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.
- 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
- 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.
- 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.
- 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.
- South Western Sydney Institute - Miller
- Schneider Electric
- BS 7671