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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.
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