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 Post subject: AF boundary Marking
PostPosted: Fri Sep 16, 2016 4:59 am 

Joined: Thu Jan 16, 2014 10:02 pm
Posts: 16
Location: pune
Hi,
As per NFPA 70E, we have 3 boundaries to mark(AFB, limited and restricted). It is easy to mark and follow only one boundary which is greater than two. Sometimes AFB is more than limited approach boundary, so we marked the AFB and follow that.

But sometimes AFB is very less for lighting boards and power distribution boards. Also it is not feasible to mark limited boundary as it is very big for small board placed in normal working area and not even AFB because it is too small to mark.

I need some suggestions on it, that whether to mark boundary to LDB & PDB's. If yes, which boundary need to mark?


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 Post subject: Re: AF boundary Marking
PostPosted: Fri Sep 16, 2016 5:53 am 
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yograj.s wrote:
Hi,
As per NFPA 70E, we have 3 boundaries to mark(AFB, limited and restricted). It is easy to mark and follow only one boundary which is greater than two. Sometimes AFB is more than limited approach boundary, so we marked the AFB and follow that.

But sometimes AFB is very less for lighting boards and power distribution boards. Also it is not feasible to mark limited boundary as it is very big for small board placed in normal working area and not even AFB because it is too small to mark.

I need some suggestions on it, that whether to mark boundary to LDB & PDB's. If yes, which boundary need to mark?


You need to mark both because depending on the task, one or both boundaries may disappear. For instance if you have everything covered up there might be no shock hazard but the arc flash hazard.

In some cases the arc flash boundary becomes less than the working distance in which case marking it is somewhat silly to do and technically at that point I just put on a warning, not a value. Limited approach boundaries are for protection of non-electrical workers and stay roughly the same for most equipment with the exception of the higher medium voltages. 70E says to mark them but plant policies often vary by for instance putting it in some kind of procedure. It's totally unnecessary in distribution work in the U.S. that falls under 1910.269 obviously because under that regulation effectively there are no unqualified employees.


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 Post subject: Re: AF boundary Marking
PostPosted: Mon Sep 19, 2016 6:33 am 
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yograj.s wrote:
As per NFPA 70E, we have 3 boundaries to mark(AFB, limited and restricted).


If you are referring to marking on a label, the Limited and Restricted Approach Boundaries are not required to be on a label according to NFPA 70E - However, the Arc Flash Boundary is.

Still, many chose to list all three on the label since they are all three required to be used depending on the task as Paul pointed out.


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 Post subject: Re: AF boundary Marking
PostPosted: Mon Sep 19, 2016 4:21 pm 
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Joined: Tue Jul 01, 2014 8:40 am
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Hi All

I agree with Jim, if you are referring to the warning label, the limited or restricted approach boundary is not required. However the arc flash boundary is. If you are referring to marking on the floor of your faciity, I am not sure why you would place markings on the floor?

Just my thought here as the nominal system voltage is required on the label according to NFPA 70E and CSA Z462. To make it easier for the workers to understand, I simply refer to the limited approach boundary as an "electrical safety zone" when I am teaching. The person who is in charge of that zone or boundary is the electrical worker. This protects the electrical worker from other workers entering or coming up close to the electrical worker and distracting. As already mentioned it also protects the other workers from potential injury. As we are aware there are specific distances for the shock boundaries indicated in 70E and Z462, however due to poor design or layout of machines and panels in a shop, one may not be able to accommodate the set distance so I inform them to do the best they can to at least establish this electrical safety zone


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 Post subject: Re: AF boundary Marking
PostPosted: Tue Sep 20, 2016 3:54 pm 
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Keep in mind also that the boundaries are based on the conductors themselves, NOT the enclosure. So for instance the 18" working distance for an MCC is based on busbars located at the back about 1" off from the back panel. With a typical depth of around 20", that means that the working distance (for arc flash purposes) is actually right at about the plane of the door, NOT outside the panel at all. When it comes to shock, similar things apply. For instance in the same panel typically the 480 VAC leads are all packed in towards the back with the control wiring located closer to the front of the panel. IF anything is exposed at all, then the restricted approach boundary (12") again is located totally within the panel so rubber gloves or insulated tools only become necessary when poking around inside the panel.

Also there is some sanity that needs to be applied to the boundaries, especially the limited approach boundary and the arc flash boundary. Let's suppose for a minute that we have a 25 foot arc flash boundary and a 10'6" limited approach boundary. Let's suppose that the panel is located inside a cinder block room with doors on both ends and in the center with 5 feet from the center door to the panel. In this case with the door closed even though the room is technically not a "panel" if the doors are closed, there are no exposed conductors outside the room. If non-electrical personnel open the door, it does become exposed so we probably want to either lock the door or put barricade tape across the doorway to keep them out. There is probably a narrow "corridor" outside the door 5'6" long but realistically barricading the door is good enough. Similarly if you are standing outside the door and the door is closed, thermal radiation is not going to pass through a solid wall nor a solid door so even though it is only 5 feet away, the arc flash boundary does not extend another 20 feet through solid steel and concrete.

But to avoid confusion, mark the panel as the Code is written with the boundaries. Understanding how far those boundaries extend in non-wide open space situations should be a part of the annual electrical safety training along with undestanding when those boundaries actually exist (not exposed=no shock hazard boundaries; not interacting with equipment in a way that has the potential to cause an arcing fault = no arc flash boundary).


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 Post subject: Re: AF boundary Marking
PostPosted: Wed Sep 28, 2016 1:52 am 

Joined: Thu Jan 16, 2014 10:02 pm
Posts: 16
Location: pune
Thank you for your help to understand the boundary marking clearly.


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