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Should there be a minimum ground clearance line to earth at the voltage level 11 kV? I am living in India and I see the trains whose voltage level is 25 kV which is large compared to 11 kV.

Strangely I observed that 25 kV is used in trains whose height is much less compared to 11 kV height (I don't know the exact height?). According to my professor's logic, the height of 25 kV line must be greater than 11 kV? Correct me if I'm wrong.

pankaj prasad
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4 Answers4

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Think about that for a moment, it is not that hard.

Why is the ground clearance needed ?

1) Prevent the high voltage arcing over to ground ?

and/or

2) Prevent people or objects from touching the lines, obviously touching any High voltage power line is an issue.

Regarding point 1), the average field strength needed to make high voltage arc over (in dry air) is about 30kV/cm

So for 11 kV about 1/3 of a cm (3.3 mm) would be needed to prevent arcing-over.

For 25 kV almost one cm (10 mm) is needed.

So obviously the height at which 11 kV and 25 kV lines are mounted has nothing to do with the arcing as the distance to ground is much, much larger than needed to prevent that.

So obviously the ground clearance has to do with point 2), so it is safety related. The fact that the voltage is 11 kV or 25 kV doesn't really matter on the clearance needed. The clearance is related to external circumstances.

Bimpelrekkie
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    Another consideration for height is how (when under heavy load or fault conditions) the lines sag due to thermal expansion. Sometimes to the point of arcing to trees and the like. – jms May 18 '18 at 13:34
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    Wouldn't it be more correct to use the distance a dry arc would self extinguish? I assume it is 1cm per 30 kV to ignite, but it would sustain after ignition at significantly lower, no? – Stian May 18 '18 at 16:34
  • @StianYttervik Give that the clearances are materially larger than either the arc distance or the distance the arc would self-extinguish, I think either number gets the message across. – Cort Ammon May 18 '18 at 19:36
  • Unfortunately, though this answer is earning upvotes, I believe that the answer is substantially incorrect or, at any rate, substantially incomplete. Upvoters are not required to have actual engineering experience in this area, but I happen to have a bit. The standards that govern pole heights are practical documents, based chiefly on accident histories. You are not wrong that accidents happen when tall things are inadvertently raised or maneuvered under power lines, but your answer does not reflect the thought processes of the committees that make the standards as far as I know. – thb May 19 '18 at 23:23
  • @thb You're right, however, my answer isn't aiming to be compete nor fully correct. Yet it does get a lot of upvotes. Why is that? That's because not everyone wants the precise and correct answer. You (could) state all the technical reasons and documents in an answer but then no-one would read it as it becomes unreadable. If you're a railway technician then sure you need that information. If like OP you just wondered about the difference in ground clearance, my (and some of the other) answer(s) are more appropriate. – Bimpelrekkie May 20 '18 at 15:29
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The height a line should be elevated to is governed by its use, and ratings. The actual air space needed around a 25kV line to avoid breakdown is only a few cms. At this low 10s of kV level, the actual voltage does not influence the line height.

In the case of 25kV lines above train tracks, access to the space under the lines is restricted. Passengers are protected by the train body. Maintenance workers on the track should have been trained to not wave long metal poles above their heads. The height has been arrived at for compatibility with the trains.

In the case of 11kV distribution, these lines run everywhere. There is little to stop people sitting on vehicles from driving under them, maybe even waving their arms over their heads. The height for these has been arrived at by a committee, trying to balance the safety (long poles give good headroom and so are safer) against cost (short poles are cheaper).

Neil_UK
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    You need very long poles to beat kites... – Solar Mike May 18 '18 at 14:34
  • It may also be worth mentioning that underground cables are safer still, provided they are buried deeply enough to avoid being accidentally dug up. They don't arc because they are covered in insulation with a much higher dielectric strength than air. – Chromatix May 18 '18 at 20:06
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On both 11kV power cables and 25kV railway OHLE installations, the supporting structure (pole or gantry) is at ground potential, which is the relevant point for insulation purposes.

If you look at the insulators between the cable and the support, you should find that the 25kV ones are longer. This is not only to meet the required dielectric strength in dry air, but across the surface of the insulator itself, which will often have a layer of dirt and/or moisture on it which is not as good an insulator as air is. That's also why insulators are typically ribbed.

Conversely, if you look at OHLE energised at lower voltages (eg. 1500V DC), it normally has much smaller insulators.

11kV power pole in Nepal

1.5kV DC OHLE, near Manchester, England

Same type of OHLE after upgrading to 25kV AC

A more modern type of 25kV OHLE

Chromatix
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  • One may also add that OHLE installations include complex suspension and tensioning arrangements that keep the amount of sag under tight control even as temperatures change, which is not the case for a 11 kV distribution line. – hmakholm left over Monica May 19 '18 at 16:43
  • @HenningMakholm That's true, but it's done to maintain the alignment of the contact wire with the train's pantograph, not for ground clearance. – Chromatix May 19 '18 at 16:55
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Your question ends up being a practical one. I do not live or work in India but have some practical experience in the matter in the United States, where the National Electrical Safety Code (the NESC, which is not to be confused with the National Electrical Code, NEC) governs the matter.

The NESC is a practical document rather than a theoretical one. If many decades of experience suggest that poles of a certain height bear, with few mishaps, conductors of a certain voltage, then the NESC allows that height at that voltage. As far as I know, for power poles, the chief controlling factor is safety during a windy rainstorm, when

  • the lines are being blown about and
  • the arcing threshold of the air is lower.

As another answerer has observed, the NESC further takes into account the heights of normal activities on the ground beneath the lines, but not really by a formula. It is a matter of experience, rather.

This is why the heights of power poles and the gaps of automotive spark plugs contradict one another by an order of magnitude. Your train is more nearly akin to the spark plug.

thb
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