# Voltage, inductance & lightning in a length of wire

I initially got the idea after watching the famous Mike Holt video on grounding.

Mike warns that longer wires to ground rods can attract more energy. So I thought he meant the lightning voltage can increase after passing through the wire (it doesn't as one of the Answers has detailed at length).

I have many surge protection devices which protect many circuits so I wanted to know the mechanisms of it. And I tried to review the physics below:

Given the inductance for a given length of wire, the induced voltage along the length is given by:

$$v=L\frac{di}{dt}$$

So the greater the rate of change in current, the greater the induced voltage.

Hence the longer the wires. The greater is the surge voltage.

From here:

• You need to draw a schematic that underpins your 2nd paragraph's statements. It's not clear what scenario you are trying to describe. – Andy aka Nov 29 '18 at 8:19
• In surge protection, the longer the wire, the more the voltage increase, this is why they suggested to use shortest leads as possible. – Jtl Nov 29 '18 at 8:21
• (1) You are not understanding my answer (based on the content you originally posted). (2) You are not listening. (3) You still haven't described a scenario (like I asked in the very first instant. (4) You are not understanding lightning properly. (5) You are disregarding lightning in your modified question. (6) You are trying to rubbish my name by implying I don't believe faraday's law. (7) You are acting this out like some drama. – Andy aka Dec 1 '18 at 9:14
• I'm not trying to rubbish your name. Sorry. I just want to understand if the surge protection industry emphasizing shorter leads is true. I'm asking this because I have all sorts of surge protective device like Metal Oxide Varistors that I even tried integrating into circuits. Or from separate box and this short lead requirement is confusing me. So I'm just confused. I will try to understand it all. Thanks for emphasizing. Maybe you were talking about lightning while I was talking of partial energy from lightning getting into wire and how many percentage would appear.. I think I'm getting it. – Jtl Dec 1 '18 at 9:18
• Here is another good video: youtube.com/watch?v=Ypo99VRxT44 – mkeith Dec 1 '18 at 22:21

Besides U=RI, there is another formula valid for the inductor which is U=L dI/dt. This formula reads like this: If current changes, voltage is proportional to that change. This is why voltage can get very high across the inductive wire because current change is huge in case of lightning.

• Wires are about 1 microHenry per meter; there is a mild naturalLog(distance) scalefactor. 10,000 meters of power line will have 0.01 Henry of inductance. The lighting bolt will arrive with 50,000 amps per microsecond of SlewRate. Multiply those 2 numbers, and we have a prediction of the line voltage at the point where the bolt hits the line. Looks to be far in excess of 1 Million Volts, right? – analogsystemsrf Nov 29 '18 at 5:22
• What would happen if lightning strikes a ground rod that is connected via 70 meters AWG 6 wire to a metal enclosure? If a person stands near the enclosure, would sparks fly to him? How near before this can happen? – Jtl Nov 29 '18 at 5:52
• It the ground rod is grounded to earth, then there would be no current flowing to the distant metal enclosure and thus no person would be harmed. – Stefan Wyss Nov 29 '18 at 19:43
• Why do EE dont believe in it that the voltages can rise the longer are the wires? Pls see andy aka replies. – Jtl Nov 30 '18 at 10:49
• I do not get your comment. If no current is flowing through the wire, then there is no voltage along the wire. – Stefan Wyss Nov 30 '18 at 11:08

In the case of lightning or surges, the longer the wire the more inductive reactance and the greater the voltage.

The inductance of free space is 1.26 uH per metre and the lightning is travelling through free space before it hits a wire like a lightning rod. The lightning rod will have pretty much the same inductance so there is no more voltage generated than that naturally produced by the lightning's electric field should it by-pass the rod or actually conduct through it.

Inductive reactance is like resistance. If the wire is longer, and more resistance, why would voltage increase?

Not really.

Remember the current would decrease from increased resistance, so voltage shouldn't increase.

Lightning has a current that is independent of what might be in its path and, the voltage it produces is going to be about the same irrespective of what it encounters i.e. it behaves more like a current source not a voltage source.

As I said in my comment, your question isn't very clear as to what scenario you are describing so if I have not addressed the scenario you have envisaged, you should draw a picture.

• @Jtl why are you giving me a reference to this article? Does it contain a schematic that might be what you are trying to understand. If so then cut and paste it into your question. I read the other answer about an hour ago. Did you not understand that the inductance of free space is the same as the inductance of a wire hence it makes no difference it being present or not should there be a lightning strike. – Andy aka Nov 29 '18 at 10:25
• Because a lightning strike has around 1000 volts per metre. – Andy aka Nov 29 '18 at 22:33
• I don’t know; maybe you can link to an article where they say this. The added pictures in your question don’t seem to clarify this at all. – Andy aka Nov 30 '18 at 7:29
• @jtl. This has to stop right now. Lightning is a massive pulse of current that flows through air (once the air has broken down) and, the presence or non-presence of a wire will not alter this. Lightning takes the shortest path it can and if something is placed in or close to its path (that increases the electric field locally) then the lightning will go around it and take the shorter path. You need to grasp this. – Andy aka Nov 30 '18 at 9:55
• I have no idea - the article is very poorly written as far as I can tell. I will also add that my original comment under your question still stands and is as relevant now as it was the other day: It's not clear what scenario you are trying to describe. – Andy aka Nov 30 '18 at 10:12

I will explain what the article is saying. See schematic. L1 represents the inductance of the long wire. I1 is the 8kA 80/20 waveform. The longer the wire, the larger L1 will be, and the larger the voltage across I1 will be when the pulse waveform is applied. That is all the article is trying to say. Yes, a longer wire has more inductance, and consequently, it will have more voltage develop AT THE POINT OF CURRENT INJECTION. Not down at the end of the line away from the current injection point.

simulate this circuit – Schematic created using CircuitLab

Now, where did the 8kA waveform come from? The article doesn't talk about that. I suppose it could be from a direct lightning strike, or (more likely) it could be induced in the wire due to a strike to a nearby object.

I don't think there is anything wrong with the article. I thought it was interesting.

• Ok. But why did Andy aka seem hostile to the idea? His reasoning seemed to be air had same inductance as wire. So since lightning voltage doesn't increase from the kilometers of distance from the sky..it wont increase from passing through a wire. Hmm.. maybe say 30% only of lighning energy goes to the wire and the voltage will depend on how long is it? I wonder if Andy aka agrees to this and can resolve the conflict with his idea and that of surge protection world. – Jtl Dec 1 '18 at 9:11
• If what you said was correct. Then I guess what is wrong with the wordings of surge protection industry is saying there is voltage rise of 1000 volts per meter. It's as if voltage gets bigger when it passes through every length. Imagine water pressure increasing after passing through longer hose with same diameter. It would make sense if the voltage increase is at the point of current of injection. Hope others who has other ideas can share too. – Jtl Dec 1 '18 at 9:43