Question: Arc flash labels often list incident energy with one or two decimal places. How many are necessary?

Answer: From a mathematical standpoint, you can display as many decimal places as you wish but let’s avoid competing with the number of decimals are in Pi.

What is practical? It is important to remember that IEEE 1584 describes incident energy as an estimate. This is due to many factors that influence the actual exposure—such as the exact point of the arc within the equipment, the exact electrode gap at that moment which can vary as conductors melt or be blow apart as well as many other system-specific conditions.

In practice, most labels use anywhere from one to two decimal places. It is important to keep in mind that adding additional digits gives a false impression of increased accuracy, even though the underlying calculation remains an approximation

This may be blasphemy and self-defeating since most of what I do for a living are power studies, driven by arc flash studies and the need for labeling, but I think it is really a rough estimate at best in many cases (see my video on industrial control panels and my video on pad mounted transformers for two examples.). There are just too many variables to be able to accurately calculate IE at the working distance for every installation ever conceived. I don't blame the IEEE. It is obvious a lot of work has been put into testing and developing the formulas. What I have seen of the test setups though, they are necessarily generic. There is no way to duplicate every type of equipment with every type of method to initiate a fault with every range of available fault currents and every point within a enclosure and every bus gap etc. It is impossible.

I have also been privy to dozens of studies performed by dozens of other engineers and all of them had problems to say the least. Not saying mine are perfect, or done "the right way" every time without mistakes. I am just pointing out there is a lot of potential for errors when doing these studies. There needs to be a better way.

I think the industry should push more towards making safer equipment and affordable retrofits for mitigating risks to increase safety as well as stricter codes. Of course, I know that cost is a major factor there that cannot be ignored. I know for instance, there is fully insulated MV gear (PremSet) that looks to me to have a very low potential for shock and arc flash hazards, at least compared to uninsulated gear. That technology doesn't seem to be widely adopted or in use though.

Not really sure what the answer is. Would like to hear others opinions.

Thanks for your thoughts! Opinions are usually all over the place.

I tend to favor one decimal place. PPE tends to have zero decimals but if the calculation uses zero places, was the calculation rounded up or rounded down? Also 1.2 cal/cm^2 is often cited by standards and has one decimal place so do what do you do if you round 1.45 down to 1.0.
Quite a bit to consider.

My choice would be 0 decimal places with all numbers rounded up to the next whole number.
I am amazed at the people that will want 39.99 shown on the label because it is less than 40.

Other than the old 1.2 cal rating nothing uses any decimals so do we need to keep doing thing the way we did before we knew better.
Training is supposed to be updated regularly, so doing the things the way grandpa taught us doesn't continue to make sense.

This forum needs a "Like" button.