Order of performance of the laboratory work

To choose the necessary circuit breaker we have to use certain indications. For circuits with long-tem overloads, for example, circuits of engines’feeding, the protectness is carried out with inertial and thermal circuit breaker. At the same time the choice is carried out using the rated current of consumers. For circuits of engines with low capacity, in order, 100 W as a rule the thermal circuit breakers are used. In these engines the starting current does not exceeds 3…4. To protect circuits with higher capacitance the inertial fuses are used.

At the stricter choosing of thermal circuit breakers for the circuits which allow starting or short-time overloads ampersecond characteristics of circuit breakers are used (Fig. 6.7). At that time the maximally effective possible value of load current is calculated for the time of its changing Т, for example, for the time of the engine starting:

,

where i isan instant value of the load current.

Then the magnetuted and Т on the axes of ampersecond characteristics are made. Thus the point A is found.

Fig. 6.7

The closest characteristics to the point A that pases upper this point, refers to the device that must be chosen. The value of the integral can be defined graphically according to the curve of the current changing in the transient mode i = f (t). If it is necessary to protect the feeder of the engine with a fuse, the following formula is used:

І_зах=І_пуск /1, 5.

So the fuse should be chosen to let it be in 1.5 time less that the starter of electrical current engine. But in this case the circuit of the engine is protected only from a short cirle.

Example. To choose a protective device for two plafons ПС-45 feeder. The capacity of the lamp is Р =25 W, the voltage U= 24 В.

Solution. The load current of the feeder is І_ном =2 Р / U =2·25/24=2, 1 А.

Thus, we choose the fuse ПВ-5.