1 Ordinary circuit breaker selection This article refers to the address: http:// Overcurrent protection circuit breakers for distribution (lines), electric motors, and household appliances, etc., the ability to withstand overload currents (including the starting current and starting time of the motor, etc.) for protected objects (such as transformers, wires and cables, motors, and household appliances) There is a difference, the circuit breakers have different protection characteristics. 1.1 Selection of circuit breakers for distribution A circuit breaker for distribution refers to a circuit breaker dedicated to distributing electrical energy in a low-voltage power grid, including a power supply main circuit breaker and a load branch circuit breaker. When selecting this type of circuit breaker, special attention should be paid to the following selection principles: (1) The long-delay action current setting value of the circuit breaker ≤ the allowable current carrying capacity of the wire. In the case of wire and cable, the wire cable is allowed to allow 80% of the current carrying capacity. (2) The return time of the motor with the 3 times long delay action current setting value ≥ the starting time of the motor with the maximum starting current in the line. (3) Short delay action current setting value I1 is: I1=1.1 (Ijx+1.35kIed) (4) The instantaneous current setting value I2 is: (5) The time period of short delay depends on the segmentation of the power Distribution System. The general time period is 2 to 3 levels. The short delay time difference between each stage is 0.1~0.2s, which depends on the action precision of the short delay mechanism of the circuit breaker. The return time should ensure the selective action of each level. After selecting the short delay step, it is best to check the thermal stability of the object to be protected. 1.2 Motor protection type circuit breaker selection The miniature circuit breaker (MCB) cannot be used to protect the motor and can only be used as an alternative fuse to protect distribution lines (such as wire and cable). The motor has a 5~7 times Ied at the moment of starting, and the starting current lasts for 10s. Even if the C characteristic is set to 5~10 times Ied in the electromagnetic trip current, it can ensure that the inrush current is avoided when the motor starts. However, for thermal protection, the action value of the overload protection is set at 1.45Ied, which means that the MCB can be tripped when the motor is subjected to more than 45% of the overload current. This is for the stator winding of the motor that can only withstand <20% overload. It is extremely easy to damage the insulation between the windings, but it is affordable for the wire and cable. Therefore, in some cases, if you need to use MCB to protect the motor, you can use ABB's unique MCB that meets the K characteristics of the IEC947-2 standard, or use MCB external heating relay to protect the motor from overload and short circuit. 1.3 Selection of household protection circuit breakers MCB is the most widely used terminal protection appliance in building electrical terminal power distribution units. You should select the maximum short-circuit capacity as you would a molded case circuit breaker and a frame circuit breaker. MCB is designed and used for 50~60Hz AC grid. For DC circuit, it should be converted according to the magnetic tripping action current provided by the manufacturer and the power frequency variation coefficient; when the ambient temperature is greater or less than the calibration temperature value, The MCB's rated current value must be adjusted according to the temperature and current carrying capacity correction curves provided by the manufacturer. The voltage on the low-voltage side of general industrial and civil building distribution transformers is mostly 0.23/0.4kV, and 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 different capacity transformers, the short-circuit current at the low-voltage feeder terminals is different. Generally speaking, for residential buildings, small shopping malls and public buildings, due to the low-voltage power grid supplied by the local power supply enterprise, the cable or overhead conductor of the power supply line has a thin section, and the electrical equipment is far away from the power supply. 4.5kA and above are selected. The MCB of the breaking capacity can be. For users with special or 10kV variable power distribution stations, the MCB with rated breaking capacity of 6kA and above should be selected because the cable cross section of the power supply line is thicker and the power supply distance is shorter. For similar occasions such as variable substation (lighting used in the station, power supply directly taken from the low voltage main busbar) and large-capacity workshop substation (for workshop power equipment), it is necessary to Select MCB with 10kA and above breaking capacity. Pay special attention to the specific selection: MCB's rated breaking capacity is measured under the upper terminal incoming line and lower terminal outgoing line. Compared with the A characteristic, the B characteristic 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 lines; the C characteristic is suitable for most electrical circuits, which allows the load to pass through. High short-time peak current and MCB does not operate, C characteristic allows peak current <5In, for line protection of fluorescent lamps, high-pressure gas discharge lamps, power distribution systems; D characteristics for high peak currents (<10In The circuit breaker device can be used to protect the primary circuit and solenoid valve of the control transformer and the local lighting transformer under AC rated voltage and frequency. 2 leakage circuit breaker selection 2.1 The choice of ordinary leakage circuit breakers. The following principles should be followed when selecting an earth leakage circuit breaker: 2.2 Selection of four-pole circuit breaker Whether to use a four-pole circuit breaker can follow the following principles: (2) A four-pole circuit breaker shall be used for the double-supply conversion circuit breaker with leakage protection. Two upper-level circuit breakers with leakage protection, and the lower-level power conversion circuit breaker should use a four-pole circuit breaker; (3) A four-pole circuit breaker shall be used for the power switching circuit breaker between two different grounding systems; (4) Four-pole circuit breakers are strictly prohibited in the TN-C system; (5) TN-S and TN-CS systems generally do not need to be equipped with a four-pole circuit breaker. However, some special cases of the TN-S system (severe three-phase unbalance, high zero-order harmonic content, etc.) do not require four-pole circuit breakers for further study; (6) The power supply line breaker of the TT system shall adopt a four-pole circuit breaker; (7) Four-pole circuit breakers should be used when there is a neutral line in the IT system. 3 use of circuit breakers We must pay attention to the use of circuit breakers during use: (1) After the circuit is connected, check that the wiring is correct. It can be checked by the test button. If the circuit breaker can be correctly disconnected, the leakage protector is installed correctly, otherwise the line should be checked to eliminate the fault. After the leakage protector is put into operation, the user should check whether the circuit breaker is running normally through the test button after a period of time; (2) The leakage, overload and short-circuit protection characteristics of the circuit breaker are set by the manufacturer and cannot be adjusted at will, so as not to affect the performance; the function of the test button is that the circuit breaker is energized after a certain period of new installation or operation. Check the running status of the state. Press the test button, the circuit breaker can be disconnected, indicating that the operation is normal and can continue to be used; (3) If the circuit breaker cannot be disconnected, it means that the circuit breaker or the line is faulty and needs to be repaired; (4) The circuit breaker is disconnected due to a fault (leakage, overload or short circuit) of the protected circuit, and the operating handle is in the trip position (middle position). After troubleshooting the cause, the operating handle should be pulled down (ie, placed in the “minute†position), so that the operating mechanism can be “re-locked†before the closing operation can be performed (please pay attention to the three positions of the circuit breaker operating handle). Different meaning); (5) After the circuit breaker is disconnected due to short circuit, it is necessary to check the contact. If the main contact is burnt seriously or there is a pit, it needs to be repaired; (6) The four-pole leakage circuit breaker must be connected to the neutral line to make the electronic circuit work normally; (7) The load wiring of the leakage circuit breaker must pass through the load end of the circuit breaker. No phase or neutral line of the load is allowed to pass through the leakage circuit breaker. Otherwise, the artificial leakage will occur and the circuit breaker will not close. Caused "missing". In addition, in order to protect lines and equipment more effectively, an earth leakage circuit breaker can be used in conjunction with a fuse. (1) The isolation of the neutral line of the power supply line breaker is not to prevent the neutral line overcurrent caused by the unbalanced current in the three-phase circuit or the personal electric shock hazard caused by this overcurrent, but to eliminate the edge The electric shock risk of the electrical maintenance personnel caused by the fault potential introduced by the sexual line; (2) In order to reduce the occurrence of “zero-break†accidents in the three-phase circuit, it is necessary to avoid installing unnecessary circuit breaker contacts on the neutral line, that is, to ensure the installation of four-pole circuit breakers as little as possible under the conditions of ensuring electrical maintenance; (3) Regardless of whether there is a total equipotential bonding in the building, the TT system power supply line breaker should be simultaneously isolated from the neutral line and the phase line, but the TN-S system and the TN-CS system building with total equipotential bonding Electrical equipment is not required; (4) The RCD in the TT system should be able to disconnect the phase line and the neutral line at the same time to prevent electric shock accidents caused by two faults, but there is no such requirement for the leakage protector RCD in the TN system; (5) Except for the power conversion circuit breaker with leakage protection function, other power conversion circuit breakers do not need to isolate the neutral line; (6) Regardless of the grounding system, the single-phase power line breaker should be able to disconnect the phase and neutral lines at the same time.
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Where: Ijx———the line calculates the load current (A);
k——— the starting current multiple of the motor;
Ied——-motor rated current (A).
I2=1.1 (Ijx+klkIedm)
Where: kl - the impact coefficient of the motor starting current, generally take kl = 1.7 ~ 2;
Iedm—the rated current of the largest motor.
The short-circuit current of the low-voltage distribution line is related to the electrical cross-section of the power supply line, the wire laying method, the short-circuit point and the length of the power supply, the capacity of the distribution transformer, and the percentage of impedance.
In the case of special conditions, the terminal incoming line and the upper terminal outgoing line are required. Because of the arc extinguishing when breaking the fault current, the MCB must be derated, that is, the rated breaking capacity must be based on the relevant derating factor provided by the manufacturer. To convert. The protection characteristics of MCB are divided into four characteristics A, B, C and D according to IEC898 for users to choose: A characteristics are generally used in applications requiring fast and no-delay tripping, that is, for lower peak current values ​​( Usually 2 to 3 times the rated current In) to limit the allowable short-circuit current value and the total breaking time. This feature can be used to protect the MCB from the overcurrent protection and mutual inductance measurement circuit of the electronic component. Features are used in applications where faster tripping is required and peak current is not very large.
(1) The rated voltage and current of the circuit breaker shall be greater than or equal to the normal working voltage and current of the line equipment;
(2) The leakage current to be protected by the line shall be less than or equal to the specified leakage protection current of the circuit breaker;
(3) The limit breaking capacity of the circuit breaker should be greater than or equal to the maximum short-circuit current of the circuit;
(4) The rated current of the overload release is greater than or equal to the maximum load current of the line;
(5) It has a short breaking reaction time and can protect the line and equipment.
(1) According to IEC 465.1.5, the circuit breaker between the normal power supply and the standby generator shall use a four-pole circuit breaker; 