Conversion of 20kA Short circuit current at 3sec to 1 sec?

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SS_EE

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Hi all,

Can anyone let me know how do we convert a short circuit current at 3 second to 1 second? I know we need to use I^2*t but I am not getting the right value as it must be higher at 1 second
Appreciate your help
 
SS_EE said:
Hi all,

Can anyone let me know how do we convert a short circuit current at 3 second to 1 second? I know we need to use I^2*t but I am not getting the right value as it must be higher at 1 second
Appreciate your help
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Not sure what you mean by “convert” a short-circuit current. Can you clarify what you are trying to do?


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what would be the short circuit current for the bus at 1 sec if it has a short circuit rating of 20kA for 3 second?
 
SS_EE said:
what would be the short circuit current for the bus at 1 sec if it has a short circuit rating of 20kA for 3 second?
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The steady state short-circuit current is 20-kA regardless of time or breaker settings. Short-circuit current magnitudes only vary in the first few cycles as it transitions from sub-transient to transient to the final steady state value. Your question doesn’t make sense. Are you trying to determine the magnitude of fault current that would correspond to a breaker trip time of 1 sec? If so, it depends on the breaker make/model and settings if it’s adjustable. I suggest referencing the published TCC or back-solving if you know the TCC equation.


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20 KA FOR 3 SECONDS, is the. main bus short circuit withstand rating. and. is. fixed. as. the design. Min feeders have a short circuit withstand rating of 1 second when properly sized for the available short circuit. Look up BUSSMANN.
 
You can not convert it. You do not have the proper information in order to construct a time current damage curve.

Your bussing can tolerate a maximum of 20kA for a maximum of 3secs. Your job is to provide a protective device whose curve falls to the left of these two values.
 
See IEC 60865-1 Short-circuit currents_Calculation of effects
 

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If you know Ithr for 1 sec you may calculate for 3 seconds since Ith<=Ithr and Tk>Tkr. Other way it is not permitted.
 
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