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 Post subject: Seemingly Artificial High IE values
PostPosted: Fri Jan 05, 2018 11:11 am 
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Hello all, first time poster here. I am working on an arc flash analysis for a client using SKM power tools. There are a few instances in which I believe I'm getting artificially high IE values. It seems that SKM "cuts off" the over-current device's trip curve right at the available fault current at that device. As a result, the tripping time is maxed out at 2 seconds (when I believe the trip time should be much less).

Does anyone else have experience with this?


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 Post subject: Re: Seemingly Artificial High IE values
PostPosted: Fri Jan 05, 2018 11:37 am 
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Location: Rutland, VT
what is the trip time for the arcing current on the TCC for the overcurrent protective device? Is it greater than 2 seconds?

Welcome to the forum!

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 Post subject: Re: Seemingly Artificial High IE values
PostPosted: Fri Jan 05, 2018 12:09 pm 
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The available fault current at the OCPD is just over 13kA. From the TCC, I would say the trip time for the OCPD is approximately .0225 S (so significantly less than 2 S).

Included are three images: A one-line showing available fault currents, a one-line showing incident energies, and a TCC diagram of the two OCPD's.

Thank you for your help!


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 Post subject: Re: Seemingly Artificial High IE values
PostPosted: Fri Jan 05, 2018 12:47 pm 
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Hi,
I'm not overly familiar with SKM as I primarily use EasyPower but there may be a setting that is defaulting to 2 sec. In EasyPower, there is some options besides using the TCC for the tripping of the device, so you may want to check that.

The other possibility which is in EasyPower and SKM may be the same but the OCPD can be ignored in the analysis by a check box. This is helpful to ignore a device if it is overdutied so the next upstream OCPD would be the first tripping device.

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 Post subject: Re: Seemingly Artificial High IE values
PostPosted: Mon Jan 08, 2018 9:14 am 
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Hello,

1. Is the 13644A Isc 3P referenced to the primary or the secondary of transformer #US/SS-1B-TR?

2. Looking at the TCC it looks like SKM thinks the available fault current at #US/SS-1B-MAIN is less than 1.1kA at 4160V. That's where the blue curve cuts off abruptly. The arcing fault will always be less than the available fault current due to the resistance of the arc itself, so the arcing fault current should be less than 1.1kA referenced to 4160V. If you double click on the blue curve there is a set of options called "SC Flag and Termination Options" under "Flags" tab. I'm guessing "Maximum Fault Level" is selected. You can choose "Do Not Terminate Curve..." to see the entire curve, but again, everything above the maximum fault is irrelevant because it won't be possible to have any fault (arcing or bolted) level higher than that (theoretically).

3. Also in the TCC options if you click on the blue curve, under the "Arcing Fault and UD Flags" tab you can check the "Show Arcing Fault Current for Worst Case Incident Energy" and select the buss you're analyzing. If no lines show up then that device is not the one which is actually interrupting the arc. Check and see if you're considering the Protective Device Line Side value, in which case the upstream (red) device will actually be interrupting an arcing fault.

Marking equipment for the incident energy at the line side of any local overcurrent protective devices is a good idea because those are almost always significantly higher/worse than the load-side. If, when someone opens up this equipment, the line-side conductors are exposed/accessible/in the same compartment it's a good idea to label for that worst case incident energy value.


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 Post subject: Re: Seemingly Artificial High IE values
PostPosted: Mon Jan 08, 2018 10:11 am 
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It looks like the TCC is at 4.16 kV, the primary voltage. The low side fuse (480V?) is shown at primary current levels and that is why the cutoff is so low. It also looks like the IE is calculated on the source side of the secondary fuse, with the fuse operating time over 2 seconds. IE on the load side of the secondary fuse would be based on the 0.02 sec fuse clearing time and would be much lower.


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 Post subject: Re: Seemingly Artificial High IE values
PostPosted: Sat Jan 13, 2018 7:23 am 
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jghrist wrote:
It looks like the TCC is at 4.16 kV, the primary voltage. The low side fuse (480V?) is shown at primary current levels and that is why the cutoff is so low. It also looks like the IE is calculated on the source side of the secondary fuse, with the fuse operating time over 2 seconds. IE on the load side of the secondary fuse would be based on the 0.02 sec fuse clearing time and would be much lower.


The area between the transformer secondary and the first OCPD (fuse or breaker doesn't matter) is always problematic because the only protection is on the primary side of the transformer. Trying to address it from the high side is difficult. One approach is to use a protection relay with a current threshold where once it recognizes that magnetization has passed (say once current drops below 100% of rated), switch curves to a tighter one that follows damage curves rather than the magnetization curve which can now be ignored. A second approach is to mount bushing CT's on the transformer secondary side and use a 50/51 relay to trip a breaker on the primary side thus giving the effect of a breaker mounted directly on the transformer bushings and making the high arcing current zone all but disappear. Suggestions to use Bay-O-Net fuses or current limiters fail to recognize that these fuses don't really offer much in the way of secondary protection. Another option is to simply work around it. Recognize that any activity around the first overcurrent protective device (fuses in this case) should only be worked on using the primary side disconnect. It takes longer and at some point the rules for normal operation of equipment (no arc flash hazard) should be applied to avoid all kinds of crazy silly attempts to perform even diagnostic work for shops that refuse to perform a risk assessment using available risk assessment standards.


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