How to correctly choose low-voltage circuit breakers and main factors

Low-voltage circuit breakers have overload and short-circuit protection functions. According to their structure, they can be divided into frame circuit breakers (ACB), molded case circuit breakers (MCCB), and miniature circuit breakers (MCB). The overload protection of MCB is realized by the inverse time characteristic composed of two metal sheets with different thermal expansion coefficients into one metal sheet. The larger the overload current, the shorter the action time. The short-circuit protection is The electromagnetic release is realized. Circuit breakers play a very important role in protecting circuits and electrical equipment. So how to choose a low-voltage circuit breaker correctly, and what factors need to be considered when choosing a low-voltage circuit breaker?

1. Select the circuit breaker according to the use environment

1. For dusty environments, dust-proof circuit breakers should be selected;

2. For chemically corrosive environments, anti-corrosion circuit breakers should be selected;

3. The altitude of the installation site is more than 2000m. Air cooling and the decrease of dielectric strength need to be considered, and the circuit breaker used at high altitude should be selected;

4. For explosion and fire hazardous locations, the requirements of the corresponding environment and regulations for circuit breakers shall be met;

5.In high temperature (ambient temperature greater than 40 degrees Celsius) and humid (relative humidity greater than 90%) places, you should choose anti-humidity products

2. Select according to normal working conditions©

1. The rated frequency and rated voltage of the circuit breaker should be compatible with the nominal value of the circuit. In some cases, the maximum or minimum voltage that may appear in the circuit under normal conditions is also considered.

The setting current of the circuit breaker is less than or equal to the allowable continuous ampacity of the conductor to protect the circuit. If the setting value of the circuit breaker is greater than the allowable continuous ampacity of the conductor, during overload operation, the conductor may generate more heat than the allowable maximum during normal operation. Temperature, over time, will accelerate the aging of the conductor insulation and deteriorate the insulation performance. When an overvoltage occurs, an insulation breakdown may occur and cause an electrical fire accident.

Three, choose according to short-circuit conditions

1. The short-circuit protection circuit breaker of the distribution line should cut off the power before the thermal and mechanical effects of the short-circuit current on the conductor and the connection cause harm;

2.The circuit breaker should be able to break the finite value of the expected short-circuit current periodic component at its installation location, and the expected short-circuit current should be determined by calculation or measurement. When the sum of the rated currents of the motors connected near the short-circuit point exceeds 1% of the short-circuit current. The influence of the reverse current of the motor should be considered.

Fourth, the selectivity between superior and superior

1. The upper and lower circuit breakers installed on important distribution lines should have selective action characteristics and coordinate with each other. Selectivity means that when the line fails, only the circuit breaker at the first end of the line is tripped, and the upper-level circuit breaker does not trip, so as to minimize the power outage. The methods for achieving selectivity between upper and lower circuit breakers are: current setting value, time, and energy.

2. For non-important loads, partial or non-selective cutting can be used. Partial selectivity means that the selectivity between the upper and lower levels can sometimes be achieved, and sometimes cannot be achieved, which is related to the fault current of the system.

5. Check the sensitivity of circuit breaker short circuit protection

1. When the short-circuit protection appliance is a circuit breaker, the minimum short-circuit current at the end of the protected circuit should not be less than 1.3 times the setting current of the instantaneous or short-delay overcurrent release of the circuit breaker. Taking the coefficient of 1.3 mainly takes into account the calculation error. The sensitivity of the short circuit protection of the circuit breaker is to be checked so that when the circuit or equipment is short-circuited, the circuit breaker can act instantaneously within the agreed time to remove the faulty system and protect the safety of people and property.