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Electric Fence - Can't figure this out

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haze10

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I have an electric fence for my horses. The charger is a Parker McCrory SE4:
**broken link removed**

I wanted to make a simple circuit tester to show the fence was on. So I took a T5 8W fluorescent bulb. Solder a wire to one terminal on each end, with one lead going to a ground rod. I made two of these, as I wanted one close and one far.

The SE4 comes with a voltmeter. It was reading 12.9KV without no bulbs attached. Standing near the charger, I attached one bulb and it went to 12.8kV and with the 2nd bulb on it read 12.7kV. Great, not much load on the circuit.

I attached one bulb about 100ft from the charger. Voltage read 12.8KV. I walked down the fence about 1000 ft and attached the 2nd bulb. Voltage dropped to 2.8kV. What gives.

I removed the 1st bulb to see what would happen, the far bulb still loaded down the charger to 2.8KV. I thought maybe something was wrong with the 2nd bulb, so I removed it, and put the 1st bulb in its place, same thing, 2.8KV.

So its not the number of bulbs, its where it is placed on the fence. The only thing I can think of is the the farther fence has more resistance, but should it make this much difference.

Any ideas of what is happening electrically. Do you think a series resistor to the bulb would help keep the voltage up.

thanks
 
I suspect 1000 feet of (not) copper wire, installed to standards that are proper for a horse, has poor electrical connections or quite a lot of resistance compared to copper. The first thing to do is see if you can guarantee clean, solid connections all along the fence. If that doesn't fix it, you are going to need copper horse fence wire.
 
The ground was soldered to a 24inch long x 1/2" od pipe. The hot connection was wrapped tight to the fence. It was not a connection problem, because it was installed identically at the charger and worked fine. Distance really isn't even 1000 feet, its more like 500 feet. The wire is a triple strand stainless steel braid with polyester rope, probably around 24 to 28 awg.

I would think the increased resistance due to distance would work in favor of maintaining a high discharge voltage.
 
Probably the second position for the lamp has a much better ground connection than the first. How much the high voltage is pulled down is a function of the total resistance through the lamp. In the first location, if the ground resistance is high, then it doesn't pull the voltage down as much.
 
First, a couple of assumptions.

1, the output from the "fence power supply" is a pulse rather than a continuous voltage.

2, the meter in the power supply uses some sampling technique to read the fence voltage.

Second, a bit of theory.

When the power supply pulses the fence wire, the pulse travels along the fence at (nearly) the speed of light, 300,000,000 meters per second.
When the pulse encounters an impedance discontinuity in the wire, some of the pulse will be reflected back to the source.
The type of impedance discontinuity will determine the polarity of the reflection, if the discontinuity has a higher impedance than the surge impedance of the fence wire then the reflected pulse will have the same polarity as the incident pulse.
But if the discontinuity has a lower impedance than the surge impedance of the fence wire the reflected pules will be of the opposite polarity to the incident pulse.

Third, what I think could be happening here.

The flourescent bulb is a load and will have a lower surge impedance than the fence line
The reflected pulse will return to the power supply, the round trip distance is 2000ft so the reflected pulse will arrive back at the power supply after 0.5µs.
If the voltmeter samples the reflected pulse, the voltage will read low.

If some of this seems a bit far fetched, google Time Domain Reflectometer for more information.

As the man said in the film " That is all I have to say about that"

JimB
 
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