Fuse and switch electric appliance with the use of
Fuse when in use, often need and other electrical switch used in conjunction, such as some of the common control circuit, a fuse, circuit breaker, contactor, thermal relay, cable ( wire ), motor. The circuit breaker as the power switch circuit for remote control motor, contactor, thermal relay protection for motor, motor feeder cable and contactor does not have current failure, and the contactor, thermal relay, motor feeder cable and motor itself is responsible for short-circuit protection circuit breaker. This paper is mainly to discuss the in and other electrical switch when used in combination, how to choose fuse.
This combination of equipment in each of a plurality of electrical components are predetermined special protection area. Low expansion over current protection by thermal relay responsible, high overcurrent protection section and below the circuit breakers rated breaking capacity of short circuit current from the circuit breaker instantaneous tripping device breaking, so can play breaker itself superiority. Only after the appearance of greater short-circuit current cases fuse to action. Then, the breaker was instantaneous release for circuit breaking, to ensure that each pole are being cut off. Therefore the selection of fuses, circuit breakers, contactor and a thermal relay combination, the need for the electrical components of the effective protection scope and characteristics are closely configured, figure 1 is the fuse protection element with various properties of the complexes.
Figure 1: three-phase asynchronous motor control electrical protection characteristics matching
A ) motor electrical control coordination between B ) fuse and circuit breaker with
1 - the starting characteristics of motor 2 - thermal relay 3, fuse characteristics
4 - contactor breaking capacity of bearing capacity characteristics of 6 5 - electric cables - breaker tripping characteristic
Figure 1 analysis:
1) each element should be in the motor protection characteristic of inverse time characteristic curve is below, and in the motor starting characteristic curve. In the current period, fuse time current characteristics of heat release characteristics of steeper than. The cable conductors and over current protection is ideal, while the motor over current protection is in need of a time delay characteristics. In the current period, when the current is just over the instantaneous trip current, circuit breaker fuse fast response ratio, but when the current is further increased when the fuse, fuse and circuit breaker speed than the speed, when the current is large, fuses and limit the expected short-circuit current role, as shown in figure 1-A.
2) the thermal relay, such as common JR series thermal overload relay and the fuse time current characteristics must be able to meet the motor starting from zero speed to full running time delay characteristics.
3) the fuse must also protect the thermal relay from may exceed its rated current of 8 times and above the current failure.
4) the fuse must also in case of short circuit protection contactor, breaking contactor can not breaking big electric current, the contactor contact in any case without fusion, or only slight welding phenomenon. Contact breaking capacity, is generally 10 times the rated current value.
5) fuse and circuit breaker with, fuse main breaking big short-circuit current, the fuse breaking range is in the intersection outside of the short circuit current, and the point of intersection within the fuse characteristic curve in circuit breaker characteristic curve above, by the circuit breaker breaking at the point of intersection within the over current and small multiple short circuit current, as shown in figure 1-b. That is, if the fuse and circuit breaker not cooperate, and other appliances with, as long as the fuse in circuit breaker characteristic curve characteristic curve can be below, both without crossing point.
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