What Principles Should Be Followed When Selecting Fuses?
Many non-professionals find choosing a drop out fuse confusing due to the wide variety of types available. How should one choose the right fuse? What principles should be followed?
(I) Selection of Fuse Type: The type of fuse should be selected according to the application. Knife-contact fuses are generally used for power grid distribution; screw-type fuses are generally used for motor protection; and cylindrical cap fuses are generally used for lighting circuits. For protecting thyristor components, semiconductor fast-acting fuses should be selected.
(II) Selection of Fuse Specifications
1. Selection of Fuse Rated Current
(1) For loads such as transformers, electric furnaces, and lighting, the rated current of the fuse should be slightly greater than or equal to the load current.
(2) For power transmission and distribution lines, the rated current of the fuse should be slightly greater than or equal to the safe current of the line.
(3) When used for short-circuit protection in motor circuits, the starting conditions of the motor should be considered, and the rated current of the fuse should be selected according to the length of the motor's starting time. For motors with short starting times, the rated current of the fuse can be determined by the following formula: IN_fuse = Ist / (2.5~3), where Ist is the starting current of the motor, in amperes. For motors with longer starting times or frequent starting, the rated current of the fuse is determined by the following formula: IN_fuse = Ist / (1.6~2). For the main busbar supplying multiple motors, the rated current of the fuse can be calculated as follows: In = (2.0~2.5)Imemax + ∑Ime. Note: In is the rated current of the fuse; Ime is the rated current of the motor; Imemax is the rated current of the motor with the largest capacity among multiple motors; ∑Ime is the sum of the rated currents of the remaining motors. For the protection of the motor terminal circuit, a type aM fuse should be selected, and the rated current In of the fuse element should be slightly greater than the rated current of the motor;
(4) For the protection of the main circuit of the capacitor compensation cabinet, if a type gG fuse is selected, the rated current In of the fuse element is approximately equal to 1.8~2.5 times the calculated line current; if a type aM fuse is selected, the rated current In of the fuse element is approximately equal to 1~2.5 times the line current.
(5) For selective protection between upper and lower levels of the line, the ratio of the rated current In of the upper-level fuse to the lower-level fuse should be equal to or greater than 1.6 to prevent cascading failures and expanding the scope of power outages.
(6) For fuses protecting semiconductor devices, the fuse is connected in series with the semiconductor device, and the rated current of the fuse element is expressed in RMS value, while the rated current of the semiconductor device is expressed in forward average current. Therefore, the rated current of the fuse should be calculated according to the following formula: IRN ≥ 1.57 IRN ≈ 1.6 IRN where IRN represents the average forward current of the semiconductor device.
(7) Derating At an ambient temperature of 20°C, we recommend that the actual operating current of the fuse should not exceed the rated current value. When selecting a fuse, environmental and operating conditions should be considered, such as the degree of enclosure, airflow, connecting cable size (length and cross-section), and transient peaks; the current carrying capacity test of the fuse is performed at an ambient temperature of 20°C, and its actual use is affected by changes in ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse, and the shorter its lifespan. Conversely, operating at lower temperatures will extend the lifespan of the fuse. (8) In power distribution lines, it is generally required that the rated current of the upstream fuse be 2-3 times greater than that of the downstream fuse to prevent cascading failures and expanding the scope of power outages.
2. Selection of Fuses
(1) UN fuse ≥ UN line.
(2) IN fuse ≥ IN line.
(3) The maximum breaking capacity of the fuse should be greater than the maximum short-circuit current on the protected line. Fuses are mainly used in industrial and mining enterprises and daily life to protect low-voltage electrical equipment. Due to their use in different electrical equipment, the principles for selecting their capacity and size vary greatly. In practice, they must be selected and configured strictly according to the regulations. Otherwise, they will lose their intended protective function.
