Circuit Breakers

Question: What is a Circuit Breaker?

Answer. Circuit Breaker is a mechanical device designed to close or open contact members, thus closing or opening an electrical circuit under the normal or abnormal conditions.

Question: How does a Circuit Breaker different from Switch?

Answer. A switch is just a device when can be able to open and close the circuit during normal operation. On the other hand circuit breaker has the ability to open and close the contacts during abnormal or fault conditions. This circuit breaker has the potential to break and make heavy short circuit currents. Auto-reclosures in the circuit breaker has the ability to re-close after certain designed duration to verify whether the short circuit was cleared.

Question: What is meant by making capacity of the circuit breaker?

Answer. The making capacity of the circuit breaker when closed on a short circuit is the peak value of the maximum current wave (including dc component) in the first cycle of the current after the circuit is closed by the circuit breaker.

Question: What is meant by Current Chopping?

Answer. Current Chopping is a phenomenon of current interruption before the natural current zero is reached.

Question: Why is current chopping not common in oil circuit breakers?

Answer. Current chopping is not common in oil circuit breakers because in most of the oil circuit breakers the arc extinguishing power is proportional to the magnitude of current to be interrupted

Question: Why are Isolation switches provided along with the circuit breakers?

Answer. Isolation switches are provided for separating the disconnected element from the live portion of the system for the purpose of safety.

Question: Why is asymmetrical breaking current higher than the symmetrical breaking current in a circuit breaker?

Answer. Asymmetrical breaking current is the RMS value of the combined sum of the dc and ac components of the short-circuit current at the instant of separation of the breaker contacts while symmetrical breaking current is the RMS value of the ac component alone. Thus asymmetrical breaking current is higher than the symmetrical breaking current.

Question: What is recovery voltage in circuit breaker?

Answer. Recovery voltage in circuit breakers is defined as the power frequency RMS voltage which appears across the breaker contacts after the transient oscillations die out and the final extinction of the arc has resulted in all the poles of the circuit breaker. It is the voltage developed after the arc extinction is completed

Question: When does an arc extinct?

Answer. If the rate of rising of re-striking voltage (RRRV) is less than the rate of rising of dielectric strength between the breaker contacts arc will not sustain and extinct.

Question: What is re-striking voltage?

Answer. The Re-striking voltage may be defined as the resultant transient voltage which appears across the breaker contacts at the instant of arc extinction

Question: For a Resistive Load and Inductive load considered for switching operation of a circuit breaker, Switching of which load is difficult and Why?

Answer. In case of switching resistive load, current and voltage will be in phase i.e, when during current zero voltage also will be zero in case of a resistive load. Hence the voltage at the breaker terminals during arc extinction is zero or minimal. On the other hand, for switching inductive load during current zero voltage across the breaker will be maximum. This results in re-striking of arc once again. Therefore switching inductive load will be difficult.

Question: Name the materials used for the contacts of vacuum circuit breakers?

Answer. Copper-Bismuth, Copper-lead, Copper-tellurium, Silver-bismuth, Silver-lead, and Silver-tellurium are some of the alloys employed as contact materials in the vacuum circuit breakers

Question: Why current chopping considered as the serious drawback in a circuit breaker?

Answer. The current chopping is considered as the serious drawback because it sets up high voltage transient across the breaker contacts.

Question: For EHV applications which type of circuit breaker is mostly preferred?

Answer. For Extra High Voltage (EHV) applications SF6 gas insulated switchgear is preferred.

Question: What are the duties of Circuit Breakers?

Answer. Some of the duties of the circuit breakers are listed below:

  1. Interruption of small inductive currents
  2. Switching of unloaded transmission lines and unloaded cables
  3. Switching of capacitor banks and reactors
  4. Interruption of terminal faults
  5. Interruption of short line faults
  6. Asynchronous switching

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