100% vs. 85% vs ??

I hate to start a new thread but I did not see this directly discussed. Doing arc flash calculations I have a main 2000 amp switchboard that is a HRC 1 at both 100% and 85% arcing current. My question is why stop at 85%. If the actual fault level is just a little lower, causing the arcing current to be around 80%, it knocks me out of the instantaneous portion of the breaker curve and quickly changes the HRC to "extreme" instead of HRC 1. So if I am to assume some variable can easily cause this situation to happen, which seems vary plausible, then what do I do? To make a very general statement, it seems that almost every piece of equipment should be HRC extreme as you just have to determine what variables will cause your fault current to occur just before the instantaneous portion of the breaker curve.

I always plan on the available bolted fault current being lower than any value I have been given.

I select multiple fault current source values, (i.e. high, middle and low utility values, and all motors off) as well as letting the software compare the 100% and 85% results.

Multiple Scenarios

In SKM and other programs, they call these Operating Scenarios. Most programs will allow you to run different scenarios and then produce labels that have the maximum arc flash energy.

As JDB pointed out, you need to calculate the energy level at 100% and 85% Iarc current for different operating conditions of your facility. Be aware that it may be very difficult trying to get the minimum value of utility fault current. They (utility) usually will provide you with the maximum but not the minimum. Furthermore, verify that the maximum fault current they provide you is for the transformer that is actually installed, not the maximum fault current if they install in the largest transformer in their inventory. As you can see from your calculations, it makes a big difference.

Arc Tolerance

I use SKM and have similar situations. Using the default options (85%), most of AF amps were cleared by the instantaneous settings and yielded Cat 0 values. However, when viewed as TCCs (Time Current Curves) many AF amps were very close to the instantaneous.

In SKM, Arc Flash Evaluation, Options, Arcing Tolerances, I changed -15% to -20% or 25% and re-ran the study. The new values were either Cat 2 or Cat 3.

I created a Custom Label that has both the required calculated Incident Energy value (using -15% Arcing Tolerance) and the requirement to use a higher PPE. I also modified the "Flash Hazard Boundary" similarly.

You can put anything you want on the label, as long as it includes the basic requirements.

One must realize that there is an infinite number of AF energy calculations that you can perform. If you change arcing fault current or the working distance, you'll get different results. You have to settle on one method or situation and stick to it. That is why we use the defaults set in SKM when we run our studies. These appear to be the "generally accepted" by the majority of consultants and protection engineers. The one area that we deviate on is selecting the upstream mis coordination option. See the attached screen shot.

Our company has their own standards which have been vetted by our legal department. We felt it would be better, in a court of law, if we could defend our choices.

We do use a 2 sec cut-off, and <=240V, but our cleared fault threshold is greater than 80%.