MCCB vs Fuse-switches 10

[STONE85]

Good morning,
With regards to the use of fuse switches and MCCBs what is the process in selecting one over the other. I know they each have their advantages and disadvantages but does it come down to the load they are protecting?

Thank you

 

[jraef]

It’s more of a religion… fuse people swear that fuses are the path to enlightenment and circuit breakers are blasphemous, breaker people are exactly the opposite. The truth, as in all things, is not in the extremes but in the middle.

Fuses are better at current limiting where you have high available fault currents, plus they can be selected to act much faster than breakers when protecting electronics in particular. But fuses can allow single phasing on 3 phase loads, plus they are more subject to the vagaries of human poor decision making, i.e. the fuse blows, so put in a bigger fuse. Or worse yet, the “no-blow” replacement fuse, aka a pipe or bolt that fits into the fuse holder.

MCCBs open all poles simultaneously so no chance of single phasing, they can be reset rather than having to have a spare available, and can have adjustments to help with coordination. The down side of breakers is mostly just in their time to act on a fault compared to the best fuses, and having a more limited range of interrupting capacity. You cannot got an MCCB that is good for 200kAIC, whereas you can get a fuse with that. How often do you come across an installation with 200kA available? Not very often. In my 40+ years in industry, I have never seen more than 100kA available, and that was once.

That said, rules on listing of power electronics for Short Circuit Current Ratings have changed recently in North America to now harmonize with the similar IEC rules to where we will see more and more requirements for high speed fuses, even if you have circuit breakers. Keep your eyes open for that when dealing with things like VFDs, Servo drives, UPSs and other power supply like devices.



" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

Dear Mr. STONE85 (Industrial)
Mr. jraef has made an excellent summery fuse vs breaker.
I can add only a little that is not mentioned:
1. Fuses
a) Fuse, is a 200 years old technology, no/little room for further improvement/innovation,
b) "Single-phasing" is happening every day, every where (domestic, commercial, industry including power generation). Nothing/little had been done to prevent/avoid it, by activate an alarm or automatically switching it off,
c) One time action. You have to replace it when blown due to over-load=(low kA) or short-circuit=(high kA),
d) Single current rating. The rated current is [not] adjustable,
e) Single tripping characteristic. The tripping curve is [not] adjustable, f) Generally, [not] designed with remote monitoring or switching ... etc.
2. Breakers
a) Breaker, is a 80+ years old technology, with very much room for further improvement/innovation,
b) All b) to f etc) drawbacks of fuse are fully over-came by modern designed breakers.
The safety, immediate switch back if tripped on(low kA), convenient of replacement and availability on the market place are evidence of the trend of no return.
Che Kuan Yau (Singapore)

 

[Mbrooke]

Fuses have the advantage in that they can easily be selectively coordinate down to 0.01 seconds under any fault current up their maximum interrupting rating. Another advantage is that they provide superior withstand ratings under high fault currents that would otherwise damage conductors and equipment if protected by breakers.

@Che12345: I partially disagree that breakers have more room to evolve and fuses do not. The logic in breakers can get more sophisticated, but you will never obtain sub-cycle clearing with a breaker as you would with a quality current limiting time delay fuse.

 

[waross]

All fuses are not created equal.
Code fuses; Just a fuse.
High interrupting capacity fuses; Current limiting.
Dual element fuses; Motor overload protection as well as short circuit protection.
Fast acting fuses;.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

There are low-voltage breakers that will interrupt in under a half cycle. Fuses do have a place, generally in small circuits, but breakers, both self-contained and relay controlled, are vastly more versatile. Show me a fuse than can trip on 3I0 or one that can trip on differential current.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[Mbrooke]

I was not aware of breakers that can make trip decision and interrupt in less than a cycle. Can you link me a page? I'm not doubting you- just want to learn

 

[davidbeach]

Current limiting breakers, all low-voltage, from many a vendor, can only be current limiting if they interrupt in less than a half cycle.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

I haven't seen one for a long time, but then I have not been in a position to see one for a long time.
Are breakers with internal HRC fuses still current? The internal fuses had a striker pin that extended when the fuse cleared. The striker pin hit a common trip bar that opened the breaker and cleared the unfaulted phases.
The breaker would not reset, but would trip free until the blown fuse was replaced.
Most of these breakers never cleared on a fault, but, in the event of a fault taking out the fuse, the cost of the fuse was often negligible compared to the damage that may have resulted had the fault not been cleared immediately.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[RRaghunath]

All (almost) the pros and cons have been covered by my learned colleagues. I just want to add a demerit with MCCBs - selectivity with different makes.
Fuse curves are well standardised making it easy to coordinate among different makes.
There is no standard protection curve in MCCB protection units that is valid for MCCBs belonging to different manufacturers. As a result, it is difficult to ensure selectivity while coordinating among different makes.
Having said that, the recently launched protection units are coming with IEC/IEEE curves and hopefully, we will not have the concern in future.

 

[edison123]

Very informative thread. Thanks.

Muthu

http://www.edison.co.in

 

[STONE85]

Thank you all for the replies. This has been most helpful.

 

[che12345]

Dear Mr. STONE85 (Industrial)
1. @Mbrooke
" ... Fuses have the advantage...can easily be selectively coordinate down to 0.01 seconds under any fault current up their maximum interrupting rating. Another advantage is ... provide superior withstand ratings under high fault currents that would otherwise damage conductors and equipment if protected by breakers...
@Che12345: I partially disagree that breakers have..."
Che. LV ACB, MCCB and MCB manufacturers publish:
i) "discrimination" tables indicating the [selectivity limitation]. It may be Full[/b i.e. [up to the max interrupting rating] , or ii) partial,
ii) "Current-limitation" data that the breaker (opening operation) [before the short-circuit current has reached its first peak].
2. @waross
" ... I haven't seen one for a long time, but then I have not been in a position to see one for a long time. Are breakers with internal HRC fuses still current? The internal fuses had a striker pin that..."
Che. No. Those products are > 70years old, obsoleted. Replaced by "current-limiting" breakers good for 200kApeak [without fuses].
3. @ RRaghunath
"...All (almost) the pros and cons have been covered by my learned colleagues. I just want to add a demerit with MCCBs - selectivity with different makes... Fuse curves are well standardised making it easy to ..."
Che. Agreed.
Che Kuan Yau (Singapore)

 

[Mbrooke]

@ David & Che12345: I could be biased here, however from my point of view current limiting breakers can not clear as fast as a fuse can:

https://youtu.be/3dckmSgp1nw?t=271

Being honest I have no idea what the internals of an IEEE current limiting breaker look like, but going by the time current curves has left me with the above notion.

 

[davidbeach]

In the current limiting region, the interruption happens in less than a half cycle. Magnetic forces in the hairpin routing blow the contacts apart even before the latch releases. Multiple current limiting breakers in series may all "burp" during the fault with not all of them actually tripping. Once the current falls below the current limiting region, they, like current limiting fuses outside the limiting region, can be quite slow.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[unclebob]

To waross,

yes Tripac breakers still exists, 200 kA rated. Smaller breakers (10 to 225A) may get the limiter add-on if required.

 

[Mbrooke]

@David, I get that. But IMO a CLF will clear faster than a current limiting breaker reducing the I2R heating of external components. Could be fuse manufacturer bias has gotten to me.

 

[che12345]

Dear Mr. Mbrooke
"...I was not aware of breakers that can make trip decision and interrupt in less than a cycle. Can you link me a page? I'm not doubting you- just want to learn ..."
Che. 1. LV ACB and MCCB with "current-limiting" characteristics have been around on the market for more than 25+ years; manufactured in Italy/brand A..., France/brand S...and Germany/S... etc.
2. I made a general comparison (as a rough indication) fuse vs current-limiting breaker; based on 400V with short-circuit 50kArms. These values may differ from different manufacturers. In both cases, if without SCPD, the current would reach 2.3 x 50kArms=115kApack.
a) Fuse
i)Rated 100A; current-limiting to 12kApack; let-through 60 (kA)2[sup][/sup]s,
ii) Rated 400A; 25kApack; 1000 (kA)2[sup][/sup]s,
iii) Rated 600A; 35kApack; 2500 (kA)2[sup][/sup]s.
b) Breaker [current-limiting type]
i) Rated 100A; 0.6kApack; 30 (kA)2[sup][/sup]s,
ii) Rated 400A; 9kApack; 35 (kA)2[sup][/sup]s,
iii) Rated 600A; 10kApack; 35 (kA)2[sup][/sup]s.
3. Observation:
a) fuse technology has little innovation,
b) breaker technology has advanced and there are very much room for innovation.
Che Kuan Yau (Singapore)

 

[che12345]

Dear Mr. Mbrooke
Further to my earlier post dated 26 August 21 04:01, I arrived at a conclusion on fuse vs current-limiting breaker. This is based on 400V, 50kArms short-circuit current level with fuse/current-limiting breaker ratings 100A, 400A and 600A respectively. This phenomenon should hold for voltages higher/lower than 400V; with short-circuit currents of lower/higher than 50kArms; fuse/current-limiting breaker rating of lower/higher than 100A-600A.
i) Refer to the data on "current limit (kApack)" where current-limit breaker (kApack) < fuse, i.e. the current-limit breaker (opens and stopped the current flow) is [FASTER than fuse],
ii) Refer to the data on " let-through (kA)2s " where the current-limit breaker (kA)2s < fuse, i.e. the current-limit breaker (let-through energy is [LOWER than fuse].
Che Kuan Yau (Singapore)

 

[jraef]

All of the wonderfulness of fuses goes right out the window when a replacement is not available...

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden