NEC 110.34

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SDW7613

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Electrician
Hello I need help understanding the electrical working clearances for 4160V
based on the 3 conditions. I was just reaching out to see if anybody could
help me get a clear understanding of the code for working clearances. We
are installing 4160 switchboard/ VFD in a existing room and we have the
distances met for condition 1 of the NEC but if we have to go with either
condition 2 or 3 we will have to modify the room.
 
You are saying you have 3 feet available but not 4. Is that right?

I have seldom seen a room that met Condition 1. But I created that condition for one project by having the contractor install an insulating mat on the railing opposite the panel. So I would say that much depends on what is on the opposite side.
 
We have 4 ft right now. We need 5.5 ft to pull the Igbt cells out of the front of drive. Behind the drive there is 4160 V switch gear for air compressors. We have 4 ft there as well.
 
SDW7613 said:
We have 4 ft right now. We need 5.5 ft to pull the Igbt cells out of the front of drive. Behind the drive there is 4160 V switch gear for air compressors. We have 4 ft there as well.
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Can you pull the IGBT cells out while the gear is energized and the 5kV is exposed?
 
You have to power the drive completely down to change the igbt cells and there is a safety interlocks on the doors.
 
SDW7613 said:
We have 4 ft right now. We need 5.5 ft to pull the Igbt cells out of the front of drive. Behind the drive there is 4160 V switch gear for air compressors. We have 4 ft there as well.
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Do the VFDs have energized parts that can be accessed from the side that faces the switchgear? If not, then you are in Condition 2. For a 4160V switchgear (2400V to ground), you only need 4 feet of clearance between the enclosure of the switchgear and the enclosure of the VFD.
 
There is not any hinged doors to access the drive on the backside you would have to mechanically take panels off to access the energized equipment inside.
 
SDW7613 said:
Yes sir that is correct.
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Then why are you trying to apply working distances for energized equipment?

I am not against doing so, I just want to know that the reason for it has been rationalized.

I worked on one design where my energized working distance was negatively impacted by 1" due to a HVAC duct. We had the choice to expand the size of the prefab equipment house or to justify that the MV components could not be accessed except when deenergized. The distance for the LV instrument compartments were not at issue.
 
Okay that clears up a lot of concern. It was brought up in our last discussion at work. I stated the distances we currently had for the equipment based off of the working distance code. Most of the people In the discussion were not familiar with working distances and wanted the engineering firm to double check us before we bolt the drive to the concrete. The main point is we will not access the drive when energized due to the interlocks and it was cleared up in this thread.
 
jim dungar said:
So the item will be deenergized when you need to work on it.
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You still may need to energize it, when you test for voltages to make sure it is working correctly. Also, you need to test for voltages to verify it is de-energized. If it fails to de-energize, you now have exposed live parts. Even though it is equipment that you would try to de-energize before turning a wrench on its components, it still is good practice to build with work space in mind.

I would like to see some insight as to what the intent of the phrase "likely to require service or examination while energized" is, and how the line was intended to be drawn. I would guess that this was added to focus on service, distribution equipment, and major load equipment, so that the rule doesn't propagate to impractically requiring work space around every light switch/every receptacle/every light fixture/etc. Loads you would shut off at the breaker before doing any work. And so the rule doesn't propagate to equipment that would be in attics and crawlspaces, where it isn't possible to achieve working space.
 
Carultch said:
You still may need to energize it, when you test for voltages to make sure it is working correctly. Also, you need to test for voltages to verify it is de-energized. If it fails to de-energize, you now have exposed live parts.
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This is MV equipment which is often constructed with internal interlocks which prevent the covers from being opened or removed until it has been deenergized. The testing for deenergization usually occurs at he incoming line or main switch compartment. It is this energized area that needs to have or exceed the minimum working clearances.

Also don't forget that Hot Sticks are often used when checking medium voltage so additional working clearance distance may need to be provided.
 
jim dungar said:
This is MV equipment which is often constructed with internal interlocks which prevent the covers from being opened or removed until it has been deenergized. The testing for deenergization usually occurs at he incoming line or main switch compartment. It is this energized area that needs to have or exceed the minimum working clearances.

Also don't forget that Hot Sticks are often used when checking medium voltage so additional working clearance distance may need to be provided.
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Thanks for clarifying that. Is there a standard for how much space you should leave, in anticipation of hot stick work?

A related question, is what's the difference between 110.31 and 110.34 in terms of which one governs MV working space? Does the fact that it is a fence, rather than any other condition 2 back-surface (e.g. concrete block wall), mean that 110.31 applies instead of 110.34? Does 110.31 apply for permanently-exposed live parts, while 110.34 applies when all equipment is normally covered by a dead-front?
 
If you read the first few paragraphs of section of 110.31, you can see from the context what they are trying to accomplish with table 110.31. The first couple paragraphs of section 110.31 essentially say you have to create a barrier to prevent unauthorized access to the electrical equipment. If you use a fence to create that barrier for an OUTDOOR installation, then you must follow the fence specs given in these paragraphs AND use TABLE 110.31. IF you provide equipment that meets the requirements of 110.31 (D) (enclosed to prevent foreign objects, locked equipment doors and/or non standard nuts, etc.) then you would not need to apply table 110.31. Practically, this means outdoor substations with overhead wire/cable feeds to live terminal substation style power equipment would always need to follow table 110.31 clearances to the fence. Since you implied that your installation is indoors, I don't think table 110.31 would apply.
 
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