By EECTECH FUSE | 14 July 2020 | 0 Comments
Application difference between fuse and circuit breaker
What EECTECH brings to you this time is the difference between the application of fuses and circuit breakers! Come take a look!
It needs to be explained that it is fair to compare the fuse with the switch and compare with the circuit breaker. Therefore, this article uses the NH-gG fuse and its switch suitable for the power distribution system to explain. NH refers to a general term for low-voltage fuses with high breaking capacity, and NHgG fuses are called high-breaking (HRC) fuses.
1. Ease of use and maintenance
In low-voltage distribution terminals, electrical failures can be caused by overload or other reasons. Miniature circuit breakers can be closed after overloading, and their operation is simple. Therefore, it is more appropriate to use small circuit breakers as protective appliances. For the fuse switch, the fuse must be replaced before it can be put into use again. Sometimes, there is a case where there is no suitable fuse to replace.
But for low-voltage distribution systems and ordinary low-voltage cabinets maintained by professionals, circuit breakers do not have these advantages. First of all, the failures in these places do not occur frequently; secondly, there is a great danger of closing the circuit breaker directly after the failure, especially short-circuit failures. When different faults occur, the status of the fuse is also different, which provides a basis for judging the cause of the accident.
2. Short circuit protection
Generally, the NH-gG fuse has a breaking capacity of more than 100kA, which greatly exceeds that of most circuit breakers, while the breaking capacity of ordinary circuit breakers is between 25 and 35kA. If the breaking capacity is increased, the price will increase exponentially. The current-limiting effect of the fuse is very strong, and the circuit breaker is also difficult to compare. When the fault current has not reached the high value, the circuit has been cut off by the fuse, so it can provide safety protection for electrical equipment, cables, and motors to avoid them. Suffering from the damage of electric power and thermal effect during short circuit can greatly reduce the dynamic stability and thermal stability requirements of the system caused by the short circuit current. A circuit breaker is a mechanical device, and its breaking capacity and speed are limited by the action process of mechanical parts, which is why the breaking capacity of the circuit breaker is lower than that of a fuse. In general, the circuit breaker is not as good as the current limiting characteristic of the fuse, and the speed of cutting off the fault current is not as fast as the fuse.
In addition, when the operating voltage is 400V, 500V and 690V, the breaking capacity of the fuse is almost not affected, and the circuit breaker is dwarfed in this respect. When the operating voltage is higher, the breaking capacity of most circuit breakers will be significantly reduced. In general, the breaking capacity at 690V is 30% smaller than at 400V. Taking a certain brand of molded case circuit breaker as an example, its breaking capacity is 50kA at 400V, but only 10kA at 690V. It is precisely the high breaking capacity of the fuse. In Europe, it is often used as a backup protection for a miniature circuit breaker or a molded case circuit breaker with a low breaking capacity, and there is no arcing when the fuse cuts the short circuit fault.
3. Overload protection
For the motor system, the thermal relay is used to protect the overload, not the aM fuse or circuit breaker. The overload protection function of the circuit breaker is not applicable here. It cannot be used to explain that the fuse has no overload protection function, and the fuse The type switch can solve the problem of phase loss of the fuse.
For gG fuses with a rated current greater than 16A, the agreed fusing current is 1.6In. Some people think that it is sometimes difficult to meet the overload protection formula I2≤1.45Iz. The wire and cable cross-sectional area should be larger than the circuit breaker with the same current setting. Point out that this is a shortcoming of the fuse. This problem needs to be dialectically viewed. Choosing the smallest cable cross-sectional area according to the current carrying capacity brings troubles for future expansion. Fuses and switches are mostly used on the power supply side of the distribution trunk. The cable cross-sectional area is selected according to economic current density. The cost is slightly increased, but the cable operating cost is reduced, and the increased cost of construction can be recovered within a certain period, and the aforementioned problem of troublesome expansion is solved. It should be noted that for the overload protection formula I2≤1.45Iz, the implementation of the German DIN standard gG fuse can meet this formula.
4. Upper and lower level selectivity
Selectivity for gG fuses in accordance with the current standard GB13539 (equivalent to IEC60269) is easy to achieve, as long as the ratio of the setting current of the upper fuse and the lower fuse is not less than 1.6. For example, a fuse with a gG rated current of 100A guarantees complete selectivity for a fuse with a gG rated current of 160A. Even for some foreign products, the ratio of the setting current of the upper fuse and the lower fuse can be 1.25, and it can achieve full selectivity for the upper and lower stages.
For the upper and lower circuit breakers, it is more difficult to achieve the same function. Technically, in most cases, the ratio of the setting current between the two circuit breaker specifications that can achieve upper and lower selectivity is much larger than the ratio between the fuses. In this way, increasing the current level of electrical components at all levels will increase the cross-sectional area of the cables and wires, and the two circuit breakers at the upper and lower levels must also be the same generation of products produced by the same manufacturer.
In particular, as long as it is a gG fuse that meets the current standard GB13539, even if the upper and lower fuse is not the same brand, it will not change the selectivity, which is stipulated by the product standard.
The engineering standards require that the upper and lower protection electrical appliances used in low-voltage distribution lines should be selective in their actions and coordinated between all levels. However, the actual project often chooses non-selective circuit breakers, which cannot achieve the selectivity of the upper and lower levels. If you choose a selective circuit breaker, one can easily cost tens of thousands of yuan, compared with a total of two or three thousand yuan for fuses and switches. The cost is too high. Therefore, relevant electrical experts in my country call on the majority of electrical practitioners to pay attention to this problem, why not consider the use of fuses and switches?
5. Reliability
The basic working principle of the fuse is to connect the fuse in the line, and only one overcurrent can make the fuse blow to protect other devices in the line. The fuse is a static protection device, and the entire product is sealed. Even in the most complicated environment and without repair and maintenance, the fuse can provide long-term and reliable protection of the circuit. The reaction of the fuse is carried out in accordance with physical laws and energetic principles, and there is no aging problem, so as long as the circuit fails, the fuse can always be opened. The simplicity of the technical design of the fuse and its switch and the physical principle of its function ensure the reliability in time.
On the contrary, the reliability of the complicated mechanism of the circuit breaker will be affected after long-term use. In the process of breaking the current, the circuit breaker is all mechanical action, it is easy to produce mechanical wear and mechanical displacement, resulting in unreliable and unstable operation. And every time the circuit breaker trips, the performance will be reduced, and it must be maintained by a professional or even the contact of the circuit breaker needs to be replaced. After repeated trips, its protection performance may be difficult to meet the protection requirements. The relevant European standards stipulate that the circuit breaker trips 5 times and must be replaced forcibly. This is also one of the reasons why fuses occupy most of the European market. In the high-voltage system, after the circuit breaker is tripped, it needs to be repaired according to the power regulations, and the equipment damage is serious to be replaced, but in the low-voltage distribution system, China does not have a standard that requires the circuit breaker to be replaced.
The above is the sharing of the difference between the application of fuses and circuit breakers brought by EECTECH to you, I hope to help you.
Leave a Reply
Your email address will not be published.Required fields are marked. *