Molded Case Circuit Breaker (MCCB) is a device used for overload long-time delay and short-circuit instantaneous tripping, featuring rated ultimate short-circuit breaking capacity, current-limiting breaking capacity and other characteristics.
Rated Ultimate Short-Circuit Breaking Capacity (Icu)
There are two types of breaking capacity indicators for circuit breakers: Rated Ultimate Short-Circuit Breaking Capacity (Icu) and Rated Service Short-Circuit Breaking Capacity (Ics). As a characteristic parameter, Ics does not simply consider the breaking capacity of the circuit breaker, but serves as a breaking indicator—meaning it can still ensure normal operation after breaking short-circuit faults several times. For molded case circuit breakers, there must be sufficient Icu to break the short-circuit current and trip the switch. According to regulations, a molded case circuit breaker is qualified if its Ics is greater than 25% of Icu. Currently, the Ics of most circuit breakers on the market ranges from 50% to 75% of Icu, so for power distribution systems with low power supply requirements, only Icu needs to be considered.
Current-Limiting Breaking Capacity
When a short circuit occurs in the circuit breaker, the contacts open quickly to generate an electric arc, which is equivalent to inserting a rapidly increasing arc resistance in the line, thereby limiting the increase of fault current. The shorter the breaking time of the circuit breaker, the closer Ics is to Icu, and the better the current-limiting effect. This can also greatly reduce the adverse effects of electromagnetic effects, electrodynamic effects, and thermal effects caused by short-circuit current on the circuit breaker and electrical equipment, extending the service life of the circuit breaker.
Short-Circuit Protection
Short-circuit protection refers to instantaneous tripping in the event of a short circuit. It is important to promptly adjust the protection setting value after load changes to prevent frequent tripping due to an excessively small setting value (which affects power supply quality) or failure to effectively protect lines and equipment due to an excessively large setting value.
Overload Time-Delay Protection
Overload time-delay protection means that when the load current exceeds the limit range of the equipment (posing a risk of burning the equipment), the protection device can cut off the power supply within a certain period of time. Overload involves a heat accumulation process, so the protection action does not need to be too rapid. The protection should not act for short-term overcurrents.
Isolation Function
The isolation function requires that the leakage current of the circuit breaker after opening does not cause harm to personnel and equipment. The switch performance degrades after multiple short-circuit trips, and the leakage current will increase. For the human body, a leakage current below 30mA is safe. In harsh environments, a leakage current exceeding 300mA lasting more than 2 hours may damage insulation, leading to phase-to-ground short circuits and subsequent fires.
Leakage Protection
There are two types of leakage protectors: thermal-magnetic and electronic. In comparison, electronic leakage protectors have the advantages of small size, high precision, and high sensitivity, but their anti-interference ability is relatively poor. Currently, electronic leakage protectors dominate the market. When the leakage current reaches the setting value, the executive circuit receives the induced voltage signal from the secondary side of the zero-sequence current transformer, drives the changeover contact to output a leakage protection signal, and causes the release to act and cut off the power supply.
The rated leakage tripping current of general terminal switches is 30mA, and that of the upper-level branch switches is 300mA. Arc faults with high fire risk are difficult to be effectively cut off by short-circuit protection, while leakage protectors can reliably disconnect ground faults, preventing personal electric shock and phase-to-ground short-circuit faults.