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Some 20 years ago, when I was 11 or 12 years old, I attended a weekend 4-H camp about electricity. I recall one of the instructors there informing us that the cheapest way to protect our electronic equipment from lighting strikes was to tie 2 or 3 knots in the power cord, claiming (if I recall correctly) that in the case of a lightning strike, these knots would form an electromagnetic field that would prevent the lightning's current from passing through to the electronic equipment.

Is there any shred of truth to this claim? I find it hard to believe that a couple simple knot-coils could protect against the tens of thousands of volts that come from a lightning strike.

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    Presumably the intent is to produce a inductive load and smooth out the current spikes. The principal is sound, but I suspect that when you work the numbers it'll be like spitting in a hurricane. Might help with messy power due to a a shaky rural grid, though. – dmckee Aug 19 '11 at 20:00
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    That is absolute bunk. The only thing remotely close to that is the practice of laying out excess wire in infinity shaped coils, which has the effect of cancelling any emf. I've done this on machine installations to reduce the risk of an emf causing noise problems in nearby signal cabling. In the situation you are talking about, you have foreign current in the wires themselves. No amount of curling, twisting or knotting will stop the effects of lighting. – Captain Claptrap Aug 19 '11 at 22:32
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    "As Seen On TV" lightning arrester kit for all your appliances! Knot tying instructions included, only $19.95......... – Moab Aug 20 '11 at 4:17
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    If such a trick would work, the cable in the building would be already twisted that way, wouldn't they? – user unknown Aug 20 '11 at 6:38
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    @vartec to tune it you would want to attack the frequency spectrum of the noise, not the carrier. But spikes imply high frequency, so what you want is an inductive low-pass filter and there is presumably a wide range of time constants that will give you some help. – dmckee May 25 '12 at 17:31
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It depends on the strike and what you mean by protection. It's a safe bet that it won't work in many cases because it's not grounded.

There are different interpretations I can think of for this claim, as lightning surges come in different ways, for example:

  • A lightning strikes your TV antenna or home directly
  • A lightning hits the grid creating a much minor surge

In the first case, you definitely need more protection than the second. Tying knots in either the coaxial or the power cord will not make your TV protected against picking up lightning from the antenna or the house itself.

A lightning contains a lot of energy. When a lightning hits a TV antenna, that energy has to go somewhere. If your TV is attached to the antenna, it goes through it and then to the ground. Whether there are knots or not, energy is conserved and your TV is fried.

Dr Bob Howlett, Reader in Electrical and Electronic Engineering, University of Brighton: Whether you plug your television in or not when there’s lightning around – it’s an interesting question. I think it’s one of those questions where the answer really is, it doesn’t make too much difference.

-- Lightning on TV

Clearly if unplugging a TV set doesn't make a difference, then tying a knot won't either.

Diverting energy to ground is the most important form of protection and this is obtained by grounding your equipment and your house properly, and installing a lightning rod.

Can we create an effective minor surge protector by tying knots in a cable?

The answer is that it probably won't work -- again, commercial surge protectors work by sending the extra current to the ground instead than your TV. Knotting a cable does not connect it to the ground.

Spark Gaps

They work by having a gap of air between line and ground. When a voltage goes over a specific level, the gap is filled with a spark which sends energy to the ground, short circuiting. Some versions can be directly connected to ground.

Gas Tubes

Another kind of spark gaps, but filled with a specific gas for better performance.

MOVs

Metal Oxide Varistor is a voltage dependent resistor

  • At low voltages it is open circuit (megohms)
  • At some specified higher voltage, the resistance falls to a low value (a few ohms or less)

While of course this is a general overview of how surge protection actually works, it doesn't prove that knotting a cable doesn't work, but it shows that it's unlikely to work because it's not grounded.

Sources (they are the slides I'm summarizing here):

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