breakers vs. fuses

[thekman]

Is there a situation where ok to use a breaker in place of a fuse? For example, the 3 legs of power coming into a 3-phase pumping unit. I'm unable to find anything mentioning this substitution.

What about using a breaker on the leg of a small transformer, I couldn't see the harm, but have never seen such a thing, and don't have a real good answer as to why not to, other than 'it will look funny'.

Aside from codes, local or otherwise, Is it as simple as trip/interrupt rating matching?

 

[rbulsara]

Either fuses or circuit breaker can be used to protect a circuit. There may be some pros and cons from the application point of view but either is acceptable.

If you are asking, if you can mix and match fuses and breaker for in a single overcurrent device of a multiwire circuit, the answer is No. If a circuit breaker is used it is required to open 'all' ungrounded conductors of the circuit simultaneously, both manually and automatically (NEC 240.15 in the USA).



Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

It might help if we knew what type of equipment or systems you are referring to. In some cases, the UL listing of a package system, particularly HVAC and refrigeration, is based on use of fuses, so fuses must be used, even though a breaker would probably be fine.

Otherwise, I agree with Rafiq.

David Castor

http://www.cvoes.com

 

[jraef]

Its more a personal philosophy rather than a hard engineering decision. For every benefit of one, there is a detractor and vice versa. Fuses are better at limiting faults, but fuses subject 3 phase loads to the possibility of single phasing. Breakers are resettable after clearing a fault whereas not having a replacement fuse can cause significant downtime. But the time it takes a breaker to clear a fault subjects the downstream equipment to significant stress. The back and forth can go on for a long time.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[Eleceng01]

Not knowing the application does make it difficult to answer, but another consideration might be arc flash hazards. Just one more thing to consider (assuming it applies here).

 

[ScottyUK]

Fuses are better at limiting very large faults. Most real-world faults don't get into the current-limiting region.


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If we learn from our mistakes I'm getting a great education!

 

[TheBlacksmith]

I'm a little rusty on my switchboards, but the Navy used to have circuit breakers with an "LF" suffix. They incorporated fuses into the breaker. The breaker was used for normally making and breaking the circuit whilst the fuses could interupt large fault currents. The fuses had atrigger that would trip the breaker if they blew; so fuse interupts large fault, then breaker opens to eliminate single phasing. Don't know if this style is still in use/available.

 

[ScottyUK]

Are you showing your age? Must be when breakers had relatively low fault-breaking capability. Today's breakers are good for 100kA, and a few reach up to 150kA: there aren't many systems with fault levels which are too high for a modern breaker. Fuses are good for 200kA though, and have lower energy let-through so they very much have a place in the modern world.


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

Blacksmith, that was a navy desgination, I still deal with some Navy breakes. But all OEM's have had the same style breakers with fuses for higher AIC's and single pahse protection.

ABB K-line - fused= KDON
GE AK series - fused= AKU or AKRU

Just a couple examples, not needed as much anymore due to higher AIC's on more modern breakers as ScottyUK mentioned.

 

[TheBlacksmith]

So I'm old. I did have the second highest graduating average in the history of the Navy Electrican's Mate Class A school in 1977. And the Navy generally adopts a new technology about the same time the rest of world drops it.

 

[rhatcher]

I will add to Zogzog's post: for Westinghouse/Cutler Hammer the designation is DBL or DSL with the 'L' meaning that current limiting fuses are added. Of course, the examples given (KDON, AKU/AKRU, and DBL/DSL) are all for current limiting air circuit breakers. The Navy 'LF' (ie. AQB-LF400) designation is for molded case circuit breakers that are similar to the civilian Westinghouse 'Tri-Pac' circuit breakers.

And yes ScottyUK, I am also an old timer, at least in experience.

 

[ScottyUK]

LOL. Nothing wrong with getting old as long as you enjoy the ride.

I don't think there was much uptake of breakers at high fault levels in the UK until the breakers became capable of breaking the faults directly. There are a few examples but not many that I can think of. Even now there is a bias toward fuses in many of the old industries like steel and power.


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If we learn from our mistakes I'm getting a great education!

 

[alehman]

The fuses installed on those breakers are typically made specifically for the purpose and are intended for their current-limiting function only. They don't add protection for lower level faults. I think they're still available.

One big issue I see with fuses used alone is that for fault currents not in the current-limiting range, they may be very slow to respond, sometimes resulting in quite high arc-flash levels.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.