Transmission Line Clearance 1

[timm33333]

I am trying to figure out which rule of NESC addresses the horizontal clearance between 161kV transmission line and an industrial facility (like a well pump). NESC rules 233(B) and 234(B) talk about horizontal clearances, and 233-3 talks about clearance envelope, but these are not clear.

Also does the right of way (ROW) of transmission line has anything to do with horizontal clearance? For example if the ROW in rural area for 161kV line is 120 feet, it would mean that the ROW would extend to 60 feet on both sides from the center of the pole. Does it mean that the minimum required horizontal clearance (from 161kV line to the industrial facility) would also be 60 feet? Thanks

 

[bacon4life]

I don't recall anything about ROW in the NESC. There are many examples of power lines installed at the edge of the ROW. There is some property rights debate about whether lines must stay inside the ROW under normal conditions, NESC light/medium/heavy, or extreme wind. There is also debate about legality of one entity placing a structure impacting the ability of the adjacent land owner to fully utilize their property.

In my state the unqualified worker safety rules (WAC 296-24-960) tend to be much more restrictive that NESC rules. For example, a worker on a 4 ft wide scaffold repainting the a wall can reach another 6 ft outside the scaffold while holding a tool, resulting in a total required horizontal clearance of around 25 ft to a 161 kV line. These rules were developed in response to numerous accidents where construction workers contacted energized lines.

Although technically the clearance is only required while both a worker and an energized line exist, it is unlikely the utility is going to be willing to plan to de-energized the transmission line to make a safe work zone for a construction worker. Failing to leave adequate electrical clearance for routine building maintenance shifts the responsibility for evaluating electrical hazards to non-electrical workers like painters and window washers.

 

[timm33333]

If we have to follow NESC: then which rule (or figure) of NESC should be used to find the minimum clearance between the “lowest phase conductor of 161kV line” and the “nearby structure/mechanical equipment”?

 

[bacon4life]

Rule 233 applies to the clearances between wires, so it is not applicable.

Rule 234 applies between an energized line and a mechanical structure. Although the NESC allows structures directly under energized lines if it meets the vertical clearance requirement, many utilities prohibit structures directly under a transmission line.

Section 5 of RUS DESIGN MANUAL FOR HIGH VOLTAGE TRANSMISSION LINES has some guidance about right of way, as well as recommending an addition 1.5 feet of horizontal clearance beyond the NESC minimum when designing a line.

 

[davidbeach]

an addition 1.5 feet of horizontal clearance beyond the NESC minimum

Goes along with my adage that if the inspector has to pull out the tape measure you didn't have enough clearance, regardless of the actual measured distance.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Padlock01]

Hi timm33333
I have attached a copy of our Utility "ESKOM" OHL clearances in RSA perhaps helpful?

 

[stevenal]

234B1b, Table 234-7, and 234G.

 

[timm33333]

Thanks everybody for information. I think 234(B)(1)(b) is for clearance from other supporting structures like the base of a surge arrester or CT. I think the correct rule would be 234(C)(1)(b), where a mechanical equipment (well-head) would come under the category of “other installations except bridges”. The minimum clearance is 1.4m, and as per 234(G)(1): it is to be increased by 10mm for every 1kV in excess of 22kV. So the total clearance from 161kV line to the well-head would be 2.79 m.

 

[stevenal]

Same table and clearance either way. Don't forget to consider wind.