Abstract: Using the framework of various types into the line circuit breaker as the circuit switches the main switch and distribution, of course, has many advantages, but the ACB has the high price and bulky, contact protection poor, weak, so as the line the main switch, is not the only option. This paper introduces the main switch voltage into the line a reasonable choice.
        Keywords: circuit breaker switch framework of the low-voltage distribution system into the line of the main switch, the designer usually the preferred framework for all types of circuit breaker, and the occasion will be more intelligent selection of ACB. Into the line of low-voltage power grid capacity should be combined with the main switch, load properties, the use of places, from the technical and economic point of view, comprehensive analysis and comparison, a reasonable selection of different types of low-voltage circuit breakers.

1, low into the line of the selection of the main switch
        As low pressure into the line of switches and other power distribution circuit switch close to the distribution transformer, "transformer, so the requirements of the electrical parameters of the switch is very high. First and foremost, within the scope of protection occurs in the symmetric and asymmetric short circuit on the expected short-circuit current, Line switch and power distribution circuit should be able to reliably switch breaking. that is asked all kinds of circuit breaker short-circuit breaking capacity not less than the maximum expected short circuit current, to ensure safe operation of power supply circuit. Second, the main switch and distribution into line electrical circuit switch in the short-circuit breaking current, the action should be selective, that is, when a short circuit in a power distribution branch, into the circuit breaker should be 0.1 ~ 0.4s delay after the action, and slip distribution Molded Case Circuit Breaker is instantaneous, and to ensure the normal supply of non-fault circuit.

    In general, low-voltage circuit breaker with a multi-level, multi-species characteristics. The so-called multi-level refers to the similar products of high, medium and low distinction, to meet the needs of different industries; many varieties to meet different use of the environment, the needs of different conditions. All walks of life from the current application and development of the situation, the ACB does not need to pursue all of our products are high performance, but according to the actual conditions, choose a different performance products. According to the information for a long period of time, our country will exist ACB general type, high, high-performance type of the three grades of products.
        ACB general type represented by DW16, overload protection features have the long delay and instantaneous secondary protection, does not have the selective protection. It features layout, fixed installation, the side handle operation, simple structure, easy maintenance, low price, relatively low breaking capacity for selective protection does not require general industrial systems, general civil construction and the rural distribution system into the line as the main switch or power distribution circuit switch.
        High-type ACB to DW15, ME (DW17), 3WE, AE, AH, and other representatives. They have three levels of protection performance, selective protection. Characterized by positive operating handle can be fixed, taking the installation, affordable, high breaking capacity for the main switch and distribution into the line circuit switch.
        High-frame circuit breakers force to DW45, M, F, 3WN6, AE-ss, CW1, HSW1, RMW1 series represented. Can release with various protection features, including intelligent release, enabling the computer network communications. It features no arc, small, accessories complete, modular structure, the overall box plastic structure, beautiful appearance, protection performance improvement, high breaking capacity.
        Various types of circuit breaker used as the framework into the line of the main switch and the power distribution circuit switching, of course, has many advantages, but the ACB has the high price and bulky, contact protection poor, weak, so the line as the main switch is not the only option. The distribution function based MCCB have low price, small size, exposure protection is good, installation of compact, beautiful appearance. Not generally use the past as a molded case switch into the line of the main switch, mainly because the old plastic molded switch by structural constraints, small capacity, short-circuit breaking capacity is low, there is no difference between selective and short-tolerance. However, with new technologies, new processes, the continual emergence of new materials, molded case circuit breaker of the fourth generation of the rated current, breaking capacity, short-term tolerance, optional features and other technical parameters have been greatly improved and enhanced. Is to limit the arc current increases, in particular the technical, VJC technology and rotating double break interrupter system technology, the switches are smaller, breaking capacity greatly increased. Molded Case Circuit Breakers of the new generation of intelligent release has the function of short-circuit short delay (eg, low-voltage electrical plant in Tianjin TM30 series), Schneider NS series of electronic trip unit also has this feature.

2, the choice of Attention
        Into the line in the rational use of the main switch at the same time, there are several noteworthy issues are:
        (1) The capacity of low-voltage side of transformer short-circuit current value, both short-circuit capacity is based on the high side a little early when ∞ calculated value, calculated in this way short-circuit current is too large, in the short-circuit current calculation of the actual project, the system still should be considered impedance. In addition, when 2 sets of parallel operation with the capacity of the transformer, the short-circuit current value can be increased to double the line voltage switch should be used to attract attention, to ensure reliable and breaking into the outlet switch fault short circuit current, a single distribution transformer capacity of not more than 1600kVA.

    (2) was elected as the incoming line with the molded case switch the main switch, general power line access can only be motivated, then qualify under the load, if the next into the line connected power supply, then the load on the outlet, easily breaking short-circuit current in the caused further breakdown arc. Therefore, the current into the line, the need to reduce short-circuit breaking capacity, the general need to reduce the use of 20% to 30%.

    (3) of the intelligent selection of ACB and the entire power distribution system should be designed consistent positioning, and the entire distribution system to match the design grade. I believe that, if the main project integrated automation, intelligent design is relatively high grade, high voltage power supply system has been designed to protect the integrated automation computer, and the central computer through a communications network card and connected form the intelligent system, already has four remote conditions. Low-voltage distribution system into the line of the main switch and the PC circuit and circuit breaker MCC should be used in high-performance type of intelligent circuit breaker, high voltage power supply system in order to achieve the centralized controls, switches and electrical outlet into the circuit to achieve functional improve the protection of more complete and more reliable protection with the realization of higher and lower levels.

    Power supply system for general industrial and civil construction power supply system, no need to use a lot of intelligent ACB. If a county water plant to transform the old distribution system when a 315kVA transformer, low voltage switch into the line of a selected frame CW1-2000/630A intelligent circuit breaker, apparently not necessary. Another example of a city mall and a commercial hybrid commercial and residential high-rise buildings of 2 units into the line of low voltage switches and mother of the switch, are the company selected ABB F Series with remote monitoring and intelligent communication frame circuit breaker is not necessary.