The selection of fuses is mainly based on the protection characteristics of the load and the magnitude of the short-circuit current. For small-capacity motors and lighting branch lines, fuses are often used for overload and short-circuit protection, so a relatively small melting coefficient of the fusible element is desirable. Lead-tin alloy fuses are usually selected. For larger-capacity motors and lighting main lines, short-circuit protection and breaking capacity should be emphasized. Fuses with higher breaking capacity are usually selected; when the short-circuit current is very large, fuses with current-limiting function should be used.
The rated current of the fuse element can be selected as follows:
When protecting stable loads without a starting process, such as lighting circuits, resistors, and electric furnaces, the rated current of the fuse element should be slightly greater than or equal to the rated current in the load circuit.
The fuse current for protecting a single motor operating continuously can be selected based on the maximum starting current, or it can be selected using the following formula:
IRN ≥ (1.5～2.5)IN
Where IRN: rated current of the fuse element; IN: rated current of the motor. If the motor starts frequently, the coefficient in the formula can be appropriately increased to 3-3.5, depending on the actual situation.
Protecting multiple motors operating continuously (power supply main line):
IRN ≥ (1.5～2.5)IN max+ΣIN
IN max: Rated current of the single motor with the largest capacity. ΣIN: Sum of the rated currents of the remaining motors.