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This website, https://circuitglobe.com/overhead-ground-wire-or-earth-wire.html, said for effective shielding, we should keep the protecting angle as small as possible, the angle between 20° and 30° is quite safe, and it should not be kept above 40°.

Why? I think the larger the protecting angle becomes, the longer will the \$L\$ be, that is, the range that shield wire can protect will become larger. Isn't it good to let \$L\$ become longer? Why does the website say we should keep the protecting angle as small as possible?

enter image description here

feetwet
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shineele
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  • Alan Greenwood wrote a classic text you may be interested in. It thoroughly covers this topic (statistics/probability based). “Electrical Transients in Power Systems”. Wiley sells paperback version cheap. It is always a balance between protection, and economics. It is not economically justified to perfectly protect. – relayman357 Apr 04 '20 at 14:35
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    I would have made the diagonal lines dashed, they are not wires, they are only reference lines. The shield wire runs perpendicular to the page. – Mattman944 Apr 04 '20 at 14:54

2 Answers2

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The article you've linked to states:

The shielding or protective angle is the angle between the vertical earth wire and the phase conductor which is to be protected. Usually, the angle between the vertical through the earth wire and the line joining the earth wire through the outermost phase conductor is taken as a shielding angle.

enter image description here

Figure 1. The rolling ball lightning protection model rolled against the protecting angle model. I have assumed that the tower is 20 m tall. (Combined image mine.)

The lightning protection angle is a simplification of the lightning rolling sphere method of analysis. To get 40° lightning protection angle may require you to assume a large sphere diameter which will reduce the safety factor of your analysis. 60 m seems to be a common value used and if my assumption of 20 m for the pylon is close then you can see that the pylon design is almost a perfect fit for the 60 m sphere.

I know only a little about the subject so wait a few days before accepting this answer if you are so inclined. That way you will encourage others to answer and may get some other insights.

Transistor
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  • You sure do know a lot of things.. I hope to learn from you. – John Cortex Apr 30 '21 at 09:09
  • @JohnCortex, have a look at https://greymattersglobal.com/rolling-sphere-method/ and do an image search for building+rolling+ball+lightning+protection and you should get some more insight. – Transistor Apr 30 '21 at 12:39
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I think the larger the protecting angle becomes, the longer will the L be, that is, the range that shield wire can protect will become larger. Isn't it good to let L become longer?

You've got this back to front.

The parts highlighted in red should not protrude left or right more than an amount that might cause the angle to become greater than 40 degrees: -

enter image description here

So preferably the phase conductors should be right below the top lightning protection earth wire. But clearly that would cause a voltage breakdown problem. So they have to be spaced away from the central frame of the pylon and if that spacing is such that the angle is greater than 40 degrees then the earth wire would have to be raised vertically to make the angle acceptable.

It's the "shadow" angle formed by the top earth wire that "protects" the electrical conductors that cannot be greater than 40 degrees (and preferably 30 degrees or less). That shadow angle is not something that you can design; it's a physical property of the earth wire suspended above a vulnerable area.

feetwet
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Andy aka
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