Question about working distances

[EmpireWind]

Hi all,
Seeking some guidance regarding working spaces, with regards to depth.
If the depth calculated by table 110.26(A)(1) is less than the arc flash boundary (which can be considerably greater), that what distance takes precedence?
Is there any guidance on this?
Thanks,
FG.

 

[jim dungar]

These are two independent distances.

Working clearance primarily comes into play during installation as it involves the physical location.

Arc flash boundaries,for PPE selection, come into play when an arcing event is possible after the equipment has been installed and energized.

 

[EmpireWind]

Agreed. But if you have a design situation where there is a wall or another panel (say panel B) behind the panel (panel A) that is going to be installed and if you do an arc-flash calculation which shows that the arc flash boundary is greater than the working distance depth allowed per 110.26(A)(1) , which one do you follow?
If working distance per table say is 3 feet and arc flash boundary is 10 feet, do you place panel B at 10 feet depth or stick with the 3 feet? What is the guidance there?

 

[kwired]

Agreed. But if you have a design situation where there is a wall or another panel (say panel B) behind the panel (panel A) that is going to be installed and if you do an arc-flash calculation which shows that the arc flash boundary is greater than the working distance depth allowed per 110.26(A)(1) , which one do you follow?
If working distance per table say is 3 feet and arc flash boundary is 10 feet, do you place panel B at 10 feet depth or stick with the 3 feet? What is the guidance there?

You always need the working clearance, the arc flash boundary is used to tell you what PPE you need if you are going to be within that boundary while energized, whether you are working on that equipment or not.

 

[jim dungar]

Agreed. But if you have a design situation where there is a wall or another panel (say panel B) behind the panel (panel A) that is going to be installed and if you do an arc-flash calculation which shows that the arc flash boundary is greater than the working distance depth allowed per 110.26(A)(1) , which one do you follow?
If working distance per table say is 3 feet and arc flash boundary is 10 feet, do you place panel B at 10 feet depth or stick with the 3 feet? What is the guidance there?

For installation follow NEC 110.26 for working clearance requirements. Ignore any arc flash boundaries.

For work on previously installed energized equipment wear the appropriate PPE based on the distance you are from the equipment.

 

[EmpireWind]

So when one is designing an electrical room layout, your are saying that use NEC 110.26 dimensions for depth between equipment and ignore the arc flash boundary, but in the event of an arc flash blast, the person will be pushed into the panel behind if it is within the arc flash boundary. So how do you account for that?

 

[jim dungar]

...but in the event of an arc flash blast, the person will be pushed into the panel behind if it is within the arc flash boundary. So how do you account for that?

How do you know that will happen?
There are no industry accepted methods for determining the amount of 'blast force' associated with an arc flash event.

Arc flash boundaries are based on thermal effects of an event. It is very common to find low level faults that exist for a relatively long time resulting in large amounts of arc flash incident energy and corresponding large boundary distances. But it is also possible to have very large arc flash events with small boundaries due to the fast operation of protective devices.

 

[EmpireWind]

Thank you for the clarification. This is what I suspected but needed clarification due to different understanding of how the two are applied within the design team.