Electric fence voltage and earthing, explained
Installers talk about voltage, joules and earthing, and energizers show readings that are hard to make sense of. This guide explains the ideas in plain language, so you can follow what an installer tells you. It does not set out the technical limits in the standard.
How an electric fence works
An electric fence is not live all the time in the way a household plug is. The energizer sends a short, high-voltage pulse along the live strands at regular intervals, and between pulses the fence is quiet. When someone touches a live strand while standing on the ground, or touches a live strand and an earth strand at the same time, the pulse passes through them and back to the energizer through the earth. That brief shock is the deterrent.
The energizer also watches each pulse. If the voltage drops, because a strand is cut, shorted to the wall or loaded by vegetation, it raises an alarm. That is why a security fence is both a barrier and a sensor.
Voltage and energy are not the same thing
Voltage is the electrical pressure of the pulse; it is what lets the pulse jump through clothing and dry skin. Energy, measured in joules, is how much punch each pulse carries. A fence can show a high voltage and still have little energy behind it, or a modest voltage with more energy. SANS 10222-3 sets detailed requirements, for example for warning signs and energizer output, and a registered installer works to them. We do not quote the figures here; ask the installer to explain how your energizer meets the standard.
Why the earth matters so much
The pulse has to get back to the energizer to complete the circuit. On most security fences, that return path runs through earth spikes driven into the ground and connected to the energizer, and sometimes through dedicated earth strands on the fence itself. If the earth system is poor, the circuit is weak. The fence may show low readings, give false alarms or deliver a weaker deterrent.
How well the earth works depends on the ground and the weather. Dry ground conducts less well than moist ground, which is one reason some fences behave differently in a dry Highveld winter than after summer rain. Rocky ground can make it harder to drive spikes to a useful depth. The installer decides how many spikes your fence needs and where to put them, and should test the earth as part of any installation, repair or certificate inspection.
What drains voltage
- Vegetation. Every leaf or branch touching a live strand leaks some of the pulse to ground. A lot of growth along a long fence can pull the voltage down a long way.
- Shorts. A strand touching the wall, a bracket, another strand or a metal gate.
- Broken insulators. A cracked insulator can let the pulse leak into the bracket and the wall, especially when wet.
- Poor joints. Badly joined or corroded wire adds resistance.
- Length. A very long fence on an energizer meant for a shorter one will read lower at the far end.
- A weak earth. As above, the return path matters as much as the live strands.
Reading your energizer
Many energizers show the fence voltage or a simple good, low or fault signal for each zone. The useful thing is not one reading on its own, but how it changes. If the reading on a zone falls steadily over weeks, vegetation or a slowly failing insulator is a likely cause. If it drops suddenly, look for a short or a break. If it swings with the weather, think about the earth or insulators that leak when wet. Note the readings now and then, and give them to the installer.
Lightning and surges
A long run of wire on top of a wall is exposed to lightning-induced surges, and Highveld summers bring plenty of storms. A good earth system and appropriate surge protection help protect the energizer. If your energizer failed after a storm, see energizer and battery replacement.
Earthing, compliance and the certificate
Earthing is one of the things a registered installer checks when testing a fence against SANS 10222-3. Any new, repaired, restrung, extended or upgraded electric fence must comply with SANS 10222-3, and the user or lessor must hold a valid Electric Fence System Certificate of Compliance, which only a registered Electric Fence System Installer may issue (source: ECA(SA)). If a certificate inspection finds an earthing problem, it has to be fixed before the certificate can be issued. This is general information, not legal advice.
What to ask an installer
- How many earth spikes does my fence have, and is that enough for its length and the ground?
- What reading should each zone normally show, and what reading means there is a problem?
- Is my energizer suited to the length and number of zones?
- Is there surge protection, and where is it?
- What maintenance will keep the readings steady?
Next steps
If your readings are low or erratic, see electric fence repairs and our guide on why an electric fence stops working. For regular checks, see maintenance and servicing.
Frequently asked questions
Why is my fence voltage low?
Common causes are vegetation touching the strands, a short, a broken insulator, a poor earth, or an energizer that is too small for the fence.
Is a higher voltage reading always better?
Not on its own. The energy behind each pulse and a sound earth matter too. Ask the installer how your energizer meets SANS 10222-3.
How many earth spikes does a fence need?
It depends on the fence and the ground. The installer decides and should test the earth system.
Can dry weather affect my fence?
Yes. Dry ground conducts less well, which can weaken a marginal earth system. An installer can test and improve it.