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Which short circuit current do you plot in your time current curve graphs?
Poll ended at Tue Nov 19, 2013 7:34 am
Symmetrical RMS 100%  100%  [ 21 ]
Asymmetrical RMS 0%  0%  [ 0 ]
Symmetrical Crest 1/2 cycle 0%  0%  [ 0 ]
Asymmetrical Crest (peak) 1/2 cycle 0%  0%  [ 0 ]
Other 0%  0%  [ 0 ]
Total votes : 21
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ekstra   ara
 Post subject: Which short-circuit value to use in coordination?
PostPosted: Tue Nov 12, 2013 7:34 am 
Sparks Level

Joined: Sat Apr 04, 2009 9:50 pm
Posts: 104
Location: San Antonio, TX
When performing an AFHA some clients are interested also on a coordination study.

In the time current curve (TCC) graphs it is customary to show the available short circuit current (ASCC) for each over current protection device (OCPD) to determine the end of the plotting of the TCC. This ASCC also could determine if two OCPD's are coordinated or not.

The question is which of the many versions of ASCC should we include on the TCC graph?

The symmetrical RMS?, The asymmetrical RMS?, the asymmetrical crest (peak)?

The choice of ASCC will determine if two OCPD's are coordinating or not.

Coordinating time intervals (CTI's) for medium voltage OCPD's are more difficult to satisfy the longer the protection curve extends downward, so the asymmetrical peak would be the worst case.

I have done some research on this subject and I have NOT found any CLEAR guidance or explanation regarding the precise type of ASCC that should be included in the TCC graphs. At the present moment I am plotting the asymmetrical RMS.

Can anybody provide a good, respectable technical reference referring to this issue?

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PostPosted: Wed Nov 13, 2013 8:45 pm 
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Joined: Wed Jun 10, 2009 5:00 pm
Posts: 263
Location: Toronto
Most, if not all time-current characteristics, are published for symmetrical RMS current. One should use available short circuit current RMS values to determine protection device operating time using such characteristics and performing co-ordination studies. Applying asymmetrical or crest values to time-current characteristics is like comparing apples to oranges. Time-current characteristics should not be confused with peak let-through characteristics that are indeed plotted in asymmetrical peak units.

Michael Furtak, C.E.T.

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