Arc Flash / Level 4 on QO Panel / Need Sanity Check

[PSU Electrical]

Conducting an Arc Flash Study for a client that of which I'm nearly complete with. Everything looks pretty good except for one anomaly and I need a sanity check. At one point in the system I have a 480VAC, 250A, MLO, 3-Phase I-Line Panel with a Category 0 Incident Energy and Isc value of 8.76kA at this bus ? I have no problem with this. This panel has a 125A breaker that feeds 75kVA Transformer (480-120/208, Delta-Wye) to a 225Amp Distribution Panel (typical step down). This distribution panel has a 225A MCB (120/208, 3-phase, 4wire) and is a Square D, QO Panel. At this point in the system, my calculated Short Circuit (Isc) is 2.67kA which I feel is fine due to cable and transformer impedance knocking this down. My concern is the Incident Energy Level is a Category 4. Now I understand that the Incident Energy calculated is a combination of the Short Circuit Current available and the Protection Device Clearing Time and based on the TCC curve of the Main Breaker on the QO Panel, this makes sense. Even if you use the OCPD upstream (the 125A in the I-Line) you still have a Category 4. I guess my question or sanity check is this ? could having too low of a Short Circuit Rating (2.67kA) actually be creating a higher Arc Flash level due to a long clearing time? It just seems odd that you can have a Category 0 Level on a higher voltage and amperage rated panel and need much more PPE for a run of the mill QO house panel. Am I crazy?

Some background ? using ETAP Version 12.0.0, Assuming 3-phase, Sym ? Cycle fault. Below is a link on the curve of the MCB on the QO Panel, Page 18 of 20:

http://www.global-download.schneider-electric.com/85257689000007EE/all/351DF9D93A8C9966852577F30065B403/$File/powerpact%20q%20-%20class%20-%20734.pdf

 

[JoeStillman]

I think the low current CAN cause the higher incident energy. The available SC current is not high enough to trip the instantaneous element and the time delay is what is killing you.

As a though experiment, you could try setting the transformer impedance to zero and see if that lowers your incident energy. It seems counter-intuitive, but incident energy is governed by TCC curve of the device in question.

Can you set the I_inst lower on the MCB?

 

[PSU Electrical]

I went back into the model and set the transformer impedance at nearly zero (it won't allow a calculation in ETAP with Zero). The Isc value came up near the 8.76kA as it should have and the incident energy level did come down to a Category Zero. If you look at the trip curve for the QO Breaker, a fault with 8.76kA would in fact trip near instantaneously so this does make sense. I think my mind is finally settled. It's just a unique situation if you put into perspective - we have a 480V, 250A I-Line panel with a Cat 0 PPE required and 6 feet to the right of it, we have 208V, 225A QO panel, run of the mill distribution panel that they should not work on hot.

 

[jim dungar]

I think my mind is finally settled. It's just a unique situation if you put into perspective - we have a 480V, 250A I-Line panel with a Cat 0 PPE required and 6 feet to the right of it, we have 208V, 225A QO panel, run of the mill distribution panel that they should not work on hot.

This is an extremely common occurrence.
This is why many, the majority of(?), companies follow the IEEE 1584 whisy-washy comment that says in effect " <240V systems fed by a single transformer <125kVA do not need to be analyzed'. Instead they 'evaluate' these transformer secondaries using the Task Tables and label them as Cat 0 (1.2 cal/cm? max), although more recently I have seen Cat 1 (4.0 cal/cm?max) being used.

Hopefully we will have more guidance from IEEE in its next edition. I am positive the wording will change, I just don't know if it will be more definitive.