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# Tie Cord in Knot to Prevent Lightning Surges???

Posted on 2005-05-11
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Hello, some time ago, a friend of mine, who's an electrical engineer, told me that if you plug all your devices into a power strip, and then tie it's power cord in a knot before plugging it into the wall, that it would cause high power surges, such as from lightning, to seek a path of easier resistance in it's search for ground, and would thus spare your equipment from surge.

I've preached this for a few years now, and possibly have some evidence of equipment being spared while others which were not protected in this way failed, but that could also be due to any number of other reasons, all having to do with the shortest path to ground.

Is this actually correct? Can anybody by any chance provide "proof" one way or the other?

500 Points for this one!

--Jon
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Question by:CoastalData

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Based on the reasoning you just gave, I would say making a loop of the power cord does NOT increase resistance, because resistance is a function of the cross-section of the wire, the conductivity, and the length, and none of these are affected by looping it.  It WILL increase the inductance, but I doubt a loop is going to dissuade a million volt (or thousand volt) surge from wrecking your equipment.  The best designed lightning protection costs enormous sums of money and is capable of absorbing a huge amount of energy, but even then is not guaranteed to stop a nearby strike - there's just too much current at too high a voltage to stop.
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Hi Joan,

Wow, that sounds interesting. I'm looking forward to the proof.

1) A knot is close to a coil with only one turn of the wire.
2) A coil (inductivity) will effect a voltage surge ...
3) I am waiting for the math on this solution :-)

======
Werner
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Assisted Solution

http://www.telos-systems.com/?/techtalk/surge.htm

it seems to confirm your theory also suggest 3 knots not 1.
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and this one has an alternative to the knot, coiling the cord around a pipe.
http://www.overclockers.com/tips1132/index03.asp
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and look here, down at the 3rd post for another persons experience with knots
http://www.segwaychat.com/forum/topic.asp?TOPIC_ID=848
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Callandor

The power involved is the least of the problem. The critical part is the dI/dt.

======
Werner
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Hey Everyone!

Does standing in a filled bathtub while holding the powerstrip in your teeth help?

wgarneau
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Seriously though - the vast majority of power surges come from telephone lines rather than power lines anyway, so even if a knot in the power strip helps your lamp, it probably won't help your computer.
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Usually in a High voltage strike, such as a lightning strike... the electrons will skim the surface of the conductor instead of passing thru the conductor... possibly the purpose of the knot is to mis-direct this particular halo effect and blow the knot.

Jeff
--signature edited--
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Let me say this. Putting a knot in the cable will in all practical application do nothing for the ability of it to protect against power surges. This is due to the fact that you are not increasing the length and coiling of the cable to any point at which a toroid effect will enable the smoothing of voltage fluctuation (eg. Line conditioner/ power correction). Also the fact that the cable run still has the insulation separating it from the other coils with heavily impare the creation of the previous effect.

Cross sectional area of the cable will indeed affect the resistance depending on whether the cable has multiple core wires. This will only affect the voltage drop over the distance of the cable and how much current the cable is effectively able to carry without physical damage to its structure.

Also, any power surge capable of "blowing the knot" will definitely leak enough current past the knot to blow any household appliance or power board without very good power protection.

Moral of the story is that putting a knot in the cable wll not help at all. This is why people have made Line conditioners, UPS' with power protection and correction!

The only thing putting a knot in the cable will assist with is...... shortening the cable.
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I don't know where your friend when to engineering school, but remind me not to send my son there.

Putting the cord in a know will have no practical impact.

I could see where this could come from.  Putting a significant inductance in series with the power line would help to prevent surges, and indeed power strips with surge protection, and UPS', do just that (among other things).

In theory, a "knot" forms an inductor from the power cord wire.

The problem is, the amount of inductance is insignificant.  I mean, it's truly insignificant, it's probably not even measureable.  Basically, this would be like the city of New York putting down one single grain of salt after a snowstorm and then telling the population that "the city street crews have salted the roads".  There is a tiny degree of truth in the principle, but the extent of the action taken will have no practical effect whatsoever.

[As to my own credentials, click on my userid]

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Hello, Wow, thank you all for your input!!!! I've been reading along all day, but have not had a chance to post back untill just now.

I think that the way this was explained to me, some years back, was similar to what Jeff says, just above, but that ultimately the power surge will take the easiest path to ground it can find, and if the cord is knotted, the surge *might* choose some other path of lesser resistance.

Certainly I agree, however, that anything that blew the knot would certainly arc and fry the equipment on the other side of the knot.

The concept of the coil effect is interesting, though likely impractical, however, I want to read up more on the links posted above by nickg5.
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i think the real protection is done by the power strip itself, as most of the modern power strips comes with metal oxide varistor that 'shorts' to gound pin when the voltage is over, say 200vac.

no need to tie a knot....
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I remember reading this in one of the better pc mags (a few years ago) as a stopgap if you didn't have a surge protector. They said the knot had to be tight, that it would stop normal powerline spikes (though probably not lighting).
I was also told this by an old Electrical Engineer (he helped build and install radar when it first came out. He also has some good stories about pre-punch card computers).   : D
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All in all, buying a UPS might be a far better use of your time.

Just so you know, Current, does not change instantly when an inductor is used(hence the L dI(t)/dt) and as such  useful when dealing with "Current Spikes". Voltage does not change instantly across a Capacitor, therefor it will help with "Voltage Spikes".

However, for them to deal with lightning, the inductor and capacitor would need to be the size of a large room
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The "coil" formed by a knot is of course a one turn inductance, but it has effect only on current going in all the conductors in the cable simultaneously, in the same direction. This is not a normal state of affairs and it should occur only if there's a lightning spike in both live and zero leads that go through your equipment to ground. Not very likely to happen, unless the equipment has some kind of separare grounding lead instead of the normal grounding through the mains plug. And it presupposes some severe kind of insulation breakthrough in the power supply. Current going "both ways" in the mains cable (as it normally does) will not be affected by this "knot coil" as the magnetic fields of each lead will counteract, annulling the effect of the "coil". An inductive spike protector could be constructed by separating the leads in the mains cable and inserting individual coils in each of live and zero lead.
/RID
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Hi Rid, from most of what I've read, they're saying that a one-turn coil just wouldn't ever provide enough inductance in the line to protect it much, if at all, even though the theory does have merit. Furthermore, from what I've read, what the effect would be, if you did somehow provide enough inductance, would be for the inducted cord to blow out, and thus not be able to transmit the power surge to the devide... Like a fuse?

But it seems that you're saying to take advantage of the actual merit of this, you would maybe somehow create an inline device, which seperates the leads, and applies inductance to each?

Ultimately, however, I think the cut and dried answer seems to be that No, simply tieing a knot in a power cord will probably not provide any significant power surge protection of any kind.

Objections, anybody? I still *want* to think that it's true, but evidence suggests otherwise.

If no objections, then I think a point split is in order -- Comments on how they should be split?

--Jon
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Accepted Solution

A one turn inductor won't do ANYTHING measureable, and any of you who think that it will are buying snake-oil.

Surge protectors (or at least high-end ones) have real inductors in them (many turns with ferrite cores), but the primary element, as chiingliang correctly stated, is an MOV (metal oxide varistor) that shorts out the surge.

The knot idea probably started in the April edition of some magazine.
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Did I hear someone say that they wanted to see the math?   Here it is... There will be a short quiz later

http://www.uic.edu/classes/eecs/eecs520/textbook/uslenghi.html
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Hi Watzman, maybe it was there, too, but I heard this years ago, and was challenged on it the other day, so I had to check with the Experts to get the Real Deal!!!!

I think the split, then, should go between Tsuro Hata, who was first to post along these lines, and humorously mentioned the shortening of the cord, yourself, and chiingliang, who've helped me, stubborn as I am, to understand why this is pulp science!

Thanks again!!!
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Hey Everyone!

We should be talking about surge protection from where power surges are most likely to come - telephone lines.  Most surges come from the phone lines rather than the power lines.  Most houses have all kinds of equipment (like a big fuse box) protecting the inside of their house with electrical.  For phone equipment?  Nada.

But knots aren't a viable solution.  Get a surge protector that includes phone protection...

I've you've ever seen a computer fried by lightning, one of the things you'll notice is that the modem is cooked.  I've actually seen melted chips on modems...

wgarneau
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A one turn coil (knot) will not provide enough inductance to make any difference.

A coil made up from leads carrying opposing current will not provide any inductance at all.

The knot will only have a *theoretical* impact if the current runs in the same direction in all the leads in the cable forming it. If we're talking GHz the reactance would be measurable. In a surge protector it is totally negligible.
/RID
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Hi Rudy,

Yes, actually, I am very interested in the math, but it is not my strongpoint, so I'd need a little more direction than just that!

If anybody'd care to lay it all out, I'd be happy to post another question for, say, 250 points? Nothing too intense, just enough to that us mathematical flunkies can look at it, and come away feeling like we've learned something!

--Jon
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Thanks Coastal... I didn't really expect to earn any points.  Maxwell's equations are pretty much the answer to all things electrical but, even though I remember taking electromagnetics courses in college I cannot even begin to claim that I understand them.
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It was certainly an entertaining thread, if nothing else.
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The more that I think about this, the more that I actually believe (seriously) that this started as a "April Fools joke" in the April issue of some technical magazine, and that someone did not realize that their leg was being pulled.

There have been many of these, many technical magazines publish such articles every April.  I remember, about 40 years ago, reading an article in QST (an Amateur Radio technical monthly magazine) about how you could increase an Antennas effectiveness by changing the wire size mid-element.  Since the thinner wire and the thicker wires were "in series", the current flowing through both had to be the same, therefore the electron density flowing in the smaller, thinner wire would be greater, which would produce a stronger magnetic field.  The article sounded perfectly plausible and there were calculus differential and integral equations, construction details, photographs, test reports .... just like a real article.  The test antenna had been dubbed "Lirpa 1" (Lirpa =April spelled backwards).

Such articles are common, and appear every April.  And the "symbology" of using a "knot" to keep something from getting "through" the wire, like it was a garden hose, is "just too much".  But the fact that inductance truly would help to inhibit surges (although by itself it's not sufficient, even with a significant inductor), and that a coil of wire is an inductor, gives one the basis for a "plausible" story.

FWIW, almost every PC power supply has both a significant inductor (not huge, but significant) and an MOV inside the power supply as part of it's construction.  It's standard stuff, even in most of the relatively cheap power supplies.
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