electing a circuit breaker based on its supply …What?
In my opinion, from all the considerations in selecting a circuit breaker, this is the most important of all.
Why?
This is the only rating which, if incorrectly done, will render the circuit breaker as a safety device useless.
The rated breaking capacity ensures that if a dead short occurs below the circuit breaker the circuit breaker
must be capable of managing the thousands of amps that will flow, and disconnect the supply quick enough.
But what will be the result of 10000 Amp flowing in a 100 Amp circuit?
One outcome is that the circuit breaker contacts instantly fuse together and it does not matter what the tripping mechanism does the circuit will stay closed, meaning the circuit breaker cannot trip.
The bottom line is…the rated breaking capacity (kA) rating of a circuit breaker must be always higher then that of the supply Prospective Short Circuit Current at that point.
But nothing is every that easy now, is it.
In the IEC 60 364 – 4.4.5.5 stipulates that the circuit breaker must be higher rated then the supply but then it says:
“unless it is in a cascaded system.”
Now what?
See here for a further explanation.
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