When it comes to miniature circuit breakers, most electrical personnel are likely familiar with them. However, when it comes to choosing between A, B, C, and D miniature circuit breakers, many electrical professionals may only have a vague understanding, and some may be completely unaware.We all know that miniature circuit breakers, abbreviated as MCBs, are widely used circuit breaker products,whose main function is to provide protection for electrical distribution devices in buildings. They are suitable for overload and short-circuit protection of circuits with an AC rated voltage of 230/400V at 50/60Hz and a rated current of up to 63A, and can also be used for infrequent switching of circuits under normal conditions.Miniature circuit breakers (MCBs) are mainly used in various places such as industrial, commercial, high-rise, and residential buildings. Thecommon tripping characteristics of circuit breakers are A, B, C, and D types. So the question arises, how do we correctly choose A, B, C, and D miniature circuit breakers?
There are generally four types of tripping characteristics for circuit breakers, namely A, B, C, and D:A-type circuit breaker: 2 times the rated current, rarely used, generally for semiconductor protection (usually using fuses)B-type circuit breaker: 2-3 times the rated current, generally used for purely resistive loads and low-voltage lighting circuits, commonly used in household distribution boxes to protect household appliances and personal safety, currently used less frequently.C-type circuit breaker: 5-10 times the rated current, needs to trip within 0.1s, circuit breakers with this characteristic are most commonly used to protect distribution lines and lighting circuits with larger connection currents.D-type circuit breaker: 10-20 times the rated current, mainly used in environments with high instantaneous currents, generally used less in households. Suitable for systems with large inductive loads and high inrush currents, commonly used to protect equipment with high inrush currents.The so-called multiple currents: refers to the resistance to inrush currents. The switch does not trip within a certain time. Its characteristic is to avoid inrush currents.Selection of low-voltage circuit breaker tripping types: The tripping types of circuit breakers include overcurrent tripping, undervoltage tripping, and parallel tripping, etc.Overcurrent trip units: can be divided into overload trip units and short-circuit current trip units, with long delay, short delay, and instantaneous types; overcurrent trip units are the most commonly used.The setting value of the operating current of the overcurrent trip unit can be fixed or adjustable, usually adjusted by a rotary knob or lever. There are two types of electromagnetic overcurrent release: fixed and adjustable. Electronic overcurrent releases are usually adjustable.The breaking capacity of the circuit breaker: refers to the ability to withstand the maximum short-circuit current, so the breaking capacity of the circuit breaker must be greater than the short-circuit current of its protective device.Overcurrent trip units by installation method: are divided into fixed installation and modular installation. Fixed devices are processed as a whole with the circuit breaker at the factory. After the product leaves the factory, the rated current of the release is not adjustable, while modular installation releases can be adjusted at any time, providing great flexibility.Instantaneous type: 0.02s, used for short-circuit protection;Short delay type: 0.1-0.4s, used for short-circuit and overload protection;Long delay type: less than 10s, used for overload protection;The commonly used DZ series air switches (miniature circuit breakers with leakage protection) have the following specifications:C16, C25, C32, C40, C60, C80, C100, etc., where C represents the tripping current characteristic of C, that is, the tripping current, for example, C20 represents a tripping current of 20A, with a tripping characteristic of C curve. A C20 circuit breaker is generally selected for a 3500W water heater installation, while a C32 circuit breaker is generally used for a 6500W water heater installation.
Circuit breakers are used to protect wires and prevent fires, so they should be selected based on the size of the wire rather than the power.If the selection of the circuit breaker is too large, it cannot protect the wire. When the circuit breaker is overloaded, it still does not trip, posing a safety hazard to the household.1.5 square wire with C10 switch.2.5 square wire with C16 or 20 switch.4 square wire with C25 switch.6 square wire with C32 switch.For air switches with motors as loads, D-type characteristics should be selected to avoid the high starting current of 5-8 times the motor starting current.
C-type circuit breakers suitable for motor circuits?It is well known that ordinary circuits such as lighting circuits use C-type miniature circuit breakers; motors and other power circuits use D-type circuit breakers. So can C-type miniature circuit breakers be used for motor circuits?Let’s take a look at the differences between C-type and D-type miniature circuit breakers?C-type miniature circuit breaker: includes overload protection and short-circuit protection, with a short-circuit protection tripping value of 5-10 times the rated current;D-type miniature circuit breaker: includes overload protection and short-circuit protection, with a short-circuit protection tripping value of 10-20 times the rated current;The two types of overload protection are the same, with the only difference being the tripping range of short-circuit protection.Generally, ordinary loads have no starting current, that is, the starting current is equal to the rated current; the starting current of a three-phase motor is about 6-8 times the rated current.
For example: a 4kW three-phase motor with a rated current of 9A, the starting current is calculated at 10 times, which is 90A. Generally, a D-type 16A miniature circuit breaker is selected as the protection device. Based on the operating current calculated at 10 times, the short-circuit protection operating current is 160A, which can avoid the motor’s starting current.If a C-type 16A miniature circuit breaker is selected as the protection device, the short-circuit protection operating current calculated at 5 times the operating current is 80A, which cannot avoid the motor’s starting current.Does this mean that C-type circuit breakers should never be chosen?Of course not. From a technical perspective, if a C-type 25A miniature circuit breaker is selected as the protection device, the short-circuit protection operating current calculated at 5 times the operating current is 125A, which can avoid the motor’s starting current.From an economic perspective, taking Schneider’s C65 series micro-fault as an example, the price of c65n 3P c25A is about 130 yuan, while the price of c65n 3P d16A is about 158 yuan. Clearly, from a price perspective, the C series is slightly lower.Consideration: Generally speaking, the principle for selecting circuit breakers is that the rated current of the circuit breaker should be greater than the load current, and then choose C-type or D-type based on the nature of the load. D-type is specifically designed for motor-type loads, but this does not mean that C-type circuit breakers cannot be used; it just requires adjusting the calculation method, and we need to be flexible.Differences and Applications of Miniature Circuit Breakers: For miniature circuit breakers, 1P, 1P+N, and 2P are usually used for switching control of single-phase electrical equipment.Differences: To reduce costs, 1P can be used, but high-end circuit breakers must have leakage tripping functions. To prevent accidents caused by confusion between live wires and neutral wires during maintenance, the upstream power supply must be cut off; To avoid this issue during maintenance, 1PN can be used; The same 18mm module circuit breaker housing has differences in the internal installation of 1P and 1PN. In the event of a short-circuit accident, the former must have a higher “limit breaking capacity” than the latter, as space is an important factor affecting breaking capacity. Therefore, for more important, frequent maintenance and operation, and power circuits that are prone to failure, it is best to use 2P (which is more expensive). When using 1P, the lighting distribution box must have a leakage tripping function, and at least the incoming line (or outgoing line on the upper layer) should use a leakage circuit breaker. Ordinary socket circuits can use 1P+N completely, but if you want to add leakage protection, it won’t work because 1P+N circuit breakers cannot be assembled with leakage protection accessories and other electrical accessories.
Three-phase circuit breakers are generally divided into three types, namely 3P, 3PN, and 4P:Differences:3P: Pure three-phase electrical equipment only uses three wires, and trips when there is a ground or phase-to-phase short circuit without single-phase loads; otherwise, when there is a load on the N line, the N line circuit breaker acts as a leakage current;3PN: Four wires L1, L2, L3, and N can use three-phase or single-phase electricity after passing through the transformer coil, and the leakage switch does not act regardless of whether the three-phase load is balanced; it only acts when there is leakage, i.e., when there is single-phase grounding or phase-to-phase short circuit;4P: The use method of four wires L1, L2, L3 passing through the transformer coil is the same as that of 3PN, with the difference that 4P disconnects the N line, while 3P keeps the N line.Four-pole circuit breakers are divided into four types: A, B, C, and D:A: The N pole does not have an overcurrent trip unit, and the N pole is always connected together, not closing or opening with the other three poles.B: The N pole does not have an overcurrent trip unit, and the N pole can be opened or closed with the other three poles.C: The N pole has an overcurrent trip unit, and the N pole can be opened or closed with the other three poles.D: The N pole is equipped with an overcurrent release device, and the N pole is always connected, not closing or opening with the other three poles. In the case of using four poles, it is necessary to specify which product to choose, as it is the same as four poles, but whether an overcurrent release device is installed on the N line has different functions and uses. Overcurrent trip units installed on the N line can be used for three-phase four-wire distribution lines primarily composed of single-phase loads, and can also be used in special cases where a large amount of harmonics is generated, such as gas discharge lamps, thyristor dimming, speed control lines, etc. Generally, equipment circuits can choose N line circuit breakers without overcurrent trip units. In fact, although A and D are called four-pole circuit breakers, their N poles are always connected and will not close or open with the other three poles. Therefore, this is commonly referred to as “pseudo four-pole” molded case circuit breakers, which are 3PN and have no essential difference from three-pole molded case circuit breakers. The only advantage over three-pole molded case circuit breakers is that in a complete cabinet, the line connection may be more convenient. Therefore, this circuit breaker can only be used for three-phase loads and only for a small number of single-phase loads (if there is a control power supply, then use a complete 220V system).Conclusion: Choosing the wrong type of A, B, C, or D circuit breaker not only fails to provide protection but can also cause significant problems. This is currently one of the most confusing issues in design and use and should be taken seriously.