×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Arc Flash Study Questions

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.

RE: Arc Flash Study Questions

I usually check the incident energy (IE) with 85%Ia and 70%Ia and go with the higher one. The panelboard would likely have a different IE value than the MCC especially if the panelboard is served by a protective device inside the MCC or if the panelboard is a lower voltage and a transformer is involved.

RE: Arc Flash Study Questions

IEEE 1584 requires the 85% calculation at 1000 V and below. There are no exceptions to this.

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

(OP)
Thanks dpc and wroggent for the prompt response. Luckily, we do not have any 208V panelboards supplied from a 125 kVA or greater transformer.

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

The arcing current is not constant. The incident energy is proportional to i^2*t. A lower arcing current may result in a longer tripping time which could result in more energy being released. Why 85% was chosen as a test point and not 75% or 83.480749% I have no idea. Probably comes from lab tests.

RE: Arc Flash Study Questions

The 85% calculation was added because the test data show a fairly wide range of arcing current for the exact same test configuration, due to the variability of the arc. The resistance in the arc is a major factor in the current magnitude at lower voltages. They are just trying capture more of the data points.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources