Basic knowledge for operation of an electric fence system
Basic physics or electric fences
Some basic technical knowledge is needed to understand how electric fence systems work.
In order to better understand the topic, we will use the example of a garden hose pipe:
We will assume that water is flowing through the hose.
This water flowing through the hose represents the electric current.
The water pressure present in the hose represents the electrical voltage. The higher the pressure, the more water flows through the hose. The water pressure (voltage) is present even when no water can flow (no flow of electricity) because, for example, a nozzle is fitted to close off the end of the garden hose. The closed nozzle is comparable to a large electrical resistance. When the sprayer is opened, the water can once again flow unimpeded. However, putting a kink into the garden hose will immediately reduce the flow rate of the water. (comparable to putting a knot in the conducting material.) We have now created a new source of resistance. In this case, however, the resistance is not as great as the closed spray head and a small amount of water (current) can still flow.
The electrical voltage [U] indicates a difference in charge between two polarities. A source of voltage, e .g. a battery or a mains socket, always has two terminals that each have a different charge. The negative terminal has an excess of change (electrons), whilst the positive terminal has too few electrons. This difference in numbers of electrons is known as the electrical voltage (potential difference). The voltage can be used to describe the size of the potential in an electric circuit. The size of the voltage is specified in volts.
When building an electric fence system, the energiser is connected to a battery or mains socket to provide it with a voltage source. The electron-rich negative terminal (fence connection) is then connected to the energiser using the fence conducting material (stranded wire, rope, tape, wire) using the fence connection cable (or the other way around). The positive terminal that has too few electrons (ground connection) is connected to the ground via a grounding cable and post. This creates and electrical voltage in the electric fence system, specifically, between the conducting material rich in electrons and the ground with few electrons.
Electric fences are special in that the energiser only creates this voltage for very short periods. The voltage appears as pulses. This means that the energiser only generates a voltage around every 1.2 seconds for a milliseconds.
Technical data for energisers frequently give the "output voltage" and "voltage at 500 ohms resistance".
The output voltage value, also known as the open circuit voltage, is not particularly relevant in practice as it specifies the output when no fence is connected.
The voltage at 500 ohms resistance represents the theoretical voltage when a fence is connected and an animal or vegetation is touching it.
It is a legal requirement that the fence voltage does not go above 5000 volts when touched by a person or animal (500 ohms).
When the two terminals are connected with each other (closing the circuit), the charge difference levels out. The extra electrons at the negative terminal flow across the connection (the electrical conductor) to the positive terminal (discharge). This flow of charge is known as electrical current [I] and indicates the transfer of electrical energy. The potential (the voltage) reduces when current flows. The more electrons that flow through the conductor per second, the larger the current strength (specified in amperes). A current can of course only flow unimpeded when the connection between the two terminals is sufficiently conductive.
The extra charge in the conducting material in the fence must attempt to flow back to the low-electron positive terminal (ground connection) on the energiser in order to balance the potential difference. Creating a connection between the charged conducting material and the ground closes the circuit. This connection can be created by vegetation or an animal touching the fence, for example. The charges can then flow through the animal, into the ground, to the grounding post and from there through the ground connection cable and into the positive terminal on the energiser. The animal perceives this flow of current as an unpleasant irritant and keeps its distance from the fence. Since voltage is only applied in short pulses, the flow of current also lasts only a short time – just a few milliseconds every 1.2 seconds. This is described as an electric shock. The animal or person is able to move away from the fence during the pause between pulses. Whenever a current is flowing, the voltage on the fence reduces. This can happen if, for example, tall grass or bushes are making a connection between the fence and the ground. For this reason, you must always try to avoid having any vegetation or other disturbances on the fence. This will ensure the full voltage and resulting shock strength is guaranteed. Vegetation touching the fence can be compared to having holes in a fire hose. If there are too many holes in the hose, no water can flow. Similarly, with too much vegetation, the fence offers no protection.
Electrical conductivity of a material specifies how well it can move charge (electrons) from the negative to the positive terminal. The electrical conductivity therefore shows how many free charge carriers are available in a material.
Electrically conductive materials including for example: copper, gold, aluminium, iron, brass, tin, lead, zinc, nickel, graphite, stainless steel, hydrochloric acid, sulphuric acid, salt solutions in water.
Non-conductive materials (insulators) are for example: PVC, recycled plastics, certain types of wood, teflon, epoxy resin, paper, stone, glass, paint, oils, air, rust.
Material with good conductive properties are used in various parts of an electric fence system. Good conductors are used for the conducting fence material. There are different quality levels for these. Various conducting materials are available depending on the animal and fence system used. Polywire with high-grade steel conductor, electric fence rope with copper conductor, wide tape with stainless steel conductors, smooth aluminium-zinc wire and galvanised steel wire are just some of the possibilities.
Good conducting materials are found in the fence and ground connection cables and the grounding stake should of course also be made from a good conductor such as galvanised steel, for example.
Non-conducting materials are used for insulators in the electric fence system. These are intended to guide the conducting material along the fence without creating a connection with the ground. This prevents any disturbances to the fence current and provides a constant voltage.
Plastic posts can be used for quick and simple fence construction. These are also made from non-conducting material and therefore do not provide a connection to the ground.
When the charge carriers inside a conductor are prevented from moving the electrons, this is known as electrical resistance [R]. This occurs when, for example, free charge carriers collide with atoms and thus have their flow interrupted. Resistance impedes the flow of current. Each component in an electric circuit provides some resistance and therefore has an influence on the current and the voltage. Electrical resistance can be seen as the opposite to electrical conductivity. The higher the resistance in a material, the worse its conductivity. The amount of electrical resistance is specified in ohms.
There are several points within an electric fence system that present a resistance to the flow of current. Particular attention should be paid to the following points when putting up the fence system: Each of the conducting fence components presents a resistance. This also means that the longer the fence, the higher the resistance. Materials that are good conductors such as smooth aluminium wire have a very low resistance and are particularly suited for moving a charge over longer distances. Simple polywire with just a few high-grade steel fibres is more suited to shorter fences. A further source of resistance is the animal itself The wool of a sheep is an extremely poor conductor and thus presents a very high resistance. To ensure the animal actually feel the flow of current when touching the fence, there needs to be a suitably large voltage on the fence. Animals with short fur, on the other hand, have a lower resistance.
Another very important point is the grounding. The ground must be a good conductor. The means that the ground cannot be too dry and must have a good connection to the energiser using one or more grounding posts and connection cable.
It is essential that you avoid having any knots in the conducting material or any connections where the material has been wound! In a knot, not all conducting wires will properly touch each other. Where, for example, a torn fence tape has been knotted together, there will be a large resistance in the fence. This will weaken the flow of electricity!
Technical devices often have information about Energy [W]. Energy is a measure of how much work someone or something can do (e.g. an energiser). The more energy a device has, the more work it can do.
The energy is a physical state variable that cannot be measured, but can be calculated:
Energy(joules) = voltage(volts) x current(amperes) x time(seconds)
With a fence energiser we usually refer to the "charge energy" and the "discharge pulse energy". The charge energy is the what the energiser takes from the mains socket or battery and consumes itself.
In order to compare the strengths of different energisers, we look at the pulse energy.
The discharge pulse energy is the maximum energy that an energiser can deliver to the fence. This value indicates the total energy of an electric pulse on the fence and shows the performance of the energiser. The pulse energy corresponds to the shock strength of the fence. The higher it is, the less of an issue vegetation is and the more secure the fence. When the fence is touched by someone (500 ohms), the pulse energy must not be more than 5 joules.
The amount of energy per time period is often given. This is then referred to as electrical power [P]. The power is an additional basic unit for characterising technical devices:
Electrical power(watts) = voltage(volts) x current(amperes)
electrical power(watts) = energy(joules) / time(seconds)
We use the electrical power value to assess the power consumption of an energiser.
Efficient energisers consume little electricity and are therefore more cost-efficient and environmentally friendly. E.g. if a very high-performance mains energiser such as the NV9000 only consumes 10 watts per hour. This is much less than the power consumption of a conventional light bulb.
Modern 9 V and 12 V energiser also come with power-saving circuits that adapt power consumption to the behaviour of the fence.
Summary of the important formulas for electric fences:
[U]Voltage(volts) = Resistance(ohms) x Current(amperes)
[I]Current(amperes) = Voltage(volts) / Resistance(ohms)
[R]Resistance(ohms) = Voltage(volts) / Current(amperes)
[W]Energy(joules) = Voltage(volts) x Current(amperes) x Time(seconds)
[P]Power(watts) = Voltage(volts) x Current(amperes)
Power(watts) = Energy(joules) / Time(seconds)