Agriculture: Electric Fence - How to not kill things?

[jwhitmore]

As a beginner, in lots of electronics, I look at things and wonder if I have the knowledge, in theory, to make said item. Even if you don't want to make one in practice.

With that in mind I recently had a run in with an electric fencing unit. I've not opened it, as yet, but having received a shock from the ignition system on my motorcycle I think I know how it works. Probably not the the optimum design but I assume that you connect the DC battery to a coil, which builds up a magnetic field and then you pull that field sustaining current flow, that produces a large voltage, (depending on turns) trying to sustain the field.

I think I was happy with that but then I wondered how do you engineer it so that you don't kill things, or do serious damage? Actually that led me to wonder can you measure the output shocking electronic pulse on a multimeter or even a scope? I assume that there's a lot of voltage there, even if the current is limited. Would you get anything sensible out of either measurement device or could they be damaged by the spike? Actually a scope probably wouldn't have the volts/division to get a measurement anyhow. How you engineer the system for optimum battery life is another question but for the moment measurement and not killing things would be my first question.

[tautech]

First understand commercial design and construction of electric fence controllers for agricultural use are bound by legislative rules that they must meet for safety.
There are rules for battery powered units and others for mains units. 10 kV is the max voltage permitted, ~1/3 of that in an ignition system but a lot more energy excepting for those ignitions used in race and drag cars.

Primarily they are only max voltages and pulse time with energy available being the the buyers choice for the application required and with increased energy available comes increased cost.
A unit I have produces 30+ Joules and cost $1k NZ.
It'll sit me on my arse 1 km from the unit. 

Simply they discharge lots of capacitance into the primary of a transformer and the secondary goes to the fence.
Typical mains units pulse 1/sec and IIRC the max pulse time allowed is 3ms.

Yep they can kill things but only if they can't get off the wire. The poor old hedgehog is the most common fatality but only when the wire is close to the ground.
Animals that might get caught or tangled are at serious risk too and care must be taken with regard to the terrain to help minimise risk. Fences across sloping terrain where if wrong footed an animal might fall against the fence need be avoided when planning electric fence layout.

Another NO-NO is the electrification of barbed wire........there have been child fatalities from it. 


No real problem measuring the parameters electric fence units can produce with 1000:1 and differential probes.

[neko efecktz]

Believe it or not but the very first agricultural electric fence controllers used ignition coils.
They were coils from a Model T motor car.
I worked for 7 years for a manufacturer of agricultural electric fence controllers in New Zealand from 69 to 76.
The first units made had a box that contained the ignition coil and a pipe about 3 feet long coming out one end,
The unit was connected to a 6 volt battery and a ball bearing would roll down the pipe and strike a momentary switch
which would activate the primary coil and the magnetic field would pull on a plate that would send the ball back up the tube.
The speed of the unit was varied  by changing the tilt angle of the pipe.
This unit was called BALL AND PIPE.
The next unit was a little more modern in that it has a normal E core lamination transformer but was still controlled by a ball bearing.
the transformer was assembled like a choke with all of the "E"'s in one direction instead of interleaved.
The "I"'s were stacked to form the plate that was attracted to the "E" section of the core.
this stack had an extension on it that would drive the ball.
This unit had a pair of rails for the ball to roll up and down on.
It was called  BALL AND RAIL.

This unit was made up until the late 60's      edited date.
The first electronic machines were built using a silicon transistor in an oscillator that charged a bank of caps up to either 200 volts or 400 volts.
The unit with the 200 volt charged caps was powered by 12 "D" cells (2 banks of 6 in parallel) and a later version that was made to hang on the wire only had 6 "D" cells
the 400 volt cap unit were either 12 volt battery lead acid type or mains powered.
The caps are discharged through the output transformer primary via an  SCR.
The transformer is assembled with all "E" lamination's in one direction and a paper gap between the "E" and "I".

Later came the very high powered units that gave one hell of a kick.
I got across one once two wires in one hand connected to the output.

This unit didn't have "E & I" laminations but instead was on an straight core about 150mm long and about 20mm x 20mm stack of laminations.
This one had to be in a custom made alloy extrusion that had large fins for cooling.
It had 4 banks of capacitors charged up to from memory 500volts
These were discharged in parallel by 4 SCRs.

My job was testing new units before packing then up for shipping.
I got distracted by someone and pulled both wires through my hand as the unit fired.
my hand went completely white and stayed whit for the rest of the day.
Over night I began to get my feeling back and some circulation slowly turning my hand pink again.

BILL.

[orin]

The 'fence' we used when we boarded a horse for a while was battery driven - solar panel on the barn roof to keep the battery topped off.  It only delivered a pulse every few seconds so didn't use much energy.  The 'wire' was really poly-something rope with stainless steel strands woven in.

There are fence testers that are little more than an LED and a resistor - one terminal goes to a ground (a short rod that you stick in the ground) and you rest the other terminal against the 'wire'.  In theory, the LED flashes each time the wire is energized.  It always worked for me and I managed not to test the fence the hard way.

One of my co-workers was thinking of turning such a fence into a ham radio antenna (Beverage) when not in use as an electric fence.  It would probably work fine for receive, but the thin stainless steel strands probably have too much resistance to try to use it to transmit.

[Richard Crowley]

Define "kill things"?  Specifically "things".  The same charge that can kill a dozen mosquitoes will go unnoticed by an elephant.  A great deal depends on the context, the conditions, and the intent.

[tautech]

Beverage consumption is responsible for many unknowingly urinating on electric fences in rural environments.
It is ONLY done once. 

When our kids were little we had a 'hot' top wire around part of our residential section.
My daughter when tiny had only experienced pain from a bee sting and the first time she had a shock it was thereafter known as the 'mean bee'. 
Great little 'mother' that she was of anyone younger than herself, she was always busy warning them of the 'mean bee' on top of the fence. Warnings from her and the adults were never enough for some little buggers, they just had to check out this fence that 'bites' much to my amusement and sometimes to the displeasure of the little buggers parents. 

 

[WastelandTek]

Define "kill things"?  Specifically "things". 

[orin]

Beverage consumption is responsible for many unknowingly urinating on electric fences in rural environments.

I should have known... at least I did capitalize "Beverage".

[jwhitmore]

Thanks to all especially the NZ stories about using Model T ignition coils. We've come a long way. As for having a fence that puts out 30+ Jules that just sounds wrong. Somebody mentioned elephants and mosquitoes.  I assume that one's for the former.

Thanks again.

[tautech]

As for having a fence that puts out 30+ Jules that just sounds wrong. Somebody mentioned elephants and mosquitoes.  I assume that one's for the former.

They're used in everyday large holdings all over NZ, primarily for cattle control.
Some of the larger manufacturers webpages of their big/grunty fence energisers:
https://www.speedrite.com/en-nz/category/110v
https://www.pel.co.nz/en-nz/category/mains

And AFAIK our longest lasting producer:
https://am.gallagher.com/nz/products/electric-fencing/power/fence-energizers/mains

[mfratus2001]

Nobody has mentioned an answer to the question: how do you keep the fence from killing things?
Current limiting is the key here. On one hand is a "punishing" zap that can kill if anyone stays connected to the fence... that would be with no current limiting (which would be careless and probably criminal on the part of the designer), on the other hand you could use a very large-value resistor (like 1Meg) to keep the high voltage and the "bite" but reduce the current and so remove the possibility of fatality. Fatality depends on voltage and current. A general minimum of 60 Volts is needed to enter the skin and reach the nerves. The resistor value needs to be fine-tuned to the application. With a single wire for a short distance, current can be low. But when you need to electrify a long or large fence, you need more current to carry the pulses.
Also the need for current is different when being driven by high-frequency pulses versus being generated from a 50/60-Hertz current from the wall, like in a neon sign transformer.

[neko efecktz]

mfratus2001
The voltage output of most electric fence controller is about 5000 volts.
Apart from building and testing the controllers for about 7 years I lived on a farm and the standard way to check if a fence was suitably alive
was to fins a long blade of grass and using the full length of it touch the live wire.
Slowly work you way up the grass until the shock was unbearable.
This would give an indication of how well the fence was working.
Low voltage Battery powered units had a reasonabley high impedance.
However we used to make a different version with less turns  in the secondary especially for horses.
Something to do with the way the muscles react when electrocuted.

larger, More powerful units were low impedance output.
however...
From memory,
the pulse rate was a maximum of 1 per second
the pulse duration was I think 3 or 4 Milli seconds.
At the time I was working for SPEEDRITE EQUIPMENT (1969 - 1976) the max power I think was about 10 Joules but later the regulations changed and as tautech said
they are up to 30 joules output but still the same max speed 1 pulse / second and 3 Milli seconds duration.
There is enough time between pulses to get clear unless the victim is snagged up in the fence.
Barbed wire has killed children by getting their cloths snagged.
also trying to climb through a fence and touching the ground / ground wire and live wire while part way through.

Snakes have died  and probable the odd Roo bouncing about and passing partly through the fence.
Bugs .
put it this war... For an electric fence to do its job of stopping cattle and other farm animals, it needs that one hell of a wallop.
One unit we made at SPEEDRITE EQUIPMENT had a center tapped transformer that gave the farmer 3 options.
full voltage going off in all directions
half voltage going off in the same direction
half voltage going off in two different directions.
This was a 10 Joule unit

BILL.