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 Post subject: trouble in comprehensive and iec standard.
PostPosted: Thu Jan 16, 2014 10:07 pm 

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Location: pune
If equipment’s (Breaker, Cable, etc.,) are build based on some standard (say – IEC 61363), how to go ahead with Arc flash hazard calculation because fault current computed by IEC 61363 method and comprehensive method will be different.


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PostPosted: Fri Jan 17, 2014 12:13 am 
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I don’t know if I fully understand your question. I use the IEC 61363-1 for my short-circuit calculations because I am doing an arc flash study for ships hence that I use the standard for ‘Electrical installations of ships and mobile and fixed offshore units - Part 1: Procedures for calculating short-circuit currents in three-phase a.c.’.

[font=Calibri]The company I work for uses SKM PTW and this is what I found on their site;[/font]
SKM stated wrote:
Why is the comprehensive short circuit study (DAPPER) used in Arc Flash and not the other short circuit study methodologies (ANSI & IEC)?
SKM stated wrote:
SKM stated wrote:

[font=Calibri]The simple answer is that the comprehensive short circuit method provides the most accurate arc flash results. Most other short circuit standards apply various multiplying factors to determine the fault current values. This is adequate for other applications, such as for specifying electrical equipment withstand and interrupting ratings and to selectively coordinate protective devices, but for Arc Flash purposes, it may not be suitable. [/font]

In Arc Flash calculations, a conservative fault current doesn’t always result in a conservative incident energy. Generally, protective devices protecting the faulted location will clear much faster when a higher fault current exists, resulting in a lower incident energy and a false sense of security.

In actual sense, a longer clearing time will result in a more conservative incident energy. Arc Flash standards (IEEE 1584 and NFPA 70E) suggest using a Low and High arcing fault tolerance to be used instead. There’s another impedance tolerance option in the software that can be used within Arc Flash as well. Making use of these tolerances and the worst case scenarios will allow the worst case incident energy to be determined.

[font=Calibri]When I switch my SC study method from IEC61363 to Comprehensive, the arc flash results do not change so that is true.[/font]

[font=Calibri]I have to say I am relatively new in the world off short circuit study’s because this was not a big subject on my school. My only experience comes from the IEC61363.[/font]

[font=Calibri]The difference That I can find is that the IEC61363 only calculates 3P faults, the comprehensive way calculates 3P, SLG, LL, and LLG faults. These have a lower current which could lead to an higher arc rating due to protection devices being set to secure against short circuits and overload not against arc flashes.[/font]

The 3P faults do not change when I switch between both ways, SKM will just use the fault current which leads to the highest arc rating.


EDIT:
SKM doesn't seem to calculate ground faults for ungrounded systems so for those circuits it will always use the 3P fault unless the 2P fault results in an higher arc rating due to the arcing time.


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PostPosted: Fri Jan 17, 2014 12:42 am 

Joined: Thu Jan 16, 2014 10:02 pm
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Thanks for the reply.


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PostPosted: Fri Jan 17, 2014 9:32 pm 

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there is one other option in SKM to unlink the fault study from arc flash result. After putting the fault current in that, the TCC curve would also unlink. so that's the problem..


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PostPosted: Mon Jan 20, 2014 2:39 am 
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Could you explain to me why you would want to unlink the short circuit study from the arc flash study?

On the internet I find
Quote:
Q: How do I determine the Arc Flash hazard for single phase equipment?
A: There isn’t a widely accepted standard (IEEE, NFPA, etc.) that discusses single phase systems in detail. In PTW, you can run a unbalanced short circuit study using the Unbalanced module.

Then enter the SLG fault current or using the 3P fault from UBSC results to the AF screen. PTW automatically unlinks the SC and link TCC. You can enter the adjusted fault current based on the UBSC results and Arc Flash program reads the trip time automatically. The results will be conservative, but having an answer is better than having no answer at all.


But after reading your initial question I assume this is not what you want to do.

Are you trying to unlink the comprehensive SC study and manually insert the IEC61363 study?

Because if you are then remember that the Iac(0,5T) shown by SKM is NOT the same as the InitSymRMS current used in the arc flash study.


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PostPosted: Mon Jan 20, 2014 8:14 am 
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ANSI and IEC (there are multiple short circuit methods under IEC, 61363 is just one of them) all use various assumptions and multipliers to calculate the short circuit current which results in different results. The "comprehensive" model uses no short cuts. It numerically calculates the assymetrical fault current given the reactive loads in the circuit, and uses actual symmetrical components impedances to determine a result. It is, as stated, comprehensive and is what results if you have a relatively powerful desktop computer capable of solving matrix algebra problems very quickly to arrive at the most comprehensive result using symmetrical components modelling. It stops just short of doing an actual time series analysis which is only used in the transient and motor starting modules of SKM.


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PostPosted: Mon Jan 20, 2014 9:43 pm 

Joined: Thu Jan 16, 2014 10:02 pm
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Got my answer. Thanks.


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PostPosted: Mon Jan 27, 2014 7:57 am 

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PaulEngr wrote:
ANSI and IEC (there are multiple short circuit methods under IEC, 61363 is just one of them) all use various assumptions and multipliers to calculate the short circuit current which results in different results. The "comprehensive" model uses no short cuts. It numerically calculates the assymetrical fault current given the reactive loads in the circuit, and uses actual symmetrical components impedances to determine a result. It is, as stated, comprehensive and is what results if you have a relatively powerful desktop computer capable of solving matrix algebra problems very quickly to arrive at the most comprehensive result using symmetrical components modelling. It stops just short of doing an actual time series analysis which is only used in the transient and motor starting modules of SKM.


yograj.s wrote:
If equipment’s (Breaker, Cable, etc.,) are build based on some standard (say – IEC 61363), how to go ahead with Arc flash hazard calculation because fault current computed by IEC 61363 method and comprehensive method will be different.

I would suggest that there are variables that may have more impact on the Arc Flash results via impact on arcing current. If the low voltage equipment is built to IEC standards it may have smaller distances between conductors. This can "increase" arcing current. Also IEC equipment, often is more compact which can increase arcing current, propensity to sustain an arc and concentrate the incident energy more as well. On the other hand, some high end IEC equipment uses a lot of insulation which may limit the probability of the arc. When you take into account the many assumptions and estimated lengths that go into short circuit calculations I would suggest that the conservative thing to do is to assume arcing current values could be wrong in either direction with the lower direction often creating more hazard if it causes significantly slower protective response.


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PostPosted: Mon Jan 27, 2014 11:14 am 
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IEC vs. ANSI designs as far as I know don't come into the picture regardless of the short circuit analysis method used. Short circuit analysis is intended to ensure that bus work is sized to withstand the potential magnetic forces involved during a bolted fault and for similar issues addressing heating. Thus a simplified approach that gets to a conservative estimate of short circuit current is acceptable for short circuit calculation purposes. The differences between IEC and ANSI in this regard have to do with the simplifying assumptions that are made.

However when considering arc flash, things are not so easy. On the ANSI and IEC standard time current curves, a reduction in current corresponds to an increase in trip time. However the curve does not match the incident energy calculation and always results in a corresponding increase in incident energy. Thus we have something of a paradox that the "conservative" results for bolted fualt purposes will under-predict arc flash. Sometimes the difference is minor. Sometimes it is substantial.

By way of example there are some cables here with have 2 conductors per phsae, 500 MCM for one MCC and 3 conductors per phase, 500 MCM for another nearly identical MCC. There has not been any indication of damage due to being undersized in the 2 conductor case after about 40 years of operation. The ANSI model predicts that the 2 conductor cables are undersized. The comprehensive model predicts that they are acceptable.


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PostPosted: Mon Jan 27, 2014 2:20 pm 
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I agree with the SKM statement above. Indeed, assumption based calculations are accurate only if the assumptions the calculations are based on are valid. In other words, proving one of or more of the assumptions wrong at the least questions the calculation results and at the worst effectively proofs them wrong.

I firmly believe in comprehensive short circuit analysis encompassing all or nearly all aspects of power system and its components. Yes of course it takes more time and effort to perform comprehensive analysis comparing to the simplified approach employed by most short circuit standards. However, you are rewarded with the numbers that can be trusted.

As an example, taking into account equipment X/R ratios in calculations results in more accurate calculations while ignoring the ratios will introduce an error into the final result. In fact, ignoring the equipment X/R ratio introduces up to 15% uncertainty in a single step of adding two impedances alone, while typical single diagram consists of dozens impedances all added together. Vern Lindberg, the author of Uncertainties and Error Propagation once said that "if the error can't be quantified then it's not engineering but only a guess". Applying different correction factors does not quantifies and does not corrects the error but only amplifies it. Therefore, cutting corners in short circuit analysis by introducing certain assumptions is workable in cases such as studying the potential magnetic forces involved during a bolted fault etc. but is not accepted in arc flash analysis as rightly noted by PaulEngr above and explained in the SKM documentation.

Check also Beware of Simplistic Fault Current Calculations article by Keith Lane about other issues arising when equipment X/R ratios are not factored in in short circuit analysis. The good news is that there are software solutions available for comprehensive short circuit analysis and they are not limited to one service provider only. Short Circuit Analytic software program is an example of such a solution and there are more resources available at the market.

_________________
Michael Furtak, C.E.T.
http://arcadvisor.com


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