Types of Breakers

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steve66

steve66

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Illinois
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Engineer
Can someone explain the difference between a molded case circuit breaker (MCCB) and an insulated case circuit breaker(ICCB).

Also, I was wondering if someone could explain the following settings for a GE breaker:

GE MVT-9
Sensor = 800
Plug = 800
Cur Set = 0.5 (400A)
LT Band = 1
Inst = 4 (3200A)

Thanks:
Steve
 
Re: Types of Breakers

Molded case circuit breaker (MCCB) = A circuit breaker which is assembled as an integral unit in a supportive and enclosed housing of insulating material, generally 20 to 3000 A in size and used in systems up to 600 Vac and 500 Vdc.

Insulated case circuit breaker (ICCB) = UL Standard 489 Listed nonfused molded case circuit breakers which utilize a two-step stored energy closing mechanism, electronic trip system and drawout construction.

Sensor = The current sensing element within the circuit breaker which provides the sensing function for that circuit breaker.

Sensor plug = A component used with the MICROLOGIC trip system to set the sensor size of a circuit breaker.

Sensor size = Maximum ampere rating possible for a specific circuit breaker, based on the size of the current sensor inside the circuit breaker. Sensor size is less than or equal to frame size.

Instantaneous pickup = The current level at which the circuit breaker will trip with no intentional time delay.

I don't know what the other stuff is.
 
Re: Types of Breakers

The circuit breaker sensor size is the maximum ampere rating possible for a specific circuit breaker. This value is based on the size of the current sensor inside the circuit breaker. (Current sensors are an integral part of the circuit breaker and cannot be removed or replaced.)
The sensor size is less than or equal to frame size. It looks like you have an 800A sensor which is most likely in an 800A frame breaker.

The rating plug varies the circuit breaker ampere rating as a function of its sensor size. Installed is an 800A rating plug. You could have 600A rating plugs, 400A etc.

Cur Set is the long time pick up. This sets the maximum amount of current the breaker will carry. In your case 400A. Some trip units would show this as .5 or 50%.

LT Band is the long time delay. This is the amount of time the breaker will hold the maximum setting of the long time pick up in seconds. So in your case it will hold 400A for 1 second before tripping. This setting is an inverse time setting. In other words as the current increases the time decreases.

Inst=4 This, as Bph gravity said, sets the value that the breaker will trip with no time delay. The numbers are multiplied by the amp rating of the plug. In your case 800A X 4 = 3200A.

The only other thing I can add is that insulated case circuit breakers do not have to be draw out construction. They can be fixed breakers as well.
 
Re: Types of Breakers

Actually the long time band will not trip in 1 second at 400A. You need to check the trip curve but I believe the MVT curve is based on a 600% overload, so your breaker will trip in 1 second at 2400A (600% overload), at 400A it is probally closer to 30 seconds.
 
Re: Types of Breakers

I think that one thing that you will notice with regard to the difference between the molded case and insulated case breakers is the short time delay.
Insulated breakers have a higher withstand, that is the ability for them to be allowed to hold their contacts together before they most be opened. As a result the short time delay adjustable settings are than molded case circuit breakers. A typical MCCB will have an available delay of 500ms max and the breaker must open its contacts where the typical ICB extends that time to .1, .3, and 6.5 seconds. However, times up to 500ms is a flat response and above 500ma commonly based on an I2t response.
When coordination is of a concern the ICB can bring some much needed headroom to the party for short circuit coordination, that is the up stream OCPD being allowed to give the OCPD closer to the fault a chance to clear first. Otherwise there has a tendency to be a race as to which one will trip first and it isn't that uncommon that the upstream breaker will trip.
 
Re: Types of Breakers

Yes, breaker coordination was the context I ran into the difference between types with.

A "race" to see which breaker opens first is kind of misleading. The smaller breaker will almost always "unlatch" first. But the current doesn't stop until a later time - the "total clearing time". During this interval, the larger breaker may "unlatch". Once a breaker unlatches, it will open shortly thereafter.

So even if the smaller breaker is much faster, the upstream breakers can still open.

Steve
 
Re: Types of Breakers

Then there is the residential load center with a 100a main when it is common for the main MCCB to trip when there is a bolted fault in a 15a branch circuit and the 15 a branch breaker does not trip.
Doen't like it, it isn't logical but it it happens.
 
Re: Types of Breakers

Although somewhat already stated, but the current setting is an adjustment that gets applied to the sensor to result in the trip (long time pickup) value. Generally this value is reached at 1000 seconds.
 
Re: Types of Breakers

Posted by templdl:

Then there is the residential load center with a 100a main when it is common for the main MCCB to trip when there is a bolted fault in a 15a branch circuit and the 15 a branch breaker does not trip.
Doen't like it, it isn't logical but it it happens.
Click to expand...
You are right, both breakers should trip. One thing that might cause this is a bad 15A breaker, or a stuck 15A breaker. I just noticed in a square D manual for F frame breakers it says:

"Operate breaker once per year"

Steve
 
Re: Types of Breakers

steve66
Operating breakers once a year is a practice that is advised by most breaker manufacturers. But, as far as defective branch breakers? Mosty likely not as if you have 10ka of available fault current going into a bolted fault all breakers in series with that foult will see it at the same time but be upstream breaker will have the benifit of the additional loads from other branch circuits to get a heads start.
It is uncommon for the branch breaker to be defective if the main trips and it does not. Sometimes both the main and the branch trip.
Often it becomes a crap shoot.
 
Re: Types of Breakers

I'm not going to try and say that doesn't happen. But according to the theory, and what is said in textbooks, that shouldn't happen if everything works the way it was designed.

The book I've been reading (Cooper's Littlefuse SPD02 - available online) makes a pretty good argument for using fuses instead of breakers, even when the breakers work the way they should.

Your experience gives the argument for fuses even more weight.


Steve
 
Re: Types of Breakers

Steve,
It is my understanding that if you have a bolted fault and high fault current, that it is very common for both the main and branch to trip where the breaker sizes are all 225 amp or less. This is exactly why the rep from Buss made the propsal for 700.27.
Don
 
Re: Types of Breakers

But, how often do bolted faults, the mother of all faults, occur. They a very rare. I wish I could lay my hands on some statistics that I had some years ago.
This is how I look at it. The most common fault is an arcing fault where currents may be so low that they may not even trip a breaker magnetically. Again, regarding arcing faults, are they L-L, L-N, or L-G. L-N or L-L arcing faults quite often involve a ground fairly quickly as the air around the arc ionizes and that's where GFCI protection is a big benefit.
So, one must consider the odds of a bolted fault happening and if including coordinated GFCI protection should be considered in the design.
Just a thought.
Dave T
 
Re: Types of Breakers

Originally posted by templdl:
But, how often do bolted faults, the mother of all faults, occur. They a very rare.
Click to expand...
I don't know if they are that rare, first a non-bolted fault happens and trips the breaker.

Than many bolted faults happen in quick succession as six different people try to reset the breaker before they call the electrician. :D
 
Re: Types of Breakers

Actually it doesn't really take a "bolted" fault to cause this problem. It just has to be a fault with enough current flow to be in the "instantaneous" part of the trip curve of the main or upstream breaker. Under that condition, both breakers will trip.
Don
 
Re: Types of Breakers

That may be true with a feeder breaker but would main and branch be tripped under the same circumstances? But then again an arcing fault may clear itself.
I'm going to have to locate that report on the odds for bolted faults. I can remember it but can't recall where I filed it.
What it comes down to is all of this stuff is conjecture anyway, what ifs, and maybes, etc. which can be argued about forever. And it is most difficult to record or even repeat an event.
 
Re: Types of Breakers

Steve, it is Cooper/Bussmann's SDP02.

I have problems with the concept of fuses always being better at selective coordination than breakers. One area where I have had difficulty providing this coordination is 480Y/277 services with ground fault protection. On the last one I did the coordination was limited to a 60A branch fuse with a 1200A ground fault relay. I would have needed a GFP with a delay of over 1 sec to coordinate with a 100A fuse.
 
Re: Types of Breakers

Oops, Bussman not Littlefuse!! There goes my commission for providing free advertising!! I guess I should at least try to get the company name right.

I don't doubt that fuses do provide better protection and better coordination. As far as coordination with GFP is concerned, it looks just as difficult to coordinate breakers with GFP. The SDP02 has a couple of pages on coordinating GFP.

But I do realize the SDP02 is biased toward fuses (that is what Bussman makes, and what they want to sell). One downside of fuses that isn't mentioned is the higher cost and larger size of switchboards needed to house them.

Steve
 
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