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In Reply to: Re: Technically speaking.... posted by Steve Eddy on March 26, 2007 at 13:37:00:
Hi.1,000,000,000R/cm3=1,000R/cm=100R/mm. (Not 1R/mm, sorry wrong typing).
Taking the example of a #18 16 strands hook up wire with extruded PVC jacket, commonly used for HV PS for tube amps. Its wall thickness is 0.925mm (UL/CSA rating). So if it were cotton wrapp insulation, its dielectric resistance would be 0.925x100R=92.5R ASSUMING the cotton wrap get the same EVEN thickness of a #18 hook-up wire designed for 600V UL/CSA rating.
So if the hook-up wire of same gauge size but with cotton wrap of same thickness instead of extruded PVC were used to carry say 350V typical tube plates HV PS, there would be a leakage DC current running from the copper conductor to the chassis neutral ground via the cotton wrap (350V/92.5=3.78mA) !!!! This would be totally unacceptable.
This would be an ideal situation ONLY if we ASSUMED even insulation thickness throughout the cotton wrap wire. Since cotton wrapp is NOT extruded like rigid PVC or Teflon materials, the current leakage would be very severe if the thickness of the cotton wrap at the contact point were futher squeezed down due to the weight of the wire itself & for whatever reasons.
Hence my statement: for signal path wiring is OK, but I would never use cotton wires for HV, period. Even for signal path use, I would make it damn sure it would NOT get close to any HV wires let alone its high dielectric constant e.
But if some find cotton wires sound better disregarding its sticky physical & electrical properties, it is their call.
c-J
Follow Ups:
DCC (Double cotton covered) copper wire was all there was in the days before varnishing and plastic extrusion.I have some interesting old radio books showing how to wind 500V coils with DCC wire. ETC.
You may do what you wish, of course, but it doesn't help others to make questionable claims, and your 1 ohm figure is nonsense!
Hi.In the good old days, before the National Board of Fire Underwriters
(NBFU), UL & CSA stepped in with the electrical standards, you could use whatever you wanted. Who would give a damn?But if you use anything electrical, not rated by UL or NBFU or CSA (in Canada) today, you are asking for trouble in case of fire.
The answer is a value in ohms, perhaps with a explanation of measurement method.My example of how cotton WAS used simply illustrates that it cannot have a inherent low resistance.
Your lecture on fire hazards etc is not releavant and just a diversion.
Your lecture on fire hazards etc is not releavant and just a diversion.
Yup. He originally said "Its resistivity is 1,000R/cm, or 1 ohm per mm. Theoretically, it can't handle any voltage at all !!!!" Now he's just doing a lot of dancing around. Not worth the time.
se
Hi.Basing on 1,000,000,000R/cm3, I deduce electrical resistance per linear mm is 100R (cube root of the above figure).
You said I was wrong. So what is the correct insulation resistance per mm thickness of cotton ???
Basing on 1,000,000,000R/cm3, I deduce electrical resistance per linear mm is 100R (cube root of the above figure).
Uh, 100 is the cube root of 1,000,000,000? I think you need new batteries for your calculator.
se
Hi.My calculation based on cotton's 100 million ohms/cm3 is 100R/mm.
You said it was "nonsense".Now you come up with less than half of my calculated value, only 42R.
So your's a "Double Nonsense".I got the impression neither of both insulation resistance numbers is correct.
Insulation resistance is in the range of mega ohms. Under a test voltage 250VDC, min. insulation resistance is 250KR. For 500VDC, 500KR, & for 1KV, 1MegaR. If a cotton wire could handle up to 500VDC,
its insulation resistance got to be in maga ohm range.So how you arrived this lowly 42R?
c-J
So how you arrived this lowly 42R?
I didn't. Deep Thought did. And it's not 42R, it's simply, forty-two.
se
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1,000,000,000R/cm3=1,000R/cm=100R/mm. (Not 1R/mm, sorry wrong typing).
Your numbers make absolutely no sense whatsoever.
You go from 10 9 ohms/cm 3 to 1,000 ohms/cm. What the hell's that supposed to mean? It's obviously not a cubic centimeter, but it can't be a square centimeter because then you have no thickness. You've divided the original number by one million. Where does that number come from?
It's nonsense.
If you want to scale it, you need to stick with three dimensions. One cubic centimeter is made up of 1,000 cubic millimeters. So you can also state the resistivity for cotton as 10 6 ohms/mm 3 .
So if the thickness of the cotton insulation is 1mm, then the resistance through an area of 1 square millimeter would be 1,000,000 ohms, not 100 ohms.
se
Hi.The insulation resitance of an insulated conductor is the electrical resistance offerd by its insulation to an impressed direct current potential tending to produce a leakage of current through the same.
For wire usually measured in megaohms per 1,000 feet.
So where come in volume & area?
Hi.Define 'electrical insulation resistance'.
c-J
Good day.se
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