Circuit Breaker

Circuit breaker opening times used in the calculator are based on generally accepted time-current data for circuit breakers of various classes. Maximum values have been selected in order to insure that calculations do not indicate too low an incident energy level.

 

Molded case circuit breakers (MCCB) from 100 through 400 amperes are assumed to have standard thermal-magnetic trip units with instantaneous trips set at ten times continuous ratings.

 

MCCB from 401 through 600 amperes are assumed to have electronic trip units.

 

Circuit breakers above 600 amperes are assumed to be low-voltage power circuit breakers with short-delay trip units set at 30 cycles.

 

When using circuit breakers with short-delay trip units to achieve coordination, the actual short-delay setting should be used if known. Formulas in Annex D of NFPA 70E or those in IEEE 1584 may be used to calculate incident energy when settings are outside the limits of the calculator. In most cases, the incident energy will exceed the capability of available PPE, i.e., over 100 cal/cm². If making a detailed study of a system protected with circuit breakers, obtain the actual opening times from the manufacturer.



Visit us on the web! www.littelfuse.com