What are the various factors affecting the selection of circuit breaker.
1 Answer

Selection of circuit breaker:

  • Function of circuit breaker are:
    • To act as switch for normal conditions
    • To automatically isolate or open the faulty part in the event of fault
  • The circuit breaker are normally used in power system at places where the power level is very high.
  • Generally they are used in high voltage transmission lines, substations, generating stations and for heavy loads in industries.

  • Since the circuit breaker are employed in places where the power level is high, whenever its contacts open it has to interrupt heavy currents both during load conditions and faulty conditions.

  • It results in large voltage across its contacts.
  • Which results into sparking at the contacts. Hence, the amount of current it has to be interrupt is an important criteria.

  • The circuit breaker for particular application is selected based on the following ratings.

  1. Normal working power level specified as rated interrupting current

  2. The fault level specified as rated short circuit interrupting current

  3. Momentary current rating

  4. Normal working voltage

  5. Speed of circuit breaker

  • The speed of circuit breaker is defined as the time between the occurrence of the fault to the extinction of the arc (when contacts are open). It is normally specified in cycles of supply frequency.
  • The standard speed of circuit breakers are 8, 5, 3 or 1 ½ cycles.
  • For 50 Hz frequency, 1 cycles $ \cfrac {1} {50} $ = 0.02 sec
  • The momentary current rating is the max. current that may flow through the circuit breaker for a small duration of time.
  • The rating of circuit breakers are selected such that its momentary current rating is less than calculated value.

  • the short circuit interrupting MVA can be determined form pre-fault voltage and short circuit interrupting current as follows:

Short Circuit interrupting MVA $=\sqrt{3}\left|V_{p f L}\right|\left|I_{f L}\right|$

where, $\left|V_{p f L}\right|=$ Magnitude of pre-fault line voltage at the fault point in kV

$\left|I_{f L}\right|=$ Magnitude of line value of short circuit current interrupting current at the fault in $\mathrm{kA}$.

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