Electric fence: how it works and what it is made of
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An electric fence is a wire on insulators fed by an energiser that sends short pulses of high voltage. Virginia Tech's extension service puts the pulse at over 5,000 volts, under 300 milliamps and no longer than 1/300 of a second; the shock comes only when someone closes the circuit to earth.
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How an electric fence works
An electric fence does not carry a steady current. The energiser releases a pulse into the fence wire again and again; according to Virginia Tech’s guide “How Does an Electric Fence Work?” by grassland agronomist Matt Booher, that pulse is over 5,000 volts, under 300 milliamps and lasts 1/300 of a second or less.
Nothing happens while the wire hangs free: the circuit is open. When an animal or a person touches the wire, current flows through the body into the soil, through the earth rods and back to the energiser’s earth terminal. That closed loop is the shock. Because the soil is part of the circuit, Virginia Tech names soil moisture as a major factor in how well a fence works, and advises keeping at least 5,000 volts on a livestock fence even in dry weather.
Electric fence materials
| Part | What it does | What to look for |
|---|---|---|
| Energiser | turns mains or battery power into high-voltage pulses | output energy in joules at a stated load, open-circuit voltage, backup battery |
| Fence wire | carries the pulse along the fence | high-tensile smooth galvanised wire; no rust or flaking coating |
| Insulators | hold the wire on posts without leaking current to them | UV-stabilised plastic that does not crack |
| Earth rods (earth electrode) | return the current through the soil to the energiser | depth and spacing as set by the energiser maker |
| Lead-out cable | links the energiser to the fence and the earth | insulated high-voltage cable, never house wiring |
| Warning signs | mark the fence for people | size and colours set in the maker’s manual |
Copper has no place in the system: Virginia Tech warns that copper joined to steel corrodes quickly and that the corrosion lowers the fence’s performance.
Electric fence energiser: joules and volts
Two numbers describe an energiser. The open-circuit voltage is what the fence reaches with nothing touching it. The output energy, in joules, is what each pulse delivers into a load; Nemtek gives it at 500 ohms, and a joule figure without its load cannot be compared. South African maker Nemtek states both for its Wizord range, which appears in searches as “Wizard 2” and “Wizord 4”.
| Nemtek energiser | Output energy | Open-circuit voltage | Power and backup | Monitoring |
|---|---|---|---|---|
| Wizord 2i | 2.0 J at 500 ohms, single zone | 7,400 V | 240 V mains, built-in 7 Ah battery, 24 hours standby | alarm on high-voltage and earth wires, gate input, radio or GSM link |
| Wizord 4i | stated differently in two places on the maker’s page | 8,000 V | 240 V mains, 7 Ah battery, 24 hours standby | alarm, gate input, radio or GSM link |
The Wizord 4i page lists 3.7 J per zone at 500 ohms in its specification table and a higher figure in its feature list. Ask the seller for the current data sheet before you compare it with another energiser.
Electric fence wire and insulators
The wire matters as much as the energiser. Virginia Tech recommends 12½ gauge high-tensile smooth wire with a Class 3 galvanised coating, which carries roughly three times the zinc of Class 1 wire, for both conductivity and life. Current travels along the surface of the wire as well as inside it, so rusty or flaking wire conducts poorly.
- Polywire and polytape contain thin metal filaments and resist current more than a single steel wire. Virginia Tech advises them for temporary fences, not long permanent runs.
- Lead-out cable from the energiser to the fence should be insulated cable made for high voltage. Aluminium-coated cable is about three times as conductive as galvanised steel wire, according to the same guide.
- Connections should use clamps or crimps rather than hand-wrapped joints, and all live wires should be linked at the start and end of each run.
- Insulators: cheaper plastic can crack with age and leak current, which is why Virginia Tech points buyers to UV-stabilised types.
Electrified fence on a wall: the maker’s rules
An electrified fence on a compound wall is what Nemtek calls an electric security fence: live wires plus a physical barrier kept electrically apart from them. The appendix of Nemtek’s Wizord 4 installer manual sets out requirements for electric security fences:
- Warning signs at least 100 × 200 mm, yellow on both sides with black lettering at least 25 mm high, at every gate, every access point and at intervals of no more than 10 m.
- The energiser’s earth electrode goes at least 1 m into the ground and at least 2 m from any other earth system.
- Barbed wire and razor wire must not be electrified.
- Two electric fences fed by different energisers stay at least 2.5 m apart.
- Leads run underground go in an insulating conduit or use insulated high-voltage cable, and never share a conduit with mains, communication or data cables.
- Gates must open without the person receiving a shock.
These are the manufacturer’s installation conditions. Planning or electrical rules where you build are a separate question for the installer.
Wire heights for livestock
A simple electric fence for animals can be one or two wires, but the height has to suit the animal so the shock lands on the face. Virginia Tech, adapting a US conservation guide, gives these heights above ground, converted here from inches to centimetres:
| Animal | Fence | Wire heights |
|---|---|---|
| Cattle | 1 wire | 66–81 cm |
| Cattle | 3 wires, at least 2 live | 46, 76 and 107 cm |
| Goats and sheep | 4 wires, all live | 15, 41, 66 and 91 cm |
On multi-wire fences, the guide advises no more than 10 inches (25 cm) between wires.
What to check when you buy electric fence parts
- The energiser’s output energy with the load it was measured at, not only the voltage.
- A backup battery and its standby time, if mains power goes off where the fence stands.
- Monitoring: whether the energiser raises an alarm on a cut or earthed wire and can signal a gate.
- High-voltage lead-out cable for every buried or indoor run.
- Earth rods and clamps in the number and depth the energiser manual asks for.
Electric fence in Zanzibar
Kitovu sells from stock in Zanzibar, and electric fence parts are not among its products. For other barriers on a wall or boundary, see barbed wire and chain link fence.
Questions
How does an electric fence work?
The energiser sends a pulse along the fence wire. A person or animal touching the wire lets the current pass through the body into the soil and back to the energiser through its earth rods, which completes the circuit and gives the shock (Virginia Tech, SPES-688P).