1. Select the breaking capacity of the circuit breaker according to the calculation of the expected short-circuit current of the line Accurate line calculation of short-circuit current is an extremely tedious task, so there are some simple calculation methods that are not very error-prone and acceptable in engineering: 1. For transformers with a voltage rating of 10/0.4KV, consider that the short-circuit capacity on the high-voltage side is infinite (the short-circuit capacity on the 10KV side is generally 200~400MVA or even larger, so the error is less than 10% considering infinity). 2. GB50054-95 "Low-Voltage Power Distribution Design Specification" 2.1.2 stipulates: "When the sum of the rated current of the motor connected near the short-circuit point exceeds 1% of the short-circuit current, it shall be included in the influence of the motor feedback current", If the short-circuit current is 30KA, take 1%, it should be 300A, the total power of the motor is about 150KW, and the feedback current calculated at the same time when starting up should be 6.5∑In. 3. The impedance voltage UK of the transformer indicates that the secondary side of the transformer is shorted (route). When the secondary side reaches its rated current, the primary side voltage is the percentage of its rated voltage. Therefore, when the primary voltage is the rated voltage, the secondary current is its expected short-circuit current. 4. Secondary side rated current of transformer = Se/1.732U where Se is the capacity of the transformer (KVA), Ue is the rated voltage of the secondary side (no-load voltage), Ue=0.4KV at 10/0.4KV, so the transformer is simply calculated. The secondary side rated current should be: 1.44 ~ .50Se. 5. According to (3) definition of Uk, the short-circuit current (three-phase short-circuit) of the secondary side is I(3) for Uk, and the short-circuit current of the secondary side (three-phase short circuit) is I(3)=Ie/Uk , this value is the AC RMS value. 6. Under the same transformer capacity, if the two phases are short-circuited, then I(2)=1.732I(3)/2=0.866I(3) The above calculation is the current value when the transformer outlet terminal is short-circuited. The most serious short circuit accident. If the short-circuit point is at a certain distance from the transformer, the short-circuit current will decrease in consideration of the line impedance. For example, the SL7 series transformer (with a three-core aluminum wire cable) has a capacity of 200KVA. When the transformer is short-circuited at the output end, the three-phase short-circuit current I(3) is 7210A. When the short-circuit point is 100m away from the transformer, the short-circuit current I(3) is reduced to 4740A; when the transformer capacity is 100KVA, the short-circuit current at the outlet end is 3616A. When the distance from the transformer is short at 100m, the short-circuit current is 2440A. The short-circuit current is 65.74% and 67.47% of 0m away from 100m. Therefore, the user should calculate the rated current of the installation (line) and the maximum short-circuit current that may occur at that location. And select the circuit breaker according to the following principles: Therefore, in selecting the circuit breaker, it is not necessary to put the margin too large to avoid waste. Second, the circuit breaker's ultimate short-circuit breaking capacity and short-circuit breaking capacity The test procedure for the ultimate short-circuit breaking capacity Icu is OTCO. The specific test is: adjust the current of the line to the expected short-circuit current value (for example, 380V, 50KA), and the test button is not closed, the circuit breaker under test is in the closing position, press the test button, the circuit breaker passes the 50KA short-circuit current, The circuit breaker immediately breaks (OPEN for O) and extinguishes the arc. The circuit breaker should be intact and can be closed again. T is the intermittent time (rest time), usually 3min, when the line is in the hot standby state, the circuit breaker is once again turned on (CLOSE for short) and the next breaking (O) (the connection test is to check the circuit breaker) Electrical and thermal stability at peak currents and wear of moving and stationary contacts due to bounce. This program is CO. The circuit breaker can be completely disconnected, the arc is extinguished, and the specified breaking damage is determined. The test of the ultimate breaking capacity of the circuit breaker is successful. The test procedure for the short circuit breaking capacity of the circuit breaker (Icu) is OTCOTCO, which is more than the Icu test program. One time CO. After the test, the circuit breaker can completely break and extinguish the arc, and if it does not exceed the specified damage, it is determined that its rated short-circuit breaking capacity test passes. After the Icu and Ics short-circuit breaking test, tests such as withstand voltage and protection characteristics are also required. Since the rated current is carried after the short circuit is broken, a re-test of temperature rise is required after the Ics short-circuit test. Icu and Ics are short-circuited or the conditions for actual assessment are different. The latter is stricter and more difficult than the former. Therefore, IEC947-2 and GB14048.2 determine that Icu has four or three values, which are 25%, 50%, 75%, and 100% Icu (for Type A circuit breakers or molded case) or 50%, 75%, 100% Icu (for Class B circuit breakers, ie versatile or frame type). The Ics value determined by the manufacturer of the circuit breaker, the Icu percentage value that meets the above criteria is a valid and qualified product. Most of the universal circuit breakers have three-stage protection functions of overload long delay, short short delay and short-circuit transient, which can realize selective protection, so most trunk lines (including the output end of the transformer) use it. As the main switch, because the main line should be cautious after replacing the fault current, the main line power failure will affect a large number of users, so two COs are required when a short circuit fault occurs, and it is required to continue to carry the rated current for a period of time, so the universal disconnection The weight of the device is biased to its Icu value; the molded case circuit breaker used on the branch circuit has completed its mission after the breaking and re-closing of the ultimate short-circuit current. It no longer carries the rated current and can be replaced with a new one. (The impact of power outages is small), generally only pay attention to its Ics value. However, both versatile and molded case circuit breakers must have two important technical indicators, Icu and Ics. Only the Ics value is slightly different on the two types of circuit breakers. The minimum allowable Ics for the molded case can be 25% Icu, and the minimum allowable Ics for the universal type is 50% Icu. Some circuit breaker application designers, when selecting the circuit breaker according to the expected short-circuit current of the line, are measured by the rated short-circuit breaking capacity of the circuit breaker, thereby determining a certain type of circuit breaker (the ultimate short-circuit capability of this circuit breaker is greater than The line is expected to have a short-circuit current, while the short-circuit breaking capability is lower than the calculated current). This is a misunderstanding. Third, the clearance and creepage distance of the circuit breaker Determining the electrical clearance of an electrical product must be based on the insulation fit of the low voltage system, while the insulation fit is based on the transient overvoltage being limited to the specified impulse withstand voltage, and the transient overvoltage generated by the appliance or device in the system must also be low. The impulse voltage specified in the power system. therefore: 1. The rated insulation voltage of the appliance should be ≥ the rated voltage of the power system 2. The rated impulse withstand voltage of the appliance should be ≥ the rated impulse withstand voltage of the power system 3. The transient overvoltage generated by the appliance shall be ≤ the rated impulse withstand voltage of the power system. Based on the above three principles, the rated impulse withstand voltage (priority value) Uimp of the electrical appliance has a great relationship with the relative ground voltage of the power supply system and the installation category of the electrical appliance: the larger the relative ground voltage value, the higher the installation category [minutes For I (signal level), II (load level), III (distribution level), IV (power level), the required rated impulse withstand voltage is greater. For example, if the relative ground voltage is 220V and the installation category is III, Uimp is 4.0KV. If the installation category is IV, Uimp is 6.0KV. The Uimp of the general molded case circuit breaker has a pollution level of 3 or 4 in 6.0KV, and the minimum clearance is 5.5mm. The actual electrical clearance of the product is greater than 5.5mm. Regarding the creepage distance, GB/T14048.1 "General Provisions for Low-Voltage Switchgear and Control Equipment" stipulates: the minimum creepage distance of electrical appliances (products) and the rated insulation voltage (or actual working voltage), the pollution level of the place where electrical products are used, and? For example, the rated insulation voltage is 660 (690) V, the pollution level is 3. The insulation material used in the product is IIIa (175 ≤ cti < 400, CTI is the tracking index of the insulation material), and the minimum creepage distance is 10 mm. The creepage distance of the general molded case circuit breaker greatly exceeds the specified value. In summary, if the electrical clearance and creepage distance of the electrical product meet the insulation coordination requirements, it will not be due to external overvoltage or the line equipment itself. The operation overvoltage causes the dielectric breakdown of the device. GB7251.1-1997 "Low-voltage switchgear and control equipment Part 1: Type test and partial type test equipment" (suburban IEC439-1: 1992), for insulation The requirements for cooperation are exactly the same as those of GB/T14048.1. Some electrical appliance manufacturers have proposed copper bars for circuit breaker wiring, and the (air) distance between phase and phase should be greater than 12mm, and some It is even suggested that the clearance of the circuit breaker should be greater than 20 mm. This requirement is unreasonable and it has exceeded the requirement of insulation coordination. For high current specifications, in order to avoid electric repulsion in the event of short-circuit current, or conductor at high current Heating, in order to increase the heat dissipation space, it is also possible to properly widen the space between the phases. In this case, whether it is 12mm or 20mm, it can be solved by the electrical appliance manufacturer, or the electrical component factory can provide the terminal block with elbow or The connection board (chip) is realized. Generally, when the circuit breaker is shipped from the factory, the arc-phase plate between the power terminals is provided to prevent phase-to-phase short circuit when the arc is ejected. The zero-arc circuit breaker has ionization molecules when the short-circuit current is prevented. Escape, also install this kind of arc shield. If there is no arc shield, the bare copper strip can be wrapped with insulation tape, the distance should be no less than 100mm. Application of four-pole and four-pole circuit breaker Regarding the application of four-pole circuit breakers, the current national standards or regulations have not made a rigid regulation on whether or not to use them. Although the design specifications of regional four-pole electrical appliances have been introduced, the debate about installing and not installing quadrupole appliances is still in progress. In recent years, the use of some areas has shown a trend of beekeeping, and various circuit breaker manufacturers have also designed and manufactured various types of four-pole circuit breakers to be put on the market. The author agrees with one opinion, that is, whether or not it is necessary to ensure the reliability and safety of the power supply, so it is basically: 1. TN-C system. In the TN-C system, the N line and the protection line PE are combined into one (PEN line). Considering safety, the PEN line is not allowed to be disconnected at any time, so the four-pole circuit breaker is absolutely disabled. 2. TT system, TN-CS system and TN-S system can use four-pole circuit breaker to ensure the safety of the maintenance personnel during maintenance, but the TN-CS and TN-S system, the N pole of the circuit breaker can only be connected to N Line, but not PEN or PE line; 3. Install a dual power supply switching place. Since all neutral lines (N lines) in the system are connected, in order to ensure the safety of the switched power switch (circuit breaker), a four-pole circuit breaker must be used. 4. For the 380V system, a four-pole residual current protector (leakage circuit breaker) should be used.
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IEC947-2 and GB4048.2 define the ultimate short-circuit breaking capacity and short-circuit breaking capacity of the circuit breaker as follows: rated ultimate short-circuit breaking capacity of the circuit breaker (Icu): according to the conditions specified in the specified test procedures, excluding The circuit breaker continues to carry the breaking capacity of its rated current capability; the rated operating short-circuit breaking capacity (Ics) of the circuit breaker: the breaking capacity of the circuit breaker to continue to carry its rated current capability according to the conditions specified in the specified test procedure.