XWUU@PURCCVM.BITNET (04/04/91)
I recently read in a reputable computer repair manual that one can gain some measure of lightning protection for an electronic device (computer, TV, etc.) if a simple overhand knot were tied loosely in the power cord. The author says he got the idea from electronic specialists and that he uses this technique on all of his equipment since the last time lightning struck near his home. It seems that his TV was the only electronic device damaged by the lightning and it was the only devive without the knot in the cord. Even his sensitive computer and test equipment were not harmed. The theory is that the lightning somehow has to "work against itself" due to the overlapping coil of the knot. Sounds like voo-doo to me. What does any one else think. Any validity to this theory? XWUU@PURCCVM
henry@zoo.toronto.edu (Henry Spencer) (04/05/91)
In article <91093.214905XWUU@PURCCVM.BITNET> XWUU@PURCCVM.BITNET writes: >The theory is that the lightning somehow has to "work against itself" due to >the overlapping coil of the knot. Sounds like voo-doo to me. What does any >one else think. Any validity to this theory? Quite possibly. "Working against itself" is techno-illiteratespeak :-) for "inductance". Lightning is a *tremendously* sharp-edged pulse, with oodles of high-frequency content, and the slightest bit of inductance is a major barrier to it. (Not that you can *stop* it -- it's just punched through hundreds or thousands of meters of a very good insulator to get to you! -- but you can encourage it to go somewhere else instead.) It's plausible that adding some inductance to the power cord might help. -- "The stories one hears about putting up | Henry Spencer @ U of Toronto Zoology SunOS 4.1.1 are all true." -D. Harrison| henry@zoo.toronto.edu utzoo!henry
dmturne@PacBell.COM (Dave Turner) (04/06/91)
In article <91093.214905XWUU@PURCCVM.BITNET> XWUU@PURCCVM.BITNET writes: >I recently read in a reputable computer repair manual that one can gain >some measure of lightning protection for an electronic device (computer, TV, >etc.) if a simple overhand knot were tied loosely in the power cord. The >author says he got the idea from electronic specialists and that he uses this >technique on all of his equipment since the last time lightning struck near his > home. It seems that his TV was the only electronic device damaged by the >lightning and it was the only devive without the knot in the cord. Even >his sensitive computer and test equipment were not harmed. > >The theory is that the lightning somehow has to "work against itself" due to >the overlapping coil of the knot. Sounds like voo-doo to me. What does any >one else think. Any validity to this theory? > It is well known that parallel wires carrying equal and opposite currents produce magnetic fields which cancel. This fact is used to explain this form of lightning protection. The knotted wire produces tightly-coupled magnetic fields which induce a reverse EMF into a part of the wire which effectively is in the reverse direction. The current in the wire (especially in the knot) cancels and protects the circuit. This is only guaranteed to work on April 1. -- Dave Turner 415/823-2001 {att,bellcore,sun,ames,decwrl}!pacbell!dmturne
nagle@well.sf.ca.us (John Nagle) (04/08/91)
dmturne@PacBell.COM (Dave Turner) writes: >In article <91093.214905XWUU@PURCCVM.BITNET> XWUU@PURCCVM.BITNET writes: >>I recently read in a reputable computer repair manual that one can gain >>some measure of lightning protection for an electronic device (computer, TV, >>etc.) if a simple overhand knot were tied loosely in the power cord. It's not a totally stupid idea. Lightning arrestors for antennas usually have a few turns of a coil made out of very heavy wire (busbar in some units) to provide some inductance to block lightning pulses. But lightning arrestors also have a spark gap (in air, or in a gas tube) to provide a high-current path to ground. With the knot approach, you're presumably relying on arc-over at the wall outlet to perform that function. This might work, if you're lucky. Very lucky. John Nagle
lsls00@iliad.kodak.com (Ilia Levi) (04/08/91)
The reason the loop in the wire should help against a lighting is that the lightning is very high frequency, and even a small inductance in the one loop coil provides a large inductive resistance.
stevem@specialix.co.uk (Steven Murray) (04/09/91)
nagle@well.sf.ca.us (John Nagle) writes: >dmturne@PacBell.COM (Dave Turner) writes: >>In article <91093.214905XWUU@PURCCVM.BITNET> XWUU@PURCCVM.BITNET writes: >>>I recently read in a reputable computer repair manual that one can gain >>>some measure of lightning protection for an electronic device (computer, TV, >>>etc.) if a simple overhand knot were tied loosely in the power cord. [Deletions] >This might work, if you're lucky. Very lucky. Come on guys! Its not going to help! When lightning hits the energy goes down anything that conducts - and leaps some gaps that don't (obviously). Unplug your gear, or if you can't get some VDR's / Gas Discharge Tubes (Yes I know you're not supposed to use them on mains) / AC-DC-AC power converters in the likely paths. There are some people out there who are going to blame you (otherwise) when this idea doesn't work. Regards Steven Murray -- Steven Murray uunet!slxsys!stevem stevem@specialix.co.uk I am speaking, but | If these are your opinions, then we are in agreement!! not for my employer.| Flames, spelling errors, complaints > /dev/null
pierson@ggone.enet.dec.com (Dave Pierson) (04/10/91)
In article <1839@eastman.UUCP>, lsls00@iliad.kodak.com (Ilia Levi) writes... >The reason the loop in the wire should help against a lighting is that >the lightning is very high frequency, and even a small inductance in the one >loop coil provides a large inductive resistance. Which matches what i have seen, reasonably authoritatively. Ferrites would be better, but the knot should help. The issue is not one of protecting against a direct stroke, (THATs a very much harder problem) but of reducing damage from surges induced by that stroke, or surges induced by those surges. Even a little inductance will block/slow the surge. The stroke is limited time event. If its resulting surge(s) and their oscillations can be damped, damage can be limited, mayhap even avoided. (fwiw, i have seen a sketch for a field expedient coaxial antenna: assume ca 2M operation: strip braid from 18" of coax. Tie knot 18" below the point where the braid was cut off. Connect to transmitter. trim free end of center conductor, OR move knot to tune. I have (k)not tried this...) thanks dave pierson |the facts, as accurately as i can manage, Digital Equipment Corporation |the opinions, my own. 600 Nickerson Rd Marlboro, Mass 01752 pierson@cimnet.enet.dec.com "He has read everything, and, to his credit, written nothing." A J Raffles