Electric fence

[PlainName]

Just getting around to acquire an energiser and a bit puzzled by the spec. Typically it says a it's good for a single wire of 8km or multi-strand (that is, two) of 3km. I would have thought that a typical fence with two runs would have them in parallel rather than series, so the length would be 8km for one or two runs.

What am I missing? Is it normal to have the strands in series? Seems a bit daft to me! Or is there something else in play?

https://www.electricfence-online.co.uk/pulsara-pc1300-hybrid-energiser-12v-230v.html

[thm_w]

Here is the actual spec, which doesn't match what you wrote:

Fence Length two-wire 7 km
Fence Length two-wire - Light vegetation 3 km
Fence length multi-wire - Recommended max 0.5 km

From what I see
- Two wire means a ground and hot wire
- Multi-wire could be 3 or more hot/ground pairs, supplied in parallel. So I don't know how the 0.5km number is calculated.

[PlainName]

Yeah, depends how you read it. The features bit says 7km (must've been thinking of output when I wrote which I assumed would be single wire. Lower down it says two wire, but then a bit lower it is 'multi-wire' at 0.5km.

I just happened to pick this one as the one I was looking at, but they all seem to have a similar reduction in length for multi-strands. A more upmarket one, the Gallaher M550 says 110 (presumed km) for single wire, 35 for two wire and 9 for multi-wire.

[thm_w]

The M550 maybe makes a bit more sense, 25km rating with one pair, divide that by 3 pairs gives 8.3km which is close to the 9km multi-pair rating. Assuming its 3 pairs because they write (3) for multipair.

There are probably so many factors they just give these ballpark numbers. If you care you can probably go and measure the voltage upon install.

In the past most manufacturers included some form of rating in kilometers or miles of fence. Many still do. In the past they tended to use a simple formulae rather than any form of testing on actual fences. Also, some did not say how low the fence voltage would be at the end of this amount of fence. So these figures were at best theoretical. More recently these numbers are being qualified and in some cases related to the load on the fence or the animals to be fenced. In practice it is also factors like the fence layout, materials and even soil type that determine the number of kilometers that can be energised. A kilometer rating is useful however, because it can most easily be related back to the buyer's property. It should be understood, however, that the figure relates to a "best case" fence, unless it is qualified as to the type of fence and worst point kilovolts used to get the figure.

https://fieldcropnews.com/2018/05/selecting-an-energizer-for-your-electric-fence/
https://www.pakton.com.au/terminology%20and%20units1.php

[CatalinaWOW]

There seems to be a dependence on vegetation, but as in many devices they aren't consistent on conditions and definition.

[johansen]

My guess is capacitive loading limits how strong the shock is.

Just for fun my sister held onto the neighbors fence wire for more than a few seconds.. he had chickens not steer...

You need something strong enough to burn through the natural growth rate of the grass while also being strong enough to deter the animals. Hence the length limit

[PlainName]

My guess is capacitive loading limits how strong the shock is.

Ah! Could be. And/or leakage since we're talking 8kV or so.

[twospoons]

Having worked for an electric fence co I can say the fence length quoted is largely marketing BS.  But marketing guys like big numbers they can stick on the outside of the box, so thats what you get. The 36J unit was marketed as supporting something like 150km of wire, but if you work it out you'd be down to about 500V at the end of a steel wire that long - not particularly effective.  Its a stupid number to use, but I guess its easier to comprehend than Joules for your average farmer.

What really matters is the total load (fence capacitance and leakage) and the  resistance at the far end of the fence. So you want clean insulators, multiple hot wires, no foliage contact, and good ground stakes (or alternating hot and ground wires ).  More energy = handles more fence load and still retains effective voltage levels.  Usual target is >5kV to get through the fur and into the animal.

[PlainName]

Thanks, I realise the specific figures are marketing best case, it was just the series/parallel apparent discrepancy that caught my eye.

(or alternating hot and ground wires )

Ah, hadn't considered that but perhaps it might make sense for our setup. Something more to think about

[rstofer]

Maybe Google can help:

https://extension.umaine.edu/livestock/pasture-course/lesson-3/electric-fence-design/

[PlainName]

Maybe Google can help:

https://extension.umaine.edu/livestock/pasture-course/lesson-3/electric-fence-design/

Or maybe not:

If you wish to install and electric fence that is one mile in perimeter, and you are charging four strands on the fence, your total fence length is four miles.

Why would you want to do that? It seems perverse to wire them in series.

[IanB]

If you wish to install and electric fence that is one mile in perimeter, and you are charging four strands on the fence, your total fence length is four miles.

Why would you want to do that? It seems perverse to wire them in series.

I don't think it said anything about wiring in series. It just said that 1 mile times four strands per mile = 4 miles total strand length, which is plain arithmetic.

Basically, all fences have leakage, so the more wire length you have, the more leakage you have. If you have no leakage you could in principle have an unlimited fence length. A given energizer can only support a certain amount of leakage before it becomes ineffective.

[tautech]

If you wish to install and electric fence that is one mile in perimeter, and you are charging four strands on the fence, your total fence length is four miles.

Why would you want to do that? It seems perverse to wire them in series.

Livestock class, leakage, vegetation, ground conductivity and wire gauge all dictate what is possible.

In a prolific growth climate like here in northern NZ there are tradeoffs we make from experience to use with electric fence design and construction along with the livestock class needing control.

Here we have some runs of 3.2mm (8g) out to breakout points where wires then run out as 2.5mm feedlines on pre-existing fence lines and/or to 2 or 3 wire fences with all wires hot !
We target ~4kV furthest from the unit for reliable control of cattle and our longest runs are approaching 2km.


Most NZ electric fence installations use earth/ground return whereas in dry climates a fence for larger stock might only have 3 wires, 2 hot and the middle the ground return. The issue with this layout is stock might already have their head and neck well through before contacting the 2 wires necessary to get a jolt.

I know from decades of livestock farming that a zap at this point normally propels an animal forward as their natural reaction is to run from danger, far more than engaging reverse gear.

As energiser have become more powerful (ours is 40J) their earth fields require more care and several long galv ground pegs to adequately transfer the current to the subsoils and the longer/deeper they go the better.

For some decades my preference has been to use a full 6m/20ft length of 20mm galv pipe, quite easily installed full depth by hand with a water supply on the top end and using an up/down motion for a water hammer effect.
When down this deep the soils are very moist which much improves conduction.

[PlainName]

If you wish to install and electric fence that is one mile in perimeter, and you are charging four strands on the fence, your total fence length is four miles.

Why would you want to do that? It seems perverse to wire them in series.

I don't think it said anything about wiring in series. It just said that 1 mile times four strands per mile = 4 miles total strand length, which is plain arithmetic.

Basically, all fences have leakage, so the more wire length you have, the more leakage you have. If you have no leakage you could in principle have an unlimited fence length. A given energizer can only support a certain amount of leakage before it becomes ineffective.

I can see that leakage/capacitance can complicate what would otherwise be a relatively simple matter, but I don't think it changes the topology. We would normally consider resistance to be the limiting factor for length, so whereas 4 miles of wire would have much the same leakage however it's set up, 4 miles in series is quite different to 4 x 1 mile in parallel.

Further, in parallel a break won't turn it off downwind of the break, whereas in series it will. Also, in parallel any point on the wire will be closer to power than ~7/8 of the points in the serial setup.

So I think it's fair to assume that when they say 4 strands of a 1 mile fence comes to 4 miles they are talking series. Otherwise it would be 2 miles, or 3 miles at worst. And my question then is why series? They must be something important that dictates that (because it seems to be not just common but the default), and even allowing for grass and animals and similar - actually, because of all that - parallel would seem to be the sensible topology.

Except... if you've got undergrowth that is now overgrowth and shorting the bottom wire, it might make sense to have that run in series from the far end, so that it's shorting the very end of the chain rather than putting a load near the start. But, surely, in that case it would make more sense to just clear the fried grass instead.

So, as I say, I am missing something...

[tautech]

Except... if you've got undergrowth that is now overgrowth and shorting the bottom wire, it might make sense to have that run in series from the far end, so that it's shorting the very end of the chain rather than putting a load near the start. But, surely, in that case it would make more sense to just clear the fried grass instead.

So, as I say, I am missing something...

Yep, exactly how I configure them when I know undergrowth is to be a problem.
Yet this is one of the advantages of the bigger energisers, the excess power to smartly burn off any plant growth that comes in contact with wire and potentially reducing fence maintenance.

In another life I did fencing contracting for some 10 years.......

[PlainName]

OK, thanks. I'll add another two 0s to our joule requirements 

[NiHaoMike]

I wonder if it might be possible to occasionally switch the fence over to a neon sign transformer or something for a minute or so to burn away the weed growth.

[tautech]

I wonder if it might be possible to occasionally switch the fence over to a neon sign transformer or something for a minute or so to burn away the weed growth.

And risk life or limb ?
The fact that it's illegal too.

[CatalinaWOW]

Burning away vegetation has its own risks.  Depending on location and time of year you may end up in the news as the one who started "The Fire".

[tautech]

Burning away vegetation has its own risks.  Depending on location and time of year you may end up in the news as the one who started "The Fire".

The time honored method to test if a fence was live was a piece of green grass held by hand and see if the fence provided a tingle to the the tester/holder.
The continued pulses inhibit plant growth and brown off the vegetation to a point where it is much less conductive.

No sparks to initiate fire are part of the process otherwise electric fences would have fallen from favour decades ago.
The much higher risk of fire is from lightning.

Let's explore this further, in an earth return fence where the ground wire is connected to the energiser, any sloppy design build not being of a proper permanent build could indeed raise a spark when the hot and ground/earth wires come together when fixtures fail and with the large energisers the continual spark erosion will in time erode through smaller diameter wire.
This also presents some risk for outrigger wires on existing fences when non-permanent/sloppy installations exist.
When existing fences use a barbed wires there is also some risk of 2 wires hooking together and current flowing into other wires not having insulator fixtures on fence posts.
In some areas concrete posts are used and of course they are quite conductive as they are also steel reinforced and as they are embedded 2' deep into the more conductive subsoils.

This can be a real issue when ppl climb electric fortified existing fences and also when larger wild fowl that can't pass through a fence might fly over it to pause on the top wire and weight it down to hook onto a barb wire.

We've had this problem with freerange turkeys and overcome it with more careful placement of hot wires, more supports and higher wire tensions/strain.

[Andy Chee]

I have nothing technical to contribute

[SiliconWizard]

I wonder if it might be possible to occasionally switch the fence over to a neon sign transformer or something for a minute or so to burn away the weed growth.

And risk life or limb ?
The fact that it's illegal too.

Dunno. I would ask Boeing's executives about that.

[PlainName]

I have nothing technical to contribute

On the contrary, saved me finding out the hard way 

[twospoons]

A strong enough fence becomes self-mowing - blades of grass touching the fence will get killed off. From memory this works for energisers of 18J or more. But you need to start with short grass!

As for parallel vs series: an electric fence is a high-voltage, low(ish) current device so capacitance/leakage matters at least as much as resistance. So total length still matters whether series or parallel - capacitance and leakage are the same in either case, but fence resilience and resistance are better with parallel wires.

eg 1km of 2mm diameter wire, 0.5m above ground, has a capacitance of roughly 9nF.  It takes almost half a joule to charge this to 10kV.

[PlainName]

As for parallel vs series

That's a good way to put it, thanks