My article in November’s SAFETY LEADER took an in-depth look at the limited approach boundary (LAB) and the restricted approach boundary (RAB) as defined in NFPA 70E, Standard for Electrical Safety in the Workplace. The arc flash boundary still needs to be discussed. So, let’s go down the rabbit hole on this often-misunderstood cornerstone of how NFPA 70E protects workers from the dangers of working around electrical equipment.

What should I do?

Article 100 defines the arc flash boundary as “an approach limit from an arc source at which incident energy equals 1.2 cal/cm² (5 J/cm²).” However, there is a qualifier at the beginning of the definition that states that this boundary only exists when an arc flash hazard does. This is like the LAB and RAB only being present when exposed circuit parts are involved. Since an arc flash can happen with doors closed, there is a big difference between exposed circuit parts and an arc flash hazard.

To make this determination, there is Table 130.5(C) NFPA 70E, which has evolved over the years from what used to be know as the arc flash PPE “yes/no” table. This table was the first step in the PPE category method for determining if a task required arc-rated PPE. The rest of the PPE category method assigned a PPE category to electrical equipment operating within certain parameters. You could look up the task in the “yes/no” table, locate the equipment you were using in the PPE category tables, determine if you needed arc-rated PPE and what category was required. However, now this table exists outside of the PPE category method and has been renamed “Likelihood of Occurrence of an Arc-Flash.”

The table still contains a range of common tasks, and then lists the likelihood of an arc flash as either “yes” or “no” based on the equipment’s operating condition. But the table doesn’t account for how much energy is available in the system. 

To determine if an arc flash hazard exists, users must consult the incident energy analysis or the PPE category method. If the table lists the likelihood of occurrence as “yes” and the incident energy is high enough, then an arc flash hazard exists, and we can move on to determining the arc flash boundary.

The magic number

First, let’s examine the shock approach boundaries. So, what is this magical number of incident energy we can use to determine if an arc flash hazard exists? Well, it is right there in the definition of the arc flash boundary: 1.2 cal/cm². After all, this is the incident energy level at which the NFPA 70E committee decided to set the arc flash boundary.

This number is important because, according to the Stoll Curve, it represents the value where we see the onset of a second-degree burn on unprotected skin. Therefore, if NFPA 70E were to set a distance from an arc flash where someone not wearing arc-rated PPE would face minimal injury, 1.2 cal/cm² or less seems like a solid choice.

If the table indicates the task could cause an arc flash and the incident energy is greater than 1.2 cal/cm² at the working distance, then the arc flash boundary is in play. Anyone within the arc flash boundary must be briefed on what hazards are present and wear the appropriately ­rated arc flash PPE based on incident energy or PPE category. 

The tricky part about this boundary is that it is a moving target. Identical motor control centers installed in several different locations throughout a plant will likely have a different arc flash boundary at each location. Several factors lead to this, but it is mainly because the available fault current is going to be different unless the installation is identical upstream.

It is easy to get the arc flash boundary confused with the two shock approach boundaries, but it is imperative to keep these concepts separate. Whichever boundary is the largest is where the minimum distance for barricades will be. Getting it right is important when it comes to protecting everyone when these hazards are present. 

The good news is that performing arc flash studies in industrial facilities is more common than it was even just 10–15 years ago, and the label on the equipment will provide the boundary. All you need to know is whether the task is likely to initiate an arc flash in the first place, which is a topic for another day.

Until next time, stay safe and remember to always test before you touch!

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