Gidagida na Gaskiya a Zabi da Karkashin Kirkirar Circuit Breaker

Yadda zaɓe da kuma yadda ake saƙo Circuit Breakers

1. Nau'o'i na Circuit Breakers

1.1 Air Circuit Breaker (ACB)
Ana ake kira molded frame ko universal circuit breaker, duka abubuwa suna cikin yanayi mai sauƙi a kan jirgin zafi. Yana da karfi ga ake gajarta masu sauƙi da abubuwan da ake amfani da su, tare da ake amfani da nau'o'i mafi yawa. ACBs suna amfani a matsayin babban wasu mai hukuma. Masu saƙo na overcurrent sun haɗa ne electromagnetic, electronic, da intelligent types. Sun ba da al'amuran daɗi biyar: long-time delay, short-time delay, instantaneous, da ground fault, da kowane al'adu ya shiga wurin adadin da ke ɗaukan jirgin zafi.

ACBs suna daidai a ƙarfin 50Hz da rated voltages na 380V ko 660V da rated currents daga 200A zuwa 6300A. Suna amfani don ƙarfin da kuma daidaita game da overloads, undervoltage, short circuits, da single-phase grounding. Wannan breakers sun ba da nau'o'i mafi yawan daidai da selective protection. A lokacin daɗi, ana iya amfani da su don kawo wata tsari. ACBs na rated up to 1250A suna daidai da motors daga overloads da short circuits a ƙarfin 380V/50Hz.

Nau'o'in da ake amfani da su sun haɗa ne main outgoing switches a ƙarin 400V na transformers, bus tie switches, high-capacity feeder switches, da large motor control switches.

1.2 Molded Case Circuit Breaker (MCCB)
Ana ake kira plug-in circuit breaker, masu sauƙi, arc extinguishers, trip units, da operating mechanism suna cikin yanayi mai sauƙi. Auxiliary contacts, undervoltage trip units, da shunt trip units suna da design modular, wanda yake ƙara da aiki mai sauƙi. MCCBs ba su daidai don kawo wata tsari daidai, kuma ana amfani da su don daidaita game da branch circuits.

Dukkan MCCBs sun haɗa ne thermal-magnetic trip units. Babbar models suna da solid-state trip sensors. Overcurrent trip units suna da electromagnetic ko electronic. Electromagnetic MCCBs ba su daidai don selective, suka ba da long-time da instantaneous protection. Electronic MCCBs sun ba da al'amuran daɗi biyar: long-time, short-time, instantaneous, da ground fault. Wasu models mafi yawan da zone-selective interlocking.

MCCBs suna daidai don ƙarfin da kuma daidaita game da feeder circuits, main outgoing switches a small distribution transformers, motor control terminals, da power switches don machinery mafi yawa.

1.3 Miniature Circuit Breaker (MCB)
MCBs suna daidai a ƙarfin electrical systems na buildings. Su sun daidai da single-phase da three-phase circuits daga 125A game da short circuits, overloads, da overvoltage. Ana samun 1P, 2P, 3P, da 4P configurations.

MCB sun haɗa ne operating mechanism, contacts, protective devices (various trip units), da arc extinguishing system. Contacts suna kawo wata tsari ta hanyar mutum ko electrically, kuma suna daukta free-tripping mechanism. Coil na overcurrent trip unit da heating element na thermal trip unit suna sama da series da main circuit, kuma undervoltage trip coil suna sama da parallel da power supply.

A ƙarfin electrical design na buildings, MCBs suna daidai don overload, short-circuit, overcurrent, undervoltage, ground fault, leakage protection, automatic transfer of dual power sources, da infrequent motor starting and protection.

2. Key Technical Parameters of Circuit Breakers

• Rated Operating Voltage (Ue)
  Nominal voltage da ake buƙaci da circuit breaker don yi aiki daidai a lokacin daɗi. A China, don systems daga 220kV, maximum operating voltage ita ce 1.15 times the system rated voltage; don 330kV da uku, ita ce 1.1 times. Breaker ya fi sanar da insulation da kuma yi switching operations a maximum operating voltage na system.

• Rated Current (In)
  Current da trip unit zaka iya carry continuously a ambient temperatures daga 40°C. Don adjustable trip units, wanda ita ce maximum adjustable current. Daga 40°C (zuwa 60°C), derating yana da kyau.

• Overload Trip Current Setting (Ir)
  Breaker zaka iya saƙo ta hanyar time delay idan current yana kadan Ir, wanda ita ce maximum current da breaker zaka iya carry without tripping. Ir ya kamata zai daɗi da maximum load current (Ib) amma zai ci gaba da permissible current na cable (Iz). Don thermal-magnetic breakers, Ir ana da kyau da 0.7 zuwa 1.0In; electronic trip units suna da wider range, da 0.4 zuwa 1.0In. Don fixed trip units, Ir = In.

• Short-Circuit Trip Current Setting (Im)
  Threshold da ake ƙara don instantaneous or short-time trip unit don kawo wata tsari ta hanyar time delay during high fault currents.

• Rated Short-Time Withstand Current (Icw)
  Current da breaker zaka iya withstood for a specified duration without thermal damage.

• Breaking Capacity
  Maximum fault current da breaker zaka iya safely interrupt, independent of its rated current. Common values include 36kA and 50kA. It is categorized into ultimate breaking capacity (Icu) and service breaking capacity (Ics).

3. General Principles for Circuit Breaker Selection

• Rated operating voltage ≥ circuit rated voltage.

• Rated short-circuit making/breaking capacity ≥ calculated load current.

• Rated short-circuit making/breaking capacity ≥ maximum possible short-circuit current in the circuit.

• Single-phase-to-ground short-circuit current at the circuit end ≥ 1.25 × instantaneous (or short-time) trip setting.

• Undervoltage trip unit rated voltage = circuit rated voltage.

• Shunt trip unit rated voltage = control power supply voltage.

• Electric operating mechanism rated voltage = control power supply voltage.

• For lighting circuits, set instantaneous electromagnetic trip current to 6 times the load current.

• For single motor short-circuit protection: 1.35× motor starting current (DW series) or 1.7× (DZ series).

• For multiple motors: 1.3× largest motor starting current + sum of other motors’ running currents.

• As main transformer low-voltage side switch: breaking capacity > transformer’s low-voltage short-circuit current; trip rated current ≥ transformer rated current; short-circuit setting = 6–10× transformer rated current; overload setting = transformer rated current.

• After preliminary selection, coordinate with upstream and downstream breakers to prevent cascading trips and minimize outage scope.

4. Selectivity of Circuit Breakers
Circuit breakers are classified as selective or non-selective. Selective breakers offer two- or three-stage protection: instantaneous and short-time for short circuits, long-time for overload. Non-selective breakers are typically instantaneous (short-circuit only) or long-time (overload only). Selectivity is achieved using short-time delay trip units with different time settings. Key considerations:

• Upstream instantaneous trip setting ≥ 1.1 × maximum 3-phase short-circuit current at downstream breaker’s output.

• If downstream is non-selective, upstream short-time trip setting ≥ 1.2 × downstream instantaneous trip setting to maintain selectivity.

• If downstream is also selective, upstream short-time delay time ≥ downstream short-time delay time + 0.1s.
  Generally, Iop.1 ≥ 1.2 × Iop.2.

5. Cascading Protection
In system design, coordination between upstream and downstream breakers ensures selectivity, speed, and sensitivity. Proper coordination allows selective fault isolation, maintaining power to healthy circuits. Cascading uses the current-limiting effect of the upstream breaker (QF1). When a short circuit occurs downstream (at QF2), QF1’s current-limiting action reduces the actual fault current, allowing QF2 to interrupt a current higher than its rated capacity. This allows use of lower-cost, lower-breaking-capacity downstream breakers. Conditions include no critical loads on adjacent circuits (since QF1 trip would blackout QF3), and proper matching of instantaneous settings. Cascading data is determined by testing and provided by manufacturers.

6. Sensitivity of Circuit Breakers
To ensure reliable operation during minimum fault conditions, sensitivity (Sp) must be ≥1.3 per GB50054-95:
Sp = Ik.min / Iop ≥ 1.3
Where Iop is the instantaneous or short-time trip setting, and Ik.min is the minimum short-circuit current at the protected line end under minimum system operation. For selective breakers with both short-time and instantaneous trips, only the short-time trip sensitivity needs verification.

7. Selection and Setting of Trip Units

(1) Instantaneous Overcurrent Trip Setting.Must exceed the circuit’s peak current (Ipk) during motor startup:
Iop(0) ≥ Krel × Ipk
(Krel = reliability factor)

(2) Short-Time Overcurrent Trip Setting and Time
Iop(s) ≥ Krel × Ipk. Time delays are typically 0.2s, 0.4s, or 0.6s, set to ensure upstream operation time exceeds downstream by one time step.

(3) Long-Time Overcurrent Trip Setting and Time
Protects against overload: Iop(l) ≥ Krel × I30 (maximum load current). Time setting must exceed allowable short-term overload duration.

(4) Coordination Between Trip Settings and Cable Capacity.To prevent cable overheating or fire without tripping:

Iop ≤ Kol × Ial
Where Ial = cable’s allowable current-carrying capacity, Kol = short-term overload factor (4.5 for instantaneous/short-time trips; 1.1 for long-time trip as short-circuit protection; 1.0 for overload protection only). If not satisfied, adjust trip setting or increase cable size.