Arc Flash Incident Energy - main breaker vs line-side fuse calculation

M

Magic Gorge

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Lexington, KY
For a double-ended SWGR I'm evaluating, the worst-case is with the tie open (non-paralleled). The main breaker trip/delay time is 2 seconds, and reported as PPE level 4. The upstream fuse trip/delay time is closer to 0.3 seconds, with PPE level 2. So, how is it that a fault at the SWGR would extend to 2 seconds without the upstream fuse tripping?
 
It is possible the software is not showing the fuse correctly. I don't know how this happens, but I think it happens during editing. I remember having to remove the CB, in this case, and then reinsert it into the one-line.
 
If this fuse is on the primary of the transformer, it's entirely possible. The fuse clearing time of 0.3 sec could be based on a fault on the primary side of the transformer.

Based on the recommendations in IEEE 1584, 2 seconds is used as a maximum fault duration. When you see an arc time of 2 seconds in the program's arc flash report, it means that nothing had cleared the fault by two seconds.

If you look at the time-current curves and compare with the calculated arcing current, it should be easier to understand.
 
D. Castor said:
If this fuse is on the primary of the transformer, it's entirely possible. The fuse clearing time of 0.3 sec could be based on a fault on the primary side of the transformer.

Based on the recommendations in IEEE 1584, 2 seconds is used as a maximum fault duration. When you see an arc time of 2 seconds in the program's arc flash report, it means that nothing had cleared the fault by two seconds.

If you look at the time-current curves and compare with the calculated arcing current, it should be easier to understand.
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This seems like the answer I was looking for, and I'm aware of the 2 second max as a practical limit. I'm just having trouble understanding what's going on with the primary fuse while the secondary MCB is letting it burn for 2 seconds. On the TCC, the fuse is undersized compared to the MCB setting (MCB has no instantaneous pickup and the fuse characteristic is to the left for most of it). So it makes sense the fuse would trip first. I know arcing fault, bolted fault etc all factor in, but can we interpret this to mean that for the worst-case arc flash at the LV SWGR, the upstream fuse will have not operated to clear the fault? Here is a snip of the detail view for both the primary fuse and MCB calculation for the LV SWGR.
1741810414726.png
voltage/Bus bolted fault/ Bus arc fault/ prot dev bolted fault/ prot dev arcing fault/ trip delay time
 
How can the fuse current on the primary side be basically the same as the 480 V current? I haven't looked at SKM reports for a bit, but for a fault on the 480 V side, the primary current through the fuse should be reduced by the transformer turns ratio. It would be unusual for a primary fuse to clear that quickly for a low side fault. Also, the arcing current through the protective device is significant less than the total bus arcing current. Is there a second source of fault current, or is this just a lot of motor contribution? I'd look at the fault currents on the one-line diagram. The total arc current is 16.5 kA but the current through breaker is only about half of that. Are you sure the tie is open in your model?

I would also take a close look at the time current curve for the low voltage breaker and make sure the settings are accurate. Even without Instantaneous, there will be a short-time trip that should clear before 2 seconds. I'd try reducing the short-time pickup to something below the calculated arcing current.
 
D. Castor said:
How can the fuse current on the primary side be basically the same as the 480 V current? I haven't looked at SKM reports for a bit, but for a fault on the 480 V side, the primary current through the fuse should be reduced by the transformer turns ratio. It would be unusual for a primary fuse to clear that quickly for a low side fault. Also, the arcing current through the protective device is significant less than the total bus arcing current. Is there a second source of fault current, or is this just a lot of motor contribution? I'd look at the fault currents on the one-line diagram. The total arc current is 16.5 kA but the current through breaker is only about half of that. Are you sure the tie is open in your model?

I would also take a close look at the time current curve for the low voltage breaker and make sure the settings are accurate. Even without Instantaneous, there will be a short-time trip that should clear before 2 seconds. I'd try reducing the short-time pickup to something below the calculated arcing current.
Click to expand...
Ah this is where I screwed up, this is the PARALLELED mode for these results. I'm not sure what you mean by the primary current, these are both referenced at 480V as they are both calculations for the same bus. Yeah I'm still trying to understand why the fuse wouldn't trip first, even at the described arcing fault. Here's a clip of the TCC, excuse the terrible editing but it's what I've got at the moment. MCB is green curve and fuse is blue curve. *edit sorry mcb is brown curve, but not much different.* I show 2 sec and 0.3 sec and approximate arcing fault points. Current is x100
1741814876690.png
 
You know what, now that I think about it, this might boil down to the "check upstream devices for miscoordination" box I did not click, as to be consistent with previous evaluations. The program isn't doing the thing I think it would because I told it not to!
 
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