Fuses and circuit breakers are common protective devices in electrical systems, and they play a key role in electrical circuits to prevent damage to circuits caused by overloads and short circuits. During the normal operation of electrical equipment, fuses and circuit breakers can effectively isolate the faulty part and protect the stability and safety of the whole system. So the question is, what conditions should be considered when choosing a circuit breaker?The following article will detail the key points of the selection of fuses and circuit breakers to help the majority of electrical personnel make a wise choice in practical applications.
01 Conditions to be considered when selecting a fuse
1) The functional role of the fuse.
It is mainly used to protect the short circuit of the electrical system, when the impact load of the system is small or zero, and when the capacity of the electrical equipment is small or the protection requirements are not high, it should be used as overload protection.
When a short-circuit fault occurs in the protected line or equipment, the melt of the fuse is immediately blown to cut off the short-circuit current and protect the line and equipment. When the protected line or equipment is overloaded, the melt of the fuse is delayed and the overload current is cut off, but due to the inconsistent delay characteristics of various types and specifications of the fuse, the overload capacity of the equipment and the line is not the same, so that the delay characteristics and the overload capacity are difficult to match. Therefore, the general fuse is only used for short-circuit protection, and does not need to be overload protected.
2) The selection of the fuse (including the selection of its melt) should consider the following conditions.
1) The voltage level of the fuse must be greater than or equal to the voltage level of the protected line or equipment.
2) The rated current of the fuse should be greater than or equal to the rated current of the installed melt.
3) The type of fuse should meet the installation conditions (indoor or outdoor) and the technical requirements of the equipment and lines to be protected.
4) The breaking current of the fuse must meet the needs of the short-circuit current calculation value at the installation site.
5) There should be selective cooperation between the fuses of the front and rear stages of the line.
6) The selection of melt current is different according to the different devices to be protected. The ratio of the rated current of the fuse selected for different electrical equipment to the rated current of the electrical equipment is different.
7) There are many models of fuses, the protection characteristics are very different, the selection should be considered in accordance with the above conditions, among which the high-voltage fuse, the total fuse of the low-voltage outlet of the transformer and its branch must be selected in accordance with the above conditions, and the protection fuse of various low-voltage electrical equipment and electrical appliances is generally only considered the rated current and peak current.
02 Conditions to consider when selecting a circuit breaker
1) Circuit breaker function.
Commonly known as air opening) is mainly used to connect and disconnect electrical circuits and electrical equipment, (should be operated with load) and has short-circuit protection, overload protection function, and under-voltage, loss of voltage protection, low-voltage circuit breaker is a complex structure, excellent performance of the switching element, can instantly cut off the short-circuit current, delay to cut off the overload current, can be operated at a distance, replaced the AC contactor, thermal relay, fuse of the combined switching circuit, safe operation, easy to use.
2) The conditions that should be considered in the selection of low-voltage circuit breakers.
1) The rated voltage of the circuit breaker should be greater than or equal to the rated voltage of the device point.
2) The rated current of the low-voltage circuit breaker should be greater than or equal to the calculated current at the device point.
3) The rated current of the low-voltage circuit breaker tripper should be greater than or equal to the calculated current at the device point.
4) The ultimate on-off capacity of the low-voltage circuit breaker should be greater than or equal to the maximum short-circuit current at the device point.
5) The ratio of the single-to-ground short-circuit current at the end of the line to the setting current of the empty-open instantaneous or short-delay tripper should be greater than or equal to 125。
6) The rated voltage of the low-voltage circuit breaker and under-voltage tripper should be equal to the rated voltage of the device point.
3) The selection method of low-voltage circuit breaker for power distribution device, motor, lighting and other adjacent electrical appliances is different, and attention should be paid to the selection.
4) Use with fuse: for the low-voltage circuit breaker with small capacity or the breaking capacity of the low-voltage circuit breaker is less than the calculation of short-circuit current, it is advisable to use the fuse in series in series to measure the power supply in the air to make up for the defect of the small breaking capacity of the low-voltage circuit breaker, and also avoid the cost of selecting a low-voltage circuit breaker with a larger breaking capacity, and have the role of backup protection, and can choose a short delay (0.1 second) circuit breaker.
Finally, the commonly used calculations in electrical design have been clarified at one time, and the results can be directly entered!
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