Short Circuit Current Calculation

[Electriman]

Here is the Problem:

I am using a software to calculate the short circuit current. The report gives me three short circuit current 3-phase for each bus, 1/2 cycle, 1/2-4 cycles, 30 cycles.

My question is which one of these must be less than
a) Bus bar bracing current
b) Breaking current of MCCB and breakers

I am saying since breakers normally act 3-5 cycle after the fault occurs so the 12/-4cycles must be selected am I right? Do we have any where in the code (NEC or IEEE) that addresses this problem?

 

[ron]

My question is which one of these must be less than
a) Bus bar bracing current
b) Breaking current of MCCB and breakers

Most low voltage equipment SCCR is based on the 3 cycle calculated number

 

[Julius Right]

IEEE 141/1993 [may be]
CH. 4.5.4 Step 4 Calculate short-circuit current.
a)First-cycle duties for fuse and circuit breaker.[half a cycle]
b) Contact parting [interrupting] duties for medium and high voltage circuit brakers.[1.5 to 4 cycles]
c) Short-circuit currents at operating times for time-delayed relaying devices.[from 1 to 6 and beyond]

 

[Electriman]

IEEE 141/1993 [may be]
CH. 4.5.4 Step 4 Calculate short-circuit current.
a)First-cycle duties for fuse and circuit breaker.[half a cycle]
b) Contact parting [interrupting] duties for medium and high voltage circuit brakers.[1.5 to 4 cycles]
c) Short-circuit currents at operating times for time-delayed relaying devices.[from 1 to 6 and beyond]

Thank you.

 

[kingpb]

Withstand rating, or device duty (ANSI C37) calculates momentary symmetrical and asymmetrical rms, momentary asymmetrical crest, interrupting symmetrical rms, and interrupting adjusted symmetrical rms short circuit currents to check the protective device close and latch, and interrupting capacities against rated.

1/2 cycle is the maximum rms short-circuit current.

The times are associated more with contact parting, protection times, and device coordination.

 

[Electriman]

Withstand rating, or device duty (ANSI C37) calculates momentary symmetrical and asymmetrical rms, momentary asymmetrical crest, interrupting symmetrical rms, and interrupting adjusted symmetrical rms short circuit currents to check the protective device close and latch, and interrupting capacities against rated.

1/2 cycle is the maximum rms short-circuit current.

The times are associated more with contact parting, protection times, and device coordination.

Is my understanding correct from IEE141 or not?

1- On the LV circuit breaker, the short circuit rating must be higher than 1/2 cycle symmetric short circuit current calculated by software.

2- On the HV circuit breaker the short circuit rating must be higher than asymmetric 1-4 cycle of the software calculated short circuit current