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Re: Using HV COAX without stripping the shield



Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi David,

Yes, this run-away effect is because there is no termination at either end of the coax and nothing to dampen energy from the coax. I believe you didn't experience it because you got lucky and the high freqencies in your system didn't excite the natural resonance due to the coax length. If the coax is properly terminated at the pig end, then no reflections can be returned to TC end and this resonance is effectively "killed". I believe Dr R when he said that stripping the insulation fixed his problem (being flashover at the HV bushing of the pig). I continue to think that transmission line effects can still happen because even without the shield there is still distributed capacitance to earth ground in the coax and the distributed inductance got changed somewhat but still exists. Bottom line, is a transmission line still exist where the characteristic impedance of the shieldless coax got changed and the propagation velocity got changed (probably because of the addition of air in the dialectric). The resonant frequency of Dr R's given length of coax probably changed as a result of stripping the shield. Anyway, this is my thinking.

Gerry R.

Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

I realize that there is the
possiblity of run-away resonant voltage due to the Blume-
line (spelling?) effect, but I have never personally noticed
this problem when using my x-ray cables with the shielding
intact.