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 Post subject: Why doesn't 1584 calculate the worst case IE on a range
PostPosted: Tue Mar 05, 2019 2:19 pm 
Sparks Level

Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
We recently found a situation where we had to look at the IE of a disconnect that was located several hundred feet away. The disconnect then got moved several feet. The difference in IE between the two locations was ~15cal and .5cal. The reason was the current was about the same as the short-time setting, one to the left and the other to right of the setting.

The point of this is we saw a huge variation in IE based on estimated data (after all isn't everything we do estimated to some degree). Such a small change resulted in huge differences in IE. One being "safer" and other being "more hazardous".

So my question is: why doesn't 1584 work on a range of values - say +/- 10% of the arcing or bolted currents and evaluate the worst case across the range instead of a select number of discrete points? It seems to me finding the worst-case on a range would prove far more valuable to the person standing in front of the equipment with a poky metal object.

Looking at the tables in 1584-2018 the coefficients are at 10^-5 or -6 when my utility contribution is +/- 20% and my cable length is equally as varied.

Just looking to find a reasonable answer to a complex problem that doesn't cost fortune but saves lives!


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 Post subject: Re: Why doesn't 1584 calculate the worst case IE on a range
PostPosted: Fri Mar 08, 2019 11:06 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
It's not IEEE 1584. And it's really not very accurate beyond about 1 to 2 digits. In your case the dramatic difference is because your circuit breaker calculation jumped significantly in opening time.


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 Post subject: Re: Why doesn't 1584 calculate the worst case IE on a range
PostPosted: Mon Mar 11, 2019 4:23 pm 
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Joined: Tue Oct 26, 2010 5:00 pm
Posts: 188
Location: Maple Valley, WA.
To elaborate a bit, when the arc flash energy is low, that is because the breaker is tripping instantaneously, the tripping time is very low. When it jumps up, that is because the breaker is tripping out in the long time delay (overload) region. This is because the increased cable length decreases the fault current which causes the breaker to trip out in the overload region.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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