Electric Fence Ratings, Terminology and Units

By Paul Thompson BE (elec) RPEQ

Introduction

Electric fences have been around now for a long time. Like most mature technologies most of the products on the market work in much the same way. You would think that it would an easy matter of comparing the features and making a choice. Not so! Comparing electric fence energisers is a point of frustration for many farmers. The terms and units used to specify electric fences are sometimes difficult to understand. There are many ways of rating them. For example: stored joules, output joules at a load, peak output joules, Kilovolts on load, maximum kilometers of fence and kilometers of wire are all used for different energisers.

So is there a simple conversion formulae between stored joules, output joules and kilovolts or kilometers? Not really, though there are some rules of thumb, for example some in the industry say one stored joule will power 10km of fence. This of course, is a simplification which ignores pulse shape and load curves, let alone efficiency. It also does not work well above about 50km.
The only easy thing to say is that in general, and to a point, more is better. To help understand this a comparison can be made to cars. Cars are rated in capacity (cubic centimeters), power, torque, time from 0 to 100km per hour, maximum speed and fuel consumption to name a few. And there are no simple conversions or simple comparison. In general having a larger motor will get you from A to B faster. But no one would confuse a truck with a sports car. When someone buys a car they look at more than just the engine capacity. They will be looking for a car that suits the job that they want done. In the same way the best measure of an electric fence energiser is how well it will work on your fence, and in particular what will the voltage be at the furthermost point.

Terms

What is an energiser

(also called energizers, chargers or controllers)

Electric fence energisers produce a high voltage on a live wire to cause a shock and enforce a barrier.

Modern "low impedance" energisers place a very short high voltage pulse on the live wire once per second and are "safe" in that the shock is too short to cause electrocution.

They work by storing some electricity in a tank (main capacitor) and dumping it onto the output terminals through an electronic switch and transformer. The amount of electrical energy stored in the capacitor can be measured in Joules, Stored Joules. Not all of the stored electricity gets out of the box, some is lost as heat in the circuit. What does get out, the output energy, is also measured in Joules, Output Joules. Obviously if there is nothing connected to the energiser there is no output energy at all. Interestingly if there is a dead short on the output terminals there is again almost no output energy because all of the energy is lost inside the unit. The graph of output energy over load is a curve which starts low, peaks, and finishes low again. The peak is the Peak Output Joules which must be quoted at some load level, i.e. 500 Ohms. This is similar to a car motor delivering peak torque at a particular rpm. The closer the peak output joules comes to the stored joules, i.e. the less lost in the unit, the more efficient the energiser is.

Kilovolts

Kilovolts is a measure of the ability of the fence to shock. A Kilovolt is 1000 volts.

Most farmers use a voltmeter to check the fence. The fence voltage, measured in Kilovolts, needs to be above about 3.5kV to be effective. Grass or scrub touching the live wire will tend to reduce the voltage on the wire so obviously a more powerful energiser should be able to keep a higher voltage on a grass loaded fence. This can be measured in kilovolts at a particular load resistance. Short fences are easier to energise than long ones. Even if the fence is well built with new, clean insulators with no grass touching the wires it still loads the energiser. This is because the live wire starts to look like a long pipe which needs to be filled with electrons as the voltage is raised. For these reasons many manufacturers rate energisers in maximum kilometers of fence or wire that can be energised. But fence layout also changes the result. For example the voltage at the end of one non branching line of fence would be much lower if the energiser was at one end compared with if it was at the mid point. One other point to make here is that some energisers are more suited to powering long lines of fence while others are more suited to powering many smaller paddocks. This is due to differing output pulse shapes and energy load curves, which can be compared to setting a car motor and gearbox combination up for high torque or high speed.

Stored Joules

Stored Joules is a measure of the amount of electrical energy stored in the main capacitor just before each pulse.

It is like motor capacity in cubic centimeters or inches

The Formulae is J= 1/2CV^2

Where J is the stored joules, C is the main capacitor size in Farads, V is the peak charge voltage and ^2 means squared.

Using stored joules alone as a measure of electric fences is just not smart. This is because it is possible to make an energiser with high stored energy (stored joules) by using a small main capacitor charged to an extreme potential. This is like over revving a small motor. It could also have a lightweight output transformer (like a weak transmission) where most of the energy is lost as heat and it could leave out the wave shaping components (like emissions control) so the pulse shape would be very short and sharp with lots of ringing. The plus for stored joules is that it can be used to roughly class an energiser. For example strip grazers are typically less than 1 joule. Small energisers are up to 2 Joules, medium from 2 to 5 and large from 5 up. Energisers above about 17 stored joules need to use some sort of feedback scheme to increase stored energy (and hence output) only if the load reduces below that of the human level of 500 ohms. This is in order to remain below the safety level which is measured at 500 ohms. This is like a governor on a motor which applies more throttle when a load is applied.

Look carefully at very large energisers, these may only dial up the higher joules into an extremely low load. Since fence wire has a relatively high resistance, these extreme loads may never actually occur. For example even a dead short circuit further away than 2 kilometers from the energiser will look like, at worst, a 100 ohm load. If the energiser is rated at over 100km and yet it will only wind up to full power for a dead short within a couple of km of the unit, you may have paid for power you will never be able to use and probably don't really need.

Output Joules

Output Joules is a measure of the amount of electrical energy transferred to a particular load per pulse.
It is like brake horse power in a motor vehicle
The Formulae is J = integral from 0-T of v(t)^2/R
Where J is the output joules, T is the pulse length, v(t) is the voltage time curve and R is the load resistance

Peak Output Joules is the maximum Output Joules for any load. The actual load should be stated. It's not much good having maximum output joules into an impossibly low load.

Is Output Joules a better measure? Well it is better, but again it is also possible to construct a large energiser with a very good peak output joules figure but with a very poor pulse shape. Worse still it may have the peak output joules occurring for a load that is out of the useful load range. Some manufacturers restrict the energiser pulse from it's natural tendency to ring (or swing negative after an initial positive pulse). Some don't bother. The ringing does not add to the peak kilovolts measured on the fence wire but it can increase the output joules by up to 30%. If we only use output joules (peak or otherwise) those units with ringing outputs look better. But they may do no better as a barrier. So if using output Joules to compare energisers you must also consider what load the figure is given at and whether the output energy is useful. This is difficult in practice.

Efficiency

The output efficiency (in electrical terms) of the energiser can be calculated as the ratio of output compared to stored energy in joules. Typical energisers have output efficiencies of 50 to 75%. I don't believe efficiency is a very useful rating for use by the general public.

Rated Kilometers (or miles) of fence

This is the theoretical maximum length of fence or wire which a particular energiser will power to a useful voltage.

Some manufacturers specify the length of the "hot" wire in cases where there is more than one live wire on the fence. In practice for large energisers the maximum length of fence or wire it can run are similar. It is assumed that the fence will be well built and not in one straight line.

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.

Other units

I have not mentioned peak output power in kilowatts, peak output current or RMS output current. Even though these could be and have also been used to rate energisers. While they are electrical specifications, they also do not relate well to the actual fence performance.

Using a table or graph of Kilovolts per load resistance is useful for energisers designed to work on small fences or fences where resistive load is the key problem to be overcome. Kilovolts per capacitance is actually a better indicator of an energiser's ability to power long well insulated fences. It is actually possible to tune an energiser design for one or the other, but not usually for both.

Pulse Length and Shape

The pulse length is simply the amount of time the high voltage pulse exists for, typically measured in micro-seconds. The Pulse Shape refers to would be seen on an oscilloscope reading of voltage with respect to time.

Pulse length by itself is not used to rate energisers, but it sometimes quoted. Shaping the pulse is one of the key factors in the quality of an energiser design. And is necessary to reduce unwanted electrical interference like ticking on phones. It is an area where a lot or research and design dollars are spent. There is no doubt that a short sharp pulse will work well on a small fence and for the smallest sub joule units that is all that is available or needed. If the pulse has to travel long distances though, the pulse shape can make a huge difference to the voltage after say 20 km of fence. This is due to complex factors which are beyond the scope of this article.

By getting the pulse shape right an energiser can do a much better job. It is possible to get up to twice the voltage at the end of a fence for the same stored joules.

Joules, safety and ever larger energisers

The safety of electric fence energisers, especially mains operated units is covered by Australian and New Zealand Standards (AS/NZS60335.2.76) and by state laws which make it mandatory that energisers are tested to comply with these standards. In Australia mains powered energisers must show an approval number to show they comply. New Zealand recently removed the requirement for compulsory testing and approval and furthermore due to the terms of the Trans Tasman Mutual Recognition Agreement (TTMRA) anything that is good enough for the New Zealand market is deemed to be good enough for ours. The standard limits the output of energisers to a level that is deemed safe. This limit is measured at 500 Ohms load. The standards state however, that children should not be allowed to play with electric fences, i.e. there is still some element of risk. This risk rises with larger energisers. In order to comply with this limit and still build bigger and bigger (stored Joules) energisers, some form of power control system is required to keep the output Joules (or current) below the legal limit at 500 Ohms and still get more out at lower loads. This is commonly known as feedback and was discovered in 1927 by Harold Black. There is a point though, at which it is better to use two smaller energisers rather than one larger one. The standard even recommends it.

Powering a long fence with one energiser soon starts to meet the law of diminishing returns. To get more power out to the further reaches of the fence more current is needed to flow down the nearer wires. More current causes a larger voltage drop (by Ohms law), which reduces the voltage available to the end etc. To overcome this some companies are selling low resistance aluminium lead out wire. Running this type of wire out for even a few hundred meters will probably cost more than a second energiser.

Let me state this clearly, at above about 10 Joules, doubling an energisers size in Joules will not double the amount of fence it can power. If a very large energiser costs about the same as two smaller ones you are better off using the two smaller ones. Also with multiple energisers a short on the fence powered by one energiser will not effect the voltage on another at all.

Good design and maintenance

It has been my experience that most farms that have problems with low voltage on electric fences are suffering from poor layout, bad insulators, poor earth or a combination of all of the above rather than from an undersized energiser.

It is not uncommon to find electric fences built with no insulators at all because people believe that hardwood is a good electrical insulator. This probably has it roots in the fact that wooden posts have been sold as insulators. These fences present a huge load on the energiser which gets worse with rainy weather and as ants infest the aging timber. Although manufacturers have for years given much good advice on correct layout there are still many who, due to position of available mains power or farm shape, run a "tree" shaped system. That is where power runs out for a long distance on one fence and then splits and splits like branches of a tree. This is the worst possible layout. Moving the energiser to the top of the "trunk" is the best solution. Or rewiring it to make it more like a star, with lines radiating out from one central point. Unfortunately it is often easier to contemplate throwing money at the problem by installing a larger energiser than in fixing the real cause. Sometimes it even works.

© Pakton Technologies 2004.