Operating Characteristics of Miniature Circuit Breakers

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. 

Sources:

1. South Western Sydney Institute - Miller
2. Schneider Electric
3. BS 7671