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 Post subject: Cleared Fault Threshold 80%
PostPosted: Tue Sep 16, 2008 2:17 pm 

Joined: Tue Sep 16, 2008 12:17 pm
Posts: 8
I am using SKM software and the default setting for Arc Flash Study, report options is Cleared Fault Threshold = 80%. Can anyone site any references to back up this assumption?
An example would be a fuse feeding an MCC with motors that clears in 1/2 cycle but it calculates the induction motors contribution for 5 cycles.
If cleared fault threshold is set to 100%, the motor fault current is additive to the incident energy through 5 cycles.
If cleared fault threshold is set to 80%, as long as the upstream fuse feeding the MCC is at least 80% of the total fault current, the motor fault contribution is only calculated through 1/2 cycle. The idea being that the arc could not sustain itself when 80% has cleared.


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PostPosted: Wed Sep 17, 2008 7:36 am 
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That's an interesting approach. Most of the software vendors I know have been attempting to come up with algorithms for motor contribution and arc flash. IEEE 1584 does not really give solid guidance on this. We know motor contribution quickly decays, it is dependent somewhat on voltage collapse at the motor terminals under fault conditions (and the arc changes that) and ANSI C37 gives "decay factors" depending on the size and type of motor. I have seen others actually attempt to apply a decrement to the motor contribution as well. I believe at this stage there is probably not a perfect answer but a lot of good attempts.

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PostPosted: Wed Sep 17, 2008 5:35 pm 
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If the goal is to accurately determine Ia for every half cycle I can see putting in the effort to model the motor decay curve. While the exercise has technical appeal, I question how important it really is.

My technical challenge is to assign an overall IE level that will protect the Qualified Person under the worse case conditions. So I do the following:

1) assume that the motor contribution is going to be present, for as long as it takes the overcurrent device to open, even if that is 10 cycles or 2 secs. Calculate Ia and then Calculate IE.

2) Take 85% of Ia with No Motor Contribution. Then calculate IE.

Which ever produces the highest IE value is what goes on the label.

As 1584 tells us, unlike a classic fault current analysis, being conservative in Arc Flash is assuming 'less' fault current that results in longer device clearing times - which typically produces higher IE values. Motor contribution in most installations is not going to be large, and in many cases even significant when the motors are small (less than 100HP). So even with Motor Contribution included longer than is practical, its doesn't change IE very much. The other thing is that a motor has one distinct difference from utility fault current - it has an on/off switch. Motors that run 24/7/365 will still be shutdown periodically for maintenance or building shutdowns. That could be the coincidental time of an Arc Flash incident. So I believe my logic covers the two worst case situations: grid maxed out with all motors contributing for the full Ia duration, or building shutdown with not one motor on but service work being performed.


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