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Selection of low-voltage electrical components [essence]

by:Shenmao     2021-06-21
General principles for the selection of low-voltage electrical appliances: 1. The rated voltage of low-voltage electrical appliances should not be less than the working voltage of the loop, that is, Ue≥Ug.           2, the rated current of low-voltage electrical appliances should not be less than the calculated working current of the loop, that is, Ie≥Ig. 3. The interrupting current of the equipment should not be less than the short-circuit current, that is, Izh≥Ich4, and the thermal stability guarantee value should not be less than the calculated value. 5. Select low-voltage electrical appliances according to the starting conditions of the circuit. For example, fuse and automatic air switch need to be selected according to the starting situation. 1. Circuit breaker selection protection: overload, short circuit, and undervoltage. General selection: 1. Circuit breaker rated voltage ≥ line rated voltage; 2. Circuit breaker rated current ≥ line calculated load current; 3. Circuit breaker release rating Current ≥ line calculated load current; 4. Circuit breaker limit breaking capacity ≥ maximum short-circuit current in the line; 5. Automatic switching instantaneous (or short-delay) tripping current of not less than 1.25 times the single-phase-to-ground short-circuit current at the end of the line; 6. The rated voltage of the undervoltage release of the circuit breaker is equal to the rated voltage of the line. Selection of circuit breakers for power distribution: 1. The long-delay operating current is set to 0.8-1 times the allowable current carrying capacity of the conductor; the return time of the set value of the long-delay operating current for 2, 3 times is not less than the maximum starting current in the line The starting time of the motor; 3. The short-delay action current setting value is not less than 1.1 (Ijx+1.35kIedm). Ijx is the calculated load current of the line; k is the motor starting current multiple, and Iedm is the maximum rated current of the motor; 4. The short delay time is checked according to the thermal stability of the protected object; 5. When there is no short delay, the instantaneous current setting The value is not less than 1.1 (Ijx+1.35k1kIedm). k1 is the impact coefficient of the motor starting current, which is 1.7~2. If there is a short time delay, the short-circuit current value is calculated at the incoming end of the lower switch whose instantaneous current setting value is not less than 1.1. Selection of automatic switch for motor protection: 1. Long delay current setting value u003d motor rated current; 2. The return time of 6 times the long delay current setting value ≥ motor starting time; 3. The squirrel cage instantaneous setting current is 8 to 15 times the rated current of the release; the winding-shaped instantaneous setting current is 3 to 6 times the rated current of the release. The selection of automatic switches for lighting: 1. The long delay current setting value is not greater than the line calculated load current; 2. The instantaneous current setting value u003d 6 times the line calculated load current. 2. Type selection protection of the knife switch: Mainly used as an isolating switch, it does not cut off the fault current, and can only withstand the electric power and thermal effects caused by the fault current. Selection: 1. Select according to the rated voltage: the rated voltage of the knife switch ≥ the operating voltage of the knife switch. 2. Select according to the rated current: the rated current of the knife switch ≥ the operating current of the knife switch. If there is a motor in the circuit, the working current should be calculated according to the motor starting current. 3. Check thermal stability and dynamic stability:   imax≥ich. Imax: Maximum allowable current. Ich: Three-phase short-circuit impulse current. 3. Fuse selection protection: short circuit, if overload protection is used, the reliability is not high. 1. Selection of the fuse melt (1) According to the normal working current, select the melt rated current ≥ the calculated current of the line (2) Check the sensitivity of the action according to the short-circuit current Idmin/Ier≥Kr  Idmin: the minimum short-circuit current of the protected circuit Kr: Fuse action coefficient, generally 42. Fuse selection for various equipment (1) Single motor circuit melt selection Ier≥Iqd/α   Ier: melt rated current Iqd: motor starting current α: calculation coefficient, depending on starting conditions (2) Selection of the melt of the distribution line Iqd1: the starting current of the motor with the largest starting current in the line Ig(n-1): the working current of the line except the motor with the largest starting current α: calculation coefficient. (3) Selection of lighting circuit melt Ier≥Ig/αm  Ig: line calculated current αm: calculation factor (4) Selection of transformer high and low voltage melt ① For transformers with a capacity of 160KVA, the high voltage melt is rated at 2 to 3 times Current selection: For transformers with a capacity of more than 160KVA, the high-voltage melt should be selected at 1.5 to 2 times the rated current.  ②The low-voltage melt of the transformer is selected according to 20% of its rated current or overload current. (5) The selection of the melt of the electrostatic capacitor bank Ier≥(1.5~2) Iec  Iec: the rated current of the capacitor bank 3. The selection of the fast fuse. The fast fuse is mainly used for silicon rectifier components, thyristor components and the interior of complete sets of equipment Overcurrent protection.  (1) Small-capacity rectifier   Ieru003d1.57IF  Ier: the effective value of the rated current of the fast fuse. IF: Thyristor forward average current (2) Large-capacity converter m: Number of parallel outgoing circuits Ki: Dynamic current sharing coefficient, generally 0.5~0.6   Ar: Maximum melting of fuse Ak: Surge of silicon element IV. Thermal relay Selection protection: overload 1, long-term work or intermittent long-term work motor thermal relay selection (1) according to the motor starting time selection tf u003d (0.5 ~ 0.7) td  tf: thermal relay can return time under 6Ie td: hot The action time of the relay under 6Ie (2) According to the rated current of the motor, Izu003d(0.95~1.05) Ied  Iz: Thermal relay setting current Ied: Motor rated current (3) According to the phase failure protection requirements, select the motor with star connection , The use of three-pole thermal relay is sufficient; for delta-connected motors, thermal relays with open-phase operation protection devices should be used. 2. Selection of thermal relay used for motor protection in repeated short-time work
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