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

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SS_EE

Member
Location
Philippines
Occupation
Engineer
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
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
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

Not sure what you mean by “convert” a short-circuit current. Can you clarify what you are trying to do?


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SS_EE

Member
Location
Philippines
Occupation
Engineer
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?
 

xptpcrewx

Power System Engineer
Location
Las Vegas, Nevada, USA
Occupation
Licensed Electrical Engineer, Licensed Electrical Contractor, Certified Master Electrician
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?

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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Last edited:

JEFF MILLAR

Senior Member
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.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
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.
 

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
See IEC 60865-1 Short-circuit currents_Calculation of effects
 

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Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
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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