Lightning Protection System w/ Inductor: Impact?

[Curiousphy]

If there's an inductor in series between a lightning rod and the ground rod, how will it affect the lightning protection ability of the system?

 

[gneill]

You'll need to be a little more specific about the makeup of the inductor. The inter--winding spacing of a typical inductor is negligible for the potentials involved in a lightning strike, and it would simply arc through/across the inductor.

So, what did you have in mind? Is there a particular reason you want an inductor between a lightning rod and its ground?

 

[Curiousphy]

That was actually a hypothetical question to understand the theory behind inductors and lightning. If the inductor was able to withstand the current, would it act more as a short circuit to the lightning induced current, or would it act more as an open circuit?

 

[gneill]

If (hypothetically) the inductor and its interwinding insulation would not break down due to the potential difference imposed by a lighting strike, then it would behave as any inductor would and present an effectively open circuit in the first instant of the lightning strike.

In the real world, there are no materials that an inductor and its insulation could be made of that would prevent an arcing across its windings for any practically sized inductor.

 

[Curiousphy]

Thank you! would the arcing likely cause the inductor to melt (say, if it was a coil of the same gauge wire sized for the lightning protection). Btw, are AWG 2 copper wires really adequate to direct lightning from lightning rod to ground rod without damage to the wire or excessive heat being created?

 

[gneill]

2 AWG is the minimum recommended wire gauge for a lightning protection conductor. Whether that is adequate to prevent melting of vaporization of portions of the wire with some particular lighting strike I don't know. Perhaps you should investigate what commercial buildings do with respect to lightning strike protection?

 

[Curiousphy]

Thank you again. What if it was a resistor instead of an inductor, would it act as an open circuit or cause arcing?

 

[gneill]

Thank you again. What if it was a resistor instead of an inductor, would it act as an open circuit or cause arcing?

What do you think?

 

[Curiousphy]

It would probably overload/melt it, and make it an open circuit, and then arc across it?

 

[gneill]

Yes, that's most likely.

Why are you asking these questions?

 

[Curiousphy]

Was wondering if lightning was AC or DC, and how it would behave with an inductor, but like you suggested any thing that increases impedance would probably get vaporized... and an inductor would not be able to pass it through in time...

 

[gneill]

A lightning strike is more DC than AC, given that it's a sudden discharge of a stored charge. However, the potential difference changes rapidly as the charge is "discharged", so there is an element of "AC-ness" about it in that the potential difference changes with time. But note that the potential difference will not change sign as a typical AC source would.

Still, the rapid change in potential difference and current could be modeled mathematically or by simulation for any combination of components in the circuit it was driving. Fortunately, mathematics and simulators don't vaporize when pressed to analyze extreme conditions

 

[Nik_2213]

Somewhat off-topic, but you could look at the 'tried & tested' protective circuits that eg Radio Hams hang on their long antennae.

IIRC, one type consists of a robust spark-gap led to an appropriate grounding rod, with an inductor on the 'shack' side to encourage the strike to go 'that-a-way' rather than continue along the feed...

Please, please, please use 'Due Care', and do not skimp on precautions.

 

[dlgoff]

A lightning strike is more DC than AC, given that it's a sudden discharge of a stored charge.

Indeed.

Positive lightning strikes tend to be much more intense than their negative counterparts. An average bolt of negative lightning carries an electric current of 30,000 amperes (30 kA), and transfers 15 coulombs of electric charge and 500 megajoules of energy. Large bolts of negative lightning can carry up to 120 kA and 350 coulombs.[39]

 

[Baluncore]

A lightning conductor should be a flat strap, not a round wire. A strap has lower inductance than a wire of the same mass. A flat strap also has lower resistance to sudden current steps.

A lightning conductor should not have sharp bends. If it does, the inductance at the bend will cause the strike to initially cut the corner, by ionising the air on the inside of the bend. The same goes for any inductance or resistance in the conductor.

A lightning conductor should not be insulated. If it is insulated the velocity factor along the conductor will be slower than along other bare conductors or surfaces nearby that will conduct the strike current. The insulation will probably be a fire hazard.

A lightning conductor should not pass through water. The dielectric constant of water also lowers the velocity factor and causes the current to spread across the water surface.

A lightning conductor should not pass through an insulated gland or tight gap. If some water wicks into the seal, the instant conversion of the liquid water to steam will explosively destroy the seal. Trees are split by lightning when water in the tree flash boils along the line of conduction.

 

[Curiousphy]

What if it was a power surge by an indirect near by lightning (say 100 meters), if the grounding path to Earth had a low impedance inductive or resistive load (say 30 ohms) in series, what power rating would the load have to support to pass the transient impulse surge safely to Earth without burning itself up?

 

[Baluncore]

… if the grounding path to Earth had a low impedance inductive or resistive load (say 30 ohms) ...

30 ohms is not a low series impedance in a lightning conductor, it is a very high impedance and demonstrates poor engineering. I would expect a flash around that impedance as the surrounding air is ionised by the voltage gradient. But you are suggesting that the series impedance is a fault in a lightning conductor and that the lightning strike is 100 metres away, so I ask you why the lightning conductor was not placed in the more exposed position where the strike eventually occurred? A lightning conductor should never be installed below a more attractive conductor.
You should not be attempting to analyse fault situations before you have understood the engineering requirements of a reliable system.

 

[Curiousphy]

Thank you. I do not have a lightning rod nor am I about to tinker with a lightning rod system. I was curious if there were any man made inductors or resistors able to withstand lightning. The answer was a resounding no and it made sense. Then, indirect power surges are quite common. I know there are high impedance inductors/resistors designed to block a power surge to protect electronics, but what if the surge went through a low impedance inductor/resistor to the ground electrode system. Are there inductors/resistors that could possibly survive such a surge current?

 

[Baluncore]

Are there inductors/resistors that could possibly survive such a surge current?

Until you specify the voltage, current, impedance and risetime there can be no sensible answer to your question.

 

[Rive]

I was curious if there were any man made inductors or resistors able to withstand lightning

As it was stated before at that current any lightning conductor has notable resistance and inductance too: to the point that even the shape of the wire matters. Lightning conductors are expected to survive lightning.

It might not fit in your image about electronic components but in the right context (for the right question) even a concrete block or a shovel of mud might count as 'resistor'.http://www.heynen.com/isabellenhutte-achieves-record-current-measurement-on-shunt-basis/ is also about a (shunt) resistor. I have a feeling that it would also survive a lightning strike.
A smaller one if you prefer something more resistor-like.

I know there are high impedance inductors/resistors designed to block a power surge to protect electronics

You know wrong. As the first line of electronic surge protection there are low impedance (! at least far lower than the load, in those circumstances) devices that offers a path to ground for the incoming high voltage surge and thus limiting the energy/voltage/current passing for the further protection barriers. For a decent protection system there are several lines of defense, all based on the limiting effect of the previous level, all aimed at different power levels.

Blocking type (series mode) defense works only at low power levels. Anything like a direct lightning strike would just bypass or vaporize such devices.

 

[Curiousphy]

Thank you all, I am understanding this a little better now. Shunt resistors that can survive 10s of Ks of amps on a persistent basis are probably the closest thing to survive a 10-50x surge? but at 1 ohm, it's almost just like a wire...