Electric fence and wooden poles without insulators? 4

[Skogsgurra]

Another day - another problem.

A guys phoned me the other week and asked me questions about an electric fence. I had helped another guy with interference from fences many years ago and this was why he phoned me. What I did was to reduce du/dt by putting a series inductor in the output from the fence "lectric" as they called it. That "lectric" was a capacitor discharge device that output 10 - 15 kV pulses with an energy contents around 30 joules. A series inductor helped reduce interference on fax and modem and the clicks that could be heard in the phone were also gone.

The energy delivered to the fence increased when the inductor was introduced. I was a little surprised at first, but when I studied the signals, I realized that the "lectric" was feeding a mostly capacitive load before and that created a high output current pulse with consequent high internal losses in the device. With the series reactor, the rising edge was about 10 us and with it was around 40 us long. So the peak current was less and the peak output voltage went from around 3 kV to slightly above 5 kV. So, all were happy and the series inductor is still there, for all I know.

Now, the problems were a little bit different. Not much, but still.

The fence were built without insulators. Using thin, flat wooden poles that were said to be suitable for electric fences. I had never seen that before and I am curious to know if those old ribs actually work in electric fence applications. There must be several hundreds of them and the environment is meadow and swamp in a mostly rainy Swedish summer. Picture shows one of the poles.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

One would think that if the posts got wet, they would burden the power supply. Most sites seem to offer insulators for wood fences; there was only one site that said: "There are specific types of wood posts designed for electric fence use without insulators."

https://www.afence.com/Electric_Fence/how_to_elecfence/elecinstall.htm

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[GregLocock]

At least for cows the typical insulator also has a necessary second feature, it moves the wire away from the fence. Otherwise when the cow rubs along the fence it has probably already damaged the fence.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[itsmoked]

Notice all the long "wet" grasses that reach the fence wire? The fences still tend to work even with lots of shorting grass stems. Sure at some point the 'grass burden' will get too high and the fence will lose it's livestock training 'quality'. A lot of e-fence works through negative reward teaching as a psychological barrier, so after the teaching period the animals avoid the 'nasty fence' automatically and it could not even be ON.

The insulation post method works for smaller confines using woods lacking in salt mineral constituents or oily woods that bead the water never allowing a mineralized conductive water path to form. Insulators often look ugly cost money and are another process to do when building a fence.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I heard an explanation that I will throw out for discussion. I have not researched or verified this.
What I was told was that the pulses were too short to allow time for the breakdown paths to ionize and conduct.
Too much induction may become counterproductive;- or I may have been fed a "line".
I remember doing some research on "Fencers". I saw claims such as;
"Will stop an elephant on 50 miles of uninsulated fence."
Probably an exaggeration, but also probably enough power for more conservative applications.
For example, that unit would probably be satisfactory on two miles or so of fence to keep bears out of the garbage dump.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Thanks, guys!

I looked at the Gallagher poles. They seem to have delivered the poles used in this fence. Special and impregnated wood. Do not know from what year, though.

Seems to be from down under and, as far as I know, that's mostly arid area. So, I think that it works there. But what about our "rain forests"? I did some simulations (yes LT-Spice can be used for any application) and got interesting results. Problem is to know what parameters to use for capacitance and leakage resistance. Anyone got a hunch?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MacGyverS2000]

One could impregnate the wood with resin to prevent uptake of water (and possibly fertilizers and other conductive salts)... but that would be expensive, and at that point you might as well use plastic. A soaked stake (depending upon species) would be like a several-kOhm resistor to ground every couple of meters.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Skogsgurra]

I will bring an old Non-Destructive Tester (12 kV 1-100 uA) on site this Thursday. Will test current flowing from wire to GND at different voltages and spraying with water to soak the poles increasingly. This is the kind of field work that keeps me going.

On top of that, the NDT is a vacuum tube monster. So I will need my 30+ year old and trusted 12/230V AC inverter. Weather forecast says calm, warm and dry. Life cannot be any better...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MacGyverS2000]

Spraying with water will not soak a pole, other than the outer skin. A pole needs to sit in a several-day-long rain/drizzle so it can soak in to give you a reasonable facsimile of how bad it can be.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Skogsgurra]

Yes, I realize that. But the last weeks have been quite rainy. So, I may get something also without spray. Spraying will probably increase the surface leakage. And that is also interesting to watch. Watching paint dry or poles soak - kind of exciting to some

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[stevenal]

Can't help but think about Ren and Stimpy.

 

[Skogsgurra]

I wonder if I am Ren - or Stimpy? And, should I be flattered? Or offended? Can't figure that one out...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[btrueblood]

Impregnation works for awhile. But see that grey flaky stuff on the outside of the post? We call that lichen here, I think that's a Swedish word for bread . Pretty sure that stuff gets somewhat conductive when it's moist, which is most of the time. Its slow growth over the years has probably caused degradation of the insulation properties of the wood posts. Or the grass has grown too long, like 'smoked has suggested.

 

[waross]

I have seen adds that suggested that the high power fence chargers could be connected to existing barbed wire stapled to existing posts.
That was touted as one of the advantages of the high power fence chargers. No need to install insulators, special posts or string new wire. A saving in both time and money.
I enjoy when you tackle a problem such as this Gunnar. I am waiting to see your results.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

I found some interesting information here:

http://www.hallman.ca/everythi.htm

Particularly these excerpts;
Newer fence chargers are mostly low-impedance output. The output from a low-and impedance fence charger is, medium to high voltage, but at much greater currents. Low-impedance fence chargers have a large reserve of current and are able to power (in other words, charge) very long fences or fences that are overgrown by green growth.
Ohms law at work here.
The lower the impedance of the fencer, the greater part of the voltage drop will be across a resistance to ground, leaving a higher voltage on the wire.
What about uninsulated fences?
If an electric fence controller produces sufficient current, it will successfully operate an uninsulated fence. However, there are drawbacks to uninsulated electric fences.
Wooden posts must be dry; a three-day "soak" (rain) will render most uninsulated electric fences useless.
Green wood or growing trees cannot be used as posts because the sap conducts the current to ground. fibreglass posts are ideal here since they act as excellent insulators.
Uninsulated fences must be short. A typical medium-priced, low-impedance fence charger can power only about 1 to 1 1/2 miles of single-wire, uninsulated fence. Even a high-priced, high-powered controller can effectively energize 4 to 5 miles of fence wire if the posts are made of wood.
Uninsulated fences often cause radio and television interference, especially after a rain. If this interference travels far enough, you might have the radio inspectors on your doorstep!

And;
Electric fence school
Shock and fence charger safety
A California professor made a very comprehensive survey of electric shock fatalities and near-fatalities over many years. Two important facts came to light.
Alternating current (AC) is about seven times more lethal than direct current (DC).
White females between the ages of four and seven are most susceptible.
DC shock is safer. This is true even with very high voltages and currents because of the all-important third factor—time. The single pulse of high voltage, high current DC shock is 'on' for about 3/100 millionths of a second only! This is many times less energy than that which might prove to be dangerous to even a sick animal or human.
CSA approval for electronic fence chargers
The Canadian Standards Association (CSA) thoroughly tests every power-line operated (AC-powered) electric fence charger sold legally in Canada, no matter where its was manufactured, for fire and shock hazard. All units approved so far have been DC shock (output) types, although they will approve an AC shock type if the shock is sufficiently weak to be considered safe.

I don't see the emphasis on uninsulated wire electric fences that I remember from 10 or 15 years ago. While they work, the uninsulated fences must have fallen short of expectations in a lot of applications.
Is there any chance of getting some 'scope traces across the wooden poles and on the fencer output?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Thanks a lot, Bill! That information escaped me. You did a thorough search there.

Scope traces? You bet! I will mix measurements and simulations in a report where I also include tests with different series inductors (if the fence owner lets me do all the tests I want to).

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[crshears]

Anecdote, probably not pertinent, but related...

Years and years ago once saw on a farm where an insulated wire had been strung around the necessary corrals / paddocks / enclosures with all sections commoned back to one point in an implement shed.

Everything normal, so far...

What made it "interesting" to me was that the insulated electric fence wire had a lamp socket holding a 15 watt incandescent bulb in series with its connection to the "hot" terminal of a 120VAC three prong plug which had in its turn been inserted into a convenient receptacle.

I was still a callow pre-teen at the time and had no concept of how bad an idea this was...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[rtronics]

Electric fence energizers are normally tested by checking pulse voltage output over a 500 ohm resistor, simulating a loaded electric fence. Rated voltage (normally about 5 kv is easily delivered.
A low powered transformer has 12.5 ohms and 60mH (@1kHz) secondary impedance and primary impedance of about 0.1 ohms, and 265 uH It would be fed from a non-electrolytic capacitor of 10 uF charged to about 600 to 900 volts through an SCR.
If you get yourself across the fence wires, you will get a nasty jolt.
As has been mentioned, once an animal is trained to the presence of the electric wire, it will keep away. However, if the fence is turned off, a cow will soon realize that, and after a couple of days, the smarter animal will just push straight through.
(farmer)Ray.

 

[gostop]

For fence leakage conductance we have used 1x10^-11mohm/m successfully. For a wet cedar fence post which we consider leaky we have used 1x10^-7mohm/m. I do not remember where these numbers came from. The closest fence I have worked on that is similar to your picture was a barb-wire fence, so the wire radius was different, but the fence capacitance of that 804meter fence was 5.68x10^-9 Farads. I have no good numbers for an individual post for you to use.