When it comes to mini circuit breakers, most electrical personnel are probably familiar with them. However, when discussing the selection of A, B, C, and D mini circuit breakers, many electrical personnel may have only a vague understanding, and some may be completely unaware.
We all know that mini circuit breakers, abbreviated as MCB, 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 in AC circuits with a rated voltage of 230/400V and rated current up to 63A, and can also be used for infrequent operation switching of circuits under normal conditions. Mini circuit breakers (also known as miniature circuit breakers) are mainly used in various places such as industrial, commercial, high-rise buildings, and residential homes. Moreover, the general tripping characteristics of circuit breakers include types A, B, C, and D. So the question arises: how to correctly select A, B, C, and D mini 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, requires tripping within 0.1 seconds, circuit breakers with this characteristic are most commonly used to protect distribution circuits 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 devices 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.
Low-voltage circuit breaker tripping type selection: The tripping types of circuit breakers include overcurrent tripping, undervoltage tripping, parallel tripping, etc.
Overcurrent tripping device: can be divided into overload tripping devices and short-circuit current tripping devices, with long delay, short delay, and instantaneous types, with overcurrent tripping devices being the most commonly used.
The setting value of the action current for the overcurrent tripping device can be fixed or adjustable, typically adjusted using a rotating knob or lever. There are two electromagnetic overcurrent release methods: 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, thus the breaking capacity of the circuit breaker must be greater than the short-circuit current of its protective device.
Overcurrent tripping devices by installation method: are divided into fixed installation and modular installation. Fixed devices are integrated 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 strong flexibility.
Instantaneous type: 0.02 seconds, used for short circuit protection;
Short delay type: 0.1-0.4 seconds, used for short circuit and overload protection;
Long delay type: less than 10 seconds, 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, i.e., the tripping current, such as 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.
The circuit breaker is used to protect wires and prevent fires, so it should be selected based on the size of the wire rather than the power.
If the circuit breaker is selected too large, it will not 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 chosen to avoid the high starting current of 5-8 times the motor’s starting current.
Is C-type circuit breaker 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 motor circuits use C-type miniature circuit breakers?
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 the short circuit protection tripping value being 5-10 times the rated current;
D-type miniature circuit breaker: includes overload protection and short circuit protection, with the short circuit protection tripping value being 10-20 times the rated current;
The two types of overload protection are the same; the difference lies only in the tripping range for short circuit protection.
Generally, under normal conditions, the starting current of a general load 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 has a rated current of 9A, and the starting current is calculated at 10 times, resulting in 90A.
Generally, a D-type 16A miniature circuit breaker is selected as the protection device. Based on the action current of 10 times, the short circuit protection action 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 action current calculated at 5 times the action current would be 80A, which cannot avoid the motor’s starting current.
Does this mean C-type circuit breakers can never be chosen?
Of course not. From a technical perspective, if a C-type 25A miniature circuit breaker is chosen as the protection device, the short circuit protection action current calculated at 5 times the action current would be 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 c65n 3P d16A is priced at about 158 yuan.
Clearly, from a price perspective, the C series is slightly cheaper.
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 select C-type or D-type based on the nature of the load. D-type is specifically designed by manufacturers for motor-type loads, but this does not mean that C-type circuit breakers cannot be used; just adjust the calculation method accordingly, and we should be flexible.
Differences and Applications of Mini Circuit Breakers:
For mini circuit breakers, 1p, 1pn, and 2p are commonly used for switching control of single-phase electrical equipment.
Differences:
To reduce costs, 1p can be used, but advanced circuit breakers must have leakage tripping functions. To prevent accidents caused by confusion of 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 internal installation of 1p and 1pn in the same 18mm module circuit breaker shell has differences. 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, more frequent maintenance and operation, and power circuits that are prone to failure, it is best to use 2p (costs more).
When using 1p, the lighting distribution box must have a leakage tripping function; at least the incoming line (or upper line of the outgoing line) should use a leakage circuit breaker.
A standard socket circuit can completely use 1P+N, but if you want to add leakage protection, it won’t work because the 1P+N circuit breaker cannot assemble leakage protection attachments and other electrical attachments.
Three-phase circuit breakers are generally divided into three types: 3P, 3PN, and 4P:
Differences:
3P: Pure three-phase electrical equipment uses only three connections, tripping in the event of a ground or phase-to-phase short circuit without a single-phase load; otherwise, when one phase N line is loaded, the N line circuit breaker acts as a leakage current;
3PN: Four wires l1l2l3n can use three-phase or single-phase electricity after passing through the transformer coil, regardless of whether the three-phase load is balanced, the leakage switch does not act; it only acts in the event of leakage, i.e., single-phase grounding or phase-to-phase short circuit;
4P: The usage of four wires L12L3 through the transformer coil is the same as 3PN, with the difference being 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 tripping device, 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 tripping device, and the N pole connects and disconnects with the other three poles.
C: The N pole is equipped with an overcurrent tripping device, and the N pole connects and disconnects 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.
When using four poles, it is necessary to specify which product to choose, as it has the same four poles but different functions and uses depending on whether an overcurrent release device is installed on the N line.
Overcurrent tripping devices installed on the N line can be used for three-phase four-wire distribution lines with single-phase loads as the main component, and can also be used for nonlinear loads that generate a lot of harmonics, such as gas discharge lamps, thyristor dimming, and speed control circuits, etc., in special requirements. Generally, equipment circuits can choose N line circuit breakers without overcurrent tripping devices.
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 commonly referred to as “fake four-pole” plastic shell circuit breaker is 3PN, which is not essentially different from three-pole plastic shell circuit breakers. The only advantage over three-pole plastic shell circuit breakers is that in complete cabinets, the connection of lines 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, use a complete 220V system).
Conclusion:
If the selection of circuit breaker types A, B, C, and D is incorrect, it will not only fail to provide protection but may also cause significant problems. This is currently one of the most confusing issues in design and use and should be taken seriously.
(Source: Jicheng Training)
『This article is copyrighted by the original author. If there is any infringement, please contact for deletion.』
Editor: Shen Binfeng
Editor-in-chief: Hu Ying
Reviewer: Li Guoqing
