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Arc Flash and Limited Approach Boundaries

While arc flash PPE and warning labels get much of the attention in electrical safety training and in electrical safety programs, it is vital to emphasize another aspect of electrical risk assessments—the arc flash boundary and the limited approach boundary.

What is an arc flash boundary?

The arc flash boundary is defined by the NFPA 70E as, “When an arc flash hazard exists, an approach limit from an arc source at which incident energy equals 1.2 cal/cm2 (5 J/cm2).”  According to the Stoll skin burn injury model, this is where a worker could potentially receive a second-degree burn if exposed for one second.

What is a limited approach boundary?

The limited approach boundary is defined by the NFPA 70E as, “An approach limit at a distance from an exposed energized electrical conductor or circuit part within which a shock hazard exists.”  We often call this the “common sense boundary” meaning that no worker should be within a certain limit when there is exposed electrical energy.

Now, how do these two boundaries fit into your electrical safety program?

Let’s look at a couple of examples:

  1. Let’s say that a qualified electrical worker is removing a panel cover from a panel-board (as pictured below) to install a new circuit breaker; however, this panel is in a hallway where there is a likelihood that a student, unqualified worker, or the general public may walk by at any time. What is the proper procedure?
    Electrical box After performing a risk assessment, it is determined that the ideal procedure would be to wait until after hours to perform the work to limit risk; however, if this is a 24-hour facility, it may be best to have an attendant stand guard and to have an electrical specific barricade set up to inform all of the risk.  Electrical specific barricades are a great way to ensure that others understand that this is simply caution tape or a general barricade, this is an electrical hazard that warrants increased attention.


electrical hazard sign

  1. Now let’s take another example to calculate where the boundaries should be placed (or where the attendant should stand). If we look at the label (as pictured below), we determine that the limited approach boundary is 42 inches and the arc flash boundary is 50 inches.  Where should the arc flash boundary be placed?  (The answer is whichever is furthest away!)


warning qualified persons only label

  1. One final consideration, a significant amount of electrical equipment is located in isolated electrical rooms. In these rooms, it is generally best practice to ensure that no unqualified workers will enter the room while the work is being performed.  If the room is in a highly isolated area, a “buddy system” is best practice to ensure that if one worker is “hung up” on the circuit while testing & troubleshooting, the other worker can assist in removing the worker from the circuit with an insulated shepherd’s hook.

How can you ensure safety?

Limiting access to areas where electrical work is being performed is absolutely vital in any facility.  Make sure that your workers have been trained and understand how and when to install barricades for an arc flash boundary, or place an attendant to ensure that your unqualified workers and the general public stay safe!