LSI Electronic Trip Breakers

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Grouch1980

Senior Member
Location
New York, NY
Hey all!
Always another question :?

So i'm looking at Time current curve for a 1,000 amp LSIG breaker that you'll see in the link below (it's the first curve / chart when you scroll down. i tried posting a pic instead, but it was coming out too small).

http://electrical-engineering-portal.com/6-adjustable-tripping-settings-circuit-breaker

What determines the slope of the line labeled 'long time delay i2t slope'? I understand the long time delay pickup is 12 seconds @ 6000 amps... but how do you determine the pickup time of 450 seconds at 1,000 amps? How do you calculate the 450 seconds? or does the slope give you this? if so, how do you calculate the slope? I'm pretty sure it's something easy that i'm not seeing.

Thanks again.
 
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zog

Senior Member
Location
Charlotte, NC
Hey all!
Always another question :?

So i'm looking at Time current curve for a 1,000 amp LSIG breaker that you'll see in the link below (it's the first curve / chart when you scroll down. i tried posting a pic instead, but it was coming out too small).

http://electrical-engineering-portal.com/6-adjustable-tripping-settings-circuit-breaker

What determines the slope of the line labeled 'long time delay i2t slope'? I understand the long time delay pickup is 12 seconds @ 6000 amps... but how do you determine the pickup time of 450 seconds at 1,000 amps? How do you calculate the 450 seconds? or does the slope give you this? if so, how do you calculate the slope? I'm pretty sure it's something easy that i'm not seeing.

Thanks again.

The slope is just the programmed curve of the trip unit, changing LTD would shift that line (at the same angle) to the right or left. A real TCC would show several parallel lines for each LTD delay setting.

In this example they are using 1000A CT's (In=1000) so at 1000A you follow the 1.0 line up until you intersect the curve, 450 sec in this example. 2000A would be about 120 sec.
 

ron

Senior Member
Hey all!
Always another question :?

So i'm looking at Time current curve for a 1,000 amp LSIG breaker that you'll see in the link below (it's the first curve / chart when you scroll down. i tried posting a pic instead, but it was coming out too small).

http://electrical-engineering-portal.com/6-adjustable-tripping-settings-circuit-breaker

What determines the slope of the line labeled 'long time delay i2t slope'? I understand the long time delay pickup is 12 seconds @ 6000 amps... but how do you determine the pickup time of 450 seconds at 1,000 amps? How do you calculate the 450 seconds? or does the slope give you this? if so, how do you calculate the slope? I'm pretty sure it's something easy that i'm not seeing.

Thanks again.

The slope is fixed. Its position up / down is adjustable
 

Grouch1980

Senior Member
Location
New York, NY
Where do I get the fixed slope of the line? Say for instance I wanted to draw the TCC curve for a breaker. How do I get the slope of this line?

I came up with this... not sure if it's the correct way... for the breaker in the example, the LTD at 12 seconds equals 6,000 amps. so i2t = 6,000 amps squared x 12 seconds = 432,000,000. this value of 432,000,000 has to stay constant throughout... so at 1,000 amps: 432,000,000 / 1,000 amps squared = 432 seconds. So it's actually 432 seconds instead of 450 seconds at 1,000 amps. So now I can draw a line from the 12 second mark @ 6000 amps up to the 432 second mark @ 1000 amps. Is this how you draw that line? Or is my calculation wrong? Or is the slope given to me somewhere and I draw the line using the slope?
 
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big john

Senior Member
Location
Portland, ME
Disregard that article. The graphics on it are confusing: The pictures do not show what the text is describing, and trip curves do not look like what is shown there.

Time-current trip curves are a series of pre-determined fixed slopes. They have two components: Pickup and delay.

Pickup is when the breaker begins to think about tripping. If the breaker is not passing enough current to reach Pickup then it is nothing more than a big switch and will not operate independently of operator input. Once in pickup the breaker operation becomes automatic.

The second component is the Time Delay. This is what determines how long the breaker will think before it decides to operate automatically.

Here is a time-current curve for the Static Trip III in your picture (see page 4).
What determines the slope of the line labeled 'long time delay i2t slope'?
If you look closely at the bottom right of those curves, you can see two sets of slopes for "Short Time" one labeled "In" the other labeled "Out." If the I2T function is not enabled, the trip unit automatically follows the "Out" slope. If the selector switch enables I2T then it follows the "In" slope.
I understand the long time delay pickup is 12 seconds @ 6000 amps... but how do you determine the pickup time of 450 seconds at 1,000 amps?
Remember that pickup and delay are two separate functions. Pickup is selected by the the "Setting" dial which tells the breaker to begin thinking once a certain amount of current has been reached. Delay is set by the dial on the right to tell it how long to think before tripping.

How do you calculate the 450 seconds? or does the slope give you this? if so, how do you calculate the slope?
If you look at the curve I linked to, you can see the different slopes for your different Long Time Delays. The only slope that is reliably in the "450 second" range would be the very top one, which corresponds to the top "30 second band" in this setting. So what that's describing is that at a fault current of approximately 1.5-1.7 times the "Long Time Pickup" the breaker will trip at 450 seconds.

That said, this is a little bit backwards: Usually you already know the current and you are trying to find the time delay. So we would say that at a current of 1.5 times the Long Time Pickup our Delay is 450-700 seconds.

To actually get down to the 30 second range of the "30 second band," you need to assume a fault current of 6 times, which shows a Delay of 30-45 seconds. This is why under the "Delay" dial is printed "@ 6x setting."
 
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Grouch1980

Senior Member
Location
New York, NY
Great info! NOW i follow it. it took me a while. so from what i'm reading, that sloped line is already given to you on the TCC charts (the long time delay slope). So you just look up the chart, and choose from the different parallel lines that make up the LTD, depending on how much you want to delay the tripping action of the circuit breaker. so i don't have to calculate anything !

Thanks Big John for the correct chart and example. I ended up cross referencing your link with mine to try to understand it all.
 
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