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Standardized Working Distance
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Author:  mountaineer [ Tue Jul 20, 2010 12:17 pm ]
Post subject:  Standardized Working Distance
I read an article recently that recommended standardizing on an 18 inch working distance for all equipment. The reason being the combination of 18, 24 and 36 inch distances would be confusing to the users and there will always be at least one task that will require an 18 inch working distance. This would also provide the worst case exposure and labeling for worst case has been recommended in several posts on the forum.

Is this typically done?
Author:  Jim Phillips (brainfiller) [ Wed Jul 21, 2010 4:18 am ]
Post subject: 
I have written several papers about standardizing the Arc Flash Protection Boundary to simplify things but standardizing the working distance is an interesting concept. The 24 inches and 36 inch working distances from IEEE 1584 are based on the bus (assumed location of the flash) would be in the rear of low voltage and medium voltage equipment (i.e. switchgear) and unless you stick your head inside the breaker cell it would be difficult to get closer. It is assuming you are in front of the cell operating or interacting with the equipment.

Standardizing / keeping it simple is always a good option but in this case it might backfire. I was working with a client recently that performed a study using the default data from one of the major arc flash programs and everything defaulted to 18 inches.

Although this is a conservative approach, they had many areas that were category 3 and 4 requiring heavier protection (in a very hot part of the United States). Much of the higher category locations included switchgear and when the working distance was changed to the higher IEEE 1584 working distances based on the type of equipment, the categories became lower.

Being conservative is good and simplifying is good but this is one case where simplifying with the three distances might be the better option.

Let me know if you have any other questions.
Author:  MIEngineer [ Wed Jul 21, 2010 5:50 am ]
Post subject: 
So is this to say that when using 18" say the HRC=4. If using a larger distance the worker could get a way with wearing only HRC=2.

Would something need to be done for the worker's arms, the insulating gloves we use do not go up to the elbow, which is about 18" on my test subject (myself). The torso will be able to be >18" away from the exposed part but not the upper arm.

Thanks for the continued great information.
Author:  Zog [ Wed Jul 21, 2010 7:02 am ]
Post subject: 
MIEngineer wrote:
So is this to say that when using 18" say the HRC=4. If using a larger distance the worker could get a way with wearing only HRC=2.

Would something need to be done for the worker's arms, the insulating gloves we use do not go up to the elbow, which is about 18" on my test subject (myself). The torso will be able to be >18" away from the exposed part but not the upper arm.

Thanks for the continued great information.


You have to be careful here, one large company in your area used increased working distances as a "mitigation" method to get some >40cal areas to <40cal, they had all different assumed working distances all over the plant, some were just silly (52"), and of course none of the skilled trades guys understood what the assumed distances meant.
Author:  Jim Phillips (brainfiller) [ Thu Jul 22, 2010 4:36 am ]
Post subject: 
MIEngineer wrote:
So is this to say that when using 18" say the HRC=4. If using a larger distance the worker could get a way with wearing only HRC=2.

Would something need to be done for the worker's arms, the insulating gloves we use do not go up to the elbow, which is about 18" on my test subject (myself). The torso will be able to be >18" away from the exposed part but not the upper arm.

Thanks for the continued great information.


I would only consider using a distance that was in line with the standards and the type of equipment. i.e. LV switchgear = 24 inches. MV switchgear = 36 inches etc. If you use 18 inches at LV switchgear, it is likely to be quite conservative and going with 24 inches which is in the standard, may be more reallistic.

However it will be quite confusing if you use an increased distance beyond what is in the standard - like something greater than a standard 18 inches at a panel. That will likely create confusion like Zog mentioned.

70E discusses parts of the body closer than the working distance may need additional protection.

Hope this helps.
Author:  mountaineer [ Thu Jul 22, 2010 5:41 am ]
Post subject: 
Thanks for the information. This forum is great - it's beneficial to read a variety of answers based on different experiences and applications of the the users. Some of the discussions really make you think and of course ask even more questions! It's a great resource.
Author:  mike01 [ Tue Sep 21, 2010 10:03 am ]
Post subject: 
brainfiller wrote:
I have written several papers about standardizing the Arc Flash Protection Boundary to simplify things but standardizing the working distance is an interesting concept. The 24 inches and 36 inch working distances from IEEE 1584 are based on the bus (assumed location of the flash) would be in the rear of low voltage and medium voltage equipment (i.e. switchgear) and unless you stick your head inside the breaker cell it would be difficult to get closer. It is assuming you are in front of the cell operating or interacting with the equipment.

Standardizing / keeping it simple is always a good option but in this case it might backfire. I was working with a client recently that performed a study using the default data from one of the major arc flash programs and everything defaulted to 18 inches.

Although this is a conservative approach, they had many areas that were category 3 and 4 requiring heavier protection (in a very hot part of the United States). Much of the higher category locations included switchgear and when the working distance was changed to the higher IEEE 1584 working distances based on the type of equipment, the categories became lower.

Being conservative is good and simplifying is good but this is one case where simplifying with the three distances might be the better option.

Let me know if you have any other questions.


But this statement confuses me what if you are working on the back of the gear?
Author:  Jim Phillips (brainfiller) [ Tue Sep 21, 2010 11:12 am ]
Post subject: 
mike01 wrote:
But this statement confuses me what if you are working on the back of the gear?


An excellent point. The 24 inch and 36 inch working distances assume the bus is towards the back of the equipment. If you are working in the back of the equipment energized (obviously not advised) the working distance could change and become closer depending on where the bus is located. I am not sure anyone considers this but may need addressed.

Any other opinions out there?
Author:  Zog [ Tue Sep 21, 2010 11:34 am ]
Post subject: 
I say 18" for working in the back of the gear. The increased distances are based on the distance from the front of the breaker to the energized stabs, or so I have been led to believe.
Author:  Jim Phillips (brainfiller) [ Tue Sep 21, 2010 11:50 am ]
Post subject: 
Zog wrote:
I say 18" for working in the back of the gear. The increased distances are based on the distance from the front of the breaker to the energized stabs, or so I have been led to believe.


I think something like that makes sense but I wonder how many people do this? (sounds like another weekly survey question!).

We just had an IEEE 1584 meeting on Saturday and we are now "officially" revising 1584. This is one of the issues I will bring up.
Author:  mike01 [ Tue Sep 21, 2010 6:48 pm ]
Post subject: 
brainfiller wrote:
An excellent point. The 24 inch and 36 inch working distances assume the bus is towards the back of the equipment. If you are working in the back of the equipment energized (obviously not advised) the working distance could change and become closer depending on where the bus is located. I am not sure anyone considers this but may need addressed.

Any other opinions out there?


Although this is not a desired working condition (back of the gear) but what about infrared testing when removing the back panels for scanning (when ports are not provided), and I have seen some projects where electrical work is conducted on the rear of the gear(switchboard UL891, and switchgear UL1556 or ANSI C37) both low voltage and medium voltage (480V, 4800V & 13200V) whether it be testing, new cable installation, etc. from the back of the gear with the gear energized, as I stated earlier not the best working condition but it happens, is multiple labeling appropriate one on the front door and one on the rear cover? Another question how does insulated bus work into the equation?

Quote:
I think something like that makes sense but I wonder how many people do this? (sounds like another weekly survey question!).

We just had an IEEE 1584 meeting on Saturday and we are now "officially" revising 1584. This is one of the issues I will bring up.


Wow after reviewing my edition of 1584 (2002) I did not see this anywhere? I thought for sure I would find something, I guess this also fall into the realm of the double egress in the NEC for large switchboard applications is double egress required from the rear access of the gear? Anyway I digress thanks for all your input and I will look for future updates. Where does it talk about the 24" and 36" distances being from the front of the gear only? thanks again a great site / forum with tons of great information. good job everyone! :D
Author:  Gary B [ Wed Oct 06, 2010 10:08 pm ]
Post subject: 
We initially standardized on 24" because we did not want the confusion and possible errors originating from wrong choices. At that time we rationalized that most AF PPE for the trunk and face rating would be no closer than 24" even though workers hands could be.

We have now revised that study to 18" on 480V MCC and 480V panelboards (but not 480V switch gear) because that is the default kicked out of our software apparently based on IEEE1584.
Author:  mahan4 [ Thu Oct 07, 2010 1:05 pm ]
Post subject: 
Back of gear energized work should be not recommended, if absolutely necessary, perhaps it would be good to de-energize the gear and estimate the distance of the bus to the person's head and shoulders, it'll probably be around 18" but it will not hurt to verify on site
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