Kadaitaccen Kula | Kirki na Kula da Tabbacin Kirki
Tsohon rating na circuit breaker ta shafi:
Rated short circuit breaking current.
Rated short circuit making current.
Rated operating sequence of circuit breaker.
Rated short time current.
Short Circuit Breaking Current of Circuit Breaker
Wani shine mafi yawan karamin short circuit da circuit breaker (CB) zai iya gama suka dace bayan ya fitar da abubuwan da ke jira.
Idan karamin short circuit ya gama da circuit breaker, za a samun wuya da stress mai kawo a abubuwan da ke jirgin CB. Idan tsari da kashi na abubuwan da ke jirgin circuit breaker ba su tana yawa daidai, ya kamata lalace da damar da abubuwan da ke jirgin CB.
Daga cikin hukumomin Joule’s law of heating, an yi wuya na farko da karamin short circuit, resistance da lokacin da karamin short circuit ya gama. Karamin short circuit ya gama da circuit breaker har zuwa lokacin da short circuit ya dace bayan fitar da circuit breaker.
Saboda stress mai kawo a circuit breaker ana so kuɗi da lokacin da short circuit, capacity na breaking na electrical circuit breaker, tana neman lokacin da ita ce. A 160oC aluminum ya zama maye da kuma ya bace strength mai kawo, wannan temperature ya zama limit na temperature rise na breaker contacts a lokacin short circuit.
Saboda haka short circuit breaking capacity ko short circuit breaking current of circuit breaker an define shi da maximum current da zai iya gama da CB daga lokacin da short circuit ya faru har zuwa lokacin da short circuit ya dace bayan fitar da CB.
An sanya halayen short circuit breaking current a RMS.
A lokacin short circuit, CB ba tana samu stress mai kawo kawai, tana samu stress mai kawo da kuma stress mai kwallon kafa. Saboda haka a lokacin da a taimaka short circuit capacity, strength mai kawo na CB ta shafi.
Don haka don yanayin circuit breaker da zai iya amfani, ya kamata a taimaka fault level a wurin da CB zai zama. Idan fault level na wani wurin na electrical transmission an tabbatar, ya zama yake da ya fi amfani da rated circuit breaker da zai da daidai.
Rated Short Circuit Making Capacity
Short circuit making capacity na circuit breaker an sanya da peak value ba a RMS value kamar breaking capacity. Na teori a lokacin da fault ya faru a system, fault current zai iya gama da twice of its symmetrical fault level.
A lokacin da a switch on circuit breaker a lokacin da system yana da fault, short circuit portion na system yana haɗa da source. Cycle na farko na current a lokacin da circuit breaker ya gama, tana da maximum amplitude. Wannan shine about twice of the amplitude of symmetrical fault current waveform.
Contacts na breaker suna da shiga wannan maximum value na current a cycle na farko a lokacin da breaker ya gama a lokacin fault. Daga cikin wannan phenomenon, selected breaker ya kamata a rate da short circuit making capacity.
Saboda rated short circuit making current na circuit breaker an sanya da maximum peak value, tana da yawa da rated short circuit breaking current na circuit breaker. Halayen normal na short circuit making current tana 2.5 times more than short circuit breaking current. Wannan tana da shi don standard da remote control circuit breaker.
Rated Operating Sequence or Duty Cycle of Circuit Breaker
Wani shine mechanical duty requirement na circuit breaker operating mechanism. Sequence na rated operating duty na circuit breaker an specify shi kamar:
Da O ta nufin opening operation na CB.
CO ta nufin closing operation time wanda tana biyo biyo da opening operation without any intentional time delay.
t’ shine lokacin da ke bayar a bayan biyu operations wanda tana da shi don restore initial conditions and/or to prevent undue heating of conducting parts of circuit breaker. t = 0.3 sec for circuit breaker intended for first auto re closing duty, if not otherwise specified.
Suppose rated duty circle of a circuit breaker is:
Wannan nufin, opening operation na circuit breaker tana biyo biyo da closing operation after a time interval of 0.3 sec, and then the circuit breaker again opens without any intentional time delay. After this opening operation the CB is again closed after 3 minutes and then instantly trips without any intentional time delay.
Rated Short Time Current
Wani shine current limit da circuit breaker zai iya gama da safely for certain specific time without any damage in it. Circuit breakers ba su dace short circuit current immediately fault yana faru a system. Akwai always some intentional and unintentional time delays present between the instant of occurrence of fault and instant of clearing the fault by CB.
Wannan delay shine saboda lokacin da protection relays yana yi operation, lokacin da circuit breaker yana yi operation and also there may be some intentional time delay imposed in relay for proper coordination of power system protection. Even a circuit breaker fails to trip, the fault will be cleared by next higher positioned circuit breaker.
In this case the fault clearing time is longer. Hence, after fault, a circuit breaker has to carry the short circuit for certain time. The summation of all time delays should not be more than 3 seconds; hence a circuit breaker should be capable of carrying a maximum faulty current for at least this short period of time.
The short circuit current may have two major affects inside a circuit breaker.
Because of the high electric current, there may be high thermal stress in the insulation and conducting parts of CB.
The high short circuit current, produces significant mechanical stresses in different current carrying parts of the circuit breaker.
A circuit breaker is designed to withstand these stresses. But no circuit breaker has to carry a short circuit current not more than current for a specified short period. The rated short time current of a circuit breaker is at least equal to rated short circuit breaking current of the circuit breaker.
Rated Voltage of Circuit Breaker
Rated voltage na circuit breaker tana neman da insulation system. For below 400 KV systems, circuit breaker an design da take so 10% above the normal system voltage. For above or equal 400 KV system the insulation of circuit breaker should be capable of withstanding 5% above the normal system voltage.
That means, rated voltage na circuit breaker corresponds to the highest system voltage. This is because during no load or small load condition the voltage level of power system is allowed rise up to highest voltage rating of the system.
A circuit breaker is also subject to two other high voltage conditions.
Sudden disconnection of huge load for any other cause, the voltage imposed on the CB and also between the contacts when the CB is open, may be very high compared to higher system voltage. This voltage may be of power frequency but does not stay for very long period as this high voltage situation must be cleared by protective switchgear.
But a circuit breaker may have to withstand this power frequency over voltage, during its normal life span.
The Circuit Breaker must be rated for power frequency withstands voltage for a specific time only. Generally the time is 60 seconds. Making power frequency withstand capacity, more than 60 second is not economical and not practically desired as all the abnormal situations of electrical power system are definitely cleared within much smaller period than 60 seconds.Like other apparatuses connected to power system, a circuit breaker may have also to face lighting impulse and switching impulses during its life span.
The insulation system of CB and contact gap of an open CB have to withstand these impulse voltage waveform amplitude of this disturbance is very very high but extremely transient in nature. So a circuit breaker is designed to withstand this impulse peaky voltage for microsecond range only.
