Kîjanê Dîmena Vêrênê ya Girêdanê Bîne Hesabkirin

Ma heta ku bîrka yek bistîne di sisteman elektrîk de were, dergeha ber bi mezin ya bîrka yek bistîne di sisteman de hatiye destnusiyaye, guhertina (CB) ra, ta dê CB werin vegirî yên. Ma heta ku bîrka yek bistîne di CB de hatiye destnusiyaye, piranên din ên CB li ser stresên mekanîk û termalên mezin bin.

Ma heta ku piranên CB nayên çendîna herêmîyek e ku bas bibe, ewa dikare ku destûrên zêdetirina garmîya mezin bike. Ev garmîya mezin dikare ku kaliteya izoleyan CB biguye.

Guhertinên CB da hamî garmîya mezin hatine. Stresên termalên guhertinên CB anjamek ji I2Rt, ku R resistanceya guhertinê ye, piştre pressiya guhertinê û rêzikê guhertinê ve. I wateya rms ya bîrka yek bistîne û t dema ku bîrka yek bistîne di guhertinên de hatiye destnusiyaye.

Ma heta ku serseriya were destnusiyayê, bîrka yek bistîne hatiye destnusiyaye ta dê vabartina CB bikar bîne. Nêzîna, t dema breaking ya guhertinê ye. Ji ber ku ev dema jî di scale de mili second, hesab kirin ku hemî xwarê destnusiyayê di serseriyan de hatiye destnusiyaye guhertinê, ji ber ku nayên dema piştrekirina û radiasyona xwarê.
Zêdetirina garmîya dikare bi formulê wek heya be,

Lê, T zêdetirina garmîya per sekund di degree centigrade de.
I wateya (rms symmetrical) di Ampere de.
A çendîna herêmîya guhertinê.
ε koyê temperature coefficient ya resistivity ya guhertinê di 20oC de.

Ma heta ku bizanin ku alûmînî ma u 160oC girtina mekanîkê yê hilbijêre û vê soft, dixwazin ku zêdetirina garmîya li vir re bi tevînê. Ev bidayekî zêdetirina garmîya mezin di bîrka yek bistîne de belciyê. Ev hedefta bi kontrolkirina dema breaking ya CB û dizaynê wêra ya çendîna guhertinê bibe.

Dergeha Bîrka Yek Bistîne

Dergeha electromagnetic developed di du parallel electric current carrying conductors de, bi formulê were verast kirin,

Lê, L girêja du guhertinên di inch de.
S dera di navên wan de di inch de.
I wateya ku hêza her guhertinê were destnusiyaye.

Bi testê werin destnusiyayê ku, dergeha electromagnetic short circuit force herî mezin hatiye destnusiyaye ma heta ku valueya short circuit current I, 1.75 times initial rms value ya symmetrical short circuit current wave bibe.

Lê, di pêwendeyên cihê de, ewa dikare ku, dergeha mezin hatiye destnusiyaye, wanî, li ser mînakî, li ser barsên rigid û di resonance de li ser barsên mechanical vibration. Testê werin destnusiyayê ku, 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.

Nêzîna, ewa şitandar ê ku error li ser side of safety bibe û ji bo hemî contingencies, ji bo ku divê ku 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.

Formula stricly 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.

Di casesên praktik de, total length of the conductor nayên infinity. 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,

is stead of L/S in the above formula.
The formula then becomes,

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.

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