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What are the Differences between A, B, C and D Miniature Circuit Breakers?

Time:2024-02-28   Author:As Beam   Browse:

Circuit breakers typically have four tripping characteristics: A, B, C, and D.


Type A circuit breakers: Trip at 2 times the rated current, rarely used, generally for semiconductor protection (usually a fuse is used instead).

Type B circuit breakers: Trip at 2-3 times the rated current, generally used for purely resistive loads and low-voltage lighting circuits, often used in home distribution boxes to protect household appliances and personal safety, currently less used.

Type C circuit breakers:Trip at 5-10 times the rated current, need to trip within 0.1 seconds, circuit breakers with this characteristic are most commonly used to protect distribution lines and lighting circuits with larger current connections.

Type D circuit breakers: Trip at 10-20 times the rated current, mainly used in environments with momentarily high currents, less commonly used in general households. Suitable for systems with large inductive loads and high inrush currents, often used to protect equipment with high inrush currents.

Miniature Circuit Breaker Type: B, C, D,  K, Z.png


Multiple current resistance refers to the resistance to inrush currents. The switch does not trip within a certain time. Its characteristic is to avoid inrush currents. Low-voltage circuit breaker tripping types include overcurrent tripping, undervoltage tripping, and parallel tripping, etc. Overcurrent releases can be divided into overload releases and short-circuit current releases, with long delay, short delay, and instantaneous characteristics. Overcurrent releases are the most commonly used.


The set value of the overcurrent release action current can be fixed or adjustable, usually adjusted by rotating or adjusting a rod. There are two types of electromagnetic overcurrent releases: fixed and adjustable. Electronic overcurrent releases are usually adjustable.


The breaking capacity of a circuit breaker refers to its ability to withstand the maximum short-circuit current. Therefore, the breaking capacity of a rotary circuit breaker must be greater than the short-circuit current of its protective device. Overcurrent releases according to installation method: divided into fixed installation and modular installation. Fixed devices are circuit breakers and releases processed as a whole at the factory. After the product leaves the factory, the rated current of the release is not adjustable. Modular installation releases serve as installation modules for circuit breakers and can be adjusted at any time, offering 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 common DZ series air switches (miniature circuit breakers with leakage protection) include the following specifications: C16, C25, C32, C40, C60, C80, C100, etc., where C represents the tripping current characteristic of the circuit breaker, e.g., C20 represents a tripping current of 20A, with a C curve characteristic. For installing a 3500W water heater, a C20 circuit breaker is generally selected; for a 6500W water heater, a C32 circuit breaker is typically used.


Circuit breakers are used to protect wiring and prevent fires, so they should be chosen based on the size of the wire, not the power. If the selection of the circuit breaker is too large, it cannot protect the conductor. If the circuit breaker is overloaded and still does not trip, it poses a safety hazard to the home.


For wires:

1.5 mm² wire matches a C10 breaker.

2.5 mm² wire matches a C16 or C20 breaker.

4 mm² wire matches a C25 breaker.

6 mm² wire matches a C32 breaker. For circuits with motor loads, a D type characteristic should be selected to avoid high starting currents of 5-8 times the motor current.

MCB Wire.jpg


Can Type C circuit breakers be used for motor circuits?


It is well known that Type C miniature circuit breakers are used for ordinary circuits, such as lighting circuits; and Type D circuit breakers are used for motor and other power circuits. So, can Type C miniature circuit breakers be used for motor circuits? Let's look at the difference between Type C and Type D miniature breakers:


Type C miniature breaker: Includes overload protection and short-circuit protection, with the short-circuit protection tripping value at 5-10 times the rated current;


Type D miniature breaker: Includes overload protection and short-circuit protection, with the short-circuit protection tripping value at 10-20 times the rated current; both have the same overload protection, with the difference only in the range of short-circuit protection tripping. Generally, general loads have no starting current, i.e., the starting current is the rated current; the starting current of a three-phase motor is about 6-8 times the rated current.


For example, for a 4kW three-phase motor with a rated current of 9A, considering the starting current as 10 times, i.e., 90A, a Type D 16A miniature circuit breaker should be selected according to conventional calculations. If a Type C 16A miniature circuit breaker is selected, it might frequently trip during motor startup due to the high starting current, affecting normal use.


Therefore, if the starting current of the motor is not high or the starting frequency is low, and the actual starting current is within the short-circuit protection tripping range of the Type C miniature circuit breaker, a Type C miniature circuit breaker can be selected. Otherwise, a Type D miniature circuit breaker should be chosen to avoid frequent tripping.


This does not mean that Type C circuit breakers cannot be chosen at all. Technically, if a Type C 25A miniature circuit breaker is selected as the protection device, the action current for short-circuit protection can be calculated as 5 times the action current, totaling 125A, which can prevent the motor's starting current. Economically, taking Schneider's C65 series miniature faults as an example, the price of a C65N 3P C25A is about 130 yuan, while a C65N 3P D16A costs about 158 yuan. Clearly, from a price perspective, the C series is slightly cheaper. In general, when selecting circuit breakers, the principle is that the rated current of the breaker should be higher than the load current, and then the Type C or D is chosen based on the nature of the load. Type D is specifically designed for motor-type loads by manufacturers, but this does not mean Type C circuit breakers cannot be used; it only requires an adjustment in the calculation method. It's important to be flexible in our approach.


Differences and Applications of Miniature Circuit Breakers

For miniature circuit breakers, 1PN, 1P, and 2P are usually used for single-phase electrical equipment control switches;







To reduce costs, 1P can be used, but advanced breakers must have leakage trip functionality. To prevent accidents caused by confusion between live lines and neutral lines during maintenance, the upstream power supply must be cut off; to avoid this problem during maintenance, 1PN can be used; there is a difference between installing 1P and 1PN within the same 18mm module breaker housing. In the event of a short-circuit accident, the former must have a higher "ultimate breaking capacity" than the latter, after all, space is an important factor affecting breaking capacity. Therefore, for more important, more frequently maintained and operated, and fault-prone power circuits, it is better to use 2P (which is more costly). When using 1P, the lighting distribution box must have a leakage trip function, and at least the incoming (or upper outgoing) line should use a leakage circuit breaker. A normal socket circuit can completely use 1P+N, but it won't work if you want to add leakage protection, because 1P+N breakers cannot be fitted with leakage protection accessories or other electrical accessories.


Three-phase circuit breakers are generally divided into three types: 3P, 3PN, and 4P


  3P: Pure three-phase electrical equipment uses three connections, tripping in the event of a ground or phase-to-phase short circuit unless there is a single-phase N line load, then the N line circuit breaker acts as a leakage current;

  3PN: Four lines L1L2L3N through the transformer coil can use three-phase or single-phase electricity, regardless of whether the three-phase load is balanced, the leakage switch does not act; it only acts in case of leakage, that is, when there is a single-phase ground fault or phase-to-phase short circuit;

  4P: The use of four lines L1L2L3 through the transformer coil is the same as 3PN, the difference is that 4P breaks the N line, 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 device, the N pole is always connected, not closing or opening with the other three poles.

  B: The N pole does not have an overcurrent trip device, the N pole closes and opens with the other 3 poles.

  C: The N pole has an overcurrent trip device, the N pole closes and opens with the other 3 poles.

  D: The N pole is equipped with an overcurrent release device, the N pole is always connected, not closing or opening with the other three poles.

 Eaton MCB Miniature Circuit Breaker.jpg


In the case of using four-pole, it is necessary to point out which product to choose, as it shares similarities with the four-pole, but has different functions and uses depending on whether an overcurrent release is installed on the N line.


An overcurrent trip device installed on the N line can be used for three-phase four-wire distribution circuits primarily composed of single-phase loads, or for non-linear loads that generate a large amount of harmonics, such as gas discharge lamps, thyristor dimming, speed control circuits, etc., that have special requirements. Generally, equipment circuits can choose a breaker without an overcurrent trip device on the N line.


In fact, although A and D are called four-pole circuit breakers, their N poles are always connected and do not close or open with the other three poles. Therefore, this so-called "pseudo four-pole" molded case circuit breaker is 3PN, with no essential difference from a three-pole molded case circuit breaker. The only advantage over a three-pole molded case circuit breaker is that it may be more convenient to connect lines in a complete cabinet. Thus, this type of breaker can only be used for three-phase loads and can only be used for a small number of single-phase loads (if a control power supply is used, a complete set of 220V systems is used).


Choosing the wrong type of circuit breaker, A, B, C, or D, not only fails to protect but can also cause significant problems. This is currently the most confusing issue in design and use and should be given attention.

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