An electric fence is a barrier that uses electric shocks to deter animals or people from crossing a boundary. The voltage of the shock may have effects ranging from uncomfortable, to painful or even lethal – in the case of security fences. Most electric fencing is used today for agricultural fencing and other forms of animal control purposes. The energizers that we use for agriculture and livestock control is not of a lethal nature and are designed with a low-impedance pulse of shorter duration and faster travel speed.
Fence energizers
Early alternating current (AC) fence chargers used a transformer and a mechanically-driven switch to generate the electrical pulses. The pulses were wide and the voltage unpredictable, with no-load peaks in excess of 10,000 volts and a rapid drop in voltage as the fence leakage increased. The switch mechanism was prone to failure. Later systems replaced the switch with a solid-state circuit, with an improvement in longevity but no change in pulse width or voltage control.
Weed burner fence chargers were popular for a time and featured a longer-duration output pulse that would destroy weeds touching the fence. These were responsible for many grass fires when used during dry weather. Though still available, they have declined in popularity.
Modern low impedance fence chargers use a different design. A capacitor is charged by a solid-state circuit – upon contact with a grounded animal or person, the charge is then released using a thyristor or similar solid-state component. Voltage is consistent due to electronic output controls, within the limits of output power. Pulse width is much narrower, often about 10 microseconds. This design works for either battery or mains power sources.
Depending on the area to be fenced and remoteness of its location, fence energizers may be hooked into a permanent electrical circuit, may be run by large lead-acid batteries (such as those used for automobiles or tractors), or may be powered by a small battery kept charged by a solar panel. The power consumption of a fence in good condition is low, and so a lead-acid battery powering several hundred meters of fence may last for several weeks on a single charge. For shorter periods dry cell batteries may be used. Some energizers are capable of being powered by more than one source.
Now for some PASTURE DIALOG – about fence voltage from fence guys. At Powerflex Fence we have been proponents of buying and using larger energizers. We still feel strongly that a commitment to providing an adequate energizer is of prime importance, however, lets put it into prospective.
One of the conversation topics discussed comes from the fact that when we are using larger energizers we get comfortable with the 9KV (9,000 volts) to 10KV (10,000 volts) voltage range. So, when we do drop down to 6KV to 8KV we get nervous. Ten to fifteen years ago we were happy with 3KV to 5KV. So how much voltage do we actually need on our electric fences?
There are many variables that come into play when designing and maintaining an electric fence system. You should consider them as you target voltage range to operate your system.
- Type of livestock. Some types of livestock require higher voltage, such as goats.
- Livestock hair coat. With longer hair or wool coats – you may need a more powerful pulse to penetrate the coat.
- Soil types and forage cover. Arid conditions may require different considerations than wet ground conditions. A thick layer of dry ground thatch may insulate the feet of lighter weight animals.
- Length of fence. Our rule of thumb is that one output joule will power around 3 miles of average agricultural fence with an average load of vegetation. (Many manufacturers claim up to 10 miles per joule – but that is in a laboratory – not on the ranch)
- Fence load. If you have clean fences with little vegetation load you may not need as much voltage. Heavy vegetation loads may require more voltage to keep the vegetation under control.
- Maintenance plan. Are you going to spray your fence lines? Will your type of livestock graze up to the fence? Are you going to occasionally weed-eat your fence lines?
- Conductors. The type of conductor, or wire, that you will use will have a bearing on your voltage. Class III hi-tensile 12.5 gauge smooth wires have little resistance and the pulse flows down it easily. On the other hand some types of poliwire products have much more resistance and will gobble up some of your voltage over longer distances.
The comments, above, are just some of the considerations you should think about when you target a voltage range.
Below are some recommendations of MININUM Voltage for different animal types:
|
SPECIES TYPE TO CONTROL |
|
MINIMUM VOLTAGE |
|
# OF STRANDS |
|
|
|
RECOMMENDED |
|
PERIMETER |
|
CATTLE |
|
4,000 To 5,000 |
|
4 to 5 |
|
HORSES |
|
3,000 to 5,000 |
|
3 to 5 |
|
SWINE |
|
4,000 To 5,000 |
|
3 to 5 |
|
GOATS |
|
7,000 to 9,000 |
|
5 to 7 |
|
SHEEP |
|
7,000 to 9,000 |
|
5 to 7 |
|
BISON / DEER |
|
5,000 to 8,000 |
|
5 to 7 |
|
PREDATORS |
|
5,000 + |
|
5 to 7 |
|
PETS |
|
3000 + |
|
3 to 5 |
|
GARDEN PESTS |
|
4,000+ |
|
2 to 5 |
These numbers above are an approximate recommendation and should be considered as a minimum voltage range. Thus, for most of the year you will probably have more voltage than shown above. If you start out with higher voltage during the winter and spring months – then when the vegetation does invade your fence (creating a load) in spring and early summer – you will still have an adequate amount of shocking power to control your animals.
It is not uncommon (depending on fence load) to drop from 10,000 volts in March to 5 or 6,000 volts in early June. Thats OK! But, on the other hand, if you go from 5,000 volts down to 2,500 volts – youre going to have a problem. We get a lot of calls regarding this scenario. If you are using a voltmeter with a fault finder or fence compass, sometimes you get nervous about readings of 20 to 40 amps. What you are seeing is often an accumulation of grass and weeds on your fence. If, as you walk your fence and the amps drop as you go, it is probably an indication that the amps showing are an accumulation of grass, etc. Watch your voltage at this time as well. If you still have good voltage, then you should probably not get so nervous. The system is working and as the summer comes on and the vegetation dries out your amps will likely drop and your voltage rise. The modern low impedance energizers will cook the juice out of the grass, but it takes some time. You will notice that the grass that is on the fence wires begins turning brown - and thats a good thing. If you can maintain voltages in line with the chart above in May and June, you are in good shape.
Adequate grounding for your energizer will also aid in helping you maintain your voltage.
This brings up another point.You must have a voltmeter! A meter that also reads amps is even better – but you need to know what is going on at the fence to know how your system is working.
And, there are always exceptions to all rules in agriculture.if you have bulls across the fence from other bulls, or open heifers – then you will want optimum voltage. Bulls seem to have an excellent memory and avoid electric fences with adequate voltage. However, if they get a whimpy shock and do get their heads together — you know the rest of the story.
I hope this helps in your understanding of how much voltage you should shoot for on your electric fence.