Selection and use of miniature circuit breakers
[Abstract] Miniature circuit breaker (MCB) is the most widely used terminal protection electrical appliance in building electrical terminal power distribution devices. According to the commonly used electrical parameters of MCB, this article proposes that it should be selected after calculating the maximum short-circuit capacity just like choosing molded case circuit breakers and frame circuit breakers; for lines of different nature, MCBs with different protection characteristics must be selected; MCB design And use is aimed at 50-60Hz AC power grid. If used in DC circuit, it should be converted according to the change coefficient of the magnetic tripping action current provided by the manufacturer and the power frequency; when the ambient temperature is greater than or less than the calibration temperature, it must be The temperature and current-carrying capacity correction curve provided by the manufacturer is used to adjust the rated current value of the MCB; the designer should select the upper and lower MCB according to the matching table provided by the manufacturer. Finally, points out the precautions when choosing MCB accessories.
Keywords miniature circuit breaker
Miniature circuit breaker (hereinafter referred to as MCB) is the most widely used terminal protection electrical appliance in building electrical terminal power distribution devices. Although MCB is a terminal electrical appliance. However, it has a large volume and a wide range. If an inappropriate MCB is selected, the losses caused are also heavy. This article discusses the correct selection method of MCB based on the common electrical parameters of MCB.
McB's rated breaking capacity The rated breaking capacity is the maximum short-circuit current that can be broken without any damage to the circuit breaker. MCBs currently seen on the market generally have several rated breaking capacities such as 4.5kA, 6kA, and 10kA according to relevant technical data and design manuals provided by various manufacturers. When we choose MCB, we should calculate the maximum short-circuit capacity in the application, just like MCCB (molded case circuit breaker) and ACB (frame type circuit breaker), and then choose MCB. If the rated breaking capacity of the MCB is less than the short-circuit fault current in the protected range, when a fault occurs, not only the faulty line cannot be broken, but the MCB's breaking capacity is too small to cause the MCB to explode, endangering the person and other electrical equipment circuits Safe operation.
The short-circuit current of the low-voltage distribution line is related to the electrical parameters such as the cross-section of the wire of the power supply line, the wire laying method, the distance between the short-circuit point and the power source, the capacity of the distribution transformer, and the impedance percentage. The low-voltage side voltage of general industrial and civil building distribution transformers is mostly 0.23/O. 4LV, the transformer capacity is mostly 1600kVA and below, the short-circuit current of the low-voltage side line increases with the increase of the distribution capacity. For distribution transformers of different capacities, the short-circuit current at the low-voltage feeder terminal is different. Generally speaking, for civil residences, small shopping malls and public buildings, due to the low-voltage power grid of the local power supply department, the cables or overhead conductors of the power supply lines are relatively thin, and the electrical equipment is far away from the power supply. 4.5kA and The MCB with the above breaking capacity is sufficient. For users with dedicated supply or 10kV substations, often because of the thicker cable surface of the power supply line and the short power supply distance, MCB with a rated breaking capacity of 6kA and above should be selected. For substations such as substations (the lighting and power sources used in the station are directly taken from the low-voltage busbar)
And large-capacity workshop substations (for workshop electrical equipment) and similar occasions where the power supply distance is short, the MCB with 10kA and above breaking capacity must be selected, and the specific design must also be verified. In addition, three points to pay special attention to are:
1. With the increase of the distribution transformer capacity in modern buildings; the use of large-capacity busbars and the shortening of the distance between the electrical equipment and the power supply and other factors, the short-circuit current at the end of the power supply line is also increasing, especially It is some high-end office buildings, office buildings, hotels and large shopping malls and other public buildings. The MCB used in such occasions should be designed with attention.
2. MCB has two product standards: one is IEC898 "Circuit breakers for household appliances and similar devices" (GBl0963-1999); the other is IEC947-2 "Low-voltage circuit breakers for low-voltage switchgear and control equipment". ! EC898 is a standard for non-electrical professionals and inexperienced personnel, while IEC947-2 is a product standard for electrical professionals. The two standards are different for the MCB's rated breaking capacity index. For the designer, the MCB must be selected according to the specific use occasion and object. If the MCB is selected according to the rated breaking capacity of IEC947-2, it should be installed in a cabinet for professionals to operate and operated by professionals, such as the lighting main distribution box on each floor and factory building; if the MCB is selected according to IEC898 , It can be installed in the operation electrical box used by non-professionals, such as the lighting switch box in the large conference hall and the factory building. These objects are all ordinary workers. Therefore, when choosing MCB, we must pay attention to the distinction, not to be confused.
3. Generally speaking, the rated breaking capacity of MCB is measured when the upper terminal enters and the lower terminal exits. In the case of special circumstances in the project that require the lower terminal to enter and the upper terminal to exit, due to the arc extinguishing when the fault current is interrupted, the MCB must be derated and used, that is, the rated breaking capacity must be based on the relevant derating factor provided by the manufacturer To convert. At present, the MCB made by some manufacturers can be wired in and installed freely on the upper and lower terminals, and the breaking capacity is not affected. However, the author believes that it is appropriate to go in and out from above if it is a last resort. According to IEC898, MCB's protection characteristics are divided into four characteristics: human, B, C, and D for users to choose: A. The characteristics are generally used for applications that require fast and no delay tripping, that is, for lower peak current values (usually 2-3 times the rated current/n) to limit the allowable short-circuit current value and the total Breaking time, using this feature can make MCB replace the fuse as the overcurrent protection of electronic components and the protection of the mutual inductance measurement circuit; the B feature is generally used in the use occasions that require faster tripping and the peak current is not very large; and A Compared with the characteristics, the characteristic B allows the peak current to pass<3In, which is generally used for the protection of resistive loads such as incandescent lamps and electric heaters and residential circuits; the characteristic C is generally suitable for most electrical circuits, and it allows the load to pass higher Short-term peak current and MCB does not operate, the peak current allowed by C characteristic <5In is generally used for line protection of fluorescent lamps, high-pressure gas discharge lamps, and power distribution systems; D characteristic is generally suitable for very high peak currents (<10In) The switchgear is 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 analysis of the above protection characteristics, it can be known that for various lines of different nature, appropriate MCB must be selected. If there is a gas discharge lamp circuit, there will be a large surge current when the lamp is started. If the MCB is only selected according to the rated current of the lamp, it will often cause the MCB to trip by mistake at the moment the lamp is turned on.
In terms of protection characteristics, the C898 standard clearly stipulates that MCB cannot be used to protect electric motors and can only be used as an alternative fuse to protect power distribution lines (such as wires and cables). In this regard, designers are often easy to overlook, and there are also some misleading places in the samples and design data manuals of some manufacturers. As we all know, the motor has a starting current of 5-7In and a duration of 10s at the moment of starting. Even if the C characteristic is set to (5-lO)In in the electromagnetic trip current, the surge current can be avoided when the motor starts; But for thermal protection, the overload protection action value is set at 1.45Jn, which means that the MCB can only trip when the motor has to withstand more than 45% of the overload current. This is for the stator winding of the motor that can only withstand <20% overload. Generally speaking, it is extremely easy to damage the insulation between windings, and it is tolerable for wires and cables. Therefore, if MCB is really needed to protect the motor in some occasions, ABB's unique MCB that meets the K characteristic in the IEC947-2 standard can be selected, or the MCB external heating relay can be used to protect the motor against overload and short circuit.
Frequency of use of McB
MCB is designed and used for 50~60Hz AC power grid. Since the electromagnetic force of magnetic trip unit is related to power frequency and operating current, MCB used under AC voltage is used for protection of DC circuit or other power frequency occasions. When, the operating current of the magnetic release is different. Generally, it should be converted according to the change coefficient of the magnetic tripping action current provided by the manufacturer and the power frequency. When AC MCB is used for DC circuit protection, due to arc extinguishing reasons, 5SX5 DC dedicated MCB similar to Siemens should be used.
The ambient temperature of McB
The overload protection of MCB relies on the thermal release. Generally, the rated current of the existing MCB thermal release is set by the manufacturer according to the IEC898 standard at a reference temperature of 30C. The operating temperature of the MCB is generally recommended to be -25C—十55C . The thermal trip unit consists of a bimetallic strip, which trips the MCB when the passing current reaches a certain set value and maintains it for a certain period of time. Therefore, the thermal release is closely related to temperature. For example, a change in the ambient temperature will cause the working temperature of the MCB to change, and the working characteristics of the thermal release will change accordingly. Since the MCB is usually installed in the distribution box, the use environment temperature cannot be constant at 30C. In actual use, the MCB in the terminal distribution box is installed closely together, and in most cases it is embedded in the wall. The internal installation results in poor heat dissipation and a large increase in temperature in the distribution circuit. Therefore, the actual operating temperature of the MCB is always about 10C~15C higher than the ambient temperature. Therefore, when the ambient temperature is greater than or less than the calibration temperature value, we must adjust the MCB's rated current value according to the temperature and current-carrying capacity correction curve provided by the relevant manufacturer. Generally speaking, when the ambient temperature is greater than or lower than the correction value 10C, the rated current value of MCB must be reduced or increased by about 5%.
MCB selective coordination
As we all know, in the power supply and distribution lines, the protection of electrical appliances must meet the "three characteristics-selectivity, rapidity, and sensitivity." The speed and sensitivity are respectively related to the characteristics of the protective electrical appliances and the line operation mode, while the selectivity is related to the coordination between the upper and lower protective electrical appliances. With proper cooperation, the accident loop can be selectively removed to ensure that other trouble-free parts of the power supply system continue to operate normally, otherwise, the reliability of the power supply will be affected. The selectivity of MCB can be divided into two areas, one is the selectivity of the overload area, and the other is the selectivity of the short-circuit area. As shown in Figure 1:
The current-time characteristic of the MCB thermal release is an inverse time curve. In the curve, t1' and t2' represent the longest non-breaking time of QLl and Q12, respectively, and t1" and t2" represent the longest of QLl and Q12 respectively. Breaking time. For a certain current, if the relationship between t1' of circuit breaker QL1 and t2" of Q12 is tl">t2", it indicates that the overload zone is selective. It has been proved by practice that if I1/I>2 of MCB in the overload zone, That is to say, it can be selective in the overload zone. When the short-circuit current flows through the electromagnetic trip system, it is very difficult to obtain selectivity between the upper and lower sides of the MCB. In order to prevent over-level tripping, generally the instantaneous trip current of QLl should be used
The ratio of the instantaneous trip current Im2 between Im1 and Q12 is greater than 1.4. When the short-circuit current is greater than 7ml, if only Q12 is to be opened, a current-limiting circuit breaker should be selected as Q12, which can reduce the peak value and duration of the current and prevent QLl from being disconnected. Of course, a delay circuit breaker can also be used器为QLl. When the short-circuit current is large, it is difficult to ensure selectivity, and only partial selectivity can be obtained. In order to make it easier for the designer to choose the appropriate MCB to ensure the selectivity, the manufacturer has a matching table recommended to the user in the design reference materials. The designer can choose the upper and lower MCBs according to the matching table.
McB accessory selection
MCB has some electrical auxiliary devices and protection accessories that can be assembled and combined with the MCB body to expand the scope of use. The most important ones are residual current operated protectors (abbreviated as RCD), shunt releases (abbreviated as ST), undervoltage release Buckle device (referred to as UR). The combination of RCD and MCB can become a residual current operated circuit breaker with overcurrent protection (RCBO for short), which can be installed in the distribution box to prevent the line from endangering personal safety when a single-phase ground fault occurs and effectively suppress electrical fires. Regarding the working principle of RCD, this article will not go into details, but here are six points for attention.
1. There should be no ambiguity in which low-voltage power distribution grounding type the RCBO is used for, because the wiring requirements in the TT, TN, and IT systems are different. For details, please refer to the "Lecture on Residual Current Protector" in "Electric World" 1996 And other related articles. But no matter how dry it changes, all live current-carrying conductors (characterized lines are also current-carrying conductors) must all be connected to RCD, while the protection line PE must not be connected to RCD, and the PE line should be connected to the metal shell of the equipment. The author believes that: in order to avoid many unnecessary false trips, the number of poles of the RCBO should be equal to the number of current-carrying conductors of the access loop.
2. The RCD rated tripping current input value should be selected according to Article 14.3.11 of JGJ/T16-92 "Code for Electrical Design of Civil Buildings". From a safety point of view, the smaller the RCD input, the better, but in fact, any electrical equipment in the power supply loop has a normal leakage current. If the ratio of the RCD is less than the normal leakage current or the normal leakage current of the loop If it is greater than 50% In, the power supply circuit cannot operate normally. Therefore, considering the reliability of the power supply, In should not be selected too small because it is mainly restricted by the normal leakage current.
3. RCD's superior and superior cooperation issues. Generally speaking, the rated residual non-operating current In0 of the RCD (according to the relevant IEC standards) is equal to 50% of In. If the RCD operating current values of the main line and the branch line are very close, it is possible that the sum of the non-operating current In0 of several branch lines is greater than the In0 of the RCD of the main line, causing the RCD of the main line to malfunction, and the difference between the two is lost. Selective. Generally, the ratio of the rated operating current of the upper and lower RCDs should be greater than 2.5. Of course, the selectivity of the RCD can also be achieved according to the difference in operating time. Generally, for the terminal distribution box, the RCD at the main power circuit breaker is mainly to prevent electrical fires. Products with In=100-300mA and time t=0.3s can be used, such as the vigiS product of Merlin Gerin. RCD on branch
Mainly to prevent personal electric shock, In=6-30mA (depending on the specific application), instantaneous products, such as Merlin Gerin vigi products can be selected.
4. For TT system, branch with RCD and without RCD
The branch circuit should not use a common ground electrode. The TT grounding system is separated from the grounding of the concave line due to the neutral point grounding. The individual line N and the PE line are not connected. The power supply line is generally long and the phase-to-ground loop impedance is large. When a single-phase ground fault occurs, the line protection device cannot be reliable. Cut off the power to the ground, it is easy to cause electric shock and fire accidents, so installing RCD in this kind of system for single-phase grounding protection is one of the effective measures. However, individual branch circuits with RCDs must have separate grounding electrodes and PE wires. Otherwise, when the circuit without RCDs has a leakage, it will pass through the PE wires to the housing of the equipment equipped with RCDs, but the RCD will not operate, which may cause electric shock. accident. Therefore, there must be an independent grounding plate and PE wire dedicated to the branch circuit with RCD, and there must be no electrical connection between them.
5. There are currently two forms of RCD produced in my country, one is electromagnetic (ELM) and the other is electronic (ELE). For ELE, I think it should be used with caution, ELE must have a stable operating voltage when working. The general EIEs on the market now have no independent operating power supply. The operating power supply is powered by the power supply controlled by the RCD. When a fault occurs, the grid voltage is often low or too high, causing the ELE to not work normally. Therefore, the designer should check the power supply voltage in the event of an accident at the RCD where the ELE is installed. If it does not meet the product's specified value, he should consider taking remedial measures or selecting the RCD of the ELM. The RCD incoming and outgoing lines of ELM can be inverted, while the RCD incoming and outgoing lines of ELE cannot be inverted.
6. For some special occasions and some special-purpose power supplies, such as chemical, petroleum, various security power supplies, emergency lighting, fire-fighting equipment power supplies, and hospital operating rooms for rescue power supplies, RCD should not be installed, and residual current can be installed if necessary. alarm system. It is important to mention that RCD is not the only measure to prevent electric shock accidents, but one of the measures. In some cases, it should be used in combination with other measures such as total equipotential or local equipotential bonding.
The accessory UR of the MCB is to trip the MCB when the power supply voltage drops below 70%; when the power supply does not return to normal, it prevents the MCB from being switched on again. It can prevent some electrical equipment from running at low voltage and damage the equipment, and it can also prevent large-capacity loads such as motors on the line from starting automatically without receiving a control signal when the power supply suddenly returns to normal, thereby improving the safety of the line. However, it is not suitable to install UR device for some special requirements occasions and general lighting circuits. Shunt trip device ST is a device that can remotely control MCB tripping.
The above two tripping devices are voltage-type coils, which can make the MCB achieve the purpose of tripping, but there is a difference between the two. UR is designed for long-term energization, while ST is designed for instantaneous energization. This is often neglected when selecting and mistakenly use ST as UR, causing ST to burn. If UR is used as ST, it is feasible in theory, but it is not economical in practice. Because UR is connected to the line for 24 hours, it will consume a certain amount of electrical power and emit a certain amount of heat. If you want to make the UR have both loss-of-voltage and shunt tripping effects, a normally closed button should be connected to the control loop, as shown in Figure 2. Please pay attention to this.