## 2 Circuit Breaker Overload Protection Working Principle and Its Service Status
### Overload Protection Service State Considering Safety Margins for Action Characteristics  
In the process of the degradation of the performance of low-voltage circuit breaker thermal disconnectors, the overload protection action time is selected as the characteristic quantity. When the action temperature rise of the thermal overload protection is \(\tau_{d}\), the circuit breaker overload protection action time can be obtained from Equations (1) and (2):  
\[t_{d}=Tln\frac{\tau_{w}-\tau_{0}}{\tau_{w}-\tau_{d}} \tag{6}\]  
According to the provisions of the national standards, the action characteristic relationship of different overload currents of the circuit breaker can be calculated, as shown in Figure 3. Where the normal overload protection action characteristic is within the range of curve 2 and curve 3, curve 2 and curve 3 are called the upper limit and lower limit of the  
Figure 2: Deformation of the bimetallic strip. overload protection action time curve, respectively, indicating the maximum and minimum action times corresponding to each overload current value. Curves 1 and 4 represent the failure of the protection characteristics, indicating that the overload protection action time no longer meets the requirements for release. In practice, the overload protection action characteristic curve will be affected by various factors, producing a curve with an error band, and the more serious the influence, the wider the error band. In order to maintain the operating state of the equipment and the overload protection action characteristics for good cooperation, it is necessary to select the appropriate action curve according to the actual situation, reduce the amount of load loss, improve the service life of the equipment, and obtain a better adjustment effect for the overload protection [33; 34]. The service processes of low-voltage circuit breaker overload protection action characteristics gradually change, so its performance degradation is relatively slow and the service state cannot be simply described as being in a normal state or fault state. Therefore, it is proposed that the action characteristics of low-voltage circuit breaker overload protection service state are divided into different states. Considering the actual service situation of the circuit breaker thermal release, the safety margin of the initial state of the low-voltage circuit breaker is defined as 1 and when the protective characteristics of the thermal release are degraded to the limit value the safety margin is 0; the expression of which is shown in Equation (7). The state grading is carried out according to the change in the safety margin, which is divided into five states. \[Y_{t,t}=\frac{t_{m,i}-t_{d,i}}{t_{m,i}-t_{c,i}}\times 100\% \tag{7}\]  
where \(t_{m,i}\) is the current overload current that is allowed to protect the action time limit value (when the trend of action time change is gradually increasing the upper limit value is taken and vice versa), \(t_{d,i}\) is the actual overload protection action time for current overload current, and \(t_{c,i}\) is the initial overload protection action time for current overload current. State 1 (healthy state): 100% \(\geq\) safety margin > 60%, the action time of overload protection meets the standard requirements and has enough safety margin to withstand the influence of external factors without exceeding the limit of overload protection action time. The current state of the circuit breaker overload protection function itself is not faulty; its overload protection action time is the shaded part in Figure 4a. Under different overload currents, the circuit breaker overload protection can ensure normal operation and realize various specified functions without any maintenance work. Figure 3: Protection characteristics and fail-safe characteristics.