BS 7671 defines circuit breaker as
A mechanical device capable of making and carrying currents under normal circuit conditions and also capable of breaking currents under specified abnormal circuit conditions such as those of short-circuits

Performance data is produced by manufacturers for all circuit breakers and is expressed in terms of the ff. 

Rated current
  • The nominal current (IN) is the continuous current rating of the circuit breaker
  • Current ratings of miniature circuit breakers (MCB) vary from 2 A to 125 A
Rated voltage
  • The value of voltage at which the circuit breaker’s short circuit performance is given.
  • Also, creepage distances and dielectric breakdown are specified above the rated voltage.
Time/current characteristics curves
  • This shows the relationship between tripping time and value of overcurrent. Other information obtained from the time/current curves are values of current that will operate the magnetic and thermal tripping mechanisms. 
There are 3 different time current characteristics of MCB's 
  • B type - mean tripping current equal to 4 times rated current 
  • C type - mean tripping current equal to 7.5 times rated current
  • D type - mean tripping current equal to 12.5 times rated current

Time-Current Curves of Circuit Breaker
source: SWSI Miller Australia
When selecting a circuit breaker, consideration must be given to the following factors:
  • maximum demand of the load
  • current carrying capacity of the circuit cable 
  • type of circuit breaker required (B,C or D), depending on the load characteristics 
  • frame size of the circuit breaker 
  • ambient temperature at the point of installation 
  • prospective fault current at the point of installation 
  • the need for backup protection. 
  1. South Western Sydney Institute - Miller
  2. Schneider Electric
  3. BS 7671

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