DC MCCB

Rated limit short-circuit breaking capacity Icu

There are two types of breaking capacity indicators of circuit breakers: rated limit short-circuit breaking capacity Icu and rated operating short-circuit breaking capacity Ics. Ics, as a characteristic parameter, does not simply consider the breaking capacity of the circuit breaker, but as a breaking indicator, that is, after breaking several short circuit faults, it can ensure its normal operation. For plastic-case circuit breakers, there should be enough Icu to break the short-circuit current and make the switch trip. According to the regulations, the Ics of the moulded case circuit breaker is more than 25%Icu is qualified. At present, most of the Ics of circuit breakers on the market are between (50%-75%)Icu, so the power distribution system with low power supply requirements only needs to consider Icu.

Limiting current breaking capability

When the circuit breaker is short-circuited, the contact quickly opens to generate an arc, which is equivalent to a rapidly increasing arc resistance in the line, thus limiting the increase of the fault current. The less time the circuit breaker is broken, the closer the Ics is to the Icu, the better the current limiting effect, and the adverse effects of the electromagnetic effect, electric effect and thermal effect caused by the short circuit current on the circuit breaker and electrical equipment can be greatly reduced, and the service life of the circuit breaker can be extended.

Short circuit protection

Short circuit protection is short circuit instantaneous trip. It is necessary to pay attention to adjusting the setting value of the protection in time after the load change to prevent the setting value from being too small and frequent tripping to affect the power supply quality, or the setting value is too small to prevent the line and equipment from being effectively protected.

Overload delay protection

Overload delay protection means that the load current exceeds the limited range of the equipment and there is a risk of burning the equipment, and the protection device can cut off the power supply within a certain time. There is a process of heat accumulation, and the protection action does not need to be too rapid. For short periods of overcurrent, the protection should not operate.

Isolation function

The isolation function requires that the leakage current after the circuit breaker is turned off will not cause harm to the person and equipment. After multiple short-circuit trips, the performance of the switch decreases and the leakage current increases. For the human body, the leakage current below 30mA is safe, and in a harsh environment, the leakage current of more than 300mA lasts for more than 2 hours, which may cause the insulation damage to occur phase short circuit and then cause a fire.

Leakage protection

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

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