Re: HV xray cable revisited

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

The black coating is "resistive" not "conductive".  If you ground one 
end and run say 60Hz AC across it, the far ungrounded end can and 
probably does get to a pretty high voltage.

For example, if the cable is ten feet long with a resistance of say 
10k ohms / foot, then the far end of the cable is 100k ohms to 
ground.  Now if we "assume" a capacitance of 10nF in the cable at 60 
Hz we have 265k ohms or leakage reactance to the outer layer.  If you 
put 15kV into the cable, the outer layer voltage is 100k / (265k + 
100k) x 15k = 4110 volts.  So it arcs to ground very well...

The conductive outer braid was meant to prevent that by providing a 
solid low resistance conductive path to ground which reduces the 
outer voltage to very near zero.

Cheers,

        Terry


At 12:00 AM 1/6/2006, you wrote:
> Hi Bart and Mike,
>
> Thinking out loud here.  If the arcing is between the center 
> conductor and the black outer shield, then there is a problem that 
> needs to be fixed.  If the arcing is between the outer shield 
> (ungrounded) and a grounded object, I would think this would be 
> expected due to the capacitance between the inner conductor and the 
> outer shield.  The outer shield has no way to get rid of induced 
> charge and would certainly arc to something grounded.  If the outer 
> shield was grounded, I would think this discharge would cease.
>
> Gerry R.
>
> > Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
> >
> > Hi Mike,
> >
> > As I understand it, the coating arcs not at the ends, but when the 
> > black coating comes in contact to a grounded object (or the ground 
> > itself). I suspect the cable is not sufficient for AC applications 
> > at the voltage your running. Check around and you'll see DC ratings 
> > on high voltage cable. AC and pulsed DC specs are much lower, so I 
> > wonder if the cable is not sufficient for the application. But, we 
> > can only make our best guess. As the cable is in your hands, 
> > possibly you could find out it's DC and AC ratings.
> >
> > Take care,
> > Bart