reduced, the action time curve has been shifted, the safety margin has been reduced, and the ability to withstand external influencing factors has been weakened. Its overload protection action time is shown as the shaded part in Figure 4b. Under different overload currents, the circuit breaker overload protection can meet the requirements. The main technical performance indicators of the equipment are within the permissible range; the overall performance has deteriorated but this will not affect normal operation and can be repaired as planned. State 3 (heavy degradation state): 30% \(\geq\) safety margin > 0%, the overload protection action time is in the limit edge state and its action characteristic performance is further degraded. Its overload protection action time is the shaded part in Figure 4c. Once affected by the external contingent factors, the overload protection action time will be out of the limit and the overload protection function of the circuit breaker enters into the faulty state, which can be chosen to be repaired. A part of the main technical performance of the equipment is degraded, there is a tendency to deteriorate, and there are a small number of bad working conditions recorded; however, it can still fulfill the main functions in the use of the time to strengthen the monitoring and pay attention to the trend of the state of health changes. State 4 (general fault state): 0% \(\geq\) safety margin > -20%, the overload protection action time does not meet the standard requirements. The overload protection action time is shaded in Figure 4d. Although it can complete the function of release, the action time is out of the standard range, which will lead to a large amount of heat being generated by the equipment or line and there is a potential danger; however, it will not cause damage to the equipment or line. The circuit breaker needs to be replaced or repaired. The major technical performance of the equipment is severely degraded, with a clear trend of deterioration and a record of poor working conditions, which may be considered for implementation of maintenance activities prior to the planned maintenance time. State 5 (severe fault state): \(-20\%\geq\) safety margin, the overload protection action time does not meet the standard requirements or even cannot be released or closed, which is manifested as a failure to act or a malfunctioning fault. The overload protection action time  
Figure 4: Overload protection action time state change diagram. is the shaded part in Figure 4e. If not dealt with in time, the normal operation of power supply lines or electrical equipment cannot be ensured, which may lead to significant economic losses; therefore, the circuit breaker should be replaced or repaired immediately. Service Reliability Analysis of Low-Voltage Circuit Breaker Overload Protection Based on the Markov Process  
The Markov model is a stochastic process that uses probability statistics to describe the state transfer of a device or system [35] and has an important feature of memorylessness, i.e., the next state of a device or system in operation depends only on the current state and is not affected by the previous state, i.e., at any time \(t_{n+1}(t_{n}<t_{n+1})\), its state \(X(t_{n+1})\) is related only to \(X(t_{n})=x_{n}\) and has nothing to do with \(X(t_{1})=x_{1},\ \ldots,X(t_{n-1})=x_{n-1}\). The relationship between the five service states of low-voltage circuit breaker overload protection can be described as a Markov process in which the next state depends only on the current state, satisfying the Markov property.