Calculation Method for Melt Current of Fuse Fuses

PublishTime： 2023-11-24 Article Source：XC News

The current of a fuse includes two aspects, one is the rated current of the fuse tube, and the other is the rated current of the melt, which cannot be confused.

1. Melt current for direct starting of a single motor IRNIRN ≥ Iq/K (A)... (l) IRN Melt rated current (A) Iq Motor starting current (A) K-coefficient or IRN=(1.5-2.5) IH (A)... (2) In the formula, IH Motor rated current (A) or 380V (AC) IRN=7PH (A)... (3) 22OV (AC) IRN=12PH (A)... (4) In the formula: PH Motor rated power (kW)

2. Multiple motors in the distribution main line (with several units), with a total fuse IRN=(1.5-2.5) IHmax+I (n-1) (A)... (5) In the formula: IHmax - The rated current of the largest motor or motor group that starts simultaneously I (n-1) - The sum of the rated currents of other motors eXCept for the largest motor group or motor group that starts simultaneously, or IRN=I (n-l)+Iqmax/2.5 (A)... (6) In the formula: Iqmax - The maximum starting current of one motor (A) or IRN=K1 [Iqmax I (n-l)] (A)... (7) In the formula: K1- load factor, usually taken as 0.4, and there are also data introducing that K1 is 0.9-l.

3. Electric lamp main fuse melt IRN=(1.3-1.5 × The rated current of the electricity meter (A)... (8) is securely installed on the output line of the electricity meter.

4. Lamp shunt fuse IRN=working current of all lamps on the shunt (A)... (9) 5. Fuse with control transformer (control circuit) IRN ≥ PH+0.1Pq/U2 (A)... (10) Where: PH - Rated capacity of control transformer (VA) Pq - Starting capacity of the largest electrical attraction coil in the circuit or the sum of several electrical attraction coils simultaneously starting capacity (VA) U2- Secondary voltage of control transformer (V) 6. Fuse without control transformer (control circuit) IRN ≥ 0.4 [IQ+IH (n-1) [(A)... (11) In equation: Iq - the starting current of the largest electrical appliance (or the suction coil that several appliances start simultaneously) in the line (A) IH (n-1) - the sum of the rated currents of the suction coils of other electrical appliances in the line (A) 7. The melt current of the load balancing control circuit IRN ≥ IH (A)... (12) In equation: IH - the sum of the rated currents of all electrical appliances load (A) 8. The selection of high-voltage fuse transformer capacity is below 100kVA, IRN=(2-2.5) × Rated current on the high-voltage side of the transformer (A)... (13) Transformer capacity above 100kVA, IRN=(1.5-2) × Rated current on the high-voltage side of the transformer (A)... (14) 9. Fuse on the low-voltage side of the transformer. The fuse on the low-voltage side of the transformer is set according to the rated current on the low-voltage side, and can also be selected based on the actual load current. For example, the rated current of the transformer is 910A, and the distribution aluminum bar is 80 × 8 (mm2), vertically placed, with an allowable current of 1320A and a temperature correction coefficient of 0.88. The actual allowable current is 1320 × 0.88=1160A, which is higher than the rated current of the transformer and can be selected.

When the current value of the fuse obtained from the above calculation differs from the standard fuse, it can be selected as close to the standard as appropriate. For example, the current on the high-voltage side of the S7-50/10-0.4 transformer is 2.89A, calculated according to equation (13) (2-2.5) 2.89=5.78-8.23 A. The high-voltage fuse standards are 2, 3, 5, 7.5, 10, 15, 20, 30, 40, 50, 75, 100, 150, 200, and 300.

Therefore, 10A high-voltage fuse can be selected.

Fig.1

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