As I visit more and more facilities across North America I continue to run into many of the same flaws in arc flash studies that have been completed across those facilities. Now I'm usually not given a copy of the model and reviewing the workmanship of another engineer, what I'm doing is walking the plant and picking up on things that were done incorrectly during the data collection or implementation phase.
In this article, I will review the three most common errors I come across and look at ways that you can make sure this doesn't happen on your arc flash model.
#1. Inconsistent incident energy levels
Let's say that you have a group of 600-volt motors ranging from 30 to 200 horsepower, all within the same motor control center (MCC). The MCC is fed from a single upstream feeder breaker, which means that each of the individual MCC starter buckets has an identical incoming energy source. When you are calculating the arc flash level (incident energy) you are trying to determine how much heat energy is available at each of the MCC buckets. In this case, they should all be the same.
The error that I've noticed is that once the labels have been applied the incident energy levels are not the same. This is a red flag and it's something that you should look for. Another example would be a group of disconnect switched all fed from the same splitter panel. Again each of the disconnects is fed from the same energy source, therefore should have identical incident energy levels.
#2. Only one label on transformers
If you are following the standard for arc flash and shock hazard warning labels than you should have both the incident energy level and the highest voltage present on the label. In the case of a transformer though, you probably should have two labels, one for the primary side and one for the secondary side. The reason for this is because typically the incident energy level is higher on the secondary side of a transformer than on the primary side however the shock hazard level is the opposite. So without two labels, you could be misled into either thinking you are faced with a lower incident energy level or a lower shock hazard than in reality.
#3. Still using categories
The arc flash categories (CAT 1,2,3 &4) were originally designed to determine what PPE to wear when you did not have the incident energy calculated. For some reason (possibly because this was built into many of the software packages for calculating arc flash) the incident energy has been reverse engineered into a category. While some argue it makes things simpler I disagree. If you know what the incident energy is then why would you want to make the number less precise?
Let's imagine we calculate the incident energy to be 8.3 cal/cm2 on a particular starter panel. If I was wearing arc flash protective clothing rated 12 cal/cm2 then I would be fine, but if the engineer decided to add a category to this panel it would show up as a category 3 which requires a minimum 25 cal/cm2. Now I'm suddenly uncomfortable. Sure, I'm probably more protected, but then you get into issues of comfort and enabling vs non-enabling behaviors. The bottom line is that I want to know exactly what the incident energy is, so once you have it calculated then leave it alone.
Arc flash studies are not something that you can take lightly and they need to be precise. Any time you notice something that just doesn't make sense then go back to the engineer and get it reviewed. Chances are that something has been overlooked.
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If you need to update your arc flash and electrical safety training why don't you change it up a bit and try out our scenario-based electrical safety simulator? Have fun!