Arc Flash Study Questions
Arc Flash Study Questions
(OP)
I am currently reviewing an arc flash study report for a small plant facility. I observed in the report the following and would like to obtain some guidance and suggestion from this forum:
The low voltage incident arc flash calculations part of the report did not consider 85%Ia as specified in IEEE 1584. If this 85%Ia is considered, it will increase the arc flash especially if the resulting 85%Ia falls under the long time TCC region of the thermal-mag MCCBs on the MCC. Almost all arc flash reports I reviewed used 85%Ia except for this particular report. However, is it required to consider 85%Ia as IEEE 1584 did not state any conditions in applying such so I see it as a compulsory requirement?
The LV load at the MCCs (600V) in the ETAP model was modeled as one single lumped load. I have a concern of the arc flash magnitude at the downstream 600V distribution panelboards supplied from the MCC as the feeder cable impedance supplying the panelboards will decrease the calculated arc flash at the line side of the panelboard MCCB. No generalization was made. Is this something I should enforce to get updated as soon as the cable schedule is available?
Thanks and more power to those who participate this forum.
The low voltage incident arc flash calculations part of the report did not consider 85%Ia as specified in IEEE 1584. If this 85%Ia is considered, it will increase the arc flash especially if the resulting 85%Ia falls under the long time TCC region of the thermal-mag MCCBs on the MCC. Almost all arc flash reports I reviewed used 85%Ia except for this particular report. However, is it required to consider 85%Ia as IEEE 1584 did not state any conditions in applying such so I see it as a compulsory requirement?
The LV load at the MCCs (600V) in the ETAP model was modeled as one single lumped load. I have a concern of the arc flash magnitude at the downstream 600V distribution panelboards supplied from the MCC as the feeder cable impedance supplying the panelboards will decrease the calculated arc flash at the line side of the panelboard MCCB. No generalization was made. Is this something I should enforce to get updated as soon as the cable schedule is available?
Thanks and more power to those who participate this forum.






RE: Arc Flash Study Questions
RE: Arc Flash Study Questions
Not sure I understand your second question. Motor loads can be grouped. We generally group 50 hp and above and less than 50 hp in separate groups to comply with ANSI C37 guidelines for breaker duty calculations. But per IEEE 1584, motors below 50 hp do not even need to be modeled.
RE: Arc Flash Study Questions
dpc, I was wondering as to 85% Ia basis of IEEE 1584 came from. It is mentioned in the standard but I would like to understand the rationale behind it. I was thinking L-L fault generalization but could be wrong.
RE: Arc Flash Study Questions
RE: Arc Flash Study Questions
As the voltage increases, the arc resistance is less dominant in the circuit and therefore the variability is less. So above 1000 V, there is no requirement (yet) for a second calculation. Most software programs do this second calculation automatically, so I'm a little surprised it was not done for your study.