Approach Boundaries can be one of the most difficult topics for me to explain in my electrical safety classes. It’s not that learning approach boundaries is above anyone’s head, but more to do with the fact that there are three different approach boundaries. People want to understand why. When I first introduce the concept, I often get these questions:
I understand the confusion. In fact, four approach boundaries were taught when I first learned about approach boundaries, until the Prohibited Approach Boundary was deleted in 2015. The complication arises from the fact that we are trying to protect ourselves from not just one, but two potential hazards—shock and arc flash. As discussed in earlier articles, shock and arc flash are different events with different injuries and different ways of causing death. Therefore, the protective equipment is different for each event.
As a result, it would be a mistake to create a blanket system that requires us to wear a full arc flash suit every time we approach electrical equipment. Afterall, the risk for shock and the risk for arc flash are not the same. A panel or task that has a high shock risk might have a low arc flash risk and vice versa.
I’ve said many times that it’s complicated to explain, but it doesn’t have to be complicated to implement. Let’s keep it simple. Simple is good for many reasons. First, it is easier to follow. That is obvious. More importantly, because it is easy to follow, it is more likely to be followed.
Take note that the Restricted Approach Boundary and Limited Approach Boundary are shock boundaries and have nothing to do with arc flash. Since these boundaries are for shock, they are determined solely on voltage which makes them very straightforward. If you have had an arc flash risk assessment performed, these approach boundaries are on your equipment labels. If not, you can refer to NFPA 70E 2018. They can be found in Table 130.4(D)(a) for AC and 130.4(D)(b) for DC circuits.
The Restricted Approach Boundary is the distance from an exposed energized electrical conductor or circuit part within which there is an increased likelihood of electric shock due to electrical arc-over combined with inadvertent movement. This boundary is only to be crossed by qualified workers and the worker must be protected from unexpected contact. This protection is usually in the form of voltage-rated gloves with leather protectors and/or insulated tools. For 50V to 150V, the restricted approach boundary is to avoid contact with the circuit parts. For 151V to 750V, it is 12 inches.
The Limited Approach Boundary is the distance from an exposed energized electrical conductor or circuit part within which a shock hazard exists. Again, this boundary is for shock and is not related to arc flash risk. It represents the approach limit for qualified employees which is 3’ 6” for voltage up 750V.
Let’s assume that you are a qualified worker and are tasked with taking a voltage reading on a 480V disconnect. The limited approach boundary will be 3’6”, which means that only you, the qualified worker doing the work, can enter. The restricted approach boundary would be 12 inches. That 12 inches would be your trigger point for shock PPE such as voltage-rated gloves and/or insulated tools.
So far so good. That’s not too complicated, right?
Now let’s introduce the Arc Flash Boundary. This is where it can get complicated. The arc flash boundary is an approach limit from an arc source at which incident energy is high enough to cause a second-degree burn. Unlike limited and restricted boundaries, which are dependent on voltage, the arc flash boundary is dependent on the incident energy or calorie rating at the equipment. Since the calorie ratings of equipment are not nearly as uniform as the voltage ratings, the arc flash boundary has a much wider range of possibilities.
With shock boundaries, we know that the restricted is always closer than the limited. The arc flash boundary, however, may be greater than, equal to, or less than either of the restricted or limited approach boundaries. To make things more complicated, since we are protecting ourselves from a different hazard, the PPE requirements also differ.
Arc Flash Boundary
• Varies based on incident energy in calories/cm2
• May be greater than, equal to, or less than restricted approach boundary
• May be greater than, equal to, or less than limited approach boundary
• Qualified Workers only
Like the Limited Approach Boundary, only qualified personnel are allowed. Once the qualified person enters the arc flash boundary, they must have arc-rated clothing rated up to or greater than the incident energy of the equipment for all body parts that are within the boundary, including face, neck, and head.
The best advice to keeping the concept of electrical approach boundaries simple is to remember these three things:
Please reach out to me with any questions or comments on this article or any of the other educational resources we have at www.mitchellandlindsey.com/resources
Thank you and be safe!
