Breaker Trip Curves and Coordination

[steve66]

I'm trying to get a 200 amp breaker with a instantaneous magnetic trip setting of 3000 amps.

View attachment page 1.pdf View attachment Page 2.pdf View attachment page 3.pdf View attachment page 4.pdf

Here I'm looking at the PDF page 4.

If I understand this right, I_n is the breaker frame size, and I_r is the trip current setting. So a 400 amp breaker with the trip setting at 0.5 would have I_n = 400 amps, and I_r = 200 amps.

Since the instantaneous ratings are based on I_n, that breaker could have its instantaneous rating set for 10x the 400 amps, which would be 4000 amps. Does that sound correct? Or would it be 10x the 200 amp setting, for a total of 2000 amps?

If the 10x 400 amps is correct, I know the tolerance only guarantees me 8x, which is 3200 amps, and still above the 3000 amps I need.

 

[ron]

"In" is the sensor value, or the trip value if the LTPU is set to 1.0.

If you have a 400A frame, 400A sensor (In) and the LTPU of 0.5 (Ir), the trip value is 200A and the instantaneous set to 10x, would be 4000A.

 

[steve66]

Ok, so adjusting the long time trip setting has no effect on the instantaneous magnetic trip setting. It seem pretty obvious once I say it out loud, but I wanted to verify.

One of the things that had me puzzled was I couldn't find a definition of In or Ir without googling it, and I wasn't sure I trusted googles results.

But now I see they are also defined right there in note #3: In = maximum dial setting of Ir.

 

[topgone]

Ok, so adjusting the long time trip setting has no effect on the instantaneous magnetic trip setting. It seem pretty obvious once I say it out loud, but I wanted to verify.

One of the things that had me puzzled was I couldn't find a definition of In or Ir without googling it, and I wasn't sure I trusted googles results.

But now I see they are also defined right there in note #3: In = maximum dial setting of Ir.

Your Note #3 means that Ir can only be set max at a setting equal to the sensor value In.

 

[ron]

One of the things that had me puzzled was I couldn't find a definition of In or Ir without googling it, and I wasn't sure I trusted googles results.

They are no universally accepted abbreviations that I am aware of for this use, so check the abbreviations each time with different manufacturers trip units.

 

[ron]

BTW, another way to achieve a higher Inst setting for a 200A breaker is to use these mission critical breakers. http://static.schneider-electric.us...ritical/LA-LH Mission Critical/0600DB0202.pdf

It was an attempt 15 years ago to provide better selectivity between 200A and 20A breakers.

We found the resulting Arc Flash Incident Energy was pretty high, so we haven't specified them in a while. Keep the IE in mind for your application too.

 

[steve66]

BTW, another way to achieve a higher Inst setting for a 200A breaker is to use these mission critical breakers. http://static.schneider-electric.us...ritical/LA-LH Mission Critical/0600DB0202.pdf

It was an attempt 15 years ago to provide better selectivity between 200A and 20A breakers.

We found the resulting Arc Flash Incident Energy was pretty high, so we haven't specified them in a while. Keep the IE in mind for your application too.

That's what I though too, but apparently the some of the mission critical breaker have been discontinued.

I believe all the remaining mission critical breakers have the same trip curves as standard breakers. They are still "selectively coordinated" with various smaller breakers. But now they rely on the downstream breakers impedance to achieve the coordination, instead of a higher mag. setting.

 

[templdl]

The thermal portion of trip curves is relatively easy to coordinate. The instantaneous looks as if it is a piece of cake as also but in real life fault currents are often unpredictable leaving you to question why the breakers didn't coordinate, that is the one closest to the fault trip and not the breaker up stream. Remember all breakers that are in series with he fault see the fault. The magnetude of the fault may be high enough to be detected by all devices. Since this element is instantaneous it is a gamble on which device will trip. Arcing faults could be low enough to to trip a breaker an not long enough in duration for a breaker to trip thermally in an acceptable length of time.
The only way to deal instantaneous trip coordination is with electronic trip units where a slight delay can be programmed in the upstream breaker of just enough length of time to allow the down stream breaker to clear the fault.
Another consideraton is ground fault detection because very often arcing faults are phase to ground and thst is where the sensitivity is.

 

[mohsan514]

Is there any free software for breakers coordination and discrimination study.. ( except Ecodial)

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[paulengr]

The thermal portion of trip curves is relatively easy to coordinate. The instantaneous looks as if it is a piece of cake as also but in real life fault currents are often unpredictable leaving you to question why the breakers didn't coordinate, that is the one closest to the fault trip and not the breaker up stream. Remember all breakers that are in series with he fault see the fault. The magnetude of the fault may be high enough to be detected by all devices. Since this element is instantaneous it is a gamble on which device will trip. Arcing faults could be low enough to to trip a breaker an not long enough in duration for a breaker to trip thermally in an acceptable length of time.
The only way to deal instantaneous trip coordination is with electronic trip units where a slight delay can be programmed in the upstream breaker of just enough length of time to allow the down stream breaker to clear the fault.
Another consideraton is ground fault detection because very often arcing faults are phase to ground and thst is where the sensitivity is.

Actually dynamic resistance is how you coordinate on instantaneous. Except that it only works consistently with tested combinations which normally limits you to one series from one manufacturer. The time tested approach is that only branch breakers have instantaneous. Feeder breakers do not use it, Of course back then we didn’t engineer for arc flash, and used real draw outs on cast frames and not cheap switchboards or ICCBs mounted on drawout frames...

Either way I disagree about the predictability of faults. Bolted faults are pretty uncommon...most are arcing faults. And those are pretty consistent when they are the dangerous kind where they escalate to 3 phase. Screwy low level equipment ground faults are in the thermal range anyway.



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[paulengr]

Is there any free software for breakers coordination and discrimination study.. ( except Ecodial)

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Yes. S&C has Coordinaide on their web site. Very easy to use. Alternatively most of the expensive software has time limits or bus limits that is plenty for simple cases. You would not model the whole plant but for coordination issues no problem, worst case install it on Virtualbox and use snapshots.

Beyond that it’s really easy to just look up the curves, print one, and hand plot a few points to sketch the others. Also good exercise for understanding what is going on.


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[mohsan514]

Thank you

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