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In the Flashpoint episode "The Farm," there's a scene where you can see one of the officers run a jumper cable from one contact on an electric fence to another, then cut the line of the fence (not breaking the circuit, mind you). Based on context clues, one could assume the fence surrounds a large, multi-acre plot of land. Another bit of information: the cable they used to keep the circuit complete was longer than the original cable on the circuit, which would mean it would have a higher resistance when the circuit was rebuilt.

I have two questions:

  1. How feasible is this?
  2. And would a (powerful, that is) fence controller be able to detect the change in resistance? Or would it be so small that it's undetectable in the grand scheme of things?

From my (limited) electronics/electrical schooling, adding the cable to jump between the contacts would change the resistance, as resistance for parallel circuits is modeled as:

$$r_{total} = \frac 1 {\frac 1 {r_{a}} + \frac 1 {r_{b}}}$$

Which means the addition of the extra cable would have an impact on the resistance of the circuit itself (although very minimally).

If I recall correctly, another one of the officers was counting down to when the first officer should splice into the circuit. I don't know enough about electric fences to come to any conclusion, but are electric fences constantly charged? Or do they have a delay between pulses?

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  • \$\begingroup\$ How much false alarms would you be willing to tolerate and how high tech should the device be? \$\endgroup\$ – PlasmaHH Sep 30 '15 at 16:09
  • \$\begingroup\$ @PlasmaHH I haven't the foggiest (hence why I ask the question). Assume the device is the highest feasible technology for this purpose in 2010, and that false alarms are minimal to none. \$\endgroup\$ – Der Kommissar Sep 30 '15 at 16:11
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    \$\begingroup\$ I would probably not go for resistance but impedance distribution and/or signal reflection detections. \$\endgroup\$ – PlasmaHH Sep 30 '15 at 16:13
  • \$\begingroup\$ @PlasmaHH I didn't even think about those. That would make more sense to use a combination of them as a detection mechanism. \$\endgroup\$ – Der Kommissar Sep 30 '15 at 16:15
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    \$\begingroup\$ Note that most electric fences don't form a "circuit". There is one wire that is run along the fence, and a series of ground rods that are sunk deep into the ground. When a person or animal touches the fence, I guess the circuit is completed, but I'm not sure that there is any way to measure impedance or resistance unless a victim is currently being shocked. \$\endgroup\$ – JPhi1618 Sep 30 '15 at 19:24
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Theoretically, yes. You can measure the amount of resistance and determine the length of the fence.

However, there are some practical limitations to this. Doing this accurately would require quite an investment while the resistance won't be stable. Even the weather would influence it. Compensating for all those fluctuations would require some pretty advanced equipment while an electric fence is essentially a very low-tech device.

are electric fences constantly charged?

No, they are usually pulsed. This has a couple of advantages, one of them making it easier to generate such a high voltage. They are continuously powered, but the circuit is not designed to sustain the charge.

As @PlasmaHH mentions, if you want to measure your fence the proper way, impedance would be more valuable than resistance. I have never seen anyone do this on an electric fence, but you can measure the length of a coax cable by sending a pulse through it. The time it takes to come back and the form of the pulse will tell you a lot about the characteristics of the cable, including the length. However, this wouldn't be foolproof on an electric fence.

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The only way a detection could be feasible is looking for changes in signal reflection. Neither resistance nor inductance will provide reliable information of bypassing activities, especially not with a mile long fence wire. However, with a sharp peaking signal (already present due to the nature of the electric fence) and analysis of the electrical reflection pattern might provide an usable information to detect bypassing. The term you want to look for is Time Domain Reflectometry. See: https://en.wikipedia.org/wiki/Time-domain_reflectometry

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I am not going to show this to my family (we meet monthly at the family farm where there are multiple electric fences), because they would laugh at it. Electric fences are designed for maximum reliability when installed by someone with only the barest understanding of electricity. As such they have no components not needed except for an oversize case to protect it from irate cows. Further the traditional wire of choice has been iron which has high resistance, and rusts which increases resistance in unpredictable ways. These days the wire of choice in nylon carbon fiber mesh which in addition to having impossible to calculate resistance, the impedance and capacitance also are affected by the weather. To build an alarmed electric fence would require not only a entirely new class of "charger" (the box that charges the fence), but different wire, insulators, poles and grounding.

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  • \$\begingroup\$ I guess I always expected that the systems for electric fences would be more sophisticated than that. \$\endgroup\$ – Der Kommissar Sep 30 '15 at 19:47
  • \$\begingroup\$ nope, sophistication loses to rugged reliability, and simplicity. \$\endgroup\$ – hildred Sep 30 '15 at 19:53
  • \$\begingroup\$ That makes sense. (As stated in the question, I have absolutely no knowledge of how they work.) Based on the answers I've gotten, the scene in the TV show is entirely accurate. \$\endgroup\$ – Der Kommissar Sep 30 '15 at 19:54
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    \$\begingroup\$ As stated earlier using a TDR method could detect a change of the fence, it will definitely not be an absolute measurement. An algorithm evaluating the reflected signal would be necessary to filter out unwanted events (e.g. discharge through animals, ...). I am pretty confident that the signal form of the reflected "test pulse" can tell you a lot what's going on. For some events the current draw might also be interesting. \$\endgroup\$ – optronik Sep 30 '15 at 21:02
  • \$\begingroup\$ @optronik What would the reflected signal look like if you had multiple paths of wire attached to one charger? Also, there isn't really a current draw unless something (or someone) is actively getting shocked. \$\endgroup\$ – BenjiWiebe Oct 1 '15 at 0:54
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Even if we ignore all the mentioned variables and assume an ideal fence, the change in resistance would be very small. Assuming the wire's total length of 4,000 ft, at a height of 6 ft. To bypass it, one would need to add only about 10 ft (5 ft down, 5 ft up). With a resistance of 1 ohm per 10 ft, the initial total resistance would be 400 ohms. Adding 10 feet would only add 1 ohm (resistance in series, not parallel), making a total of 401 ohms. This is a change of only 0.25%. Also, there is an additional complication. You can not take measurements of the line while the high voltage pulse is active. This means that to get its resistance (impedance), you have to sample the line in between the high voltage pulses.

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