agricultural production guidelines
m
Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 9.1 1999
ELECTRIC FENCING
C I Macdonald
KwaZulu-Natal Department of Agriculture
A
Summary of How the Electric Fence Functions
Materials/Equipment
Required for the Electric Fence
INTRODUCTION
Well maintained, good electric fencing is highly reliable, extremely versatile and cost effective, provided the fences are built properly and correctly with high quality equipment. It is then a pleasurable and rewarding management tool to work with (and often a great source of amusement!). By contrast, badly maintained, poorly constructed electric fencing using low quality equipment is both a liability and a curse.
The concept is not new, electric fences have been used with varying degrees of success for many years. Modern technology has now enabled equipment, especially energisers, to become far more reliable, powerful and safe to use. The variation in products available to the farmer has increased equally dramatically to such an extent that it is easy to understand why a prospective buyer of such equipment becomes confused with what he actually needs and what he wants to need.
While the advantages of electric fences are many and varied, the most important being cost effectiveness, versatility and reliability, they should not be used in handling yards or narrow passages where animals are forced to touch the fence through no choice of their own. Neither should they be used where they will be totally overgrown by uncontrolled herbage growth.
Sound, practical advice should therefore be obtained before any materials are purchased. Objectives and plans (both short and long term) should be clear and the equipment purchased must be in accordance with the buyers specific requirements. A clear understanding of how an electric fence functions is also required. A brief description therefore follows:
A SUMMARY OF HOW THE ELECTRIC FENCE FUNCTIONS
Stated very simply an energiser charges an insulated fence wire with electricity. It relies on the animal/intruder to close the circuit between the now electrified fence wire and the earth terminal on the energiser. The animal gets shocked in the process. The earth terminal on the energiser is connected either to an earth wire in the fence or an earth peg driven into the water table in the soil profile. In the latter case the current flows from the electrified wire, through the animal, through the soil moisture and back to the earth peg. This also closes the circuit. Once again the animal gets shocked in the process. Due to the design of the energisers available commercially today, the fence network is NON LETHAL - despite the high voltages used. AN ELECTRIC FENCE IS A PSYCHO-LOGICAL AND NOT A PHYSICAL BARRIER.
It will therefore be obvious that in either circumstance any limitation in the fence network (too thin a wire, a bad join or faulty insulator, dry soil under the animals feet, etc) will lessen the shock the animal/intruder receives. The object is to make this shock as great as possible. For efficient control of animals the following minimum voltages (V) must be maintained.
Sheep and goats - 2000 V
Wild Game - 4000 V
Cattle - 1000 V
Security fences - 5000 V
Pigs and horses - 600 V
MATERIALS/EQUIPMENT REQUIRED FOR THE ELECTRIC FENCE
Energisers:
Three basic types of energisers are available.
- Mains powered energisers are obviously recommended when mains power is in close proximity to the proposed electric fence. The power consumed daily by the most powerful energisers is far less than that of a 60 watt light globe.
These energisers can power more than 125 km of wire under ideal conditions. They are therefore used extensively on large farms, or on those farms with a multi-strand, (usually a 3 or 5 strand) monitored, perimeter fence integrated with the (internal) grazing management fences.
Less costly, medium output mains-powered energisers are also available. These are quite adequate for most farms where the sole purpose of the fences is grazing management. These energisers power between 40 and 60 km of wire.
Most good energisers have switches or different terminals to reduce the output once the animals are trained and are therefore also suitable for use with more sensitive animals such as horses and pigs.
- Battery-powered energisers are also available in different models which operate from a 6 v to a 32 v power source. The small output models, usually
6 v dry cell, are essentially for temporary strip grazing and therefore only suitable for short lengths of fence (less than 2 km).
The 12 v wet cell powered models are most useful as they can power up to 15 km of fence and the battery can be recharged by the farm vehicle or a trickle charger in the workshop, or using a solar panel.
Depending on the brand of energiser, the usual-sized tractor battery (90 amp/hr) lasts up to four weeks before recharging is necessary. Furthermore, these energisers are small and compact, and therefore readily portable. It is strongly felt that these energisers are far better value for money than the strip-grazing energisers, even at a slightly higher purchase price. However, it should be remembered that a 12 v battery has, of late, become a highly desirable commodity in the ‘free used spares’ market. It is therefore strongly recommended that both the energiser and the battery be housed in a ‘shock-box’ to dramatically reduce losses of batteries. Such a ‘shock-box’ costs roughly the same as a new 12 v battery.
Energisers requiring 24 or 32 v power sources are not recommended unless permanent charging facilities, be they mains, solar or wind, are available and permanently connected. Power consumption of these energisers is relatively high and batteries lose their charge within a few days. Such energisers, are however, extremely useful in areas where mains power is often interrupted or in a security-type of application when the mains supply could be deliberately cut. They have a fence capacity of approximately 40 km.
- Solar powered energisers, with their special batteries, are also available. They are expensive, specialised units suitable for remote areas and are therefore rarely seen. They power up to 30 km of fence. Nowadays they are more frequently being displaced by the larger models of the wet-cell energisers coupled to a solar panel and battery.
The size and type of energiser is therefore determined by the power supply available and length of fence to be powered. Most energisers (ranging from the smallest strip-grazing models) ‘generate’ roughly 6000 volts under ‘no load’ or ‘nil vegetation’ conditions. The difference between energisers therefore lies essentially in the higher energy output of the larger energisers and not in higher voltage output. This enables the larger energisers to maintain acceptable voltages under adverse or ‘heavy load’ conditions (caused either by longer fences or vegetation growing on the live fence wire/s). The energiser selected must therefore be capable of maintaining at least 4000 volts throughout the entire fence network under normal conditions.
It is equally important to ensure that all spares (for the energiser(s) especially) are both available and in stock when required. Some companies have been known to take over six months to repair an energiser while others can repair most faults within five minutes.
Earthing the energiser
An extremely important and interrelated facet to be considered with the energiser is the earthing of the energiser and use of earth pegs. More than 80% of problems encountered with electric fencing can be attributed to bad or insufficient earthing. Furthermore, the higher the output capacity of the energiser, the larger the required surface area of the earth. The earth peg/s should preferably be sited in a permanently damp place, e.g. near a drain or gutter downpipe, but not within 3 m of any mains-earthing (e.g. ESCOM) system.A simple test should be carried out to test the energiser earth. Proceed as follows. Short out the fence about 100 m from the energiser, using, for example, steel fence standards or some other device with good electrical conducting potential. Place one hand (or the end of one lead of a voltmeter!) on the soil and the other hand (or the end of the other voltmeter lead) on the earth peg. If a shock is felt (or a voltage reading in excess of 1000v recorded!) the earth is inadequate. The earth must be improved by adding earth pegs (usually galvanized steel spikes driven into the ground about 2 m deep, or at least into the water table) until no further shock is felt. The pegs should be interconnected with each other and the energiser earth terminal.
Wire
Electrified barbed wire is extremely dangerous and therefore not recommended under any circumstances. It is now also illegal. As regards mass, strength, conductivity and cost, 2.24 mm or 2.00 x 2.60 mm galvanised high-tensile wire is recommended as the best compromise. This wire requires very little tension to pull it straight (about 1/3 of tension for barbed wire) and this means less strain on the fencing system (e.g. insulators and corner posts). High-tensile wire is not suitable for semi-permanent electric fences. For this purpose 2.5 mm soft, galvanised, baling wire is recommended. It is extremely important to remember that copper and galvanised wire/accessories should not be mixed. (Once moist, a process termed electrolysis takes place when electricity flows through the connection. This causes polarisation between the two metals. Eventually a break in conductivity will occur.) It should be mentioned that any capacity energiser can be coupled to permanent or semi-permanent steel fences.Alternatives to soft wire exist for temporary fencing. These include nylon and metal braided cord with varying numbers of stainless steel wires in the braid, laminated tape, or similar alternatives. While these are more easily visible they stretch and deteriorate over time. They also offer high resistance to current flow on longer strains (more than 500 m) and should therefore never be used as power supply lines. Caution should also be exercised when coupling the larger capacity energisers solely to nylon braided cord. Once this braid has deteriorated, even slightly it simply burns off.
Fence posts and droppers
If possible, avoid using steel posts and droppers because of the potential of dead shorts through faulty insulators or loose wires. Timber posts and droppers can be used to good advantage. Some (the very hard heartwood types) can be used without insulators, with tremendous cost savings in the long run. They are also resistant to fire, ants and rot. Be careful though, when using them, they are relatively heavy and therefore require at least two strands of wire to hold them upright. The top strand should be as high as possible (900 mm) above ground level.Soft timbers should be treated with creosote and should preferably be used with insulators.
Fibreglass droppers and posts can be used, but they should be of good quality, treated with the correct fire-retardants and U.V. (Ultra Violet) stabilizer. Experience has shown that while fibreglass droppers and posts are generally of quite acceptable quality, the spring clips attached to the droppers to support the wire tend to rust and lose their tension relatively quickly.
Insulators
It must be emphasized that although roughly 30% of the cost of electric fencing can be attributed to (good) insulators, it is strongly felt that cutting costs by using, for example, old hosepipe or baling twine, is false economy. Such insulators can be and are used, but they dramatically increase leakage problems and therefore maintenance time, while decreasing the affectivity and reliability of the fence.It is therefore recommended that where plastic insulators are used that they be of the high density, U.V. stabilised variety. Even then, after about three years the insulators become brittle and can start to leak or ‘track’. In areas where burning is part of the management strategy, highly glazed, crack free porcelain insulators should be used.
Another option is to use fire-proof bakelite or fibreglass insulators. Invariably they are less-costly than porcelain, more resistant to vandalism and do not crack/deteriorate with age. Ensure however, prior to purchase, that the fibreglass insulators have been U.V. stabilized and treated with the correct fire retardant.
All three types of insulator are available in differing shapes, sizes and methods of attachment for their various uses.
Accessories
Various accessories are commercially available. Most are very useful but not all are strictly necessary. It is therefore suggested that where financial restraints have to be exercised they are exercised here. It is also here that it pays ‘to shop around and compare prices’, especially with items such as undergate cable and fence tensioners. Some accessories are nevertheless considered necessities and include a digital voltmeter, good quality cut-out switches and galvanised line-clamps. Beware of electro-plated line clamps, they are no substitute for hot dipped galvanised line-clamps, even at half the price! The hot-dipped clamps outlast the electro-plated clamps by many years!Another good investment is an adequately earthed lightning diverter. The point to remember is that the diverter earth must be better than that of the energiser.
Erecting the electric fence
It should be remembered that dry, sandy, granite-based soils are bad electrical conductors. By contrast heavy, wet, clay soils (by virtue of the soil moisture), are relatively good conductors. If, therefore, an animal stands on a conductive, wet soil and touches a live fence wire, the current passes through its body to the soil and eventually back to the energiser. If it touches a live and an earth wire, the current may only pass across its hide. The shock an animal receives when the current passes through its body as opposed to across its hide is obviously more effective and remembered for far longer.Bearing these factors in mind, two basic fence configurations exist. The ‘live-earth-live’ system is used in areas where there is little or no soil conductivity due to drought or soil type. As the name implies, alternate wires (anything up to six strands) are live and earth, and connected in parallel with those of similar polarity at least once every 1 000 m. Furthermore, the earth wires are connected to earth stakes along the fence, also at 1 000 m intervals and to the earth terminal of the energiser. This configuration does not rely as heavily on the energiser earth-peg network as does the ‘all live’ configuration. The disadvantage of a ‘live-earth-live’ configuration is the risk of leakage between the earth and live wires due to the possibility of trees or branches falling onto the fence, helpful neighbours hanging loose pieces of wire on the fence, or some other obstruction connecting the live and earth wires together.
An ‘all-live’ configuration is used in areas where soils are moist and/or have good conductivity. All fence wires are live and therefore without an earth return wire. The energiser to earth-peg network must therefore be ‘perfect’. The risk of leakage as in the ‘live-earth-live’ configuration is eliminated. This configuration is therefore preferred if a choice is possible. If, however, when you erect the fence, irrespective of the configuration, all wires are insulated at corner posts, the fence can be converted from the one to the other with a minimum of time and effort thereby accommodating changing soil conditions and achieving the ‘best of both worlds’.
In selecting spacings for wires, posts and droppers, the aim is to have the fence wires as parallel to the soil surface and to each other as possible, with the distance between fence wires such that the animal will jump backwards when shocked and not forwards. In practical terms this means that the animal must be shocked forward of its ears. Wires should therefore not be more than 250 mm apart, the top wire roughly 900 mm high and electrified. The lowest permanently electrified wire should not be less than 250 mm above the soil (due to vegetation growth and therefore leakage potential). For use with pigs, lambs and calves, the lowest wire should be neutral (insulated in order that it can be connected (live) if necessary) but not less than 150 mm from the soil surface. Goats may even require this strand to be earthed and strained 50 mm above ground level thereby ensuring a good earth when they attempt to force their way through the fence. Posts should not be more than 45 m apart on very even land, with two evenly spaced droppers per post. The spacings between posts and droppers will decrease drastically on rough terrain. The use of ‘tie-downs’ in hollows may be necessary. Wire should be tensioned sufficiently to ensure the fence wire is straight (not slack) and no more. Strain tension for high tensile wire should not exceed 1/3 of that usually required for barbed wire, i.e. 50 kg as opposed to 150 kg. Uninterrupted strains of up to 1 500 m, though uncommon, are quite acceptable.
Game fencing
Suitably designed, well constructed and powered electric fences are proving highly efficient in controlling wild or game animals. The same hardware (energisers, insulators, wire etc.) used with domesticated animals is used for game fences. The same principles with respect to fence construction and maintenance also apply. Particular attention must be paid to the spacings between the strands of wire and also between the posts and droppers.Game fences differ from grazing management fences in height (and therefore number of strands of wire) and often also in wiring configuration. This depends on factors such as topography and the type of animal to be controlled.
Whatever game fence is constructed the following two points should be borne in mind.
- Build the game fence in sections (or parts thereof). Electrify the completed fence as soon as possible, at least every night, and leave it electrified from then on, only switching it off to carry out repairs. Make the newly erected section of fence as visible as possible by using any (removable) white non conductive material (e.g. discarded plastic bucket lids) or ‘live lights’ to warn unsuspecting game of the presence of the new fence.
- Ensure that every time any animal ‘tests’ the fence it is well and truly discouraged, even if it means ‘overpowering’ the fence with a more powerful energiser than would normally be required for the same length of fence with domesticated animals.
Certain species of game (especially kudu and impala) that jump fences and those which charge through them (e.g. bushpig) are the most difficult to control. A 14-strand, 2.0 m high fence appears to be capable of controlling most animals, including the above-mentioned.
It must be stressed, however, that in South Africa the concept of electric fencing for game animals is in its infancy. It is also relatively specialized. It is therefore essential that before embarking on any project involving electric fencing for game, advice is sought from a reputable and experienced advisor/dealer/supplier who has the required expertise in this field.
Security Fencing
While no security fence can be guaranteed to be totally impenetrable under all conditions, the use of monitored electric fences is becoming increasingly widespread.The same hardware as for grazing management fences is used and is coupled to a ‘fence-end’ voltage monitor. The monitor is coupled to an alarm system (including, for example, a siren or radio paging device). Ideally the security fence should completely encircle the area. The energiser is connected to one end of the fence and the monitor, while physically adjacent to the energiser is connected to the other end. The energiser and monitor then utilize common earth pegs. Should the voltage at the end of the fence drop below a desired (pre-set) threshold level the monitor will then trigger the alarm such a voltage drop could be caused by physically cutting or shorting the insulated fence wire. It should be mentioned that the level of sophistication (and therefore purchase price!) of fence monitors varies greatly. Beware especially of ‘sectorised’ system monitors and the manner in which the security fence is monitored. Some sectorised systems simply do not work as such, while others at the same cost work extremely well.
Two points are therefore stressed: a) be careful when purchasing a security system, and b) the amount spent on such a system must be in keeping with the value commodity being protected.
Remember also that the essential difference between a grazing management fence and a security fence is that the latter is connected in series while the former is connected in parallel. This means that a security fence is essentially a very long, continuous, single wire encircling the protected area seven or eight times (if you have a fifteen strand, two metre high, live/earth fence). This, together with the fact that high fence voltages (6 000 v+) must be maintained under all conditions, therefore requires a larger output capacity energiser than is normally used with the usual grazing management fences.
Grazing management fences can be integrated with a monitored security fence. This practice is not generally recommended simply because if a voltage drop does occur (and the alarm sound), there is no way (other than a physical area inspection) of knowing whether the fault was in the perimeter (security) fence or in the grazing management fence network. While such integrated systems do exist (and have proved worthwhile) it is often necessary that they are used in combination with a security guard who is in close proximity to the protected area. It must be stressed therefore that while such a perimeter fence may be an ‘early warning deterrent' it can, at best, be regarded as a ‘low level security’ fence.
Problems
Problems with electric fencing can and will occur. If good fencing principles and techniques are applied when building the fence, these problems will be reduced to a minimum.The most common faults are likely to be the following:
- Leakage from:
(a) dead shorts, e.g. metal on the fence, causing a direct leak to earth,
(b) breakdown of insulation, e.g. either crackedor weathered insulators, causing a number of small leaks. Their effect is cumulative, and
(c) vegetation. This should not be suspected if a good energiser is being used, unless the fence is heavily overgrown over a reasonably long distance. Problems can arise however, in late winter and early spring with tall, dead, unburned veld-grass touching the lowest electrified strand of wire, especially in the early morning after a heavy overnight dew.
- Bad earth system: either the soil has dried out, especially around the earth peg/s, or the connections are loose or corroded. Remember, the earth system is half of the entire network! Roughly 80% of all faults with electric fences can be traced to the earth system.
- Loose or deteriorated connections, especially in the ‘live’ half of the system. (The use of reef knots or figure eight knots, in preference to double loops, and galvanised line clamps while building the fence greatly reduces such possibilities.)
- Faulty energiser: the may occur, for example, following lightning strikes and severe storms, or simply due to wear and tear.
LITERATURE CITED
ANON., undated. Insultimber Power Fencing Manual, 6th edition. Gallagher Power Fencing System, Hamilton, New Zealand.
ANON., 1979. Permanent electric fencing. Bull No. 79/6, New South Wales Dept. Agric.
ANON., undated. Electric fencing Notes, New Zealand Agric. Engineering Inst.
WILLIAMSON, M.J., HARDMAN, N.G., 1988. Personal communication. Gallagher-Poldenvale(Pty) Ltd., Pietermaritzburg, 3201.