faultfinding on electric fences

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?

Reply to
Bruce Varley
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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.

I think the pulse is less than 1ms

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

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)

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

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Reply to
Jasen Betts

"Bruce Varley"

** Correct.

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

** Don't count on it.

.... Phil

Reply to
Phil Allison

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

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Reply to
Jasen Betts

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

Reply to
who where

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