Main Characteristics of Molded Case Circuit Breaker

Rated limit short circuit breaking capacity Icu

There are two kinds of breaking capacity indexes of circuit breaker: rated limit short circuit breaking capacity Icu and rated operation short circuit breaking capacity Ics. As a characteristic parameter, ICs not only considers the breaking ability of circuit breaker, but also acts as a breaking index, that is, it can ensure its normal operation after several short-circuit breakdowns. For plastic case circuit breaker, there should be enough Icu to break short circuit current and make the switch trip. According to the regulations, the Ics of plastic case circuit breaker is qualified as long as it is more than 25% Icu. At present, most of the ICs of circuit breakers in the market are between (50%-75%) Icu, so only Icu should be considered for distribution systems with low power supply requirements.

Current Limiting and Breaking Capacity

When the circuit breaker is short-circuited, the contact opens quickly to generate arc, which is equivalent to a rapidly increasing arc resistance in series in the circuit, thus limiting the increase of fault current. The shorter the breaking time of circuit breaker is, the closer Ics is to Icu, the better the current limiting effect is. It can also greatly reduce the adverse effects of electromagnetic, electric and thermal effects caused by short circuit current on circuit breakers and electrical equipment, and prolong the service life of circuit breakers.

Short circuit protection

Short-circuit protection is short-circuit instantaneous tripping. Attention should be paid to adjusting the setting value of protection in time after load changes, so as to prevent too small setting value from frequent trips affecting the quality of power supply, or setting value too high to effectively protect lines and equipment.

Overload Delay Protection

Overload delay protection refers to the danger of burning down the equipment when the load current exceeds the limit of the equipment. The protection device can cut off the power supply in a certain time. Overload has a process of heat accumulation, and protection action does not need to be too fast. For short-term overcurrent, protection should not be operated.

Isolation function

Isolation function is to require the leakage current of circuit breaker after disconnection not to cause harm to people and equipment. After several short circuit trips, the performance of the switch decreases and the leakage current increases. For human body, the leakage current below 30 mA is safe. In harsh environment, the leakage current over 300 mA lasts for more than 2 hours, which may cause phase-to-ground short circuit of insulation damage and fire.

Leakage protection

There are two kinds of leakage apparatus: thermal magnetic leakage apparatus and electronic leakage apparatus. Compared with electronic leakage apparatus, electronic leakage apparatus has the advantages of small size, high precision and high sensitivity, but its anti-interference ability is poor. At present, electronic leakage protector occupies the mainstream. When the leakage current reaches the setting value, the executive circuit receives the induction voltage signal of the secondary side of zero sequence current transformer, drives the conversion contacts to output the leakage protection signal, so that the trip action cuts off the power supply.

The setting leakage current of general terminal switch is 30 mA, and the setting value of upper branch switch is 300 mA. Arc short circuit with high fire risk is difficult to be effectively cut off by short circuit protection, and leakage apparatus can reliably disconnect grounding fault to prevent the occurrence of personal shock and phase-to-ground short circuit fault.