Tuesday, September 3, 2013

Shocking facts about electrical fences

I ran across a basic misconception about how electric fences work on some other blog, and I think it's worth talking about the electric fences I use here, and how they're set up.  

The misconception was that "more wires means higher resistance", and the discussion was how a fence with many conductors (8 or 10) was somehow producing a higher resistance than a fence with 3 conductors.  

Voltage decreases with the length of the wire, and with the diameter of the wire.  You get the least amount of voltage drop with a larger wire, or a shorter length, or both.  

Voltage is what keeps your animals off the fence:  The higher the voltage, the more effective the shock, and (unsurprisingly) the more careful the animal is to avoid the fence.  if any of you have been shocked by a properly working 50 mile charger, you know what I mean.  I take special pains to never, ever repeat that experience!

So if you have a very long run for your fence charger, you'll be better off using the thickest wire that you can find.  Now I'm not talking about the ROPE or TAPE diameter.  Most electrical fences that aren't metal use some sort of non-conductive or marginally conductive plastic rope or tape, with stainless steel threads woven through it.  The voltage is conducted through the metal part of the fence, the stainless threads.  The size of the rope, or tape, is not important.  

That's an important distinction.  When you are looking at one kind of electric fence vs another, you want to pick the one that has the most METAL, not the biggest rope.  So  count the threads and pick the highest count.  If you are running very long distances, consider using a bare wire of the largest diameter (gauge) that you can find.  That's usually something like 12.5 gauge, but if you can find thicker, buy that for longer runs.  

The "size" of the "wire" (or I'll use "pipe" ) is the total diameter of all of the threads.  The more threads, the wider the pipe,the less resistance to voltage.   You can get this same effect, and I do, by stringing multiple conductors.  each conductor "widens" the pipe, lowers the resistance, and gives you better voltage at distance.    

How do you tell if the wire is thick enough?  Test the voltage at a distance.  You want to see at least 4,000 volts everywhere as a minimum.  With my fencing I see 18k volts at the fence charger and between 9 and 11,000 volts at various points in the field.    If my voltage drops, that's my sign that I need to go clear the fenceline.  If it never gets that high it means I should probably string another conductor or two to increase the size of the pipe delivering the voltage.  

Your voltage will decrease every time that your wire touches something; in the case of the blog I was reading, the problem that the author had wasn't that there was too many wires, it was that with the larger number of wires more of them were touching the ground and weeds.  More touching, less voltage.  When she reduced the number of wires to 3, her voltage went up because her fence grounded out less, not because fewer wires were less resistance.  

People use round rope electric fences -- I do -- or they'll use a tape, and the primary reason that they use the tape is that it's more visible to animals that might otherwise ignore it.  I use the rope because the pigs I am mostly containing hate electric fences, and will look carefully for them when they're out walking, and the rope is easier for me to coil and store.  I'm used to handling big shots of rope.  


11 comments:

Leon said...

>more wires means higher resistance

You're right of course - more wires means lower resistance but it also means longer length of wire and that drops the voltage too, so the final result is kinda the same - may be that's what they've meant?

By the the same token - will additional wire help you to increase voltage? In some cases - sure, if benefit from reduced resistance is more than the drop caused by the longer wire. But may be not - it mostly depends on the resistance of your new conductor I guess.

Jeff said...

If her resistance increased, her voltage would go up thanks to good old Ohm's law. With multiple ground outs, her resistance was actually decreasing!

Bruce King said...

Leon: Additional conductors will increase the cross-section of the wire, the thickness of the wire, and reduce voltage drop related to the distance. The length of the wire is the same. What the author was claiming was that reducing the number of conductors for the same length reduced the resistance; what really happens electrically is that the opposite is true: Fewer conductors, more resistance, less voltage.

Bruce King said...

Jeff: " If her resistance increased her voltage would go up" -- her REQUIRED voltage would have to go up to counter the resistance, but her actual voltage, the voltage delivered to the animal at the fence, goes down as resistance goes up.

You're absolutely right: With an electric fence shorted out, the resistance through the ground (or weeds) reduces, resulting in less voltage.

This is easy to test. Short out your electric fence and look at the delivered voltage: down. Remove the short: up. Add conductors: up. Remove conductors: Down.

The conductor test is from the distance run; you may have trouble measuring that easily if your runs are only a couple of hundred feet. I've got thousands of feet of fence.

She got better results with a different fence, but completely missed on how that better result was accomplished.

Andrew said...

One important thing to look for with the rope or tape (or net) fencing is the number of conductor strands therein.
Most farm stores around here only carry the 6-conductor varieties, unless you go for the big 1.5" horse tape, which is pretty unwieldy.
The 9-conductor stuff is a better deal, even though you have to special order it. Just take a good hard look at the resistance values that are usually printed on the packaging, usually in ohms/mile.

Across The Creek Farm said...

I noticed that you didn't talk about joules, which is more important (at least to me) to getting the job done. The more joules on the fence, the stronger the fencer's ability to do work - and not get shorted out. Voltage is the ouch, but joules are the thing that knocks you back 8 foot and not feel your arm for 10 minutes. We run a 15 joule charger in our heaviest brush field. It kills blackberries, tree seedlings, and other brushes and grasses that touch the fence. Just some thoughts

Leon said...

> Additional conductors will increase the cross-section of the wire, the thickness of the wire,

Absolutely

> and reduce voltage drop related to the distance

May be

> The length of the wire is the same.

How so? Let's say you have one wire 1 mile long. You know resistance of that wire, you can calculate the voltage drop - piece of cake. Now, let's say you add a second wire to that fence. Are you saying you still have 1 mile long wire? I'd think you have 2 miles of wire now. So yes - you do have less resistance now but you also have more distance.

It's been a while since I learned this stuff, so may be I make stuff up now, sorry :) But these guys seem to agree: http://www.zarebasystems.com/learning-center/all-about/fence-chargers/fence-charger-expert-tips (see Number of fence wire strands). Also, from my experience - you need a much bigger charger to get the same voltage on a 9-strand HT fence than on a single wire, all other things being equal. Definitely not 9 times bigger but much bigger, probably 2 or 3 times.

I have no idea what the original argument you refer to was about - I'm just curious to find out if I'm wrong thinking that each additional connector increases the distance.

Sure, when I think about a big fat cable made out of many wires ... yes, it seems counterintuitive to add length of each one to determine the real distance the energy travels. But then again - may be that's why they don't use multi-wire for serious amperages - it's always one large wire, right? May be it does increase the "distance" and, therefore, energy loss.

Bruce King said...

Wires that are serially linked, end to end, do increase the resistance. Wires that are parallel decrease it. Woven or twisted multistrand wire is the standard for most long cables that I have seen. The cabkle that supplies power to yiur house for instance.

Jeff said...

I find it useful to think of electricity in terms of water. It's not perfect, but a good approximation. Voltage is equivalent to water pressure, current the rate of water flow, and resistance the thickness/number of the pipe(s). If you have one 100 foot garden hose hooked up to a water tower, you will get a stream of water. If you hook up 10 garden hoses to the same water tower and let them empty in the same spot, you will get a torrent.

Think of your electric fence charger as the water tower. The amount of joules is the size and height of the tower. Your wire is like the pipes. The more pipes you have or the larger the diameter of the pipes, the more water (electrical current) you can deliver at a distance. Think of every time your fence grounds out on a blade of grass as putting a small hole in your water hose. A few holes won't affect your water pressure and flow at the end of the pipe, but adding a bunch of holes will eventually cause a problem.

Leon said...

> Wires that are serially linked, end to end, do increase the resistance. Wires that are parallel decrease it.

Yep, that sounds right and does ring a bell ... I even remember doing an experiment proving it in the school. Thanks for reminding me :) So, Zareba boys don't know what they are talking about? :) Surprise, surprise ... well, not really, their chargers are crap.

> Woven or twisted multistrand wire is

not what I was talking about. From the point of view of electricity multi-strand is just one big wire. Fence is more like multi-wire, where each conductor is insulated from the other and these are never used for big loads, I think ... but that's not really relevant anymore, must be some other reason.

Jeff, I find comparison with water very useful too.

adalynfarm said...

Current travels on the outside of the wire, so using a stranded #12 would do a better job than a solid #12, my guess is that it would probably hold up better too.