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 Post subject: IEC 61363
PostPosted: Mon Aug 20, 2012 1:59 am 
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When calculating arc flash the traditional classic method for 3phase bolted fault currents are generally used as input. In the softwares I have looked into, one can also choose IEC 60909, but none use the IEC 61363 standard.

Is there any reason why IEC 61363 should not be included in software progammes? In ship electrical systems, IEC61363 is deemed as more accurate than IEC 60909, and therefore I am wondering what short circuit standard I should use as input into arc flash calculations.

Thank you.


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PostPosted: Mon Aug 20, 2012 5:24 am 
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What software do you use that uses IEC 60909 results for arc flash calculations?

SKM Powertools can only use its Comprehensive method when running arc flash calculations (so neither IEC or ANSI methods)).


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PostPosted: Mon Aug 20, 2012 5:39 am 
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Paladin DesignBase has the following options:
1. AC Classical
2. AC ANSI/IEEE
3. IEC 60909


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PostPosted: Mon Aug 20, 2012 7:48 am 
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Are you sure the software actually uses those results for the arc flash study? The explanation I got from SKM a few years ago was that since the tests in IEEE 1584 are not based on IEC 60909 SC calculations, you cannot use them in your study. For 909's max case calculations the pre-fault voltages go up to a factor 1.1, which means that you're getting conservative (i.e. high) results. Fine for equipment sizing, but I wouldn't use it for arc flash hazard calculation.

I'm not sure what the differences are between Classical and ANSI/IEEE, but maybe others can weigh in on that.


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PostPosted: Mon Aug 20, 2012 8:32 am 
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Yes, although Paladin DesignBase does not recommend using other methods than AC Classical, the user can select IEC 60909.

As I only work with ship electrical systems, I only use IEC 61363, and therefore I am wondering how to calculate arc flash onboard ships. NFPA 70E states that is can not be used for ship electrical systems, and this confuses me even more.


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PostPosted: Mon Aug 20, 2012 5:14 pm 
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70E's reference to ship electrical systems is the same as the references to utilities which you also see reflected in NFPA 70 (NEC). The issue at hand is that there are safety and electrical installation standards that cover those installations. So the scope language limits itself to a particular area. That's not to say that you can't use it as a voluntary guidance document. It just means that it wasn't specifically intended to work with say the NESC or ABS standards.

As to short circuit standards...the short circuit calculation standards tend to give conservative (high) results. For instance many completely ignore cable impedance. Most software systems that calculate arc flash will use whatever standard is specified for calculating short circuit values and then use a completely different methodology (the most accurate available) to calculate the bolted fault value for the arc flash incident energy.


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PostPosted: Mon Aug 20, 2012 11:57 pm 
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Thanks for the replies.

I still can not see why the software companies will not allow me to use IEC 363 as input for arc flash calculations. They tell me that the IEEE 1584 standard is based on classical methods for short circuit and thus the results will not be acceptable when using IEC 363. IEC 363 is however the most accurate standard for ship electrical systems.

This tells me that neither IEEE 1584 or NFPA 70E is of any use when calculating arc flash in ship systems.

Any thoughts on this?


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PostPosted: Wed Aug 22, 2012 7:28 pm 
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Beo wrote:
Thanks for the replies.

I still can not see why the software companies will not allow me to use IEC 363 as input for arc flash calculations. They tell me that the IEEE 1584 standard is based on classical methods for short circuit and thus the results will not be acceptable when using IEC 363. IEC 363 is however the most accurate standard for ship electrical systems.

This tells me that neither IEEE 1584 or NFPA 70E is of any use when calculating arc flash in ship systems.

Any thoughts on this?


NFPA 70E does not specify HOW to calculate incident energy. It gives a list of 8 different potential methods in an annex. It does not give a greater emphasis or weighting for one vs. another. IEEE 1584 on the other hand has at least 2 different methodologies. It has a simplified approach as well as the one that most folks are familiar with. The algorithm is backed up by hundreds of tests. The calculation itself is empirically based and is essentially a curve fit to the data. Both the data set and the documentation are pretty thorough. Regardless of the method, they all use the bolted three phase fault current as an input to the algorithm. They do not specify the specific method for calculating it.

One of the biggest variations is that the assymetrical fault current is handled in various ways (or not at all) in the various short circuit calculation methods. Since the most popular power system modeling software packages used to calculate arc flash incident energy (SKM, ETAP, EasyPower) explicitly model assymetrical fault currents as well as transients and subtransients from large motors, the result is that the input bolted fault current is frequently much different from the standards that I have seen for calculating short circuit currents. The differences are usually fairly subtle but they are clearly there. They result in elevated incident energy values relative to what you might calculate by hand using inputs from a short circuit study.

This has nothing to do with the "accuracy" of any particular short circuit study. It's that we're dealing with two different issues. A short circuit study is primarily interested in the relatively short term thermal capacity of the equipment. A transient study is primarily interested in the magnitude of the voltages (for insulation coordination purposes) and currents in the system during those events that generate transients such as switching. Arc flash studies are interested in the amount of energy released in an arcing fault. They can pull data from either study but the level of detail required from a transient study is better so the default is usually to use that kind of data.

This has nothing inherently to do with ship-board or shore-based power systems. The major difference from a purely electrical point of view as I see it is that ship-based AC power systems typically operate ungrounded whereas the norm on shore tends to be solidly grounded or impedance grounded systems these days. From a pure arc flash point of view this has implications. Impedance or capacitively ("ungrounded") grounded systems have a slightly higher incident energy than their solidly grounded equivalents and this is reflected in IEEE 1584 calculations. However from a risk point of view, they also enjoy a much lower likelihood of arc flash in the first place and if you don't do a risk assessment (as opposed to a hazard assessment), you will totally miss out on the improved safety factor.


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PostPosted: Thu Aug 23, 2012 12:42 am 
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Thanks.

We do risk assessments as well as arc flash calculations, and have always used IEC 363 as input for arc flash calculations. Therefore I was a bit surprised when I got the answer from the software developers.

As stated above, IEEE 1584 requires a 3 phase bolted fault and does not specify what standard to use. In ship systems IEC 363 is clearly the most accurate method.


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PostPosted: Mon Aug 27, 2012 8:09 pm 

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Hi,

If you want to account for decaying effect of the current, you can calculate it manually using step by step method.
It is mentioned in "Generator Backup Overcurrent Protection" IEEE Trans. Industry App. The method could apply for calculating arc energy considering decrement current.
However, the decaying rate between IEC363 results and arc current could be different !


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PostPosted: Tue Aug 28, 2012 12:06 am 
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Thanks.

IEC 909 also consideres decrementing currents, so why I am not allowed to use IEC 363 as input is not clear to me. I always calculate 1/2 period of the currents, never 5-cycle, 10 cycle or steady state, and use the 1/2 period as input into arc flash calculations.

As I interpret the IEEE 1584 standard, the highest theoretical maximum 3 phase symmetrical short circuit current should be used as input. For ship systems this is calculated according to IEC 61363-1 and using the classical method or Ansi method would give levels considered for far from generator faults.

I am doing a calculation using all the standards as input, and comparing results.


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PostPosted: Tue Aug 28, 2012 12:46 am 
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Did a quick calculation on a ship system using paladin designbase, the different values are
AC classical: 65 kA
Ansi/IEEE: 64 kA
IEC 909: 68 kA
IEC 363: 53 kA

All calculations are 1/2 period. Did not expect 363 to give a SC so low compared to the others. Even though it is in the sub transient range the decrementing effect of near generator faults give a much lower SC rating.

Using the classical method will clearly give a higher arc current, but can this current be achieved in ship systems?


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PostPosted: Thu Aug 30, 2012 8:51 am 

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Beo,
pls. can you clarify where into IEEE 1584 std you find that for arf flash calulation the maximum 3 phase symmetrical short circuit current should be used as input?


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PostPosted: Thu Aug 30, 2012 9:57 am 
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Section 5.2 Arcin currents.
"Bolted fault current for three-phase faults (symmetrical RMS)"


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PostPosted: Thu Aug 30, 2012 2:52 pm 
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The IEEE equations are looking for a 3 phase short circuit current. Whether it is an IEC or ANSI based calculation doesn't matter as far as 1584 goes. You will want to look at not only the maximum current but minimum as well as in a unit or line out of service. Lower short circuit currents could create a situation where the protective device that is limiting the arc flash duration trips slower resulting in a greater incident energy.

Also, since the generation has a decrement curve, multiple arc flash calculations could be used to determine the incident energy changing over time. i.e. one calculation during the subtransient, transient and synchronous portions until the arc is extinguished. Then you total all of the incident energy. At lease one software company I know does this and we added this type of language in the draft of the next edition of IEEE 1584.

Great discussion with lots of good input. Interested to see how others model this.

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PostPosted: Tue Sep 11, 2012 2:07 am 
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Thanks!


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PostPosted: Thu Dec 06, 2012 12:53 pm 
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I know that some classification societies for shipping are currently looking into the problem with arc flash. There are some that already suggest to use IEEE 1584, but there is no information on what value of the short circuit to use as input.

Normaly the short circuit studies only contain the first half period (sub-transient region) to check that the breakers can handle the making capacity. The peak current and rms is calculated. Using the sub-transient rms value as input to the IEEE equations is quite normal I think, and as a result the calculated incident energies would be very conservative. To check the minimum currents are not normal since the generator shall deliver 3 times nominal current and the generator breakers are set according to this. Using the steady state currents would probably give more realistic results but again they would be too low since the sub-transient and transient period is not considered.

When is the next revision of IEEE 1584 expected to get released?
Can I ask what software calculates this decrementing effects of a near-generator fault?


Thanks.


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PostPosted: Sat Dec 08, 2012 8:29 am 
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SKM and ETAP have some means of accounting for short circuit current and do the incident energy calculations piece wise.


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PostPosted: Sun Dec 16, 2012 8:25 am 
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Beo wrote:
I know that some classification societies for shipping are currently looking into the problem with arc flash. There are some that already suggest to use IEEE 1584, but there is no information on what value of the short circuit to use as input.

Normaly the short circuit studies only contain the first half period (sub-transient region) to check that the breakers can handle the making capacity. The peak current and rms is calculated. Using the sub-transient rms value as input to the IEEE equations is quite normal I think, and as a result the calculated incident energies would be very conservative. To check the minimum currents are not normal since the generator shall deliver 3 times nominal current and the generator breakers are set according to this. Using the steady state currents would probably give more realistic results but again they would be too low since the sub-transient and transient period is not considered.

When is the next revision of IEEE 1584 expected to get released?
Can I ask what software calculates this decrementing effects of a near-generator fault?


Thanks.

The piece wise solution that Paul mentions is about the only option for a more accurate solution. A group of us got together about this time last year in Pittsburgh to begin the draft of the next IEEE 1584 text. In that draft we did not give specific instructions about this type of modeling, only that a piece wise model could be used. It could be based on the sub-transient impedance Xd'' as you stated, but also it could factor in the transient Xd' and synchronous Xd impedance as well to model the change in current and corresponding change in clearing time and incident energy.

As far as a time frame for the next IEEE 1584, I believe we have at least a year or two to go - maybe a bit longer. We have a few task groups working out a some of the details, a few additional tests to conduct and then getting everyone on board with the text, approval etc.

I typically provide updates at the arc flash forum when there is new information available about IEEE 1584 (and I can discuss it)

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