If you work with electricity then you are probably well aware of the hazards (arc flash and shock) but something you might not be so comfortable with is the safe limits of approach and what is referred to as the working distance. In my experience, these distances seem to back seat to the incident energy level of an arc flash or the voltage level of a potential shock hazard but they are critical in determining one's safety. In fact, if they are ignored or misinterpreted then the risk of injury is definitely going to go up.
In this article, we will go over what each of these distances actual mean and why they are important to you or anyone who is working on or near exposed energized electrical parts.
Where can I find these numbers?
Most industrial facilities have gotten around to completing an arc flash incident energy analysis and posted labels (or stickers) on all of the electrical equipment. If the engineer followed the recommendations in CSAZ462 workplace electrical safety then each label should have listed the working distance, arc flash boundary, limited approach boundary, and the restricted approach boundary.
The working distance is in reference to the arc flash incident energy and it is actually the distance that you are assumed to be standing at while working. You see, distance is a key component to calculating an arc flash, so the engineer needs to make an educated guess as to where someone might be when an arc flash happens. Since most arc flashes occur when someone is performing work on a prospective piece of equipment it only makes sense to calculate the arc flash at that assumed working distance.
The important thing to remember is that if you were working on a piece of equipment and you were closer than the assumed working distance you would be exposed to a greater arc flash incident energy level than what is posted on the label. Conversely, if you were able to take a few steps back from the equipment and continue your work (maybe with a broomstick... just kidding) then you would be exposed to an arc flash of much less magnitude.
Arc flash boundary
The arc flash boundary is the distance away from an arc flash where someone would be able to receive a second-degree burn. Sounds crazy but that's what it is. They used to call it the arc flash protection boundary until someone pointed out that it did not protect anyone from anything (well maybe third-degree burns).
There are two important takeaways from this. One, set up barricades to keep bystanders outside of the arc flash boundary. Two, add some distance to whatever the label says because I wouldn't want anyone standing where they can get a second-degree burn.
Restricted approach boundary
The restricted approach boundary is the furthest distance away from exposed energized conductors that an electrician can come without wearing rubber insulated gloves. Well, the boundary is imaginary so there really is nothing stopping them from crossing it without gloves but it's definitely not recommended.
Air acts as an insulator, so that's why you don't seem lighting bolts shooting out of the power lines while you drive down the street. But you don't necessarily need to come in contact with those power lines for it to arc across and give you a shock, you just need to come within a certain distance. This distance is the restricted approach boundary (with a little bit of a safety factor built in I'm sure).
Limited approach boundary
The limited approach boundary is similar to the arc flash boundary. If you have no business being anywhere near exposed energized parts then stay out of this area.
Really all an electrician needs to remember is every time you are exposing energized parts you should be setting up barricades to keep unwanted people away. I mean why wouldn't you want to do this anyway, it'd be great, no supervisor breathing down your neck, and no operations team asking you every two minutes if you are done yet! Just put up the barricades and be done with it.
What boundary do I use for barricades?
I know I said you need barricades for the arc flash boundary as well as the limited approach so what do you do? It's pretty straight forward. Just choose the greater of the two and I always add a couple feet for good measure.
Why do my labels have the prohibited approach boundary?
You have old labels. In the latest version of the standards (CSAZ462 or NFPA70E) they dropped the use of prohibited approach boundary. This was mainly due to the fact that it made no difference to the end user. Electricians didn't have a use for the number, it didn't change the PPE they wore or how and what tools they used. So just take a black magic marker and scratch them all out if you want.
Wrapping things up
Hopefully, now you can see the importance of understanding the different boundaries and limits of approach provided on your arc flash labels. They are much more important than most people give them credit for and you are going to want to make sure anyone who works with electricity at your workplace is familiar with their use and understands the differences between them.
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