brianafischer, on Sep 11 2006, 03:10 PM, said:
- Is there any reason, besides cost, why people use fuses instead of circuit breakers?
- The manual for the Saftronics drive states the Circuit Breaker should be 15A. Isn't this extremely over-rated, considering the drive consumes 5.2 Amps
With a motor that draws say 10 amps, you can use a current limiting fuse at just a little over that. For a circuit breaker, you'd have to set it to the inrush, several TIMES the normal current draw of the motor. Effectively, circuit breakers will barely catch short circuits and even then only after your motor has been destroyed. Current limiting fuses will protect a motor from short circuit conditions with no damage.
With arc flash considerations, things are a bit more confusing. As has been previously mentioned, in many cases, fuses are a much better choice. However, there are some special cases I've seen where a circuit breaker works better than a fuse.
There is one more safety reason why fuses are preferred over circuit breakers. Technically, you get caught in a catch-22 situation with a circuit breaker. With a fuse, they only trip when there is actually a problem in the circuit. There is no such thing as a false trip with a fuse. With a circuit breaker, false trips are a fact of life. So when the circuit breaker trips, you don't know if there really was a fault or not.
This brings up an NEC rule. You are not supposed to reapply power to a circuit until you have positively identified and removed the fault from the circuit. With a fuse, you know a fault occurred. With a circuit breaker, you can't really ever determine whether it is safe to reenergize a circuit or not.
Worse yet, utilities actually set their circuit breakers to automatically reset at least once automatically before remaining open. They are actually covered under the NESC though instead of the NEC.