“Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.”

1. Q: Why is making capacity of circuit breaker higher than making capacity of Circuit breaker?
2. Q: What are the values of making capacity of circuit breaker?
3. Q: What are the values of breaking capacity of circuit breaker?
4. Q: Define the terms ? Breaking capacity, making capacity.
   
5. Q: Distribution equipment, such as circuit breakers, fuses, switchgear, and MCCs, have interrupting or withstand ratings defined as the maximum rms values of symmetrical current.
   
6. Q: What is Circuit breakers breaking capacity ? Answer: A circuit breaker can't interrupt a circuit at the instant of inception of a short. Instead, due to the relay time delay and breaker contact parting time, it will interrupt the current after a period of five to eight cycles, by which time the DC component will have decayed to nearly zero and the fault will be virtually symmetrical.
7. Q: What is Circuit breaker's making capacity? Answer:Closing a breaker against an existing fault makes it possible to intercept the peak of the asymmetrical short-circuit current, which is greater than the rms value of the symmetrical current. For this reason, equipment is also tested at a particular test X/R ratio value typical to a particular electrical apparatus, such as switchgear, switchboards, or circuit breakers, and is designed and rated to withstand and/or close and latch the peak asymmetrical current described above.
8. Q: Why fault analysis is required? Answer: Fault analysis is required to calculate and compare symmetrical and asymmetrical current values in order to select a protective device to adequately protect a piece of electrical distribution equipment.

Reference:

1. This site has 5 sections - Substation, High Voltage, Transformers, Protection, Commissioning. It is good for having an overview and going in detail in few areas. http://www.sayedsaad.com/Protection/index.htm
2. This site has lot of material and books on electrical. http://www.blogger.com/2.%20http://www.filecrop.com/search.php?w=Practical%20Power%20System%20Protection&opt_t=1&opt_d=0&c=30&m=&size_i=&size_f=&engine_r=&engine_m=&pos=1&order=score&mod=dec
3. http://ecmweb.com/training/electrical_basics/