Re: Electric Fences (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Tue, 28 Oct 2003 09:01:55 +1300
From: Malcolm Watts <m.j.watts@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Electric Fences (fwd)

Hi Matthew,

On 24 Oct 2003, at 8:12, High Voltage list wrote:

> Original poster: Steven Roys <sroys@xxxxxxxx>
>
>
>
> ---------- Forwarded message ----------
> Date: Fri, 24 Oct 2003 21:45:57 +0930
> From: Matthew Smith <matt@xxxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Electric Fences
>
> Greetings All
>
> Some "applied HV" questions for you today.
>
> 1) I need to feed power to an electric fence through an underground
> cable.  The solid core, triple-insulated cable sold for the purpose
> seems to have a very short lifespan, with the insulation splitting after
> very little time.  I am planning to route the cable through water pipe
> to prevent penetration by stones, etc., but was wondering what would be
> the best cable to use.  Moisture in the pipe would probably cause
> ordinary mains cable to pinhole with the HV pulses.  How about some sort
> of coax?

I think the capacitance of the cable would seriously distort the
pulses reducing the voltage.

> [Solid-core polypropylene coax (NOT the foam core) is great for HV, but it
> has a fair amount of capacitance so I'm not sure how well it would work
> with pulsed or high-frequency HV.  You could always change your driver
> circuit and just charge your fence up with straight high voltage DC ;-)
> As a side note, you can even use coax cable as a high-voltage capacitor.
> SRR]

If the capacitance is high enough it might also push the discharge
energy outside that permitted by regulation for electric fences.

> 2) I would like to measure the voltage on the fence at various points,
> to calculate losses and trace faults.  (I use a traditional Megger
> tester at the moment for fault-finding.)  Since the HV is coming in
> pulses, what is the best way to measure?  I was thinking along the lines
> of a divider network with the A to D convertor of an AVR microcontroller
> sampling at about 1kHz to catch the pulses. Any thoughts?

Sound pretty complex and the sample rate is probably nowhere near
high enough to catch transients. I'd use a good old compensated
divider and analog scope.

Malcolm



> [You could probably wire up a cap and resistor with a long enough time
> constant so you wouldn't have to worry about catching pulses.  SRR]
>
> Cheers
>
> M
>
> --
> Matthew Smith
> Kadina Business Consultancy
> South Australia
> http://www.kbc.net.au
>
>
>