faultfinding on electric fences

Do you have a question? Post it now! No Registration Necessary

Translate This Thread From English to

Threaded View
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?  



Re: faultfinding on electric fences
Quoted text here. Click to load it

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.

Quoted text here. Click to load it

I think the pulse is less than 1ms  

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

Quoted text here. Click to load it

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)  

Quoted text here. Click to load it

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

--  
?? 100% natural


Re: faultfinding on electric fences
Quoted text here. Click to load it

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

--  
?? 100% natural


Re: faultfinding on electric fences

Quoted text here. Click to load it

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

Re: faultfinding on electric fences

"Bruce Varley"

Quoted text here. Click to load it

** Correct.

Quoted text here. Click to load it

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


Quoted text here. Click to load it


** 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.


Quoted text here. Click to load it

**  Don't count on it.



....    Phil




Site Timeline