1000V+ circuits, Working space, Fences vs other grounded metal behind

[Carultch]

In NEC110.31, the minimum distance from a fence to live parts is 10 ft for 1000V circuits.
In NEC110.34(A), the minimum distance from grounded parts behind equipment to live parts is 4 ft for 1000V circuits.

What is the reason for a fence requiring more working space than any other grounded conductive surface behind the equipment, such as a concrete block wall?

 

[charlie b]

Here's my take on the situation. 110.34(A) addresses the case of a worker opening a cover and thereby exposing live parts. The 4' of space to the wall in back of the worker is intended to allow the person to work with some degree of safety. But to be clear, the live parts do not become exposed until the worker removes the cover. 110.31 addresses the case of live parts that are exposed all the time (i.e., no covers to remove). The danger here is that an unqualified person (i.e., Darwin Award Candidate) might try to reach in with a pole, just for the fun of it. The 10' rule just means the person will need to get a longer pole.

 

[Carultch]

Here's my take on the situation. 110.34(A) addresses the case of a worker opening a cover and thereby exposing live parts. The 4' of space to the wall in back of the worker is intended to allow the person to work with some degree of safety. But to be clear, the live parts do not become exposed until the worker removes the cover. 110.31 addresses the case of live parts that are exposed all the time (i.e., no covers to remove). The danger here is that an unqualified person (i.e., Darwin Award Candidate) might try to reach in with a pole, just for the fun of it. The 10' rule just means the person will need to get a longer pole.

Thanks for the clarification. It makes perfect sense, too.

As a follow-up, suppose I did have a piece of 1000V equipment in a closed enclosure (only to be opened by authorized personnel), with a grounded fence that is 4 ft behind its cover. Would that be an acceptable installation, just as it would be if there were a block wall replacing the fence?

In otherwords, as long as my equipment is in enclosures, I can forget about 110.31, and plan my fence layout to achieve space per 110.34(A).

 

[charlie b]

I believe that you have that right. If there are no live parts that can be reached by an unqualified person (i.e., if all the live parts are internal to an enclosure), then the rules concerning the distance of the fence from the equipment do not come into play.