# Tying knots in power cables to prevent power surge from lightning strikes

There are plenty of references to this claim on the internet that tying knots in power cables will prevent a piece of equipment e.g. television or computer from a power surge.

How can this be debunked (or proven) using mathematics?

I stumbled across this which seems reasonable to me, but is there some way this can be proved?

The surge impedance of any line is the square root of its inductance divided by its capacitance, and electromagnetic waves travel most readily down a line where that surge impedance doesn't change. A point of changing impedance is a discontinuity that causes a partial reflection of the wave back towards its source. As an example, the end of the line is a surge impedance jump to infinity and the whole wave is reflected back (which means the wave voltage at the open end doubles!) This is also the reason why you want to use terminators on the ends of coaxial cables. Open-ended cables will reflect back the signal causing poorer picture quality and ghosting (and similar things happen for poorly made connections that have higher impedances than the surge impedance of the coax).

Knotting the line gives that part of it a higher inductance (think of the knot as a coil with a couple of turns). That means two surge impedance discontinuities (from line to knot, and from knot back to line). It seems to me (too lazy to resort to doing the math) that this is bound to reduce the magnitude (voltage and current) of a surge passing through the knot because some will be reflected back. However, I'd guess that the reduction would be small.

• Similar recent question on Skeptic.SE: Does tying a knot in a power cord provide any protection against lightning?. Aug 26, 2011 at 4:12
• Damned if you do, damned if you don't. Quote: "One just need to coil up the cable a sufficient number of times, or use too much insulation around the cable, or both. Laboratory tests have shown that ignition ... may easily occur in such cases. At one such incident it was sufficient to coil up the cable three times and cover the coil in clothing." Fire or lightning? Take your pick. Aug 26, 2011 at 14:14
• @Willie - Thanks for that link. It would have been good if they linked to the 'laboratory tests' they mention. Aug 27, 2011 at 10:06