The Electric Perimeter Security Fence
Let’s build a legal security barrier that shocks anybody that touches it, and will trigger an alarm if cut, climbed, shorted, torn down or tampered with. As a bonus, it will also notify us if the voltage falls below a certain threshold.
An electric security fence, or array, can be freestanding, placed atop a wall, or attached to an existing fence. Since most electric security arrays are added to the inside of a chain link fence, that’s the example we’re going to use, but the principles are the same.
In Figure 1 we see an array attached to the inside of an existing fence. The wires of the array start a few inches above ground level, and there’s a wire every 4 to 6 inches, to a height of 8 or 10 feet. We run the array all the way up because chain link fences are easily cut with a $10 bolt cutter.
Alternate wires are grounded to ensure that anything coming over or through the array receives a shock. If the wires remain in contact with each other for more than a few seconds, it will also trigger the alarm. If a hot wire is cut, the alarm sounds.
This also negates throwing a mattress or other insulator over the array because the fence will short out, which triggers the alarm.
A optional solid cement footer (see Fig. 1), or some rows of heavy debris along the base of the fence can be used to prevent digging under the fence.
Another way to prevent digging is shown in Figure 2. We run the array horizontally for three feet across the ground to prevent predators and other thieves from digging their way in.
In our experience, we have never had a trespasser attempt to dig under an array. They have always tried to come over it or through it. After the first shock, they leave and don’t return.
Another special case is keeping snakes out of utility installations. Our systems are used for this extensively in Texas. Contact us for snake-proofing and other specialized designs.
Wiring the Array
We saw in Figure 1 that alternate rows of the fence are hot and grounded.
The next step is that the hot wire from the energizer, out to the array and back again, must be one continuous circuit.
In a monitored electric security array we need to have continuity in the hot wire. That’s how we can constantly monitor the voltage. Figure 3 shows a simple example of this.
Figure 4 shows how this is done on a larger scale.
The grounded (green) wires can be bundled and sent directly to ground – the better the grounding, the better the security system.
Having good earth grounds on the array and components is what ensures that anybody that comes into contact with it gets a good, hard zap. This is what knocks them off the fence.
The hot (pink) wire goes back and forth in an “S” pattern, eventually making its way from the high voltage source – the fence energizer – to the Fence Hawk monitor.
This continuous “S” pattern of the energized hot wire is what makes a monitored security array work, and is the key to monitoring the voltage on the fence.
On larger installations it’s good security practice to split the secured area into smaller segments or zones. This allows dispatching security directly to the area when an alarm goes off.
It also allows you to automatically turn on lights and cameras for that area, if you are so equipped.
Third, it allows you to turn off individual zones during working hours – a front gate, for example – without turning off the entire system.
In Figure 5 the gates are wired and zoned in such a way that the entire system can remain on when the gates are left open. The array on the gates can be turned on or off independent of the rest of the system.
Other Electric Fence Sensors
We’ve been in the security business for 40 years. We installed the standard combinations of motion detectors, lights, cameras and other devices, but knew that they were easily bypassed and it wasn’t decent perimeter security.
An electric array was the logical protection device, but they’re easily defeated with a pair of wire cutters, so the voltage has to be monitored for this idea to work.
We purchased and tested every fence monitor made, but they all had the same failing – they measure a floating average voltage and are susceptible to lightning strikes, vegetation growth, failing energizers, aging equipment, rust, etc., that the other array sensors cannot detect.
The only solution was to design and build one the right way, one that is designed to be a security device.
The challenge in making an electric fence monitor is in measuring the voltage. The pulse is very high voltage, but it only lasts for a brief period of time – typically 0.00005 to 0.00030 seconds, or 50 to 300 microseconds.
We found that all the fence monitors and sensors we tested were based on a 1978 patent application, #4220949.
As an electrical engineer can tell you from Figure 6 (Fig. 3 in the linked patent abstract), it works by having the fence charger pulse energize a neon tube, which is read by an optically sensitive resistor, which gives an approximate idea of the voltage, which then charges a capacitive circuit, and the state of the capacitor is monitored. If the capacitor voltage changes fast enough, an alarm is triggered.
While better than no fence monitor at all, we found this floating voltage measurement doesn’t work well enough, or accurate enough, for a reliable security application.
Nearby lightning storms can cause large pulses on the fence, inducing a voltage change and fooling the floating circuit into thinking the fence has been shorted – triggering a false alarm.
When vegetation grows up along an electrified array the voltage drops slowly over a period of time. The sensor design in patent #4220949 is unable to measure this voltage drop. As long as the neon tube is flashing regularly, the monitor thinks all is well, and isn’t going to trigger an alarm until the voltage is quite low – way below the level that makes an electrified array an effective deterrent.
The Fence Hawk Monitor/Sensor – Made in the USA
So we designed an improved monitor and sensor we named the Fence Hawk. The state-of-the-art, computerized, patent-pending circuitry measures the actual voltage on the line. When installed, it’s adjusted to the voltage of the array – up to 25,000 volts.
When the voltage falls below a set value and remains that way for a period of time a security alarm is sounded.
When the voltage falls below an adjustable minimum voltage, usually 2,000 volts below the normal fence energizer voltage, a weed alert is generated.
Because the Fence Hawk measures and counts the voltage pulses on the fence, and allows for the occasional random or spurious pulse, it doesn’t generate false alarms.
If the pulse rhythm is interrupted by a nearby lightning strike or a person touching the array, the Fence Hawk is designed to watch carefully, and if the pulse and voltage quickly return to normal, it ignores this.
It only generates an alarm when the fence voltage is lost, or drops below a set level, for a set period of time.
Since installing our first Fence Hawk we’ve had no false alarms, and we’ve never had a perimeter violation when the array was activated and armed - because we believe a system that generates false alarms is worse than no security system at all!
Â The Fence Hawk Explained
An Integrated Security System
The Fence Hawk monitor is designed to connect to any alarm control panel. It connects like a motion or smoke detector.
One pair of wires from the Fence Hawk carries the alarm signal, the other pair signals low voltage – the weed alert.
This gives you a fully integrated alarm system, and allows your existing alarm to call the police, trigger sirens, floodlights, cameras, etc., when the Fence Hawk detects an intrusion.
No Alarm System? No Problem.
We have kits and assembled systems with sirens, phone dialers, etc., that make DIY installation affordable, quick and easy. With a phone dialer that calls you directly you won’t have monthly monitoring fees.
We provide free tech support by phone, fax, email, digital photos, Skype, etc.
The Fence Hawk can be used to activate any device. When an alarm or alert is triggered the Fence Hawk closes a set of contacts – a switch – that can be used to remotely activate any device.
We will supply schematics to wire into anything. If we don’t have a schematic for your application we’ll draw one up for you.
Visit our online store for more idea-generating examples, and a broad range of DIY electric security array construction materials.
For more information and technical specifications of the Fence Hawk, click here.
The Fence Hawk installation manual in PDF format.
For photos of a variety of installations that use the Fence Hawk, hover your mouse over the drop-down menus at the very top of the screen – Commercial, Residential, etc.
For a installation quote, or design assistance for a DIY project, please contact us. For DIY assembly tips and tutorials, see the menu items at the very top of the screen.
To purchase a Fence Hawk and integrated security packages, please visit our online store.
We ship and install anywhere in the world.