Peak let-through curves for breakers vs fuses

[jcbabb]

I'm getting confused trying to read some values from the attached let through current chart.

I am getting the available fault current from the utility, but i need to gather the following values from the chart:

1. Apparent let-through
2. RMS current
3. Peak let-through current

These values are being requested by the AHJ.

I've seen and done several of these types of exercises using let through charts from fuses. My confusion stems from the fact that this breaker let-through chart looks different from the fuse let-through charts that I've seen before. For example, on a fuse let-through chart, the smaller diagonal intersects are labeled with different values of ampere ratings. This breaker chart has them listed by voltage. My guess is that the "peak let-through current" is gleaned from where the "Maximum available let-through energy at 1/2 cycle without breaker" line intersects the available fault current. After that, I would think that the "Apparent let-through" value would come from the "up-over-down method" using the proper voltage line, and the "RMS" value would simple be the same as the available fault current.

Am I completely off base in my understanding of this chart?

 

[ron]

I can't see the chart, but I wonder why the AHJ wants to see information relative to a let through chart.

You can't use it for short circuit ratings, so it seems that he meant something else.

 

[jcbabb]

I seem to be having trouble with attachments. I have attempted to attach the chart and the form required by the AHJ.

 

[templdl]

I can't see the chart, but I wonder why the AHJ wants to see information relative to a let through chart.

You can't use it for short circuit ratings, so it seems that he meant something else.

All breakers are current limiting to some extent. But as I recall only breakers that a classified as current limiting have TC curves that include their CL capabilities. But such data as I also recall can only be used to support combination testing as such data is not to be used in the field to support the devices CL capabilities.

 

[jcbabb]

Here is a final attempt at the breaker chart.

 

[ron]

Reading the let through curves can be seen on pg 14 http://static.schneider-electric.us/docs/Circuit Protection/0600DB0301.pdf

 

[jcbabb]

Reading the let through curves can be seen on pg 14 http://static.schneider-electric.us/docs/Circuit Protection/0600DB0301.pdf

Ron,

Thanks for the reply. The chart shown on that page is what I was expecting to see, but I don't have. What I have is attached to the post right before yours. Can you comment on how to use it? It does not have ampere ratings to choose from; it only has voltages.

 

[ron]

Ron,

Thanks for the reply. The chart shown on that page is what I was expecting to see, but I don't have. What I have is attached to the post right before yours. Can you comment on how to use it? It does not have ampere ratings to choose from; it only has voltages.

I beleive it is this way ..... Start on the X axis and find the available bolted fault current. For example, I will use 100kA. Follow it up to the voltage that you are using the device at. For example, I will use 240V. Now from that point, run horizontally to the left to the line that is labeled Maximum Available Peak Let Through Current, then down to the X axis again. I get ~17kA

 

[jcbabb]

I beleive it is this way ..... Start on the X axis and find the available bolted fault current. For example, I will use 100kA. Follow it up to the voltage that you are using the device at. For example, I will use 240V. Now from that point, run horizontally to the left to the line that is labeled Maximum Available Peak Let Through Current, then down to the X axis again. I get ~17kA

So in other words, you would use it much the same as a fuse chart, using the voltage variables as opposed to the amperage variables? If so, would I then get the needed values as shown below:

1. "Peak let-through current" is gleaned from where the "Maximum available let-through energy at 1/2 cycle without breaker" line intersects the available fault current
2. "RMS current" is simply the available SC current at that point
3. "Apparent let-through" value would come from the "up-over-down method" using the proper voltage line as stated in the quoted post above

Have you ever seen a form such as the one required (attached to post #3)?

 

[ron]

Have you ever seen a form such as the one required (attached to post #3)?

I have not.