Abstract Miniature circuit breakers (MCBs) are the most widely used terminal protection devices in building electrical distribution systems. This paper, based on common electrical parameters of MCBs, proposes that, similar to selecting molded case circuit breakers and frame circuit breakers, the maximum short-circuit capacity should be calculated before selection. For different types of circuits, MCBs with different protection characteristics must be selected. MCBs are designed and used for 50-60Hz AC power grids; if used in DC circuits, the magnetic tripping current should be converted to the power supply frequency variation coefficient provided by the manufacturer. When the ambient temperature is higher or lower than the calibration temperature, the rated current value of the MCB must be adjusted according to the temperature and current carrying capacity correction curve provided by the manufacturer. Designers should select upper and lower level MCBs according to the matching table provided by the manufacturer. Finally, the paper points out the precautions for selecting MCB accessories. Keywords Miniature circuit breaker Miniature circuit breakers (hereinafter referred to as MCBs) are the most widely used terminal protection devices in building electrical distribution systems. Although MCBs are terminal devices, they are used extensively, and the loss caused by selecting an unsuitable MCB can be severe. This article discusses the correct selection method for MCBs based on their commonly used electrical parameters. The rated breaking capacity of an MCB is the maximum short-circuit current it can break without causing any damage. Currently, MCBs available on the market, according to technical data and design manuals provided by various manufacturers, generally have rated breaking capacities of 4.5kA, 6kA, and 10kA. When selecting an MCB, similar to selecting an MCCB (molded case circuit breaker) or ACB (automatic circuit breaker), the maximum short-circuit capacity for the intended application should be calculated before selection. If the rated breaking capacity of the MCB is less than the short-circuit fault current within the protected area, it will not only fail to break the faulty line during a fault, but may also explode due to its insufficient breaking capacity, endangering personal safety and the safe operation of other electrical equipment. The short-circuit current of a low-voltage distribution line is related to electrical parameters such as the conductor cross-section, conductor laying method, distance between the short-circuit point and the power source, capacity of the distribution transformer, and impedance percentage. The low-voltage side voltage of distribution transformers in general industrial and civil buildings is mostly 0.23/0.4L, and the transformer capacity is mostly 1600kVA and below. The short-circuit current of the low-voltage side lines increases with the increase of distribution capacity. The short-circuit current at the low-voltage feeder end is different for distribution transformers of different capacities. Generally speaking, for residential buildings, small shopping malls, and public buildings, since they are supplied by the local power grid, the cable or overhead conductor cross-section of the power supply line is relatively thin, and the distance between the electrical equipment and the power supply source is relatively far, so an MCB with a breaking capacity of 4.5kA or above is sufficient. For users with dedicated power supply or 10kV substations, the cable cross-section of the power supply line is often thicker, and the power supply distance is shorter, so an MCB with a rated breaking capacity of 6kA or above should be selected. For applications with short power supply distances, such as substations (where lighting and power are directly drawn from the low-voltage main busbar) and large-capacity workshop substations (supplying workshop equipment), MCBs with a breaking capacity of 10kA or higher must be selected, and verification must be performed during the design phase. In addition, three points deserve special attention: 1. With the increasing transformer capacity in modern buildings, the use of large-capacity busbars, and the shortening distance between electrical equipment and power sources, the short-circuit current at the end of power lines is constantly increasing. This is especially true for high-end office buildings, hotels, and large shopping malls; the design of MCBs used in these applications should take this into consideration. 2. There are two product standards for MCBs: one is IEC 898 "Circuit breakers for household and similar installations" (GB 10963-1999); the other is IEC 947-2 "Low-voltage switchgear and controlgear - low-voltage circuit breakers". EC898 is a standard for use by non-electrical professionals and inexperienced personnel, while IEC947-2 is a product standard for use by electrical professionals. The two standards have different rated breaking capacity specifications for MCBs. Designers must carefully consider the specific application and user when selecting an MCB. If selecting an MCB according to the rated breaking capacity of IEC947-2, it should be installed in a cabinet operated by professionals, such as the main lighting distribution box in a building or factory. If selecting an MCB according to IEC898, it can be installed in an operating box for non-professional use, such as the lighting switch box in a conference hall or factory; these users are general staff. Therefore, it is crucial to distinguish between these standards when selecting an MCB and avoid confusion. 3. Generally, the rated breaking capacity of an MCB is measured with the upper terminal inlet and the lower terminal outlet. In engineering projects, if special circumstances require the bottom terminal to be the incoming line and the top terminal to be the outgoing line, the MCB must be derated due to arc extinguishing when interrupting fault current. That is, the rated breaking capacity must be calculated according to the derating factor provided by the manufacturer. Some manufacturers now produce MCBs with both top and bottom terminals for incoming lines and free installation without affecting breaking capacity. However, the author believes that, unless absolutely necessary, top-in and bottom-out is preferable. According to IEC 898, MCB protection characteristics are divided into four types: A, B, C, and D, for users to choose from: A. Characteristic A is generally used in applications requiring fast, non-delayed tripping, i.e., for lower peak current values ​​(typically 2-3 times the rated current/n), to limit the permissible short-circuit current and total breaking time. This characteristic allows the MCB to replace the fuse for overcurrent protection of electronic components and protection of mutual inductance measurement circuits. Characteristic B is generally used in applications requiring faster tripping and where the peak current is not very high. Compared to characteristic A, characteristic B allows a peak current <3In, generally used for the protection of resistive loads such as incandescent lamps and electric heaters, and residential circuits. Characteristic C is generally suitable for most electrical circuits; it allows a higher short-time peak current to pass through the load without the MCB tripping. Characteristic C allows a peak current <5In, generally used for the protection of fluorescent lamps, high-voltage gas discharge lamps, and power distribution system circuits. Characteristic D is generally suitable for switching equipment with very high peak currents (<10In), generally used for the protection of primary circuits and solenoid valves of control transformers and local lighting transformers under AC rated voltage and frequency. From the above analysis of protection characteristics, it is clear that for various types of circuits, it is essential to select the appropriate MCB. In circuits with gas discharge lamps, a large inrush current occurs when the lamp starts. If the MCB (Mechanical Circuit Breaker) is selected solely based on the lamp's rated current, it often leads to accidental tripping of the MCB the instant the lamp is switched on. Regarding protection characteristics, the C898 standard clearly stipulates that MCBs cannot be used to protect motors; they can only be used as alternative fuses to protect power distribution lines (such as wires and cables). Designers often overlook this, and there are misleading points in some manufacturers' samples and design manuals. It is known that motors have a starting current of 5-7 In for 10 seconds at startup. Even if the electromagnetic trip current is set to (5-10) In, which can ensure that the surge current is avoided during motor startup, for thermal protection, the overload protection trip value is set to 1.45 In. This means that the MCB will only trip when the motor withstands an overload current of more than 45%. For motor stator windings that can only withstand <20% overload, this can easily damage the insulation between windings, while it is tolerable for wires and cables. Therefore, in certain situations where MCBs are necessary to protect motors, ABB's proprietary MCBs conforming to the K-characteristics of IEC 947-2 can be selected, or an externally heated MCB relay can be used to provide overload and short-circuit protection for the motor. The operating frequency of MCBs : MCBs are designed and used for 50-60Hz AC power grids. Since the electromagnetic force of the magnetic trip unit is related to the power frequency and operating current, the operating current of the magnetic trip unit will differ when an MCB used under AC voltage is used for DC circuits or other power frequency applications. Generally, it should be calculated based on the magnetic trip operating current and power frequency variation coefficient provided by the manufacturer. When an AC MCB is used for DC circuit protection, due to arc extinguishing requirements, a DC-specific MCB like Siemens' 5SX5 should be selected. The operating temperature of MCBs: Overload protection of MCBs relies on thermal trip units. Typically, the rated current of the thermal trip unit of existing MCBs is set by the manufacturer according to the IEC 898 standard at a reference temperature of 30°C. The recommended operating temperature for MCBs is generally -25°C to +55°C. The thermal trip unit consists of a bimetallic strip. When the current passing through it reaches a certain set value and is maintained for a certain period of time, the MCB trips. Therefore, the thermal trip unit is closely related to temperature. Changes in ambient temperature will cause changes in the operating temperature of the MCB, resulting in corresponding changes in the operating characteristics of the thermal trip unit. Since the MCB is usually installed in a distribution box, and the ambient temperature cannot be constant at 30°C, in actual use, the MCBs in the terminal distribution box are installed tightly together, and in most cases, they are embedded in the wall, resulting in poor heat dissipation and a significant temperature rise in the distribution circuit. Therefore, the actual operating temperature of the MCB is always about 10°C to 15°C higher than the ambient temperature. Therefore, when the ambient temperature is higher or lower than the calibration temperature value, we must adjust the rated current value of the MCB according to the temperature and current carrying capacity correction curve provided by the relevant manufacturer. Generally speaking, when the ambient temperature is higher or lower than the correction value by 10°C, the rated current value of the MCB should be decreased or increased by about 5%. Selective Coordination Between Upper and Lower Stages of MCB As we know, in power supply and distribution lines, protective devices must meet the "three characteristics"—selectivity, speed, and sensitivity. Speed ​​and sensitivity are related to the characteristics of the protective device itself and the line operation mode, while selectivity is related to the coordination between upper and lower stage protective devices. Proper coordination allows for the selective disconnection of faulty circuits, ensuring the continued normal operation of other fault-free parts of the power supply system; conversely, improper coordination affects the reliability of power supply. The selectivity of MCB can be divided into two areas: overload zone selectivity and short-circuit zone selectivity. As shown in Figure 1, [IMG=MCB Selective Coordination Between Upper and Lower Stages]/uploadpic/THESIS/2008/1/20080104093550576648.jpg[/IMG], the current-time characteristic of the MCB's thermal trip unit is an inverse time curve. In the curve, t1 and t2 represent the longest non-breaking time of QL1 and Q12, respectively, and t1" and t2" represent the longest breaking time of QL1 and Q12, respectively. For a given current, if the relationship between t1' of circuit breaker QL1 and t2' of Q12 is t1' > t2', it indicates that the overload zone has selectivity. Practical experience shows that generally, if I1/I > 2, the MCB can be selective in the overload zone. When a short-circuit current flows through the electromagnetic tripping system, it is difficult for the MCB to achieve selectivity between its upper and lower terminals. To prevent over-tripping, the ratio of the instantaneous tripping current Im1 of QL1 to the instantaneous tripping current Im2 of Q12 should generally be greater than 1.4. When the short-circuit current is large... To ensure only Q12 breaks at 7ml, a current-limiting circuit breaker should be selected as Q12. This reduces the peak current and duration, preventing QL1 from breaking. Alternatively, a circuit breaker with a time delay can be used as QL1. When the short-circuit current is very large, it is difficult to guarantee selectivity; only partial selectivity can be achieved. To facilitate designers in selecting suitable MCBs to ensure selectivity, manufacturers provide matching tables in their design references. Designers can select appropriate MCBs based on these tables. MCB accessories include electrical auxiliary devices and protective accessories that can be combined with the MCB body to expand its application range. The most important of these are residual current operated devices (RCDs), shunt trip units (STs), and undervoltage release units (URs). Combining an RCD with an MCB creates a residual current operated circuit breaker (RCBO) with overcurrent protection. Installed in a distribution box, this prevents personal safety hazards during single-phase ground faults and effectively suppresses electrical fires. The working principle of the RCD will not be elaborated here; however, six points of attention are highlighted. 1. There must be no ambiguity regarding the type of low-voltage distribution grounding for which the RCBO is used, as the wiring requirements differ for TT, TN, and IT systems. See relevant articles such as the "Lecture on Residual Current Devices" in *Electric World* magazine, 1996, for details. However, regardless of the variations, all live current-carrying conductors (including the individual conductor) must be connected to the RCD, while the protective earth (PE) conductor must never be connected to the RCD; the PE conductor should be connected to the metal casing of the equipment. The author believes that to avoid many unnecessary accidental trips, the number of poles on the RCBO should ideally be equal to the number of current-carrying conductors in the connected circuit. 2. The rated tripping current λ of the RCD should be selected according to Article 14.3.11 of JGJ/T16-92 "Code for Electrical Design of Civil Buildings". From a safety perspective, the smaller the λ of the RCD, the better. However, in reality, all electrical equipment in any power supply circuit has a normal leakage current. If the RCD ratio is less than the normal leakage current or the normal leakage current of the circuit is greater than 50% of λ, the power supply circuit cannot operate normally. Therefore, from the perspective of power supply reliability, λ should not be too small, as it is mainly constrained by the normal leakage current. 3. Coordination of RCDs between upper and lower levels. Generally, the rated residual non-operating current In0 of an RCD (according to relevant IEC standards) is equal to 50% of In. If the operating current values ​​of the RCDs on the main line and branch lines are very close, the sum of the non-operating currents In0 of several branch lines may exceed the In of the RCD on the main line, causing the RCD on the main line to malfunction, thus losing selectivity between the two. Typically, the ratio of the rated operating currents of the upper and lower level RCDs should be greater than 2.5. Of course, the selectivity of the RCD can also be achieved by the difference in operating time. Generally, for terminal distribution boxes, the RCD at the main circuit breaker is mainly for preventing electrical fires, and products with In = 100-300mA and a time t = approximately 0.3s can be selected, such as the Merlin Gerin VigiS type. The RCDs on branch lines are mainly for preventing electric shock, and products with In = 6-30mA (depending on the specific application) and instantaneous operation can be selected, such as the Merlin Gerin Vigi type. 4. For TT systems, branches equipped with RCDs and those without RCDs should not share a common grounding electrode. In TT grounding systems, because the neutral point grounding and the grounding conductor are separate, and the N and PE conductors are not connected, the power supply lines are generally long, resulting in high phase-to-ground loop impedance. When a single-phase ground fault occurs, the line protection device cannot reliably cut off the power supply, easily causing electric shock and fire accidents. Therefore, installing RCDs for single-phase ground fault protection is one of the effective measures in this system. However, individual branch circuits equipped with RCDs must have separate grounding electrodes and PE conductors; otherwise, when a leakage occurs in a circuit without an RCD, it will pass through the PE conductor onto the casing of the equipment with the RCD, but the RCD will not trip, causing an electric shock accident. Therefore, there must be a separate grounding plate and PE conductor dedicated to branch circuits with RCDs, and there must be no electrical connection between them. 5. Currently, there are two types of RCDs produced in my country: electromagnetic (ELM) and electronic (ELE). For ELE, the author believes caution should be exercised, as ELE requires a stable operating voltage. Most ELEs on the market do not have an independent operating power supply; this power is supplied by the power source controlled by the RCD. In the event of a fault, the mains voltage is often too low or too high, causing the ELE to malfunction. Therefore, designers should verify the power supply voltage at the RCD location where the ELE is installed in case of an accident. If it does not meet the product's specifications, remedial measures should be considered, or an ELM RCD should be selected. The input and output lines of an ELM RCD can be reversed, but those of an ELE RCD cannot. 6. For special occasions and power supplies with special purposes, such as chemical, petroleum, various security power supplies, emergency lighting, fire-fighting equipment power supplies, and emergency power supplies for hospital operating rooms, RCDs should not be installed. If necessary, residual current alarm devices may be installed as appropriate. It is important to emphasize that RCDs are not the only measure to prevent electric shock accidents, but only one of them. In some cases, they should be used in conjunction with other measures such as overall equipotential bonding or local equipotential bonding. The UR accessory for the MCB trips the MCB when the power supply voltage drops below 70%; and prevents the MCB from reconnecting when the power supply has not returned to normal. This prevents damage to electrical equipment caused by low voltage operation and also prevents large-capacity loads such as motors from starting automatically without a control signal when the power supply suddenly returns to normal, thus improving the safety of the circuit. However, the UR device is not suitable for some special requirements and general lighting circuits. The shunt trip device ST is a device that can remotely control the tripping of the MCB. [IMG=A normally closed pushbutton should be connected in the control circuit]/uploadpic/THESIS/2008/1/20080104093556885144.jpg[/IMG] Both of the above-mentioned tripping devices are voltage-type coils and can both achieve the purpose of tripping the MCB, but there are differences between the two. UR is designed for long-term energization, while ST is designed for momentary energization. This difference is often overlooked during selection, leading to ST being mistakenly used as UR, resulting in ST burning out. Using UR as ST is theoretically feasible, but practically uneconomical. This is because UR is connected to the line 24 hours a day, ultimately consuming a certain amount of power and generating heat. If UR is to have both undervoltage and shunt tripping functions, a normally closed pushbutton should be connected in the control circuit, as shown in Figure 2. Please pay close attention to this point.