Many people that do not fully understand interrupting ratings (AIC ratings) are under the common misconception that the AIC rating means that the circuit breaker will never allow more than that value to pass by the circuit breaker to downstream components or other circuit breakers. This is incorrect.
The AIC rating is the highest amount of current that the circuit breaker has been tested to interrupt before the circuit breaker fails or possibly explodes.
Another common misconception is that the circuit breakers AIC rating means that whatever is supplied by the circuit breaker is protected up to the marked AIC rating. This is also incorrect.
Example: In an installation where there is 35,000 amps of available fault current, many electricians will install a 42k AIC circuit breaker in the MDP to supply a downstream panelboard with 10k AIC circuit breakers and think that the 42k AIC breaker at the MDP will protect the downstream lower rated breakers against any fault current that is higher than 10,000 amps. The only way to guarantee that the combination of the circuit breakers used in this method will perform as hoped is if the combination has been tested as a series combination and marked in the series rating information provided with the panelboard.
If no one did their homework and if the combination is not a tested combination, then there is no guarantee that the downstream circuit breaker will be properly protected in the event that a fault occurs in one of its branches that draws the maximum amount of fault current available at its line terminals.
Which of the following is true?
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