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 Post subject: Realistic working distances? (longer than my arm length!)
PostPosted: Tue Nov 21, 2017 4:37 pm 
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I am questioning the IEEE 1584 default working distances. Even 24" seems extreme (my arms right up to the fingertips are 24" long), let alone 36".

18" seems much more realistic for many applications including MV switchgear.

I don't want to just assume the IEEE 1584 default values "just because." If you do use the default values, what are your reasons?

Your thoughts?


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 Post subject: Re: Realistic working distances? (longer than my arm length!
PostPosted: Wed Nov 22, 2017 4:35 pm 
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The working distances are based on the position of the face/chest area relative to the busbars at the back of the enclosure. The accepted standard "working distance" from an ergonomic view point from the hands to the body is 15" and that's the value that OSHA requires for 1910.269 analysis for overhead power lines where the assumption is that this is all rubber glove work in open air. Now if we move the center point up to the point midway between the front of the chest and the face, that adds on the 3" to give us an 18" working distance. Inside gear you've got to consider that the bus bars are all the way in the back but that's not what you work on. For instance with MCC the bus bars are about 1" from the back panel and behind the bucket which needs about another 1" of clearance and metal so the bucket is really only 18" "deep". Taking the centerline or 9" and adding 18" gets us to about 27" but since often equipment is more towards the back, 24" is about the right distance. Similar arguments apply for switchgear where the work is mostly towards the "front" so the working distance gets much larger. At a certain point though, don't overanalyze too deeply because there's a lot of estimating and guestimating going on.


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 Post subject: Re: Realistic working distances? (longer than my arm length!
PostPosted: Mon Nov 27, 2017 8:07 am 
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18" is reasonable for 480V but not 4160V. A 36" or longer hot stick would be used at higher voltages. I have seen 18" at 15kV but only with insulating blankets.


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 Post subject: Re: Realistic working distances? (longer than my arm length!
PostPosted: Thu Nov 30, 2017 6:03 pm 
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engrick wrote:
18" is reasonable for 480V but not 4160V. A 36" or longer hot stick would be used at higher voltages. I have seen 18" at 15kV but only with insulating blankets.


Medium voltage working distance is 24" in many enclosures, not 18". That's assuming rubber glove work method. If you are using a hook stick to pull fuses or something like that then obviously the working distance for that task would be very different. But for instance inside a Class E1 or E2 starter where the door interlock is disabled or some other kind of work is going on, or in some metal enclosed gear, it might be necessary to do a task such as reaching in to place a current clamp which is really only practical way to do this at least for me is to reach in and do it with rubber gloves. Obviously the best way to do a lot of activities is de-energized, but that's not always possible to do. And if you are doing the work directly on an energized line then obviously the work distances shrink to only 15" (by OSHA 1910.269) with rubber glove work method, or 18" if you are following IEEE 1584 and claim that the overhead line is utilization equipment rather than distribution which may be appropriate for some medium voltage equipment.

As an example I've worked with a site where the minimum distance for overhead equipment was 12 feet to ground. NESC allows 8 feet but in a mining area it had to be 12 feet. They pulled and inserted all the cutouts using insulated pole method (hook sticks) and the shortest pole used was 10 feet. So realistically giving the man some working distance on the pole the minimum working distance was 8 feet. All the locations were recalculated with the 8 foot working distance which dropped incident energy down to a small number, under 4 cal/cm2, and pretty much eliminated everything in terms of arc flash PPE except FR shirts and pants, when it came to operating cutouts.

I don't suggest modifying the working distance for "typical" situations but there are many cases where either the nature of the equipment or the nature of certain tasks is such that the default standards in IEEE 1584 just don't make sense. So conservatively changing the working distance to a more appropriate value should be done where it makes sense to do so, but should be done judiciously.


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