What does the actual energy profile vs. time look like if you diagrammed it out?
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| Author: | Larry Stutts [ Wed May 22, 2013 1:47 pm ] | ||
| Post subject: | Arc Flash energy profile | ||
What does the actual energy profile vs. time look like if you diagrammed it out?
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| Author: | wbd [ Wed May 22, 2013 7:13 pm ] |
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Not sure if this is a question or a statement with the diagrams to show it??? |
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| Author: | Larry Stutts [ Thu May 23, 2013 6:15 am ] |
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The diagrams show a couple of my perceived possibilities. In a short-duration event, I think they would look like the time-constant profile curves. I am not really sure in a longer tA whether the energy has an initial peak then a lower level while it is arcing or whether it is just a stretched out time-constant profile. I realize "short" is relative. In this context, I mean perhaps less than 1/2 second or so. If an incident stretches out longer than a second, it would seem there would have to be a high amplitude initial blast with a lower level of energy as the arc is sustained. I was really just considerring if we should be protecting ourselves against the peak energy or the total incident energy - which is likely dependendant on tA. Not to say that the danger of a longer-duration event should be dismissed, more that the danger of a short-duration event is greater. All the incident energy released in a short tA equates to a higher blast pressure in comparison to a drawn-out incident. |
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| Author: | PaulEngr [ Mon May 27, 2013 5:46 pm ] |
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The arc voltage drop is difficult to model but is generally held to be around 150 volts. This explains why models under 1 kv are different from over 1 kv where arc voltage is inconsequential. During an ac sinewave then the initial arcing voltage to get a restrike causes a discontinuity which creates the characteristic "square wave" pattern that AFCI's detect. This is all steady state stuff. On top of this during a fault you can get an assymetrical sinewave which is a dc offset equal to up to the ac peak voltage that decays away. The maximum amplitude depends on phase angle at the initiation of the fault so it is totally random, between zero and maximum value. The decay rate depends on system parameters but is generally easy to determine. This all results in a peak thermal power that decays rapidly (usually lasting less than about 10 cycles). ETAP models it as a system wide constant. SKM does a more detailed model with both a transient and subtransient. In very short trip times such as instantaneous trips and with highly inductive loads and poor system x/r ratio, this makes a significant contribution to worst case incident energy. But outside of a high fault current/fast trip scenario, the contribution will be negligible. IEEE 1584 does not specifically account for this. You have to do piecewise integration over time to get at this value. Validity will be questionable since there isn't model data for it, and the transients are dc superimposed onto the ac waveform, not exactly what 1584 was intended to model. Not saying that this is invalid (I have it turned on in SKM), just saying that it is not backed by experimental data. Further, this is only valid if we consider radiative heat transfer only. There is a time delay as the air heats up and expands, along with any ejected plasma or other molten/gaseous materials which transfer by conduction/convection, so a quick trip arrests this before it happens. After the arc blast passes, further heat transfer is almost pure radiation. Again so far we don't have models for the physics of this so we can't really address it yet. When we do, arc blast and plasma ejection will be modeled as well as radiation. |
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