Fused disconnect or enclosed circuit breaker?

[The Smashing Electrons]

I have a large project at the factory I work at.
I am concerned about arc flash mitigation.
One new machine requires a 200A 3P 230V branch circuit.
Seems to me I would have greater control over arc flash mitigation if I used fuses for the clear time and to limit the current and also if new technology were to arise it would just be a matter of changing fuses.
I have heard CB's (big ones) need to be recertified/recalibrated by the factory after a certain amount of trips.

Your thoughts please

 

[topgone]

I have a large project at the factory I work at.
I am concerned about arc flash mitigation.
One new machine requires a 200A 3P 230V branch circuit.
Seems to me I would have greater control over arc flash mitigation if I used fuses for the clear time and to limit the current and also if new technology were to arise it would just be a matter of changing fuses.
I have heard CB's (big ones) need to be recertified/recalibrated by the factory after a certain amount of trips.

Your thoughts please

What level of energies did you arrive at when doing your calculations?

 

[jim dungar]

For all intents, 400A and smaller fuses and circuit breakers have similar performance when it comes to AF levels.

Molded case circuit breakers are basically maintenance free unless they have electronic trip units.

 

[big john]

Fuses would allow for current limiting.

But a breaker with an electronic trip unit would allow for much faster clearing of some lower-energy faults.

The coordination would also be easier with a breaker that had an ETU.

 

[petersonra]

Most times over 100 or 200 amps a molded case breaker or molded case switch costs less than a fused switch and takes up less room.

 

[templdl]

I apologize for another long post some replies need explanation.
"I have heard CB's (big ones) need to be recertified/recalibrated by the factory after a certain amount of trips."
Really?
Have you had an opportunity to review the testing requirement that are required to be UL489 listed?
When a fuse blows it commonly is only one leaving the other two. Now you have a single phasing event that could damage the equipment.
Then it is going to the store room the get a replacement fuse. Replace the fuse and cross you fingers that it holds. If not hopefully you have another and fuses may not be cheep either.
Then, why did the fuse blow, short circuit or overload?
With a breaker it can be reset by latching to the off position and then closing it. And if u you happen to be standing there when it trips if the breaker is able to be latched right away the cause of trip is probably instantaneous as a result of a fault. It is not a good practice to close the breaker until the cause of the fault is resolved. Would it be a pleasent thing to do with a fused disconnect by replacing the fuse and the closing the switch back into a fault?
With a breaker by needing to wait a bit before the breaker can be latched is a good indicator that the thermal elements need no cool some which is an indicator of an overload.
A worst case scinereo is a bolted fault which are very rare where arcing faults are a more common.
And, if you question a breaker, breakers can the tested to varify their integrity. Try that with a fuse. You just hope that the fuse does what it is supposed to do.
A common breaker interupts in less than a 1/2 cycle. A current limiting breaker must limit the let through current it 1/4 cycle or less, time after time.
At what point does a fuse element start to clear and then completely clear? With a breaker hit the trip bar and it's less than a 1/2 cycle regardless.
There no magic with solid state trip units either other than features as they have to hit the trip bar the same as the thermal mag breakers and are no faster and require no addition maintenance.

 

[wbdvt]

Molded case circuit breakers are basically maintenance free unless they have electronic trip units.

Not quite. Please refer to NFPA 70B for recommended maintenance of MCCBs. Generally it is a visual inspection (cracks in case which would negate the structural strength in a short circuit) and cleaning (especially if dusty environment which helps prevent tracking and maintains heat dissipation), check for loose connections, and exercise the breaker to wipe the contacts and help the lubrication.

 

[big john]

"I have heard CB's (big ones) need to be recertified/recalibrated by the factory after a certain amount of trips."
Really?
Have you had an opportunity to review the testing requirement that are required to be UL489 listed?

Most "big" breakers are power or insulated case and don't fall under the providence of UL489. There absolutely are OEM described testing and maintenace intervals for those breakers.

With a breaker hit the trip bar and it's less than a 1/2 cycle regardless.
There no magic with solid state trip units either other than features as they have to hit the trip bar the same as the thermal mag breakers and are no faster and require no addition maintenance.

The vast majority of the delay in breaker operation comes not from the movement of the mechanism, but from the designed delay in the trip unit. That's why ETUs can be far superior because they are much more precise than most thermal-mag elements and can also be programmed to pick up and trip much sooner for specific fault types. Once a trip has been initiated the mechanism delay is almost inconsequential.

...Please refer to NFPA 70B for recommended maintenance of MCCBs. Generally it is a visual inspection (cracks in case which would negate the structural strength in a short circuit)....

I can't swear they dont, but I'd be very surprised if 70B actually recommended opening molded case breakers, because that would contradict many manufacturers instructions.

 

[wbdvt]

I can't swear they dont, but I'd be very surprised if 70B actually recommended opening molded case breakers, because that would contradict many manufacturers instructions.

It does and why would you not exercise a de-energized breaker? See attached NFPA 70B article 17.10

 

[big john]

It does and why would you not exercise a de-energized breaker? See attached NFPA 70B article 17.10

My mistake I thought you were advocating opening the case on a breaker, as in disassembling it. Not merely operating it.

 

[templdl]

My mistake I thought you were advocating opening the case on a breaker, as in disassembling it. Not merely operating it.

Unless the breaker has an interchangable trip most molded case circuit breakers are factory sealed which means that the factory sealed must be b broken to remove the cover which we would be in violation of its UL listing.
These breakers are maintenance free and I agree that exercising a breaker is highly recommended as a part of a maintenance schedule. The moving contacts actually rub against the stationary contacts as they close together with the added benefit of distributing the lubrication of the.moving parts.
I recommend the when ever you have the opportunity take as many breakers appart as you can to see what makes them tick before discarding them I've had the opportunity to disassemble breakers from 15a to the big 2500a breakers, TM and MCPs, current limiting and any internal accessory available. Examine the contacts and arc chutes, and see if you can identify both the thermal and magnetic elements.
Of course electronic trips are different and use something like a shunt trip too hit the trip bar in order to open the breaker.