In order to better protect circuits and products, the choice of fuses is critical. The following description is mainly from different perspectives.
(a) Select the type of fuse from which type of fuse should be selected according to the application. The distribution network generally uses blade type contact fuses (such as Ferraz shawmut Amp-trap series or Bussmann FRS-R series); the motor protection generally uses a spiral type fuse; the lighting circuit generally uses a cylindrical cap type fuse; protection and control Silicon components should be selected for semiconductor protection with a quick fuse.
(b) Select from the specifications of the fuse. Selection of the rated current of the melt (1) The rated current of the melt should be slightly greater than or equal to the load current for loads such as transformers, furnaces and lighting.
(2) For transmission and distribution lines, the rated current of the melt should be slightly greater than or equal to the safe current of the line.
(3) When used as a short-circuit protection in the motor circuit, the starting condition of the motor should be considered. The rated current of the melt is selected according to the length of the motor starting time.
For motors with short start-up time, the melt's rated current can be determined as follows: In melt = Ist/(2.5~3)
In the formula Ist - the starting current of the motor, unit: A
For motors with long start-up time or frequent start-up, the melt current rating is determined by the following formula: IN melt=Ist/(1.6~2)
The rated current of fuses at main busbars supplied by multiple motors can be calculated as follows:
In=(2.0~2.5)Imemax+∑Ime
Note: In fuse rated current; Ime motor rated current; Imemax multiple motor capacity of the largest motor rated current; ∑Ime the remaining motor rated current sum.
The end of the motor protection circuit, the choice of aM-type fuses, fuse body rated current In slightly greater than the motor rated current;
(4) Protection of the main circuit of the capacitor compensation cabinet. If a gG fuse is selected, the rated current In of the fuse is approximately equal to 1.8 to 2.5 times the calculated current of the line; if aM type fuse is selected, the rated current In of the fuse is approximately equal to the line 1~2.5 times the current.
(5) Selective protection between the upper and lower levels of the line. The ratio of the rated current In of the upper fuse to the lower fuse is equal to or greater than 1.6, which satisfies the need to prevent step-by-step actions and increase the scope of fault blackout.
(6) The fuses for semiconductor devices are protected, the fuses are connected in series with the semiconductor devices, and the rated current of the fuses is expressed in rms. The rated current of the semiconductor devices is expressed by the forward average current. Therefore, the calculation shall be made by the following equation: Body rated current:
IRN ≥ 1.57 IRN ≈ 1.6 IRN Where IRN denotes the forward average current of a semiconductor device.
(7) Derating for use at ambient temperature of 20°C, we recommend that the actual operating current of the fuse-link should not exceed the rated current value. When selecting the fuse-link, take into account the environment and working conditions, such as the degree of closure of the air flow connecting cable (length And cross-section changes in instantaneous peaks, etc.; the current carrying capacity of the fuse link is tested at an ambient temperature of 20°C. Actual use is affected by changes in ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse-link. The shorter the service life, the shorter the operating temperature will be to extend the life of the fuse link. Source: http://
(8) In the distribution line, it is generally required that the rated current of the first-stage melt is 2 to 3 times larger than that of the next-stage melt, so as to prevent the occurrence of step-over action and expand the range of failure blackout.
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 circuit.
The fuses are mainly used to protect low-voltage electrical equipment in the production process of industrial and mining enterprises and in daily life. Because they are used in different electrical equipment, the selection principle of capacity and size varies greatly. In practice, the configuration must be selected strictly in accordance with the regulations. . Otherwise, it will lose its proper protection.
1. Used in household appliances. The fuses used for overcurrent or overload protection of household appliances usually have no independent overload protection for household electricity. Only fuses are provided instead. The configuration principle is 1.05 to 1 total current when all household appliances are used. .15 times to configure.
2. Used in high and low voltage circuit breaker closing circuit. For the closing fuse of the closing circuit of the electromagnetic closing mechanism of high and low voltage circuit breakers, because the closing time of the circuit breaker is very short (ms level), according to the current inverse time characteristic curve of the fuse: the larger the current, The shorter the liquefaction time, the instant the large current (value above the inverse time characteristic curve) is ignited; the smaller the incoming current, the longer the liquefaction time, or will not be blown. It is usually configured by 1/3 (Ieî–‚1/3) of the breaker closing current.
3. Instantaneous short circuit protection for low voltage motors. For light-load start or start-up time is short, for example, start-up time is less than 3 s or the fan (motor) motor, according to the motor rated current of 4 to 5 times the configuration; for the start-up time is 4 ~ 8 s, or for example, pumps and other heavy-duty start The motor, because its starting current is up to 6 times of the rated current, its fuse size is set at 5-6 times of the rated current of the motor; for the starting process more than 8s or even longer, and the fuse is frequently started. According to the motor rated current 5 to 7 times the configuration.
4. For multiple small-capacity motors share line short-circuit protection. The configuration of common line fuse size for multiple small-capacity motors is set by the sum of 1.5 to 2.5 times the rated current of one of the largest capacity motors and the remaining rated current of all motors.
5. At the same time also need to consider the characteristics of the fuse core should also be noted the characteristics of the fuse, a variety of applications should be selected different fuse, according to national standards and IEC standards classification:
gG, gL, aM, aR, each curve is inconsistent, the selection is not the same, when used for motor protection, the selection of aM is much smaller than gG.
(a) Select the type of fuse from which type of fuse should be selected according to the application. The distribution network generally uses blade type contact fuses (such as Ferraz shawmut Amp-trap series or Bussmann FRS-R series); the motor protection generally uses a spiral type fuse; the lighting circuit generally uses a cylindrical cap type fuse; protection and control Silicon components should be selected for semiconductor protection with a quick fuse.
(b) Select from the specifications of the fuse. Selection of the rated current of the melt (1) The rated current of the melt should be slightly greater than or equal to the load current for loads such as transformers, furnaces and lighting.
(2) For transmission and distribution lines, the rated current of the melt should be slightly greater than or equal to the safe current of the line.
(3) When used as a short-circuit protection in the motor circuit, the starting condition of the motor should be considered. The rated current of the melt is selected according to the length of the motor starting time.
For motors with short start-up time, the melt's rated current can be determined as follows: In melt = Ist/(2.5~3)
In the formula Ist - the starting current of the motor, unit: A
For motors with long start-up time or frequent start-up, the melt current rating is determined by the following formula: IN melt=Ist/(1.6~2)
The rated current of fuses at main busbars supplied by multiple motors can be calculated as follows:
In=(2.0~2.5)Imemax+∑Ime
Note: In fuse rated current; Ime motor rated current; Imemax multiple motor capacity of the largest motor rated current; ∑Ime the remaining motor rated current sum.
The end of the motor protection circuit, the choice of aM-type fuses, fuse body rated current In slightly greater than the motor rated current;
(4) Protection of the main circuit of the capacitor compensation cabinet. If a gG fuse is selected, the rated current In of the fuse is approximately equal to 1.8 to 2.5 times the calculated current of the line; if aM type fuse is selected, the rated current In of the fuse is approximately equal to the line 1~2.5 times the current.
(5) Selective protection between the upper and lower levels of the line. The ratio of the rated current In of the upper fuse to the lower fuse is equal to or greater than 1.6, which satisfies the need to prevent step-by-step actions and increase the scope of fault blackout.
(6) The fuses for semiconductor devices are protected, the fuses are connected in series with the semiconductor devices, and the rated current of the fuses is expressed in rms. The rated current of the semiconductor devices is expressed by the forward average current. Therefore, the calculation shall be made by the following equation: Body rated current:
IRN ≥ 1.57 IRN ≈ 1.6 IRN Where IRN denotes the forward average current of a semiconductor device.
(7) Derating for use at ambient temperature of 20°C, we recommend that the actual operating current of the fuse-link should not exceed the rated current value. When selecting the fuse-link, take into account the environment and working conditions, such as the degree of closure of the air flow connecting cable (length And cross-section changes in instantaneous peaks, etc.; the current carrying capacity of the fuse link is tested at an ambient temperature of 20°C. Actual use is affected by changes in ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse-link. The shorter the service life, the shorter the operating temperature will be to extend the life of the fuse link. Source: http://
(8) In the distribution line, it is generally required that the rated current of the first-stage melt is 2 to 3 times larger than that of the next-stage melt, so as to prevent the occurrence of step-over action and expand the range of failure blackout.
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 circuit.
The fuses are mainly used to protect low-voltage electrical equipment in the production process of industrial and mining enterprises and in daily life. Because they are used in different electrical equipment, the selection principle of capacity and size varies greatly. In practice, the configuration must be selected strictly in accordance with the regulations. . Otherwise, it will lose its proper protection.
1. Used in household appliances. The fuses used for overcurrent or overload protection of household appliances usually have no independent overload protection for household electricity. Only fuses are provided instead. The configuration principle is 1.05 to 1 total current when all household appliances are used. .15 times to configure.
2. Used in high and low voltage circuit breaker closing circuit. For the closing fuse of the closing circuit of the electromagnetic closing mechanism of high and low voltage circuit breakers, because the closing time of the circuit breaker is very short (ms level), according to the current inverse time characteristic curve of the fuse: the larger the current, The shorter the liquefaction time, the instant the large current (value above the inverse time characteristic curve) is ignited; the smaller the incoming current, the longer the liquefaction time, or will not be blown. It is usually configured by 1/3 (Ieî–‚1/3) of the breaker closing current.
3. Instantaneous short circuit protection for low voltage motors. For light-load start or start-up time is short, for example, start-up time is less than 3 s or the fan (motor) motor, according to the motor rated current of 4 to 5 times the configuration; for the start-up time is 4 ~ 8 s, or for example, pumps and other heavy-duty start The motor, because its starting current is up to 6 times of the rated current, its fuse size is set at 5-6 times of the rated current of the motor; for the starting process more than 8s or even longer, and the fuse is frequently started. According to the motor rated current 5 to 7 times the configuration.
4. For multiple small-capacity motors share line short-circuit protection. The configuration of common line fuse size for multiple small-capacity motors is set by the sum of 1.5 to 2.5 times the rated current of one of the largest capacity motors and the remaining rated current of all motors.
5. At the same time also need to consider the characteristics of the fuse core should also be noted the characteristics of the fuse, a variety of applications should be selected different fuse, according to national standards and IEC standards classification:
gG, gL, aM, aR, each curve is inconsistent, the selection is not the same, when used for motor protection, the selection of aM is much smaller than gG.