250.80, exception

[ryan_618]

250.80 Service Raceways and Enclosures.
Metal enclosures and raceways for service conductors and equipment shall be grounded.
Exception: A metal elbow that is installed in an underground installation of rigid nonmetallic conduit and is isolated from possible contact by a minimum cover of 450 mm (18 in.) to any part of the elbow shall not be required to be grounded.

Change to read:

Exception: A metal elbow shall not be required to be grounded where it is installed in a nonmetallic raceway and is isolated from possible contact by a minimum cover of 450 mm (18 in.) to any part of the elbow or is encased in not less than 50 mm (2 in.) of concrete.

Substantiation: This change makes the exception to 250.80 mirror exception number three to section 250.86. The protection afforded by 2 inches of concrete encasement should be considered as equivalent to 18 inches of earth cover. Section 230.6 seems to agree with the concept, when it considers 18 inches of earth cover or 2 inches of concrete encasement suitable to consider service conductors ?outside of the building?.

 

[charlie]

Re: 250.80, exception

The proposal looks great although I have a little problem with the content.

The service raceway in 250.80 has no overcurrent devices in front of the metallic elbow. If the conduit were to become energized, there would not be a chance that an overcurrent device would be able to protect it. Concrete is a conductor, therefore, the exception in 250.86 should not have the same concrete envelope that you wish to apply to 250.80. In other words, I believe CMP-5 should not have permitted this part of the exception.

 

[ryan_618]

Re: 250.80, exception

As always, thanks Charlie for your opinion...you know how much esteem I hold it in.

I'll kick that around for a bit and see what I can come up with.

 

[pierre]

Re: 250.80, exception

I never really understood this exception. How does this not create a potentially hazardous situation where the earth will develop a "step potential" with the current flowing through the earth back to the source... if the elbow were to have a short with a phase conductor.

 

[charlie]

Re: 250.80, exception

Actually Pierre, you said it much better than I did. Thanks

 

[ryan_618]

Re: 250.80, exception

I think the issue is simply feasibility. How do you go about bonding the isolated metal elbow? Many people desire a metal elbow to ease pulling the conductors, but don't want the corrosion problems of metal throughout the entire raceway.

I agree with you guys, it could be a problem, but bonding every isolated elbow doesn't strike me as "practical safegaurding", as discussed in 90.1(A).

 

[charlie]

Re: 250.80, exception

Ryan, while I agree with you, I can't agree that a conductive material like concrete should be used to insulate an isolated elbow.

 

[pierre]

Re: 250.80, exception

Ryan
A thought... how about a nonconductive coated elbow, as part of the proposal?

 

[bphgravity]

Re: 250.80, exception

I spoke with a representative from the Steel Tube Institute recently for some research I am doing. He told me about some products that are in development by a few conduit and tubing companies. One major manufacturer is in the process of developing an internal PVC coated steel tubing and fittings.

This doesn't really address the proposal, however I thought it is an interesting concept that sort-of relates.

 

[hmspe]

Re: 250.80, exception

re: step potential and current returning to its source through the earth

I'm not sure this is an issue. What sticks in my mind from Mike's courses is the 25 ohm ground rod at 120V fault is 4.8A current flow. Would an elbow have a lower resistance? I also recall the curve for the voltage gradient from an energized rod in earth. The danger I see is physical contact with an energized raceway, which is unlikely with either adequate burial or with concrete encasement.