We sometimes have a need to calculate the flash protection boundary for a location with a known incident energy. (I can get into the "why" if needed)

So for example, we would have the following data:

Incident energy
Voltage
Equipment type
Working distance that the known incident energy was calculated based on
(Fault clearing time unavailable)

And we need to calculate the flash protection boundary from the above data.

We have been using an equation derived from IEEE 1584 equations to help us reverse calculate the FPB when the incident energy is already known.

I am attaching the equation and how we derived it below. Has anyone used a similar method? Any feedback?

Thanks.

Not following you here, how can you have Ei without knowing the clearing time?

FCT was known at some point of course to be able to calculate Incident Energy but I don't have it for this calculation.

Sounds like a homework question?

I don't see anything wrong with your approach. If I follow it correctly, D would be the working distance at which E was calculated.

Have you tried using it on a case where you know the fault clearing time? Calculate both incident energy and the boundary and then use your derived equation to see if it holds.

We do this sometimes, so I have no problem with the concept.

Exactly right Jghrist.




Yes, I have tried this and the derived equation holds.

FCT Calculated Both Ways

I attached a side by side calculation of the flash protection boundary using both methods.

Reverse calculating the FCT may give you a point on the TCC, but it won't give you anything else. If you need another time, because you have another fault current flowing, then you have almost nothing to base yourself on (you only know the TCC may have a decrease in time for an increase in current).

I wouldn't want to trust such a calculation, knowing the original person didn't document it properly. Too much at stake.

The OP is not reverse engineering the FCT. They said, given a specific incident energy, they are trying to determine a boundary (distance).

Right, I read a bit too fast the OP.

Doing what's in the 2 pdf, you still assume at least the X factor. Look at the thread about a new configuration for a padmount transformer by Jim Phillips to see that you cannot always assume that the X value is chosen as per the IEEE 1584 table.

Having two unkowns (t and X) makes me feel uncomfortable with any further processing of those values, unless a good sensitivity assessment is made.

I saw a label on a production equipment just yesterday where some info was obviously wrong: Ie = 0.14 cal/cm^2 at 18", HRC 0, AFPB = 43". There's something obviously wrong here. This kind of label makes me wary of assumptions from the results of a calculation, especially if part of the documentation can't be found.

If the X factor cannot be taken from the IEEE table it is not worth making any calculations at all. If the arcing gap corresponds to a table value, then so will the X factor. The problem with the padmount transformer, in the discussion you reference, is the gap is outside of the Table 4 ranges.


Could it be possible the company's policy is to establish a uniform minimum AFB in which case the label would be correct (although I would have thought they would use the 48" distance from NFPA70E)? I know of at least two paper companies that do this.

This is the OP.

I am 100% of the distance exponent. It is from Table 4.

I don't think so. I'm going back to the same plant on Friday, I'll check other panels. If I cannot find anything satisfying, I'll give a call to the company which did the study (the name is on the label).

Yeah something is obviously messed up here.

0.14 calories @ 18" and 1.2 calories @ 43". I would have expected to see it closer to 5" than 43".