coax as a capacitor

It was only a temporary fix,and was replaced later that same day.I just wondered what caused it.Every wire has a certain capacitance,and when two conductors are separated by an insulator,you have a capacitor.I figured this was the inherent capacitance of the coax discharging.RG6 has about 30Pf per foot.X 10 feet = 300Pf.

Not much,but enough to produce a visible spark.

<...visible spark...> um, er, assume 1:1 transformer is um, er, <tap, tap, tap, press equals> er, around 0.6 Joules.

Is it capacitor action or transformer action? Does it spark at both ends? Is the shield braid? Try it at nite.

I don't believe that he said it was RG58 but it may have been, he could have used RG213 which has a max voltage of 5000 VRMS which means there is a peak voltage of 7000V. I have seen RG213 used as and under gate jumper.

It would be capacitive inductance I would think. As an aside when we worked on testing surge diverters for 11KV lines it was the theory that the voltage after the breakdown of the diverter would rise at the rate of 70KV per foot, hence the positioning of arrestore at the bushing of the transformers.

I agree. It's most certainly the voltage pulses from the electric fence punching through the dielectric of the coax.

The only way I can see that capacitance could increase the voltage is if it were part of a resonant circuit and the electric fence were driving it at the resonant frequency.

A repeating pulse is not DC.

Assuming the one-per-second pulse is at 5 kV for only one millisecond when working properly then this one-hertz pulse has a 0.1% duty cycle. (I suspect the duty cycle is higher but I have made my point.)

Since the shield is not connected to the grounded return of the pulsed power source, the capacitor between the center conductor and shield is discharged (Δ0V) until the outer, black sheath breaks downs. Likely the pulsing voltage source is current-limited to prevent dead cows so the shield is not at 5 kV for a very long period of the arcing interval.

Most importantly, as long as this arcing cable is connected to the electric fence the whole fence is not working anywhere to stop cattle.

Further checking revealed that the arcing of the shield occurs between the main pulses, when main pulse is in off state,which indicates a capacitor to me.

It is a very weak pulse in comparison to the main pulse,so if I connected the center conductor to the hot side and the shield to ground it would give an additional minor shock to stubborn animals,like goats.

A repeating pulse,of same polarity is DC.It does not change polarity.It is a pulsating DC voltage.A half-wave of an AC pulse.The fence has a diode in series with the output to the fence,it is a solar powered fence.

I once put a solid state TV trippler on an old fashioned transformer type fence charger,but it was WAY too much voltage.

Around 25KV..it was arcing across the ceramic insulators to the fence post!YIKES!

It would light up a neon voltage tester screwdriver from 4 feet away,no contact.

I disconnected it immediately.

"A repeating pulse, of same polarity is DC."

Huh?

Technically, it can be both AC and DC. A spectrum analyzer would show a DC component and an AC spectrum.

Pulsed DC current changes in value, but not in direction.

It's polarity does not change.

Try this with a simple 1.5v cell.Manually connect and disconnect one wire,look at wave form on scope,you will see.

AC means:Alternating Current, meaning the polarity is alternating from positive to negative.

In AC the polarity constantly changes. see https://en.wikipedia.org/wiki/Pulsed_DC

From a mathematical point of view, you can understand a "pulsed DC" signal as the superposition of a "pure" AC signal and a DC offset.

"AC means: Alternating Current."

Exactly: use a current probe to study the current in the near end of your electric fence.

Zero Volts is arbitrary; zero Amps is not.

Current flows in one direction in order to reach 5 kV. To discharge back to 0 V the current flows in the alternate direction. A biased pulse has a DC component but a pulse is not DC.

I have been enlightened by the conversations on this forum.

I just never thought about it in this way.

I always thought of a DC pulse as rising in amplitude,then decreasing in amplitude,but never crossing zero.

And actual ZERO is undefinable,depending on how sensitive the measurement device,which is why most RS 232,488, transmit signals use a non zero for logic.

I now realize it could be interpreted as an alternating voltage if applied to a capacitor or inductor.

Thanks to all contributors!