Tie A Knot In It!

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Simple electrical protection – Rusty

Ed Note: Rusty emailed this and I thought it would be interesting to others.

While a warranty for surge suppressors and UPSs is fine, prevention is better.

Years ago, a Navy
electronics technician reasoned (rightfully, as it turned out) that the
major source of fried electronics was from the surge current (not voltage),
and that it created an accompanying magnetic field. He thought that if the
magnetic field could be suppressed (or better, reversed), the surge current
could be neutralized.

His solution was simple – in all wires – power, net, modem, etc. he tied a
simple knot in each cable:


When you see these round things on cables, sometimes all that’s inside is a knot, sometimes it’s a metal sleeve (ferrite core).

A few months after, a major lightning storm
swept through the area (Washington, DC). The Army and Air Force lost a lot
of equipment. The Navy, none.

Tie a knot in each cable. I implemented this in our office about fifteen
years ago. We took a direct lightning hit on our phone – while it fried
the telco’s box, we lost nothing.

Incidentally, a lightning strike can cause more damage with an indirect hit
than a direct hit. One of my Navy Reserve officers was the Superintendent of
Electrical Services at Tampa International Airport, the first with “Totally
Electric” terminal systems. The “People Movers”, electric shuttle cars,
would frequently fail during a lightning storm; the breakers (700 A) would
either fail to trip or fuse closed.

The cause was that the lightning strike created a rising-falling magnetic field – exactly what induces a
current in a conductor. So the airport, during a storm, became an huge
generator! The current was generated downstream of the supply breakers.


Reader Responses CONTINUED page 2…

Jelle Gerardts:

“Just wanted to say in response to this that it has a negative point one should be aware off.

As current flows through a wire a (generally small) electromagnetic field is generated.
When a magnetic field (or more accurately a CHANGE in magnetic field) is applied to a wire loop, it generates a small current. To see any visible effect, one would need a spool with several thousand winds and a cast iron core. The effect this has with DC, as displayed in high school physics, is that a light bulb will start glowing a second later then it would without the spool. When turning the power off, the lightbulbs fades out in about 2 seconds.

Since this effect only occurs with changes of magnetic field, the effect is with DC in when turning the device on and off. AC current changes polarity about 60 times each second. This way, a single loop could create a, still VERY small, distortion in the power flow. A good example is a extension cord. If you own one of the types you can roll up, its maximum wattage while wound up should differ from its maximum wattage fully unwound (My 10M cable is 1000W vs 3200W).

My knowledge of electronics isn’t sufficient to say if this could create any problems, just as I can’t explain the protective effects it has, but it might be worth knowing.”

Ian Pengelly BSc

“Whilst I agree with the theory on Rusty’s advice, this advice needs a warning with it. On cables with low current draw, there would be no problem with the knot (effectively an inductor) being there. However, on a cable with a high current flow, there will be localised heating. If you look on any cable that comes on a windup holder (ie Jo Jo leads and the like), they give ratings for wound and fully unwound, specifically for this reason.

There are also a couple of other side effects:

  • The loop will reduce the cables ability to deliver a large current when required, like the inrush current that a transformer draws when it is switched on.

  • Some devices may not start properly because of this; others, such as power amplifiers in hi-fi, will not perform (sound) as well.

The best advice given so far is the use of an isolation transformer that is of sufficient size to meet your power requirements. It works on a similar principle to the loop in the cable, but has the advantage of being completely electrically isolated from the source and it is designed to cope with an increase in temperature.”

More Reader Responses CONTINUED page 3…

Ed Note: OK, maybe it’s me, but I find these little tidbits interesting; if you do, read on:

Bill Wichers

“Saw your two responses and wanted to add something:

Both responses list
coiled cords as an example, and how they have lower ratings when wound up
then when unwound. This is not due to magnetic induction from the coil, but
rather due to the concentrated heating caused by *resistive losses* in the
wire, which produce a higher temperature rise when the cable is coiled due
to reduced cooling.

At 60 Hz line current, a knot is not going to cause any
problems from magnetic induction, and there won’t be heating issues since
there isn’t enough wire in a single knot to make a big difference. A knot
will also not effect an audio amplifier’s “sound”. Any DC-output power
supply, such as those used in audio amplifiers, will have capacitors to deal
with large surges in current demand and will not be affected by a minute
amount of inductance added to the power cord.

Much better than a knot in the cord is to wind several coils of the cord on
a piece of iron pipe. The iron pipe serves as a magnetic core which results
in an inductor of higher value and is far more effective at stopping large
transients (such as those caused by nearby lightning strikes) than a simple
knot would be.”


“The principle is inductance, as well as the mutual inductance occurring in the wire itself.

All wire has an inductive field from AC power; that inductance can be measured or calculated mathematically. Further, you do not need an iron core; the iron core contributes a gauss field only and while it has an effect. It is not necessary for all uses; see the inside of the ol’ computer – there are coils on MOBO’s and some other add in cards that uses ceramic, or even paper doped or not doped with a resin.

A localized event from a magnetic field can be sufficient, in a coiled or knotted section, that makes it higher or high enough from the wire passing through, that a return event can be created based on time and inductance. The time of the event (lightning strike) and the inductance which could be in the micro number range can capture, in a magnetic field event, the lightning event and stop the real damage, or reduce the damage passing through. Most likely, the cord will be destroyed by the event however.

One other detail: the resistance and capacitance of the wire also affects its properties.

See a good electronics tech manual, this information on inductance and gauss can be found there.”

Paul Alexandroff, aka Paxmax

“I work as an Electrical Engineer and as an Electrical Test Engineer.

I test manufactured electronic equipment according to international (such as
IEC) and national standards (such as UL).

Among the tests I preform is a “whoopass” voltage surge (not the actual
technical term, but a good description) which simulates a lightning strike
on a nearby electrical pole. I can set the severity level in terms of
voltage, energy content and rise/fall time of pulse. We are talking about
1-12 kV for this one, and equipment that fails usually goes “BANG!” with

If manufacturers knew that a knot would be a good protection against such
surges, I would expect see a LOT of knots in tested equipment, since a knot
would be very cheap! However, that is not the case for me.
I can’t say I have any other real hard data to dismiss the idea totally
either – I could run some simple tests to try the theory.

However, I’d like to say that the idea that knots and coils would impede the
mains current is completely untrue, at least for the average 110/230V 50/60
Hz user.

The coils and knots I’m talking about now are wound with an ordinary mains
cable, that houses both live and neutral wire and possibly earth wire. For any appliance in the household to work, we need a current that flows to
the “thing”, the “thing” does it’s magic, and then the current has to go
back to the source.

So, the current rushing into a live wire will also, at the same time, travel in
the opposite direction in the neutral wire. The voltage,(and current) will
every so often change its direction, since it is AC 50/60 Hz, but the current
will always go in opposite directions in the neutral and live wire. The
magnetic field created by both wires will, therefore, cancel each,other out,
so in essence no EMF is created.

The coiled extension cord gets hot because of the resistance of the wire.
The current will experience “friction” when passing through. To fit 30 feet
or so on a “handy” coil, the wires can’t be too thick. So, manufacturers
skimp on the wire size and rely instead on the cooling effect provided by a
unrolled in-the-open cord.

For those who still believe in the “EMF coil heating of cords”, I have a
simple home assignment: Unroll the extension cord (to reduce the EMF, right?),
insulate 3 feet or so with some bubble wrap or towels and load it to the
maximum. After a minute or two, you can test if the cord is hot or not. You
need to insulate at least 3 feet or so, otherwise the cable will be cooled by
the nearby non-insulated area. Since the wire is made from copper, it will
lead off the heat.”

Wray Odom

“Although the facts presented in these responses are accurate, I don’t see them as being particularly relevant. First, we are dealing with rather small currents. Phone lines, ethernet cables and coaxial TV cables in particular draw so little current that heat dissipation is negligible; but at this level of inductance, reactance is so small that even the power cord to your computer will not generate significant heat. (For the purpose of this discussion, we can define reactance as the resistance to an alternating current (AC) or to a change in a direct current.)

This brings us to the second point: Their examples feature tens or even hundreds of coils very close together. This results in a relatively high inductance leading to a high reactance. Rusty’s proposition uses a single coil – a very small inductance resulting in a small reactance. In fact, that’s what makes his proposal dubious to me. It seems unlikely that there would be sufficient reactance to dampen the pulse (Although it should be born in mind that inductive reactance is inversely proportional to the period of a pulse, or put another way, the shorter the pulse, the easier it is for even a small choke to block it.)

If you doubt this, consider all the wires you have looped and tangled behind your entertainment center. Is it generating substantially more heat than if the wires were all perfectly straight? Do you notice it significantly degrading your audio? And most importantly to this discussion, has it protected your stereo and TV from a lightning strike? I realize that tying a knot will make a tighter loop which will raise the inductance. I’m just dubious that it will be enough to make a significant difference.”

Aiden Morrison

“I must say that I’m a bit sceptical about all of this tie a knot in it stuff managing to save equipment. The anecdote about the Air Force (or was it navy) not suffering damage, and the company not losing equipment for over a decade is impressive *cough* but I have reason to think these claims need substantial substantiating :).

For one the base inductance of a conductor, just a straight piece of wire is about 1uH/m. (one micro Henry per meter) adding a loop could give us maybe a few micro henries…. if we were lucky.

Also keep in mind that every inch of wire between you and where your power is generated adds inductance, so if the loop were a true guardian of electronics it would already be redundant a few thousand times over….

Go HERE and check out the double blind test on speaker wire, made possible by people who pay $500 for a magic extension cord because it makes their music sound ‘like running downhill'”.

Aaron Rhynas

“In my experience, particularly noticeable with coaxial cable, looping the cable causes interference, and in the case of a TV signal, it makes the signal worse. I know coax is extremely susceptible to interference, but recently I noticed a similar problem with my monitor cable… It had somehow managed to get itself tangled nicely into the rat’s nest that *used to be* behind my desk, and I was getting some nice visual effects as a result. Ghosts, shadows, streaks across the screen…

When the colors started changing, I decided to check it out. The cable was tight, but when I isolated the monitor cable from all the other cables (there was a power cord and a couple other cords running directly parallel to it, right up against it), all my problems went away.”

Al Smith aka oldbrave

“This is not A NEW IDEA!

In 1956 and 1957, Ford Motor Co. used this very simple solution on
the coil wire of the Thunderbird to greatly reduce EMI to the radio
and it had no apparent effect on the operation of the Ignition system!
As a matter of fact, it’s something that knowledgeable collectors
look for in the identification of original and/or properly restored cars!”

Steve D

“Reminds me of Job I did: When I was TV Cameraman in USAF, we used a reel of power cord that was 100-200 feet on a reel. One day we did a job and did not need to unreel it, with 5K lights and 20 amps a-sucking.

With only a few feet, we smelled the rubber burning. Boy did it get hot!

From then on, we always unreeled the cable spool for AC!

Thank God we smelled it, or FIRE. But we would not leave a 5000 watt light by itself burning. The problem was getting enough power – you can only hook one up to a circuit and after that, you snake cords thru the building – if they let you.”

Ed Note: Thanks to all who took the time to contribute their expertise and experiences. I have to confess to an intense curiosity about things like this, and you never know how this little piece of electrical knowledge might come in handy some day.

Email Joe


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