MCCB Magnetic elements

[zog]

Anyone know why some magnetic elements react faster than others of a different vintage and OEM? I mean at the same current levels (Multiples of current rating). Also after looking at some different curves I notice the max clearing time for some breaker types (INST) is the same as current increases while other types show some inverse time characterstics in the INST range, I never noticed that before and am curious to why the differences.

 

[jim dungar]

Many thermal-magnetic breakers use an actual magnet and armature assembly, different designs will have different characteristics.

It is possible to have some inverse relationship in the INST region if the breaker contacts are of the 'blow apart' style, which are common in the latest generation of current limiting designs

 

[zog]

Thanks Jim, sort of what I was thinking. I am seeing this causing some coordination issues, guess that is why OEMs only promise coordination between same breaker types huh?

 

[jumper]

Any chance you could post, PM or email those curves? It is a bit out of my league, but I am curious and would like to understand better. I know a little about trip curves, but not much.

 

[jim dungar]

Any chance you could post, PM or email those curves? It is a bit out of my league, but I am curious and would like to understand better. I know a little about trip curves, but not much.

One of these is the LC breaker from Schneider Electric.
http://products.schneider-electric.us/support/technical-library/?event=detail&oid=090089268014715d

 

[zog]

Here are the 2 breakers in question, at 700A in series the 100A breaker trips before the 20A breaker.

View attachment TEB20.pdf View attachment HFB.pdf

 

[jumper]

One of these is the LC breaker from Schneider Electric.
http://products.schneider-electric.us/support/technical-library/?event=detail&oid=090089268014715d

Here are the 2 breakers in question, at 700A in series the 100A breaker trips before the 20A breaker.

View attachment 6027 View attachment 6028

Thanks!

 

[T.M.Haja Sahib]

other types show some inverse time characterstics in the INST range, I never noticed that before and am curious to why the differences.

It is self-contradictory to say instantaneous region in the magnetic trip characteristic of the breaker has also some inverse time characteristic.It should not be interpreted that way.

 

[zog]

It is self-contradictory to say instantaneous region in the magnetic trip characteristic of the breaker has also some inverse time characteristic.It should not be interpreted that way.

I agree with what you are saying, perhaps I should reword that, the max clearing time of one of those curves I posted shows an inverse time characteristic.

 

[T.M.Haja Sahib]

I agree with what you are saying, perhaps I should reword that, the max clearing time of one of those curves I posted shows an inverse time characteristic.

Then it should lie on the overload region and not on the short circuit i.e instantaneous region of the thermal-magnetic characteristic of the breaker.Isn't it?

 

[jim dungar]

It is self-contradictory to say instantaneous region in the magnetic trip characteristic of the breaker has also some inverse time characteristic.It should not be interpreted that way.

On the contrary, in the context of overcurrent protective devices the term Instantaneous means 'without intentional delay'. Afterall there is no protective device that can actually detect an overcurrent situation and interrupt the current flow 'instantly'.

 

[T.M.Haja Sahib]

On the contrary, in the context of overcurrent protective devices the term Instantaneous means 'without intentional delay'. Afterall there is no protective device that can actually detect an overcurrent situation and interrupt the current flow 'instantly'.

I do not believe such random delays need consideration,because the INST region is shown as a vertical straight line in the thermal magnetic characteristic of the breaker.If you know otherwise,kindly reproduce here.

 

[jim dungar]

I do not believe such random delays need consideration,because the INST region is shown as a vertical straight line in the thermal magnetic characteristic of the breaker.If you know otherwise,kindly reproduce here.

You are mistaken. The INST region is the horizontal portion after the vertical portion.

 

[T.M.Haja Sahib]

You are mistaken. The INST region is the horizontal portion after the vertical portion.

If you illustrate with a typical curve,it will be very helpful.Thanks.

 

[jim dungar]

If you illustrate with a typical curve,it will be very helpful.Thanks.

I will not.

You do not agree with references others post, as this thread already contains links to three different breaker time current curves.
I suggest you do your own research. Try an internet search using the phrase "understanding trip curves".

 

[T.M.Haja Sahib]

I will not.

You do not agree with references others post, as this thread already contains links to three different breaker time current curves.
I suggest you do your own research. Try an internet search using the phrase "understanding trip curves".

You probably based your conclusion on your post #5.I am sorry to say some error in your statement.That diagram shows a band of curves,thereby giving an illusion that INST region is horizontal.No,it is not.Please See page no.36 in link below
http://www3.sea.siemens.com/step/pdfs/mccb_2.pdf

 

[jim dungar]

There is no error in my statement.

The curve you reference is showing the adjustable magnetic range of the breaker as it transitions from the Long Time portion (thermal element) to the Instantaneous portion.

As I said previously, in a protective device the term Instantaneous refers to 'no intentional time delay'. The portion that marketing literature refers to definitely has a time delay associated with it. The Instantaneous portion of the breaker's trip curve actually occurs to right hand side of the vertical curve.

One of my engineers, a graduate of the Indian Institute of Technology, Kharagpur, had no problem understanding trip curves the first time I worked with him. In fact, you and I have spent more time discussing this topic than he and I have had.

 

[T.M.Haja Sahib]

In fact, you and I have spent more time discussing this topic than he and I have had.

I still have valid points,but unless you are interested,I am not willing to continue.

 

[zog]

I still have valid points,but unless you are interested,I am not willing to continue.

This is from your link and supports my (And Jims) statements, it also answers my actual question, so thanks for helping me find an answer, even if it was on accident.

Quoted from Page 33 -
"The maximum clearing time (time it takes for breakers to completely open) decreases as current increases. This is because of the blow-apart contact design which utilizes the magnetic field built-up around the contacts. As current increases the magnetic field strength increases, which aids in opening the contacts."

 

[T.M.Haja Sahib]

This is from your link and supports my (And Jims) statements, it also answers my actual question, so thanks for helping me find an answer, even if it was on accident.

Quoted from Page 33 -
"The maximum clearing time (time it takes for breakers to completely open) decreases as current increases. This is because of the blow-apart contact design which utilizes the magnetic field built-up around the contacts. As current increases the magnetic field strength increases, which aids in opening the contacts."

Sorry,Zog,but there is one more detail to be taken into account by you.There is short time delay over load region that occurs just before the INST region in the breaker characteristic.The two were clubbed together and simply called instantaneous trip component in that link.Study the following lines in the same page 33.

This circuit breaker has an adjustable instantaneous trip point from 900 A to 2000 A, which
is 4.5 to 10 times the 200 A trip unit rating. If the trip point adjustment is set to minimum (900 A), and a fault current of 900 amps or greater occurs, the breaker will trip within 1 cycle (16.8 ms). If the trip point setting is set to maximum (2000 A), and a fault current of 900 amps occurs, the breaker will trip between approximately 12 and 55 seconds. A greater fault current will cause the breaker to trip faster.

See the underlined sentence.If the fault current is above 900A for the INST setting of 900A,you can not determine the exact trip time for each value of fault current above 900A from the same characteristic of the breaker.But if it is less than 900A,trip time may be determined from the breaker characteristic for such values of fault current.The same holds for each value of trip setting from 900A t0 2000A.

Am I clear?