How an Electric Fence Works
An electric fence is simply one or more wires carrying a pulsing electric charge. Any animal coming into contact with the wire or tape receives an unpleasant, but harmless, electric shock. Unlike conventional fences, an electric fence is not so much a physical barrier as a psychological or mental one. The animals 'learn' not to approach the fence.
The Electric Fence Energiser generates high voltage intermittent pulses. Each pulse consists of a short one second burst of intense electrical energy released into the fence medium (wire or tape) through the lead-out cable coming from the energiser in a controlled way. Any animal touching the live wire receives a harmless shock by completing the electric circuit back to the energiser, via the ground earth system, whilst its feet are in contact with the ground.
To work effectively an electric fence must have:
* Sufficient voltage on the fence line to penetrate the resistance of an animal's skin.
* Enough energy (power) in the energiser to electrify the actual distance of wire.
* A good earth system using earth stakes pushed well into damp ground.
Measuring Energiser Performance
Energiser performance really relates to the ability to deliver a sufficiently high voltage (shock) onto a fence line. This means that different performance is required of a small portable energiser designed to power 500m (550yds) of wire, than from a high power energiser which can handle over 200km (120miles).
HOW CAN PERFORMANCE BE MEASURED?
The measure of energy an energiser will give out in output joules at specific loads. Output joules are more relevant as an indication of fence performance than stored joules.
The shape of the electrical pulse produced by the energiser also influences its performance on the fence line. Pulse-shape at voltage under load/distance is very dependant on the fence conditions and less relevant than stored energy. This can be quoted in single wire terms of known electrical resistance (ohms).
The performance of an energiser in distance terms can be quoted on the basis of the length of a single stand wire of known electrical resistance with ideal insulation and fence construction. Distances are normally given from EU guidelines set under ideal conditions. For realistic conditions, we suggest that at least 20-30% is taken off this value. Fence distance is given as a total length of all strands of the fence added together e.g. a 4-strand 1km fence is a 4km fence.
Questions and Answers on Electric Fence Energisers
Q. HOW IS A FENCE LINE ENERGISED?
A. By successive, high voltage, short duration pulses of electrical energy at approximately one second intervals.
Q. WHAT IS STORED ENERGY?
A. The energiser's electronic circuitry takes electrical energy from the power source (mains or battery) and accumulates it in the storage capacitors. This energy is the stored energy and is measured in joules.
Q. DOES STORED ENERGY INDICATE THE ENERGY AVAILABLE AT THE FENCE TERMINALS TO ELECTRIFY YOUR FENCE?
A. No. It simply indicates the energy that can be stored in the capacitors.
Q. WHAT IS OUTPUT ENERGY?
A. Triggered by electrical circuitry, the stored energy is discharged through an output transformer within the energiser to the energisers fence terminals. This is the output energy and is measured in joules. The effective output energy of this pulse is dependent on these factors;
The stored energy of the energiser.
The efficiency of the output circuit.
The fence load connected to the terminals.
Q. WHAT SHOULD I LOOK FOR WHEN CHOOSING AN ENERGISER?
A. The best performance characteristics in the size range to suit your needs i.e. fence length and type.
This means: The maximum output pulse energy (output joules) available at the fence terminal of the energiser. The highest available voltage under heavy fence load conditions (normally given as a 500ohms load) i.e. not pure wire resistance (resistance is lower in plain galvanised wire than in electric tape for horses).
Definition of Basic Electrical Terms
a.c:- Alternating current or mains power supply. Your local supply will be within 220/240volts a.c.
d.c:- Direct current or battery power supply.
Joules:- Electrical energy. One joule is the energy required to produce one watt for one second. Watts x Sec
Voltage:- Electrical pressure. One volt is the pressure necessary to cause one amp to flow through a resistance of one ohm. Ohms x Amps = Volts
Watts:- A unit of power. A flow of one amp at a pressure of one volt = one watt. Amps x Volts = Watts
Amps:- Electrical rate of flow. One amp is the current that results when a pressure of one volt is applied across a resistance of one ohm. Volts/amps=ohms
ohms:- Electrical resistance. One ohm is the amount of resistance that will limit the flow rate to one amp when a pressure of one volt is applied. Volts/amps=ohms
Short or Shorting out (short circuit):- In electric fencing this implies a large energy loss from the fence line to earth, such as an earth wire touching a live wire or long lengths of electrified wire lying on the ground, vegetation touching a live wire etc. A total short circuit = zero ohms (e.g. broken post insulator).
Load (loss of voltage):- Anything that draws power from the energiser puts a load on the fence line. This can be in the form of leakage or shorts and is measured in ohms. The lower the resistance of the load, the greater is its effect.
Leakage:- In electric fencing this is a small energy loss from the fence line to earth such as grass growing over the fence line, faulty insulators etc.
Low Impedance:- In effect, the internal resistance of an energiser. All modern energisers are low impedance which means maximum energy transferred to fence.