I am learning Etap and when I modeled some known equipment the Ka rating for some of the circuit breakers is lower than calculated for the short circuit study. How would you handle under rated PD’s in the arc flash study?

I would not label those instances other than to use a "DANGER" label w/ no other information. Then, an engineering solution must be found to ensure the breakers/equipment in question have an appropriate interrupting rating or appropriate bracing.

Mike

Thank you!

That question can still stir up quite a bit of discussion. Below is a survey that I conducted several years ago on the subject.


Overdutied Equipment and Arc Flash Studies Survey

When devices fail the equipment evaluation the analysis takes on a new level of complexity. You need to verify that it fails beyond a doubt because you're telling the client that they need to spend money for a fix that will only be a problem on the rare occasion of a fault in the system. And if there is a fault it could be could be extremely dangerous.
So you need to exhaust all options in the review with specific attention to NEC 240.86 Series ratings.
And you also need to make sure your calculation is appropriate and uses the correct fault current and X/R ratio for the contribution as well as accurate cable lengths and sizes.
I often say that anyone can buy a power systems analysis package and press the button for an equipment evaluation, but it takes proficiency to interpret the results.
Good luck. It's good work.

Another item to note when encountering overdutied equipment in a study is that this is not only an NEC violation but also an OSHA violation. I think a lot of people are unaware that it is also an OSHA violation.

OSHA 1910.303(b)(4):
Interrupting rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.

One thing to consider when performing the required risk analysis is the likelihood of an occurrence before mitigation can be implemented.

Equipment is considered as being overdutied based on a three phase bolted fault. What is the likelihood that this type of fault occurs in an operating electrical system that has no maintenance or rewiring performed on it? Arcing fault currents are often significantly lower than bolted fault currents, so it may be the equipment is adequately rated for clearing these faults.

Overdutied equipment needs to be mitigated.

Over analyzing the questions. Yes, faults are often less than 3 phase bolted. But you have to design for worse case. Overdutied equipment should be changed, or a method to reduce fault current implemented.