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 Post subject: IEEE 1584 Arc Flash Calc for Ungrounded Systems
PostPosted: Fri Feb 20, 2009 11:33 am 
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The IEEE 1584 Incident Energy (IE) calculation for 208 to 15 kV Systems has a K2 factor in the exponent of a multiplier in the equation. This number is equal to zero for ungrounded & high-resistance systems, and equal to -(0.113) for grounded systems. What this does mathematically is reduce the multiplier for grounded systems, thereby showing an increase in the Incident Energy buildup for an ungrounded or HRG system. (relative to the grounded system)
I've worked the IE calculations for two identical systems, one with a delta secondary, and the other a wye secondary. The inicident energy is approximately 30% higher for the ungrounded/hrg system.
I've checked those calculations using SKM, and got the same results - since I'm using the IEEE 1584 equation method in SKM.
So I'm confident that these results are accurate.

I found an IEEE response to this question, more directly it was in response to the questioner citing several IEEE sources that advocated HRG for Arc Flash safety reasons, and asking the same question I'm asking here 'Why is the IE higher for an ungrounded system'. That response follows:
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Interpretation Response #3
I agree with your assessment that IEEE std 1584 seems to be contrary to statements in the other standards you cited, but appearances sometimes deceive. Each statement you quote from a standard is correct and your two calculations show the difference in results that I would expect.

The IEEE 1584 statement is based on three-phase testing with ungrounded and grounded systems and analysis of the results. This is because it is recognized that arc power is higher in three phase faults than in line-to-line or ground faults and single phase faults often escalate to become three-phase faults. IEEE 1584 considers only that worst case.

The other two standards you quote are discussing only ground faults, based on the fact that most faults begin as ground faults. If fault current can be limited, the fault is not likely to escalate to become a three phase fault. Arc fault energy will be quite small by comparison to the three phase fault case.

However, even with resistance grounding, a three phase fault is still possible and so it must be considered when calculating incident energy.


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But this doesn't answer the question: Why is the incident energy higher, by equation, for an ungrounded, or high resistance grounded system - for a three phase fault?

If anything there is an obvious reduction in ground-faults, and all that's left is three-phase faults. I know the calculation is only based on three phase faults.
But why would a three-phase fault for an ungrounded system have a higher incident energy than the same system when grounded??

There must have been a reason why the equation represents a higher IE for an ungrounded system.

Any thoughts are appreciated.

John M


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PostPosted: Fri Feb 20, 2009 12:43 pm 
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The IEEE 1584 equations are empirical, not theoretical. The equations give a higher energy for ungrounded systems because the tests showed a statistically significant increase in energy. No theoretical basis for the increase is given. Just guessing, but the arcs may be a different length because of the grounding and the proximity of a grounded enclosure.


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PostPosted: Wed Feb 25, 2009 2:45 pm 
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tx for the response

jghrist,
Thanks for the response.
.. makes sense.. not the formula, the explanation!
John M


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PostPosted: Wed Feb 25, 2009 4:05 pm 
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Great discussion.
There are a few things happening here. I understand that the arc length does enter into this and I believe some current flows back on the ground (grounded system) via the conducting plasma. This is because try as you might, even though a three phase fault is theoretically balanced, it is not perfectly balanced in reality due to it's erratic nature. The arc also can conduct through the box between phases.

People do say using high impedance grounding helps with safety even though the formulas state the high z case will yield a higher incident energy which implies it would be worse. The basis for this claim is that a very high percentage (I've heard 80% or more) of faults on grounded systems are line-to-ground faults. A high impedance system restricts ground faults to only a few amps so the 80% of faults / arc flash that would be single phase can not occur or turn into a three phase arc flash. However if the arc flash begins as a three phase event, it could yield a greater incident energy than the grounded system.

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PostPosted: Thu Feb 26, 2009 12:34 pm 
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I was hoping you'd respond Jim! Thanks!
I've copied your response in italics below so I can address it completely:

There are a few things happening here. I understand that the arc length does enter into this and I believe some current flows back on the ground (grounded system) via the conducting plasma. This is because try as you might, even though a three phase fault is theoretically balanced, it is not perfectly balanced in reality due to it's erratic nature. The arc also can conduct through the box between phases.

People do say using high impedance grounding helps with safety even though the formulas state the high z case will yield a higher incident energy which implies it would be worse. The basis for this claim is that a very high percentage (I've heard 80% or more) of faults on grounded systems are line-to-ground faults. A high impedance system restricts ground faults to only a few amps so the 80% of faults / arc flash that would be single phase can not occur or turn into a three phase arc flash. However if the arc flash begins as a three phase event, it could yield a greater incident energy than the grounded system.


I think your "conducting plasma" comment presents an opportunity for Incident Energy (IE) reduction in field testing for grounded systems due to distributed or attenuated arc. Is that what you're presenting there?

Your second paragraph presents the case for a significantly lessened probability of arc flash incidents with an ungrounded or High-Resistance-Grounded system, thereby making it inherently safer. And the "however" points back to paragraph one - which gives a plausible explanation for a lower IE on the grounded system.

I'm totally in agreement with your post, and I appreciate your insightful explanation. Our company VP has been bothered by the higher IE for HRG systems because he's designed dozens and dozens of Data Center HRG systems, fully expecting that he's saved lives in the process by making safer systems. But when he got ahold of IEEE 1584 and saw the reduction multiplier for grounded systems he began to question his design philosophy. It looks to me like he can rest comfortable that there's likely to have been a huge reduction in the number of incidents, with perhaps a 30% increase in IE for the HRG systems.

Thanks again.

John M


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PostPosted: Mon Feb 13, 2012 11:57 am 
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This is another trap for those of us who try to do things by understanding the theories underlying the application. This is a great example of how IEEE1584 (and NFPA70E) could be made more practical (and therefor useful) by simplifying the calculations, not making them more complicated as the committees seem intent, even if a little bit of accuracy were sacrificed.

There is no explanation available that would satisfy an theorist (though the above responses take a good shot at plausible reason), yet we are supposed to administer these codes to tradesmen (??!!).


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PostPosted: Mon Feb 13, 2012 2:00 pm 
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Gary B wrote:
This is a great example of how IEEE1584 (and NFPA70E) could be made more practical (and therefor useful) by simplifying the calculations, not making them more complicated as the committees seem intent, even if a little bit of accuracy were sacrificed.


Not sure I follow you here. The reason that the calculations are un-intuitive is because they are based on empirical data, ie curve-fitting hundreds of tests. As long as this remains the most accurate method they'll stay that way too (and perhaps become even more so because of extra parameters taken into account). Sacrificing accuracy to get 'nice' looking equations is not something I'd be happy with.

As for understanding the underlying theory (and putting that in the standard) I agree that would make it more intuitive, but it's not going to change the way the hazard is calculated.


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