Miniature circuit breakers (MCBs) generally refer to small circuit breakers suitable for AC 50/60Hz, rated current up to 60A, and rated voltage 230/400V. They are mainly used for line overload and short circuit protection. Due to their simplicity and practicality, they automatically disconnect power in the event of circuit overload or short circuit, thus protecting the circuit. Therefore, they are one of the more common electrical devices and are commonly found in various power-using locations. However, there are some situations where this type of circuit breaker is not suitable or cannot be used. Below is a brief explanation of the situations in which it is not recommended or not possible to use this type of MCB.
First, circuit breakers are mainly used for overcurrent protection in circuits. When an overload or short circuit occurs in the circuit, the current becomes too large and exceeds the current carrying capacity range. The circuit breaker then cuts off the circuit to prevent the fault from escalating further. A key aspect here is overcurrent protection.
Once you grasp this point, it's easy to understand why it's not recommended to use it directly in motor circuits: First, because sometimes the motor fails before the current flows, or sometimes it fails without the current flowing. In this case, the circuit breaker, as a circuit protection device, either doesn't work or has a certain lag, and doesn't have the characteristics of real-time and timely circuit protection.
Secondly, the selection of circuit breakers is generally based on 1.2 to 1.3 times the total current. However, for motors, the starting current must be considered, which is generally much higher than 1.3 times. This is contradictory. Therefore, based on the above analysis, circuit breakers are not suitable for use in motor protection circuits.
Secondly, these small circuit breakers offer no protection against electric shock or electrical imbalance, so they should never be used as protection devices against electric shock or electrical imbalance, because circuit breakers only operate during overload and short circuits, and leakage current is not within their operating range.
Finally, it's worth mentioning that when this type of miniature circuit breaker is used in residential circuits, it is often used in conjunction with a residual current circuit breaker to effectively balance the protection of the circuit and the protection of personal safety.
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