circuit breaker vs fuse 11

[cgossett]

Do fuses offer better protection for a motor power circuit than circuit breakers? Are fuses typically faster acting than circuit breakers?

 

[dpc]

This is a real can of worms. The answer will depend on who you ask.

The current-limiting fuse will limit the maximum current within the first 1/2 cycle and may be somewhat faster for high-level faults, such as a cable fault.

So, why do most starters have MCPs or breakers?

Breakers are three-phase - reduced risk of single-phasing the motor.

Breakers don't have to be replaced after each fault.

Breakers can have adjustable magnetic trip units and even electronic trips, including ground fault.

For low-level motor faults, breakers will generally operate faster than fuses.

Breaker characteristics can be tested non-destructively.

Breaker have a wide range of available options such as shunt trip, auxiliary contacts, remote trip indication, etc.

Although there are some large industrial facilities that use fused starters exclusively, the large majority of MCC starters in the U.S. utilize mag-only breakers (motor circuit protectors).

Hope that helps.

dpc

 

[cgossett]

Thanks for the quick help.

 

[jbartos]

Suggestion: It appears that the question is somewhat ill-posed or remote from the best motor protection concept and intent, which are presently "smart" electronic overloads in conjunction with breakers or fuses. The fuses and mag-breakers are the second best or partially best since they protect motor for shorts only, if the overload relays are applied.

 

[redtrumpet]

I thought the question was well-posed. It was remote from motor protection because he is not asking about motor protection. He referred to the motor power circuit and which was faster-acting, breakers or fuses. Remember that the power circuit includes the feeder cable as well as the motor.

 

[jbartos]

Suggestion: If one adheres to NFPA 70 National Electrical Code, one must treat the motor and its branch circuit, feeder circut, and control circuit as one entity for its proper design and protection (Article 430).
The motor feeder shall have its short-circuit and ground-fault protection.
The motor branch-circuit shall have short-circuit and ground-fault protection.
Motor control circuit shall its overload protection.
Clearly, in view of these safety requirements, I see the above postings somewhat lightly covering the subject.
"Motor Power Circuit" certainly needs some clarification since it can mean motor branch circuit or motor feeder circuit. MCC are very special arrangements that somewhat circumvent the motor feeder circuit protection and leave the motor branch circuit protection only, where the branch circuit is very short in the MCC compartment. Two question above postings weaknesses, the following is presented:
One counterexample: The electronic overload relays may have a feature that reacts to single phasing; therefore, the circuit breakers and fuses compared in this respect result in the circuit breaker single phasing protection as being redundant.
Another weakness of the above posting is that the mag-trip circuit breakers are permitted in conjuction with motor overload relays (often in MCCs only), which is not stated above. This could lead to a motor overload and fire hazard.
One major disadvantage of circuit breakers in comparison with fuses was not mentioned, namely, the circuit breakers can malfunction more likely than fuses because of their complex mechanisms and possibly electronics.
Clearly, the above postings need elaborations to be on the safe and comprehensive sides.

 

[scud27]

My opinion is that fuses are the most convenient and cheap way of protection also taking into account that they really limit the peak of current but...

I wouldn't say that a CB has a complex mechanism or, at least, a CB is structured in such a way that if it breaks it will open the 3 phases together - whether it is an thermal magnetic or an electronic based one. Also, remember that short circuit is not protected by the release but by a different phenomenon that is the reflex of the shaft. In real life, the contacts will open in 3 to 10 msecs all together since, even though only one phase feels the SC, they are mechanically connected.

Fuses have the bad idea to become old in different ways. This means that you might have a single fuse that bore more current than the others and wrecks, thus causing a bi-phase operation of teh motor and possibly burns it. This is the real problem of fuses.

By the way, each philosopy is right: when you need to protect an expensive motor, use fuses if you worry about short circuit and CB (maybe with a motor protection release) if you worry about overload and locked rotor.

And remember that, for really high currents, CBs are often cheaper than fuses...

 

[wiresketch]

One interesting point not addressed in the prior postings is fault current potential. Small breakers (100 amp or less) typically are 10,000 K AIC (Amp Interrupting Current),such a breaker close (feeder conductor lenght) to a large main service could possibly see fault currents 4x the magnitude of the breaker AIC and cause the breaker to lock and consequently trip at a much slower rate.Every application could possibly be better served by one device or another.

 

[jbartos]

Suggestion to scud27 (Electrical) Oct 18, 2002 marked ///\\My opinion is that fuses are the most convenient and cheap way of protection also taking into account that they really limit the peak of current but...
///Agree up to but...\\I wouldn't say that a CB has a complex mechanism
///In terms of part count, it is definitely more complex than a fuse or even three fuses.\\ or, at least, a CB is structured in such a way that if it breaks it will open the 3 phases together - whether it is an thermal magnetic or an electronic based one.
///This has become somewhat less important with the advent of electronic or smart overloads. Therefore, the emphasis on the simultaneous opening of the three poles of the circuit breakers is becoming old fashined.\\ Also, remember that short circuit is not protected by the release but by a different phenomenon that is the reflex of the shaft. In real life, the contacts will open in 3 to 10 msecs all together since, even though only one phase feels the SC, they are mechanically connected.

Fuses have the bad idea to become old in different ways. This means that you might have a single fuse that bore more current than the others and wrecks, thus causing a bi-phase operation of teh motor and possibly burns it.
///Again, this reasoning is somewhat oldfashoned by the application of electronic or smart overload protective relays.\\ This is the real problem of fuses.
///Used to be, since the electronic or smart overload relay sense the singlephasing.\\By the way, each philosopy is right: when you need to protect an expensive motor, use fuses if you worry about short circuit and CB (maybe with a motor protection release) if you worry about overload and locked rotor.

And remember that, for really high currents, CBs are often cheaper than fuses...
///It may be. However, if the circuit breaker clears three short circuit faults, it loses the UL label and shall be replaced. How many places are really replacing the circuit breakers after three shorts? I have not seen many markups on circuit breakers indicating the it cleared one fault, two faults, etc. When it comes to fuses, the short may damage one fuse only if the electronic overload trips on singlephasing. Then, the fuse protection is not that expensive.\\\

 

[PUMPDESIGNER]

I have to add a different viewpoint into this discussion.
The comments above were pretty good, they cover most of the design considerations of the fuse vs. breaker arguments.
One thing left out is that fuses are by nature "fail safe", i.e., they will definitely open by burning out the element whereas breakers can weld together and are not as fail safe as fuses.

That being said, I strongly favor breakers as follows:

My viewpoint is one of OEM manufacturing. Our desire is to ship product and not have problems, no warrantee or support costs if possible. We also want the customer to have a long and happy experience with the equipment. Fuses definitely hurt that possibility, circuit breakers are a HUGE improvement on that and sometimes thrill the customer that after an electrical surge they can just "reset".

During construction and set up fuses get abused. Frequent starting and stopping not only of our machinery but also other surges coming in on the line due to construction work on site. This fatigues the fuse elements, causing accelerated aging, and therefore premature failure.

Fuses are expensive, especially on three phase systems where all 3 fuses really should be replaced each time. Expensive in time to go buy the fuses, order them in, stock them, etc.

In certain areas of the country power surges are constant and sometimes trip the breakers due to MOVs and other current/voltage limiting devices downstream of the fuses/breakers. South Florida, mountain areas, Texas Gulf Coast, and other areas have extremely high lightning frequency.

Any OEM shipping with fuses will encounter field support problems that they either ignore, pay for, or enjoy due to their charging for the service albeit at the cost of an unhappy customer.

One question, whoever stated that circuit breakers clear the fault faster please elucidate more on this and show sources. I am not contradicting, just inquiring out of ignorance of this issue.

Thank you.
Richard Neff
pump@shadow.com Richard Neff
Irrigation Craft

 

[PUMPDESIGNER]

Jbartos makes a couple of good points about circuit breakers losing their ability to clear a fault after tripping. There is no easy solution to this. If ground faults and short circuits occurred more often this issue would definitely come up more often. Fortunately ground faults and short circuits are infrquent in most situations.

Any suggestions out there as to what to do about the weakening of circuit breakers after tripping numerous times?

But jbartos is incorrect to imply that 3 fuses should not be replaced when one fuse opens under certain circumstances. When 1 fuse in a 3 phase load opens, the remaining 2 fuses are stressed instantly. In most cases, no one wants to come back again later to replace another fuse just after he replaced one already. And who will know exactly what happened and if the other two fuses are damaged? Repeat service is embarassing, costly, and a real pain you know where. Sooo... a true professional technician proud of his work is likely to replace all 3 fuses every time. Richard Neff
Irrigation Craft

 

[busbar]

Molded-case circuit breakers, when operating at somewhat less than their interrupting rating, have an unpleasant habit of "venting" nice blue plasma at their seams.

 

[jbartos]

Suggestion: I agree that the replacement of all fuses, in case that one cleared the fault, is the safe way to do the electrical service business.
I also encountered experiences in Europe, where IEC fuses have an objective criterion to leave them or replace them, namely there is a little cap on the front of the fuse, held by a spring. If the fuse is deficient the cap is relased by the spring. If one must follow rules and replace one fuse and leave others in according to the objective cap indicator, what shall one do? Break the rules? Or get an experience from a different part of World?

 

[scud27]

Naturally I did not want to start up a dilemma on fuses and CBs... that will always exist.

Somebody up there said that CBs are complex. They surely are much more complex as a structure than fuses but let me tell you that the mechanical technology inside a CB is not complex at all.

If we want to talk about electronic releases, they are surely not my favourites... Apart from the fact that they are however expensive (and if used with fuses, they will however raise the installation price - but I am not discussing prices), I can tell you that the electronic faults happen much more often than the mechanical faults (this can at least be said on CBs: I worked on that!). When I said that, in case of fault a CB will open phases together, I said something natural: the biggest manufacturer cannot sell monophased interrupting CBs... this happened 30 to 40 years ago... and when the first MCCBs where however supported by fuses downunder.

My opinion for a perfect protection is indeed a combination of the two: fuses and CBs together. But this will get the installation redundant and expensive...

As per pumpdesigner request, I can grant you that - at the max shortcircuit power - a energy limiting CB (with maybe 2 contacts in series or parallel) will open in about 3 to 5 msecs. Fuses, too, will burn as fast as this.

As per the fact that after 3 SCs you have to replace the CB, you are right, Jbartos! But... indeed this is due in case of SCs at the limit, this is not true in case of OL while a fuses must always be replaced - and possibly with his companions. The standard said 2 and now 3 SCs because after the first SC there will always be somebody that will try to reset the system. If the CB cannot stand the 2nd SC there will be risk for his life. It is supposed that he will not try again... As far as the 3rd is concerned, this habitude comes from the navy installation where you have spare parts but you want to be sure that you will finish your trip before you really have to change it: afterwards the habitude has taken place as a standard also for normal installations.

After a fault, you should however always open the CB and check where the fault was since otherwise it will be back again.

By the way, the best way to know if you have to replace a CB is to open it and check the contacts. If they are in bad conditions, change the CB. If they are clear or almost, it was an OL and you can use the CB again.
...and, yes! never buy a CB that you cannot open and easily watch into otherwise you will not be able to easily know what happened!

 

[jbartos]

Suggestion to the previous posting: It is somewhat difficult to open a molded case circuit breaker that is completely encapsulated without any screws to open it.

 

[scud27]

Dear Jbartos, if you are talking about miniature CBs or MMS you are perfectly right. They usually cannot be opened without breaking them since they have no screws.

As far as MCCBs or ACBs are concerned, the most important ones can all be opened (somtimes screws are very well hidden... e.g. look under the manufacturer label...).

What you cannot always do in MCCBs is to look inside the poles since some manufacturer seals them telling you that they need no maintenance. This is purposely made so that you have to change the complete CB also in case of strong OL preventing you from looking inside.

Indeed, you can (and should be able to!) maintain also MCCBs and you can change arcing chambers, contacts, etc. That is what I meant.

 

[jbartos]

Suggestion: It depends on a manufacturer, type and size of circuit breaker to be able to open it and service it. However, as far as the UL label is concern, the circut breaker service is supposed to be performed by qualified test laboratories.

 

[scud27]

I do agree on what you write but... what should you do if your fuses burn or if your CB trips while in the middle of the sea? Do you replace them or do you try to understand what happened by looking into the CB and look for the fault so that it does not happen again?

This is the main point I want to emphasize...

 

[jbartos]

Suggestion to the previous posting: Unquestionably true. However, there is other way to go about blown fuses, namely, one may use many fuses filaments. Probably, more than the circuit breaker can handle with damaged contacts, even on high seas.

 

[carnage]

why hasent anyone made a setup for three phase where the current flows through the circuit breaker through the fuses and into the motor? then you have a voltage sensor sensing the voltage across the fuses (each indipendantly) if one fuse blows the voltage across it becomes=to source voltage tripping the voltage sensor which opens the circuit beaker to all three phases. viola the best protection money can buy.
so maybe it isn't practicle but it would work.