Short Circuit Calculation

[Electriman]

Friends,

I am calculating the short circuit ratings for a plant and I have below questions. My calculation is based on IEEE 141.

1- At the starting point, I have the utility grid. I have requested utility to give me their fault data at my PCC (Point of Common Connection) and they gave me the single phase and three phase short circuit currents along with phase impedances. My question is here: as you know we have 1/2 cycle short circuit current, 1.5-4 cycle short circuit current, steady state short circuit current, symmetric and asymmetric. I was wondering if any one knows the value that I have given reflects which one of these items. Do I need to change it when I put it in the model to find out my short circuit current ratings at the equipment?

2- My second question is when I calculate the short circuit current rating for LV CB I know that I have to find out 1/2 cycle short circuit current at the equipment based IEEE141 CH. 4.5.4. but I was wondering if I have to find out symmetric value or asymmetric value.

3- My third question is similar to my second question but for HV and MV CB. I know that I have to look at 1.5-4 cycle short circuit but don't know again is it symmetric or asymmetric?

I appreciate your response in advance.

 

[Bugman1400]

If you are performing a coordination study then use the 1/2 cycle asymmetric value (should be the worst case with highest value). If you are performing an arc flash study, you want to ask the local PoCo for the subtransient, transient, and synchronous impedances.

 

[Electriman]

If you are performing a coordination study then use the 1/2 cycle asymmetric value (should be the worst case with highest value). If you are performing an arc flash study, you want to ask the local PoCo for the subtransient, transient, and synchronous impedances.

Thanks for the response. I am performing this to select short circuit rating of the equipment.

 

[kingpb]

If your simply looking for the equipment ratings than you are wanting to do duty cycle calcs in accordance with IEEE C37. The X/R ratio becomes important when evaluating the LV equipment in order to possibly make adjustments in equipment selection.

Duty cycle looks at mom. symm, and asymm rms, mom. asymm crst, interrupting sym rms, and interrupting adjusted symm rms.

 

[Bugman1400]

If your simply looking for the equipment ratings than you are wanting to do duty cycle calcs in accordance with IEEE C37. The X/R ratio becomes important when evaluating the LV equipment in order to possibly make adjustments in equipment selection.

Duty cycle looks at mom. symm, and asymm rms, mom. asymm crst, interrupting sym rms, and interrupting adjusted symm rms.

Can you be more specific about the IEEE C37 standard? There are a lot of standards under the C37 section.

 

[Electriman]

If your simply looking for the equipment ratings than you are wanting to do duty cycle calcs in accordance with IEEE C37. The X/R ratio becomes important when evaluating the LV equipment in order to possibly make adjustments in equipment selection.

Duty cycle looks at mom. symm, and asymm rms, mom. asymm crst, interrupting sym rms, and interrupting adjusted symm rms.

Let me clarify in an example:

In my case on 480 VAC, my switchgear is rated 22 kA. The short circuit results I got is:
Symmetric RMS 1/2 Cycle=19704 A
Asymmetric 1/2 Cycle= 24940 A

Does the equipment fail?

What if I have the same Scenario in 2.4 KV?

 

[kingpb]

Let me clarify in an example:

In my case on 480 VAC, my switchgear is rated 22 kA. The short circuit results I got is:
Symmetric RMS 1/2 Cycle=19704 A
Asymmetric 1/2 Cycle= 24940 A

Does the equipment fail?

What if I have the same Scenario in 2.4 KV?

Off hand and without looking at the calcs I'd say you need higher AIC rated equipment because you need to use the asymmetrical value.

But also look at IEEE C37.13 for the info because related to the effect of X/R.