Circuit Breaker vs Fuse

[brianafischer]

I am designing a control panel that includes a VFD and a SCR Power controller. The components/control panel has the following specs: 3 phase 480VAC VFD: 3HP Saftronics CV104003 (5.2 Amps) Power Controller: Omega SCR39Z (22.5 Amps) I have 2 questions: 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? Thanks!

[TimWilborne]

It has to do with the inrush on power up when the capacitors are charging

[BobLfoot]

With the new emphasis on Arc Flash standards many companies are using fuses rather than breakers to reduce worker exposure to arc flash. A 15 Amp fuse has a significantly lower arc flash than a 15 amp breaker.

[Snerkel]

Also for SCR you should look at using Semiconductor Fuses for the best level of protection.

[TimWilborne]

I hadn't thought about that. Could you elaborate on what type of fusing it should be?

[brianafischer]

The SCR39Z has a 40A internal fuse. My plan is to use a 30A circuit breaker. on the 3 phase 480VAC circuit with a 16kW heater and SCR39Z: 16kW * 1.25 = 20kVA 3 Phase x 20kVA x 460VAC = 25.13A 25.13A x 1.25 ~ 30A circuit breaker Edited September 12, 2006 by brianafischer

[Snerkel]

Using a breaker as you suggest will be fine. I assume the SCR is being used for a heater system? If it is then you will probably want to look at over temperature protection that controls a contactor in series with the SCR. In normal operation the contactor remains on and the SCR controls the heating, if an over temperature is detected then the contactor is used to isolate the heating circuit. Another idea for the over temperature protection is to use a circuit breaker with a shunt trip such as this http://new.kimcontrols.com/item/G3P030STDC?man=3 Putting 24v onto the shunt trip turns off the circuit breaker so an external circuit (such as over temperature) can be used to "trip" the breaker.

[brianafischer]

I just checked out the data sheet from here the shunt trip option seems like a good idea to save panel space.

[paulengr]

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.