General principles for the selection of low-voltage electrical appliances:
1. The rated voltage of the low-voltage electrical appliance should not be less than the working voltage of the circuit, ie Ue≥Ug.
2. The rated current of the low-voltage electrical appliance should not be less than the calculated working current of the loop, ie Ie≥Ig.
3. The breaking current of the equipment should not be less than the short-circuit current, ie Izh≥Ich
4. The thermal stability guarantee value should not be less than the calculated value.
5. Select low-voltage electrical equipment according to the starting condition of the circuit. For example, the fuse and the automatic air switch need to be selected according to the starting condition.
First, the selection of circuit breakers
Protection: overload, short circuit, under voltage
General selection:
1. The rated voltage of the circuit breaker ≥ the rated voltage of the line;
2. The rated current of the circuit breaker ≥ the line calculates the load current;
3. The rated current of the circuit breaker release ≥ the line calculates the load current;
4, the circuit breaker limit on-off capacity ≥ the maximum short-circuit current in the line;
5. The automatic switching instant (or short delay) tripping setting current of the single-circuit short-circuit current at the end of the line is not less than 1.25 times;
6. The rated voltage of the circuit breaker undervoltage release is equal to the rated voltage of the line.
Selection of power distribution circuit breakers:
1. The long delay action current is set to 0.8 to 1 times the allowable current carrying capacity of the wire;
2. The return time of the motor of the 3 times long delay action current setting value is not less than the starting time of the motor with the maximum starting current in the line;
3. The short-delay action current setting value is not less than 1.1 (Ijx+1.35kIedm). Ijx calculates the load current for the line; k is the motor starting current multiple, and Iedm is the maximum rated current of one motor;
4. Short delay time according to the thermal stability check of the protected object;
5. When there is no short delay, the instantaneous current setting value is not less than 1.1 (Ijx+1.35k1kIedm). K1 is the impact coefficient of the motor starting current, which is 1.7 to 2.
If there is a short delay, the short-circuit current value is calculated at the input end of the lower-stage 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 = motor rated current;
2. The return time of 6 times long delay current setting value ≥ motor starting time;
3. The instantaneous setting current of the squirrel cage is 8-15 times of the rated current of the trip unit; the instantaneous setting current of the winding shape is 3-6 times the rated current of the trip unit.
Selection of automatic switch for lighting:
1. The long delay current setting value is not greater than the line calculation load current;
2. The instantaneous current setting value = 6 times the line calculates the load current.
Second, the selection of the knife switch
Protection: It is 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. According to the rated voltage: the rated voltage of the knife switch ≥ the working voltage of the knife switch.
2. According to the rated current: the rated current of the knife switch ≥ the working current of the knife switch. If there is a motor in the circuit, the operating current should be calculated according to the motor starting current.
3. According to the thermal stability and dynamic stability check:
Imax ≥ ich. Imax: Maximum allowable current. Ich: Three-phase short-circuit inrush current.
Third, the fuse selection
Protection: Short circuit, if overload protection, the reliability is not high.
1, the choice of fuse melt
(1) Select according to normal working current
Melt rated current ≥ line calculation current
(2) Verify the sensitivity of the short-circuit current
Idmin/Ier≥Kr
Idmin: minimum short circuit current of the protected line
Kr: fuse action coefficient, generally 4
2, various types of equipment fuse selection
(1) Single motor circuit melt selection
Ier≥Iqd/α
Ier: melt rated current
Iqd: motor starting current
α: calculation coefficient, depending on the fuse characteristics of the starting condition
(2) Selection of distribution line melt
Iqd1: Starting current of the motor with the highest starting current in the line
Ig(n-1): line operating current other than the motor with the highest starting current
α: Calculated coefficient.
(3) Selection of lighting line melt
Ier≥Ig/αm
Ig: Line calculation current
Mm: calculation coefficient
(4) Selection of high and low pressure melts for transformers
1 Transformer with a capacity of 160KVA, its high-pressure melt is selected according to 2~3 times rated current; the transformer with capacity above 160KVA, high-pressure melt is selected according to 1.5~2 times rated current.
2 Transformer low-pressure melt is selected according to its rated current or 20% of overload current.
(5) Selection of electrostatic capacitor bank melt
Ier ≥ (1.5 ~ 2) Iec
Iec: rated current of the capacitor bank
3, the choice of fast fuse
Fast-acting fuses are mainly used for internal rectifier protection of silicon rectifier components, thyristor components and complete sets of devices.
(1) Small-capacity rectifier
Ier=1.57IF
Ier: The rated current of the fast fuse.
IF: thyristor forward average current
(2) Large capacity converter
m: number of parallel paths
Ki: dynamic current sharing coefficient, generally 0.5 to 0.6
Ar: Fuse maximum blow
Ak: Silicon component surge
Fourth, the selection of thermal relay
Protection: overload
1. Selection of long-term work or intermittent long-term work motor thermal relay
(1) Select according to motor starting time
Tf=(0.5~0.7)td
Tf: the return time of the thermal relay at 6Ie
Td: the operating time of the thermal relay at 6Ie
(2) Select according to the rated current of the motor
Iz=(0.95~1.05)Ied
Iz: Thermal relay setting current
Ied: rated motor current
(3) Select according to the requirements of phase failure protection
For the star-connected motor, a three-pole thermal relay can be used; for a delta-connected motor, a thermal relay with a phase-breaking operation protection device should be used.
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