faultfinding on electric fences

[Bruce Varley]

Spent a day recently with a colleague on his farm, tracking down shorts and
leakage on electric fencing. There are a couple of natty devices, including
a little handheld box that you just hook onto the wire, and it indicates the
current and voltage. My colleague was using the magnitude of the current as
we worked our way along the wire to infer the distance to the fault, his
theory was that more current means closer. In fact, the current did seem to
vary as we moved along, I wasn't watching closely enough to see whether it
related clearly to the fault location.

My understanding is that electric fences are energised in pulses of a few
KV, with a PRF of a second or so. Assuming a pulse width in the low
milliseconds without a lot of high frequency content, then for a fencing
setup spanning not too many Km, transmission line effects should be fairly
negligible, and the line current should be fairly uniform along the fence.
Is this correct, or do transmission line effects actually play a part in
what you measure? What sort of pulse waveforms and pulse lengths do fence
controllers typically deliver?

 

[Jasen Betts]

On 2012-11-05, Bruce Varley <bv@NoSpam.com> wrote:

Spent a day recently with a colleague on his farm, tracking down shorts and
leakage on electric fencing. There are a couple of natty devices, including
a little handheld box that you just hook onto the wire, and it indicates the
current and voltage. My colleague was using the magnitude of the current as
we worked our way along the wire to infer the distance to the fault, his
theory was that more current means closer. In fact, the current did seem to
vary as we moved along, I wasn't watching closely enough to see whether it
related clearly to the fault location.

if the fault is sparking then yes the peak current will be seen near
the fault.

if the fault is resistive only then currnt will gradually fall as it
is approached and be significantly lower after it has been passed.

My understanding is that electric fences are energised in pulses of a few
KV, with a PRF of a second or so. Assuming a pulse width in the low
milliseconds without a lot of high frequency content, then for a fencing
setup spanning not too many Km, transmission line effects should be fairly
negligible,

I think the pulse is less than 1ms

most of the energy will be in the Kilohertz somewhere
even for a unipolar pulse

and the line current should be fairly uniform along the fence.

for the current ot be mostly uniform the fence must be terminated with the
right impedance (or be infinitely long)

(for perfectly uniform the fence must aslo be lossless)

Is this correct, or do transmission line effects actually play a part in
what you measure? What sort of pulse waveforms and pulse lengths do fence
controllers typically deliver?

yes, at the scale of a farm it looks more like distributed capacitance

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[Phil Allison]

"Bruce Varley"

My understanding is that electric fences are energised in pulses of a few
KV, with a PRF of a second or so.

** Correct.

Assuming a pulse width in the low milliseconds without a lot of high
frequency content,

** Expect frequency components up to the top of the audio band.

then for a fencing setup spanning not too many Km, transmission line
effects should be fairly negligible,

** Some transmission line effects show up with cables of a few metres long,
even at audio frequencies.

All cables ( twin, twisted or co-ax ) are transmission lines and if
unterminated act as simple capacitors, if shorted they act as inductors.

Only if terminated with their characteristic Z, are they resistive.

and the line current should be fairly uniform along the fence.

** Don't count on it.



..... Phil

 

[Jasen Betts]

yes, at the scale of a farm it looks more like distributed capacitance

at the scale of a small farm anyway, some farms are plenty big enough
to see transmission-line effects.

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[who where]

On 5 Nov 2012 11:23:55 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

yes, at the scale of a farm it looks more like distributed capacitance

at the scale of a small farm anyway, some farms are plenty big enough
to see transmission-line effects.

?? I've used Beveridge antennas 100m-200m long on HF quite
successfully, set up as a fence.