Fuses VS MCCBs For Motor Circuit Protection 5

[nightfox1925]

I am presently inviting everyone to provide their personal technical views with the regards to a comparison between fuses and MCCBs use for motor circuit protection. I am assuming LV motor application. One of the considerations I am into in selecting between the two is short circuit rating...fuses have higher short circuit ratings and are fast acting compared to MCCBs. Hence, I normally prefer fuses for higher short circuit requirements on the MCCs. I am trying to create some comparisons based from individual preferences. This is some start...any other views?

respectfully...

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[itsmoked]

Hi 02101972;

I would never use fuses for a motor, n e v e r.

Reason: I have seen well over a hundred motors destroyed specifically by fuses. One blows, the motor is single-phased and dies.

I cannot remember seeing a motor that died because of MCCBs.

Fuses are so lousy for motors that most of the refrigeration equipment makers void their warranties the instant you get the fetid things near their compressors. They ship the compressors, often, with application specific MCCBs, ones rated like 43A or 31A.

Fuses are cheap to install but if the load is hard and hot like all forms of compressors they really end up costing you much, much more. How much does it cost to when you have a semi-hermetic compressor burn out and it instantly creates acids that fatally contaminates the entire system? Or your drive-in reefer full of caviar or ice cream takes a dump on you? Or your restaurant walk-in is suddenly getting illegally hot?

If you are cheap or your motor isn't running on the edge like compressors do, or the machine has a human standing next to it whenever it's running, then fuses might be okay.

Oh and heatered overloads are just about as murderous in my experience. The new electronic overloads that detect a lost phase and dump the starter within 1 or 2 seconds could make fuses more tenable to me but in the past I have rarely seen them in the field.

Anyway that's my opinion and I'm sticking with it.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rbulsara]

02101972:

Are you getting paid for preparing this analsis and report?? Have you done any research on this? What have you found so far?

This comparision is provided by all fuse manufacturers and CB manufactures in sufficient detail.

Hidden point is this forum is not meant to be used to have others do the job for you.

 

[jraef]

I don't see anything wrong with asking for opinions.

My opinion is that it is very application dependant. I tend to lean towards MCCBs for motor circuits as well, but I have seen motors destroyed when breakers get their contacts welded or otherwise fail to open, especially in high fault current systems. I did a lot of work in the Pacific NW where they have many areas with 65kA or 85kA available fault current, yet the electrical distributors only stock MCCBs rated for 25kAIC because they are cheaper. The end result is a surprising number of what I called "breaker bombs".

By far though, if the expected user's maintenance dept. is in the least bit lacking in intelligence, avoid fuses as much as possible. I have seen far far more motors destroyed by someone "patching" the problem of a blown fuse by using the wrong size replacement or putting in a piece of pipe, especially in far flung places or by swing shift crews.

http:/

http://www.jraef.com

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[nightfox1925]

rbulsara, I am asking opinions from people who directly encounter both types of starter protection. I do have checked several manufacturers' specifications and other articles, each of them provides features and refer to certain standards for compliance. What I don't have are personal technical opinions of technical people who encounter both the advantages and problems that associate them. I am not getting paid for such query either.

jraef and itsmoked. thanks for views and opinions...

With due RESPECT...

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[nightfox1925]

rbulsara, if you have some important link of such comparisons from manufacturers (as you have stated), feel free to pass it on. You're opinions are also very much welcome.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[nightfox1925]

In design, we normally specify MCCBs of the current limitting types where available fault currents are very high.

It is true (in the plant we are working on)that some maintenance folks use wrong types of fuses because sometimes they ran out of stock on some fuse ratings. They tend to justify as for temporary replacements but later on takes them for granted and leave them there. Most of the time, fuse stocks come very late due to purchasing delays.

For me, as long as they are well maintained and propely rated, they are very much recommended compared to fuses. However, some old plants still use fuses for LV starters.



GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[waross]

Please remember that there is no comparison between "code" fuses and dual element, motor specific fuses. I have had bad luck with standard fuses and good luck with dual element fuses. The bad luck wasn't that bad but the applications had different loading cycles than the compressors that itsmoked works with. I have always considered a motor failure due to single phasing to be as much an overload relay issue as a fuse issue.
respectfully

 

[ScottyUK]

I guess the question relates to AC motors, but for the sake of completeness, fuses are perfectly acceptable on DC machines. They are arguably preferable to breakers on DC systems with high available fault level from a protection point of view, but have the downside of all fuses that they require replacement rather than resetting.


----------------------------------

I don't suffer from insanity. I enjoy it...

 

[DaveScott]

02101972,

Off the top of my head, hope it helps:

Adv MCCB (depending on model):
Easy and quick to reset (if initial problem is solved);
Easy to change setting, and adjust discrimination;
Phase loss detection;
Low voltage detection;
Motor modelling;
Can be used as a starter, and isolator (in an imperfect world...)

Disadv MCCB:
Easy and quick to reset (can damage motor by repeated reset);
Easy to change setting;
Fault current high = high cost;
Bad contact = overheating = early trip;
Mis-use as starter and isolator.

Adv Fuse:
If correctly sized: function is certain;
Replacement is slower (encourages proper solution);
Fault current capability;
Faster in certain applications.

Disadv Fuse:
Indication of failure not always visible (depends on type);
Fuses can be weakened by previous faults (by not changing the whole set);
Spare parts requirement can be troublesome if there are repeated faults;
Sometimes difficult to match to the load characteristics.

General:
Depending on application: either method could be more expensive.

 

[nightfox1925]

Thanks for the valuable inputs. I am presently making some comparisons in tabular form and I have also contacted some maintenance folks to share their views as well. I will post such table soon.

Any other views?


GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[Electic]

Here is another consideration:

In specific instances we have been challenged to coordinate the required, oversize MCP for an group B energy efficient motor (set at 17X FLA to allow starting) with a secondary main PCB dialed down to limit arc flash exposure. Selective coordination is important at this site.

We have not compared how this might coordinate with fuses but on a new design I would model both ways to see if the 'unachievable' might be achieved.

 

[unclebob]

I did a service call yesterday for a 350 hp 575 V burnt motor.
It has been re-wound. Cause of burning: faulty softstart.
So the motor was started (multiple times without success)with one phase missing. The thermal
overloads didn't open the contactor. Being an high efficiency motor, large current toasted the winding...I suggested the addition of a phase loss relay.