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 Post subject: Bolted short circuit current vs IEC61363
PostPosted: Wed Jul 02, 2014 12:47 pm 
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Joined: Wed Jul 02, 2014 12:34 pm
Posts: 1
Hello all,

I need some help.
I'm trying to understand the arc flas calculations but I'm running into a problem of definition.

I'm designing electrical systems for ships. These are systems which are supplied by generators.
For the short-circuit calculations I'm following IEC61363.

Whith this method, you calculate a lot of different short circuit values:
Ik"
Iac(0.5T)
Ip(0.5T)
All these values vary with time.

The IEEE 1584 is talking about the bolted short circuit current.

My question is, which one is the bolted short circuit current?

Many thanks in advance
Maarten


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 Post subject: Re: Bolted short circuit current vs IEC61363
PostPosted: Wed Jul 02, 2014 11:32 pm 
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Joined: Fri Jan 03, 2014 6:57 am
Posts: 66
Location: the Netherlands
Well I use SKM and in that case the short circuit current used for the arc flash calculation is not in your list. SKM uses the initial symmetrical RMS current calculated with the comprehensive short circuit study. It uses the comprehensive study because that current is more accurate then the IEC61363.

Quote:
Why is the comprehensive short circuit study (DAPPER) used in Arc Flash and not the other short circuit study methodologies (ANSI & IEC)?
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.

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.
Source: http://www.skm.com/faq_comp_af.html

In the IEC61363 the Ik’’ is the initial symmetrical RMS current. Ik’’=Ip/(2*sqrt2) see page 33 of the IEC61363.
But I find that using a factor 1,8 instead of 2 is more accurate (1,8 is commonly used for installations with faults close to the source.

An example for a 2330kVA generator:
Initial symmetrical RMS current (comprehensive) = 15185A
Initial symmetrical RMS current (IEC61363 Ik’’) factor 2*sqrt2 = 13541A
Initial symmetrical RMS current (IEC61363 Ik’’) factor 1.8*sqrt2 = 15046A
Arc flash bolted fault current = 15185A

If you have any more questions feel free to ask.


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 Post subject: Re: Bolted short circuit current vs IEC61363
PostPosted: Mon Jul 07, 2014 1:22 pm 
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The comprehensive short circuit method provides the most accurate arc flash results while short circuit standards apply various multiplying factors to determine the fault current values. Also, the disparity between calculation results from ANSI, IEC and the comprehensive method can be explained by different approximations and assumptions adopted by different methodologies. For example, some techniques ignore equipment X/R ration in the analysis and apply different correction factors to equipment ratings. Note that disregarding the equipment X/R ratio introduces up to 15% uncertainty in a single step of adding two impedances alone while applying different correction factors does not quantifies and does not corrects the error but only amplifies it. You may also want to check Beware of Simplistic Fault Current Calculations article by Keith Lane about other issues arising when equipment X/R ratios are not taken into account in short circuit analysis.

For arc flash analysis, you are also looking for a total available short circuit current (including contributions from up- and downstream generators and motors) as well as a part of available short circuit current through protection device. The total available short circuit current is used in incident energy and arc flash boundary calculations while the part of available short circuit current through protection device is used to determine arc duration as a function of upstream protection device time-current characteristics. Please have a look on ARCAD's Short-Circuit-Analytic version 1.0 (SCA V1.0) for an example of short circuit software program meeting the above criteria.


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