Bus Duct Short Circuit Ratings

[philly]

I'm looking at short circuit ratings for 480V (635V Max) Non-Seg Bus Duct with the following ratings:

2-second Withstand: 63kA
1-second Withstand: 89kA
Momentary Peak: 170kA
Momentary Asymmetrical: 100.8kA

When comparing these ratings to the calculated available LV fault duty do you use the 2-second rating or the 1-second rating. For a particular location the available short circuit current is calculated at 6700A. If this is to be compared to the 2-second rating than this Bus Duct would be overdutied?

The 2-second rating listed above is the maximum rating available for this Bus Duct. Has anyone run across a similar case with BusDuct being overdutied?

Why does the LV Bus duct have a momentary rating associated with it? I have always though that this momentary rating was reserved for MV equipment which has both interrupting and momentary ratings and that LV equipment only has an interrupting or withstand rating?

 

[gadfly56]

I'm looking at short circuit ratings for 480V (635V Max) Non-Seg Bus Duct with the following ratings:

2-second Withstand: 63kA
1-second Withstand: 89kA
Momentary Peak: 170kA
Momentary Asymmetrical: 100.8kA

When comparing these ratings to the calculated available LV fault duty do you use the 2-second rating or the 1-second rating. For a particular location the available short circuit current is calculated at 6700A. If this is to be compared to the 2-second rating than this Bus Duct would be overdutied?

The 2-second rating listed above is the maximum rating available for this Bus Duct. Has anyone run across a similar case with BusDuct being overdutied?

Why does the LV Bus duct have a momentary rating associated with it? I have always though that this momentary rating was reserved for MV equipment which has both interrupting and momentary ratings and that LV equipment only has an interrupting or withstand rating?

If your 2-second withstand is 63kA, that's 63,000A. That's almost 10X your available SCC (6,700A), unless you dropped a decimal point.

 

[philly]

If your 2-second withstand is 63kA, that's 63,000A. That's almost 10X your available SCC (6,700A), unless you dropped a decimal point.

Yup I apologize I dropped a decimal point. Available fault duty is 67kA. This is on secondary of 3MVA transformer so fault current is pushing threshold of secondary ratings. LV Switchgear is rated for 85kA.

 

[shortcircuit1]

Yup I apologize I dropped a decimal point. Available fault duty is 67kA. This is on secondary of 3MVA transformer so fault current is pushing threshold of secondary ratings. LV Switchgear is rated for 85kA.

As far as i know i think you should compare your calculated SC with 2 second SC rating. Look at it this way..1 cycle=0.016 seconds. So for 2 seconds it would be 120 cycles...which is a your symmetrical rating when they specify for 2 seconds.

Look at EATON's busducts...

 

[shortcircuit1]

Yup I apologize I dropped a decimal point. Available fault duty is 67kA. This is on secondary of 3MVA transformer so fault current is pushing threshold of secondary ratings. LV Switchgear is rated for 85kA.

Yes your busduct is overdutied....

 

[philly]

Look at it this way..1 cycle=0.016 seconds. So for 2 seconds it would be 120 cycles...which is a your symmetrical rating when they specify for 2 seconds.

Right I get that part. Just trying to figure out why they give both a 1-sec and 2-sec rating. And why they provide a momentary rating.

Also does it mean that the upstream device must clear the fault within 2-sec in order to meet that rating? So lets say that there is a 60kA fault on this particular bus, does that mean that the upstream device must clear this fault in less than 2-seconds? In this case the upstream device is on primary of transformer so trip time will be delayed for faults on secondary of transformer where bus is located.

 

[Ingenieur]

For a detailed sc study you need a curve, 3 points
basically
peak or subtransient the 170k
asymmetrical or transient 100k and 80k
symmetrical or steady state the 63k
the actual corresponding fault i points need to be less
this is primarily determined by system avail fault i, x/r, motor contribution, etc

 

[wbdvt]

did you model the system using the available fault current from the POCO or was it determined by infinite bus values?