Electric fencing

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What is electric fencing?

Electric fencing refers to a barrier system that uses electrical current to deter the passage of humans or animals.  While the same principles apply to those fences designed for humans or animals, this text is focused on electric fencing for animals.

A standard electric fence consists of a number of features:

• an electrical power source,

• an energiser box to convert the electrical current provided by the power source to a voltage appropriate for the application required,

• an electrical conductor, normally a wire, to carry the current along the line of the fence,

• fence posts to hold the conductor in the correct position,

• Insulators to isolate the conductor from the fence post,

• An earthing system to carry the current back to the energiser when an animal makes contact with a live conductor.  This is most commonly the ground itself, but alternatively, it can be an earth wire.

Electric Fencing Design

The information provided above is meant as a general background to fence design and establishment.  When designing an electric fence for use on the ground a significantly higher level of detail is required.

Electric Fencing Kits

The use of electric fencing kits is the simplest way to ensure that you have a system that is appropriate for the purpose required.  These come in a variety of forms tailored to the target animal and size of fence required. Typical examples of electric fencing kits include domestic pet fences, bird aviary protection fences, heron fences and horse paddock fences.  These kits will provide the customer with all the materials required to construct a fence, and will normally include; and energiser, an earthing system, conductor wires, posts and insulators.  Extension kits are often available to increase the size of a fence through the addition of posts wire and insulators.  Electric fencing kits are usually only applicable to small scale projects.  Once a fence increase in size, a bespoke design is usually the best way forward.

Bespoke Design.

When considering the design of an electric fence, it is normal to start with the type of fence energiser required.  This can be determined through the use of the fence calculator shown in figure 1 below.  This calculator takes into account a the variables that determine the energiser requirements.  These variables comprise; the power source, the target animal, the overall length of the fence, the nature of the conductor and the overall number of lines.  The resulting number provides the customer with the required no. of joules that the appropriate energiser should be capable of storing.   There will normally be a number of possible energiser options for the customer and a choice can be made based upon cost and value for money.

 

T

x

A

x

F

x

M

x

L

=

No. Of Joules

Type of Energiser

Animal Type

Fence Length

Material

Lines

Stored Energy

Mains energiser

1

Cows

1

500 metres

0.5

High Tensile Wire 2.5mm

1

1 line

1.25

Battery energiser

0.2

Sheep

1.5

2500 metres

2.5

High Tensile Wire 1.6mm

1.5

2 lines

1

Horses

0.9

Powerline Wire

3

3 lines or more

2

Wild

1.5

Turboline Wire

1.5

Rabbits

1.5

Powerline Tape

3

Poultry

1.5

Turboline Tape

1.5

Powerline Rope

3

Turboline Rope

1.5

Netting

3

Turbo Netting

2

Example

Mains energiser

Sheep

5000m

High Tensile

3 lines or more

1

x 1.5

x 5

x 1

x 2

= 15 Joules

 figure 1: Fence Calculator

 

Power Sources

The electrical power source for the electric fence can be the mains, 240 volts in the UK, or a 9 or 12volt battery.  The choice of power source for an electric fence is dependent upon the location of the fence, the target animal, and the overall power requirement of the fence.  If the fence is located close to the mains source and is permanent in design, then a mains based energiser would be appropriate.  Mains powered electric fences have the capacity to deliver larger shocks and provide that shock over a greater distance of wire. Mains powered fences can also maintain that level of shock even if there is a heavy load on the fence (caused for example by vegetation touching the conductor wire and shorting out the fence). If it is not practical to couple the energiser to the mains, or more flexibility is required then a battery powered energiser would be the better option.  The batteries used to power energisers are normally of the ‘car battery’ type, but nowadays theses are quite sophisticated and are based on either a lead acid or alkaline system.  It is of course necessary to charge these batteries over time, and this can be achieved either by swapping the discharged battery for a new one and charging it from the mains, or by using a solar panel to maintain the charge.  The size of battery required will depend on the design of the energiser used which is in turn directly related to the target animal, and length of fence.

Energisers

Energisers are usually sealed units which convert the relatively low voltage power source to the significantly higher voltage current that is sent down the electric fence conductor wire.  Normal voltages are in the order of 6,000 to 10,000 volts.  While this may seem dangerously high, the current carried is very low.  Also the energiser sends a short pulse of power down the electric fencing every second, so safely limiting the power of a single shock.  The most effective way to reflect the power of an electric fence energiser is through its output energy.  Generally, the higher the output energy;

• the greater the length of wire the energiser can supply,

• the higher the voltage of the shock,

• the greater the capacity of the energiser to cope with earth leaks,

• the larger the animal the energiser box is suited to.  As a rule the smallest energisers are suited to domestic pets, while larger models are suited to horses, stock and wild animals.

The energiser box us usually connected to the electric fence itself using what is known as a ‘lead out cable’.  This is a wide diameter steel cored cable which is highly insulated.  The wider the diameter of the steel core the lower the power loss between the energiser box and the fence.  Where energisers are powered from the mains, it may be a number of kilometres to the location of the electric fence, so this power loss has the potential to significantly reduce the effectiveness of the electric fencing system.  The high degree of insulation is required to reduce voltage loss along the lead out cable and maximise high voltage at the fence. 

Electric fencing Conductors.

In its simplest form the conductor is a strand of galvanised steel wire.  However due to the different demands required from a conductor, the range of electric fencing conductors available is considerable.

Galvanised steel wire is often used for permanent electric fences where cattle, pigs or larger wild animals are the target animal.  It is also very cost effective over longer fence lengths.  Specialist wire is now available with alloy coatings to reduce power loss over the length of the fence.  Plastic coated wire with lines of graphite set into the plastic allow for a permanent electric fence that is safe for horses.  Where temporary electric fencing is required, a stranded wire is more appropriate as it is easily dispensed and rewound onto rolls.

Strands of wire are also incorporated into nylon rope or tape.  These provide conductors that are flexible, easy to handle, log lasting and are highly visible, a concern in particular where horses are involved.  The mixed construction also makes them well suited to horse fencing.

The final form of conductor available is a form of netting that is suited to smaller animals such as sheep.  This is generally constructed from nylon line interwoven with steel wire to carry the current.  Mesh sizes vary but generally apertures are in the order of 150mmx70mm at the base rising to 150x150mm at the top of the net.

Electric Fencing Posts

The type of post used is dependent upon the nature and purpose of the electric fence.  In general timber or steel is used where fences are permanent, plastic or reinforced plastic posts are used where fences are temporary. 

Steel posts either box section or angle iron may be used where the fence longevity is the key.  Steel posts also allow for a greater fence height than a timber post would.  Normally steel posts are secured with concrete in the ground.  Applications where steel fences are appropriate are where wild animals are the target, or in zoos etc.  Concrete posts may also be used in place of steel.  Steel and concrete posts have pre-drilled holes to take the insulators. This may limit their flexibility in some circumstances. Timber posts are generally used where a 10-15year life span is sought, but expenditure is minimised.  Timber posts are normally associated with electric fencing for stock, horses or deer.  Timber posts are typically driven into the ground rather than concreted in.  Timber posts are suited to a wide variety of insulators.  Timber posts are generally softwood treated with tanalith or wood preservative.  However there is an exotic hardwood timber post on the market that is extremely durable in its own right, but also has very high insulating qualities.  In this case it is not necessary to use insulators to attach the conductor to the post.  Instead standard fencing staples may be used.  As with all timber products it is essential that timber posts are sourced from a sustainable suppler such as one with FSC accreditation.

Where  temporary electric fencing is required, plastic or steel reinforced plastic posts are typical.  These are by their nature self insulating.  In order to facilitate insertion into the ground they normally have a 150mm section of steel bar exposed at the base to push into the ground.  The plastic posts have pre-moulded wire retainers at approx 150mm intervals running up their length.  An alternative to the plastic post is the small section steel post with a plastic insulation sheath.  These will also have an integrated wire retainer built into them.

Electric fence Insulators

A wide variety of insulators are available, but virtually all consist of an insulated wire retention part and a fixing system.  In the case of a permanent fence, the wire retention part may be captive, in that the wire is threaded through the retainer during the construction of the fence and cannot be removed.  This ensures that the conductor wire remains within the insulator.  Alternatively where a temporary or semi-permanent fence is required the retainer may be of a quick release type to allow quick construction or removal of the fence.  These do not offer the same level of retention as the captive insulators.  The nature of the fixing system will be dependent upon the posts used in the fence construction.  If box section or angle iron posts are used, threaded bar and nuts will be used to hold the isolator in place.  Timber posts are generally suited to a screw in or a nail on insulator.  In the case of temporary fence posts clip on or wind on insulators are available.

Electric Fencing Earthling Systems.

This is often the most overlooked and ignored part of an electric fence, but is as important to the success of a fence system as any of the other components.  It is often taken for granted as a large part of it comes for free, the ground in which the fence sits constitutes the largest part of an earthing system for most electric fences.  To ensure that the best earth loop is established, a number of factors have to be taken into account.  The first is the earth system linked directly to the energiser box.  This ensures that the best possible contact is made between the energiser and the ground and will generally consist of 1-3 steel rods driven up to 1.5m into the ground, connected to the energiser by insulated cables.  Where optimum performance is required, these steel rods can be set in a bentonite and salt mixture which enhances the conductivity between the rods and the ground.  In certain circumstances it may be advantageous to increase the conductivity of the earthing system further.  This may be the case where soil conductivity is particularly low such as in arid conditions, or zoo animals are the target species.  In these cases, a direct earth wire or series of earth wires may be run along the fence. If steel posts are used then these may be earthed directly to the energiser as well.  The installation of an efficient earthing system will ensure the effectiveness of the entire electric fencing system.

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