Fuse Vs. Circuit Breaker

[jkim780]

I know this has been discussed before but I need more clarification. On NEC 2002 handbook Arc.440 reads:

"When an air conditioner is listed by a qualified electrical testing laboratory with a nameplate that reads 'maximum fuse size', the listing restricts the use of this unit to fuse protection only and does not cover its use with circuit breakers."

Can anyone tell me why? Is that something to do with motor type?

 

[nvcape]

Re: Fuse Vs. Circuit Breaker

I chased down why units are UL listed only for copper wire. It's money, they were only tested for copper!

 

[charlie b]

Re: Fuse Vs. Circuit Breaker

Part of your answer lies in the last sentence of the paragraph that you have quoted. The code wants you to follow the manufacturer?s installation specs as closely as possible. Part of the answer lies in the fact that fuses and breakers operate under different principles, and therefore provide different protection for the air conditioner?s motors. nvcape might have the rest of the answer: perhaps the manufacturer did not want to spend the extra money to have the equipment tested twice (i.e., once with fuses, once with breakers).

 

[charlie]

Re: Fuse Vs. Circuit Breaker

There is also the school of thought that fuses are much better than circuit breakers.

And there is the school of thought that circuit breakers are much better than fuses.

They both have advantages over the other.

 

[templdl]

Re: Fuse Vs. Circuit Breaker

Can we test a fuse for calibration or are we to assume that it will work as manufactured. Breakers are tested after they are manufactured and can be retested after being applied for calibration. Can a fuse be retested to assure that it will protect?

 

[iwire]

Re: Fuse Vs. Circuit Breaker

Maybe I am a little naive here but I do not see a way a fuse can not work unless some basic laws of physics (I think that would be the right term) suddenly change.

If you make a batch of fuses out of a specific material of a specific cross sectional area and one of them tests correctly how would the others not perform in the same way.

A breaker has moving mechanical parts that the tolerances on are a trade off between cost and accuracy.

 

[tom baker]

Re: Fuse Vs. Circuit Breaker

I have instructed many classes on motors and motor controls. the issue of fuses vxs circuit breakers comes up, the folks from the large industrial facilites will always agree that fuses offer better motor protection than circuit breakers, however overfusing and spare fuses are issues.
But I discussed this with a senior engineer from Dow Chemical, he stated they always use fuses for motor protection. They are a big company with a lot to loose from a motor failure.
Another important consideration for current limiting fuses is the reduction in fault current and compliance with 110.16 will be easier. The 2005 NEC will almost certainly require calorie labeling at switchgear, I have already seen protective clothes calorie rated.

 

[insptr]

Re: Fuse Vs. Circuit Breaker

fuses and circuit breakers have different trip characteristics so depending on how a company wants their units protected will determine which protective device to use. Motors are protected with thermal overloads the circuit breaker or fuses are for short circuit and ground fault protection.

 

[rpmlube]

Re: Fuse Vs. Circuit Breaker

If you go to Bussman fuse web page they have a lot of information on fuses. They also have motor circuit protection tables covered in nec article 430.

 

[templdl]

Re: Fuse Vs. Circuit Breaker

With regards to motor circuit protection the clearing time of the protective device must also be considered. Motor faults commonly start as low level faults which a dual element fuse may take many cycles to clear. With an adjustable MCP, if correctly sized and set, will pickup and clear the fault in less than one cycle. Setting the MCP to a trip point just above nuisance tripping on motor startup can provide the closest protection.

 

[charlie b]

Re: Fuse Vs. Circuit Breaker

Originally posted by templdl: Can we test a fuse for calibration or are we to assume that it will work as manufactured?

Originally posted by iwire: If you make a batch of fuses out of a specific material of a specific cross sectional area, and one of them tests correctly, how would the others not perform in the same way?

Bob has half of the answer (except that you need to test a larger sample then just one). The other half comes from the quality control measures taken by the manufacturer. So long as the manufacturing standards can assure consistency in the product, and so long as the fuses that are occasional selected at random for destructive testing function as designed, then the user can have confidence that the fuses installed in their facility would do their job. You can get a surprisingly high confidence level from a surprisingly small sample set, if all of the samples pass the test.