The Electrified Perimeter Security Array
An array can be freestanding, placed atop a wall, or attached to an existing fence. Since most electrified security arrays are added to the inside of a chain link fence, that’s the example we’re going to use, but the engineering design is similar for all.
In Figure 1 we see an array attached to the inside of an existing fence. The wires of the array start at 3″ above ground level, and there’s a wire every 3″ to 6″ (usually 6″). Even though the existing fence offers some protection, we run the array all the way up because chain link fences can be easily cut with a $10 bolt cutter.
Alternate wires are grounded to ensure that anything coming over or through the fence receives a shock. Otherwise, a trespasser with heavy rubber shoes might not feel the high voltage shock, and an animal that tries to cross the fence by jumping on it wouldn’t feel the shock, either.
A solid cement footer, or some rows of heavy concrete or other debris along the base of the fence can be used to prevent digging under the fence.
Another alternative to prevent digging is shown in Figure 2. In this example we ran the array three feet along the ground inside of the fence, to prevent livestock from digging their way out.
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.
More sketches for creating arrays atop walls, around corners, connecting to gates and specialized applications can be found by using “Search” at the top of this page, and examining the photos in this folder:
Wiring the Array
We saw in Figure 1 that alternate rows of the fence are hot and grounded. The other important step is that the hot wire from the energizer, out to the array and back again, must be one continuous circuit.
In a livestock electric fence one usually runs multiple wires out to the end of the fence. But in an electric security array we need to have continuity in the hot wire. That’s how we can constantly monitor the voltage. Figure 3 gives a simplified example of this.
In Figure 4 we see the how this is done:
The grounded (green) wires can be bundled and sent directly to ground – the better the grounding the better the security system.
The hot (pink) wire goes back and forth, back and forth, in a snaked or “S” pattern, eventually making its way from the high voltage source (fence energizer) to the Fence Hawk monitor.
This continuous “S” pattern of the energized “hot” wire is what allows a monitored security array to work, and is essential to the installation.
We’ve found that having the top wire as a grounded wire helps prevent false alarms by diverting falling branches and protecting small birds that land on the fence. Plus, when an intruder jumps up and catches the wire it will stretch and contact the hot wire below it, ensuring a good hard Zap! that knocks them off the fence.
On large installations it’s good security practice to split the secured area into smaller segments or zones. This allows you to dispatch security directly to an area where an alarm has sounded. 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 6 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.
A set of magnetic contacts are used on the gates and wired into the alarm system, to notify security if the gates are open or closed at any particular time.
The Fence Hawk Sensor
We’ve been in the security business for 40 years, and never a month went by that someone didn’t ask about perimeter security. We did the standard combinations and installations of motion detectors, lights, cameras and other devices, but knew that they were easily bypassed and just 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 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 existing fence monitors and sensors were based on a 1978 patent application, #4220949.
As an electrical engineer can tell you from Figure 8 (Fig. 3 in the patent abstract), the solution was to have the fence pulse flash a neon tube, which is read by an optically sensitive resistor (which gives an approximate idea of voltage at best), which then charges a capacitive circuit, and the state of the capacitor is monitored. If the capacitor voltage changes fast enough (in either direction), 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 that the voltage drops slowly over a period of time. The sensor 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.
To solve this we designed an improved monitor and sensor that 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 – a minimum of 6,000 or 7,000 volts, up to 25,000 volts.
When the voltage falls below a set value – factory preset at 350 volts – and remains that way for a period of time – factory set for 6 seconds – 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 maintenance alarm is generated.
Because the Fence Hawk measures and counts the actual voltage pulses on the fence, and allows for the occasional random, spurious pulse, it doesn’t generate false alarms.
If the pulse rhythm is interrupted by a nearby lightning strike or an animal coming into contact with 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 - a system that generates false alarms is worse than no security system at all!
For more information and technical specs on the Fence Hawk, click here.
A method of maintaining electrical contact in a sliding gate security array:
A short video about the Fence Hawk and how it works: