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Miniature circuit breakers selection


Miniature circuit breakers (hereinafter referred to as MCB) is the most widely used terminal protection device in building electrical terminal distribution equipment. Although MCB is a terminal appliance. However, it is a very large amount. If an inappropriate MCB is chosen, the loss will be heavy. This article based on MCB common electrical parameters to talk about the correct selection of Miniature circuit breakers method.

MCB rated breaking capacity rated breaking capacity is to ensure that the circuit breaker without any damage to the premise of breaking under the maximum short-circuit current value. Now seen on the market MCB, according to various manufacturers to provide technical information and design manuals, generally 4.5kA, 6kA, 10kA several rated breaking capacity. MCCB MCCB (MCCB) and ACB (Frame Circuit Breaker) should be used as the MCB. When calculating the maximum short-circuit capacity of the MCCB, select the MCB. If the rated breaking capacity of MCB is less than the short-circuit fault current within the scope of protection, the fault of MCB can not only break the fault line, but also cause the MCB explosion due to the too small breaking capacity of Miniature circuit breakers, endangering the body and other electrical equipment lines Safe operation.

The short-circuit current of the low-voltage distribution line is related to the electrical parameters such as the conductor cross-section of the power supply line, the laying route of the conductor, the distance between the short-circuit point and the power supply, the capacity of the distribution transformer, and the percentage of impedance. General industrial and civil distribution transformer low-voltage side of the voltage is mostly 0.23 / O. 4LV, most of the transformer capacity of 1600kVA and below, the low-voltage side of the line current with the distribution capacity increases. For different capacity distribution, low-voltage feeder short-circuit current is different. Generally speaking, for residential buildings, small shopping malls and public buildings, due to the low voltage power grid supplied by the local power supply department, the cable or overhead conductor of the power supply line has a small cross section and the power equipment is far away from the power supply. 4.5kA and MCB can break the ability above. For those who have dedicated or have 10kV substation users, often due to the power supply lines Ping Ping thicker, shorter power supply, should be rated 6kA and above MCB rated breaking capacity. For similar substations such as substations (lighting used in the station, power supply taken directly from the low-voltage busbar) and substations with large capacity substations (for the workshop), it is necessary Use 10kA and above Miniature circuit breakers breaking capacity, the specific design must also be verified.

In addition, three points of particular attention are:

  1. With the increase of distribution transformer capacity in modern buildings, the use of large capacity busbars and the shortening of the distance between the power supply and the power supply make the short-circuit current at the end of the power supply line also continuously increased. Is some high-end office buildings, office buildings, hotels and shopping malls and other public buildings, such occasions used in the Miniature circuit breakers, should be noted in the design.
  2. Miniature circuit breakershas two product standards: one is IEC898 “household devices and similar devices with circuit breakers” (GBl0963-1999); the other is IEC947-2 “low voltage switchgear and control equipment, low voltage circuit breakers.” The EC898 is for non-electrical professionals and inexperienced personnel while IEC947-2 is for product standards that are operated by electrical professionals. Two standards on the MCB rated breaking capacity indicators are different, for designers, we must look at the specific use of the occasion and the object to choose MCB. According to IEC947-2 rated breaking capacity to choose MCB, should be installed in the cabinet for professionals to operate and operated by professionals, such as the floor, the plant’s lighting distribution box; if by IEC898 to choose MCB , For installation in non-professional use of the operating box, such as large conference rooms, factory lighting switch box, these objects are used by the general staff. Therefore, the choice of MCB must pay attention to be distinguished, can not be confused.
  3. In general, Miniature circuit breakersrated breaking capacity is measured in the upper terminal into the line, the lower terminal outlet status. In the project if the special circumstances under the requirements of the terminal into the line, the terminal outlet, due to cut off the fault current arc reasons, Miniature circuit breakersmust be derating use, that is, the rated breaking capacity must be provided by the manufacturer of the derating factor To convert. Now some manufacturers manufacture MCB, both upper and lower terminals can enter the line and free installation, breaking capacity is not affected, but I believe that, as a last resort should be carried forward. Miniature circuit breakers protection features According to IEC898, Miniature circuit breakers is divided into A, B, C, D four characteristics for the user to choose:

(1). A characteristics are generally used in applications that require fast, no-delay tripping, ie, for lower peak current values (typically 2-3 times the rated current / n) to limit the allowable short- Current value and the total breaking time, the use of this feature can make MCB alternative fuses as electronic components over-current protection and mutual inductance measurement circuit protection; (2). B characteristics are generally used for faster tripping and the peak current Not a big application; Compared with the A characteristic, B characteristic allows the peak current <3In generally used for incandescent lamps, electric heaters and other resistive loads and residential line protection;

(3). C characteristics are generally applicable to most of the electrical circuit, which allows the load through the high short-term peak current and the MCB does not operate, C characteristics allow the peak current through <5In generally used for fluorescent lamps, high pressure gas discharge lamps, Power distribution system line protection;

(4). D Characteristics Switchgear generally suitable for very high peak currents (<10In) and are generally used for the protection of the primary wiring and solenoid valves of control transformers and local lighting transformers at AC rated voltage and frequency.