Ji bo Dîrokên Berdîna Qurtbûnê Berekê?

Kesayış Deyişînin Hesabkirin?

Kesayış Deyişînê Pêşnûmakirin

Kesayış deyîşî di dema ku hewç bihastî dibêje, ya ku dergahên kesanrê hatine girîng bikarhat. Kesayış deyîşî ya ku ji elektrikî sistem de derbas diket, ya ku girîng deyîşî di sisteman de derbas diket.

Dema ku hewç bihastî dibêje, girîng deyîşî di sistem de derbas diket, ya ku kesanrê (CB) têne girîng deyîşî. Di vê derbasde, yekê CB naxistî, CB yên têne girîng mekanîkî û termîkî serbexser in.

Eger partên kefanan CB-yan dest pê kanî çihaz ne, wan zaf dike, ku bêdîsa kesanrê têne girîng bike. Serbexseriyên termîkî yên contactên CB an jî proportional e I2Rt, ku R resistanceya contact, I rms valueya kesayış deyîşî, û t dema derbas deyîşî.

Pas dibêjekan, kesayış deyîşî derbas diket ta unita interrupting CB, kesanrê bikat. Demek, t dema breaking timeya kesanrê ye. Lema ku vê dema zi bas heye, her heat ê di dibêj de dibû di conductor de hatine girîng bikarhat.

Zêdetina wêna dikare bi formulê berdibandin,

Lema, T zêdetina wêna per second di degree centigrade de.I current (rms symmetrical) di Ampere de.A cross-sectional area of the conductor.ε temperature coefficient of resistivity of the conductor at 20 ^oC.

Aluminum lêzani di 160°C de, ji ber vê ku pir hesas e ku zêdetina wêna bin 160°C bikarhat. Ev berdewam zêdetina wêna yê kesayış deyîşî, ku dikare bi control kirdina breaking timeya CB û design kirdina dimensionên conductor bikarhat.

Kesayış Force

Electromagnetic force developed between two parallel electric current carrying conductors, is given by the formula,

Lema, L length of the both conductors in inch.S distance between them in inch.I current carried by each of the conductors.

It is experimentally proved that, electromagnetic short circuit force is maximum when the value of short circuit current I, is 1.75 times the initial rms value of the symmetrical short circuit current wave.

However, in certain circumstances it is possible that, forces greater than these may develop, such as, for instance in the case of very rigid bars or due to resonance in the case of bars liable to mechanical vibration. Experiments have also shown that the reactions produced in a non resonating structure by an alternating current at the instant of application or removal of the forces may exceed the reactions experienced while the current is flowing.

Thus it is advisable to error on the side of safety and to allow for all contingencies, for which one should take into account the maximum force which could be developed by the initial peak value of the asymmetrical short circuit current. This force may be taken as having a value which is twice of that calculated from the above formula.

The formula is strictly useful for circular cross-sectional conductor. Although L is a finite length of the portions of conductors run parallel to each other, but the formula is only suitable where the total length of each conductor is assumed as infinite.

In practical cases the total length of the conductor is not infinite. It is also considered in mind, that, the flux density near the ends of current carrying conductor is considerably different than its middle portion.

Hence, if we use above formula for short conductor, the force calculated would be much higher than actual. It is seen that, this error may be eliminated considerably if we use the term. It is stead of L/S in the above formula.

The formula, represented by equation (2), gives error free result when the ratio L/S is greater than 20. When 20 > L/S > 4, formula (3) is suitable for error free result.

If L/S < 4, formula (2) is suitable for error free result. The above formulas are only applicable for circular cross-sectional conductors. But for rectangular cross-sectional conductor, the formula needs to have some correction factor. Say this factor is K. Hence, the above formula ultimately becomes.

Although the effect of shape of cross-section of conductor reduces rapidly if spacing between the conductor increases the value of K is maximum for strip like conductor whose thickness is quite less than its width. K is negligible when shape of cross-section of conductor is perfectly square. K is unity for perfectly circular cross-sectional conductor. This holds true for both standard and remote control circuit breaker.