Protecting equipment from lightning???

[Wittenborn]

Hello all!

A guy I work with just read a book concerning computer hardware/software, and how to protect them from lightning. The author suggested that if you tie the cord in three knots (between the outlet and the computer) it will protect the computer from an electrical surge from lightning.

The idea is that when a high level of voltage travels through the cord and reaches one of the knots, it will block the high voltage from passing through the knots. He said to use three knots incase the first set don't get the job done.

Being an aerospace guy, I don't know whether or not to believe this, but I figured that this is a good place to pose the question.

Does this suggestion have any merit??

Regards,
Grant
Aerospace Engineer

 

[buzzp]

Thats laughable in my opinion. The guy is full of beans.

There is no benefit at all by doing this to protect from lightning.

 

[rbulsara]

Just let us know which aircraft will be using this technology!!!!! for our safety!

 

[jghrist]

The logic is that the knots (like loops in wire) make a large inductance that block the high frequency surge current. The fallacy is that the damaging voltage is not caused by lightning current flowing to ground through the computer cord, but by current flowing to ground somewhere else.

 

[Wittenborn]

jghrist,

Thanks for your explaination. If I understand you correctly, the damage to the computer isn't caused by the direct voltage from the lightning, but rather the voltage going to ground via a path other than the computer cord? Then how does my computer get damaged? Sorry for the ignorance...


buzzp,

"There is no benefit at all by doing this to protect from lightning."

Why not?


rbulsara,

"Just let us know which aircraft will be using this technology!!!!! for our safety!"

I ain't tellin!

Actually, we try to dissipate lightning in aircraft, and try to route it out of the airplane via a path OTHER than the electrical and avionics systems... This is why we use static wicks on the trailing edges.




Regards,
Grant
Aerospace Engineer

 

[GOTWW]

Other than side-flash, I believe you have a good point. If you office and computer are outside of the nfpa 780 sideflash distance I do not see voltage surge on nearby grounded objects having an effect. (but if I am wrong, please feel free to blast awaY!) The knots are for inductance, often power and telecom is barried with a snake shape near the riser from buildings, wind generator towers etc., this two is too increase the inductance as well.

 

[jghrist]

If the voltage between the power supply wire and the computer case is higher than the insulation level of the power supply, then there will be a flashover and damage. The voltage can occur because of surge current directly on the power conductor somewhere or be induced by nearby stroke current. It may be that the voltage of the ground wire that connects to the computer case increases because of surge current flowing into the earth through the grounding system. A lightning stroke going to earth causes voltage differences all over the place because of the inductance of different paths to ground.

The only thing that the knots in the power cord would do is restrict surge current flowing in the power cord. There will be no surge current flowing in the power cord unless something in the computer has flashed over, in which case it is too late to prevent damage.

 

[Wittenborn]

Good explanation jghrist, Thanks



Regards,
Grant
Aerospace Engineer

 

[GOTWW]

But, wait, will not the added inductance lower the rise time of said "voltage-rise" between power leads and ground. And if the width of said rise is narrow, then a lot of the higher freq energy components are filtered out. How long a surge are we talking about? lightning is on the order of 20mS or 1.3 cycles at 60Hz. The power supply should have 600V BIL.

 

[mersin33]

Theoretically I don't know...depending on the steepness of the surge, duration of the surge, etc this may even be within the wave theory.

Practically however, after going through real inductive medium like transformers, conduits with many other cables carrying current, etc (unless of course, the lightning striked right next to the cord and induced a surge capacitively), I cannot see what additional benefit would the inductance of three knots do in terms of suppressing current (dimensionally, it would feel like a conductor for the surge anyways).

 

[Wittenborn]

Sounds like we need some experimentation?



Regards,
Grant
Aerospace Engineer

 

[GOTWW]

Per NFPA-780, max bend radius of down conductors is 8 inches, maximum rate of rise is 1:4. Now I do not know how tight you tie your knots, but...

 

[jghrist]

Wittenborn,

First string the power cord through a key before tying the knot. Use the second knot to tie a kite string to the cord. Go fly the kite during the next thunderstorm. Then ...

 

[buzzp]

Why isn't there any benefit?

Well first off we need to define what this is suppossed to help and under what conditions. I believe we can rule out a direct strike (using the cord as path to ground) since the cord will be vaporized immediately.

I don't really care to take the time to explain why something so off the wall will not work. Ill leave you to figure that out by conducting some experiments. One loop is not going to do anything for you. The inductance of the wire itself is likely more effective at reducing 'transients' than one knot.

 

[GOTWW]

Buzzpee, The rise time of lighting is on the order of 50nS, that is 20MHz of bandwidth. The The knots will have significant inducance in this band, whereas your cable alone will be a twin-lead transmission line, the common mode is unlikely as for the neutral ground bond. Waving your hands does not cut it, we what facts!

 

[sreid]

The amount of inductance added to the line cord with three loops of wire is not very much. To really protect against lighting strikes one usually needs inductors, capacitors (line filters), fuses, MOVs, spike catchers and spark gaps.

Most electronics is protected to some degree against some level of line spikes. A power strip with a circuit breaker and a MOV provides some additional protection.

 

[jbartos]

Suggestions to Wittenborn (Aerospace) May 28, 2004 marked ///\\A guy I work with just read a book concerning computer hardware/software, and how to protect them from lightning. The author suggested that if you tie the cord in three knots (between the outlet and the computer) it will protect the computer from an electrical surge from lightning.
The idea is that when a high level of voltage travels through the cord and reaches one of the knots, it will block the high voltage from passing through the knots. He said to use three knots incase the first set don't get the job done.
///There may be some analogy applied from the Lightning Air Terminal downwires that are supposed to form a smooth run from the air terminal to ground rods so that the lightning does not have to negotiate any turns thus not causing any path to some other conductive surfaces and paths to the ground (which would be unwelcome).\\Being an aerospace guy, I don't know whether or not to believe this, but I figured that this is a good place to pose the question.
///Real smart.\\Does this suggestion have any merit??
///In highly unlikely case that the lightning would actually travel along the cord, there would be some merit. However, the lightning peak power is so destructive that the cord knots will provide very little or negligible protection. Normally, the lightning elevates the potential on all conductive paths, surfaces and soil surrounding the direct lightning hit to the ground.\\\