Intelligent Fencing Systems :: Electric Fence Alarms; Design Strategies for Security Fences

To design a security fence you have to know the terms:

Low Impedance Charger: Between "low impedance" and "high impedance", the rule of thumb is, the lower, the better. Since few manufacturers actually state the actual output impedance, this a rather vague term. Opt for "low impedance" charger when possible.
Joules: This is like watts, and is used extensively to rate the output of fence chargers. This is essentially the voltage, times the current, times the TIME or length of pulse. This is only a rough guide to zapping strength. The pulse voltage plus the output current is much more a determining factor. This is rated as the actual voltage drop with a 500 ohm load. The human body under normal conditions is usually about 20 to 30 times that resistance.
Pulse Voltage: The is the peak output voltage and the most important consideration for security fencing. Voltage is equivalent to electrical pressure and the higher the pressure, the more painful the shock. You also have to look at the voltage delivered when someone grabs the fence.
Range: 15 mile or 50 mile charger ... Anything over a 100 yards is kind of redundant, as guard or police response to an alarm will never know where to go to. However, this can be a rough guide as to zapping power.

To recap,

Obtain the highest voltage output fence charger that you can obtain within your budget. Check for the lowest voltage drop under load (500 ohm standard)
Break your perimeter in sections, so your response people (guards or police) have some idea where to respond to, in the event of a break or intrusion. You will need a charger and voltage monitor for each fence section. We have found that a fence or section about a 100 yards or 100 meters is about the maximum reliable response section.
Consider, if you have 100 yards/meters of fence and are charging 12 strands, that is over a kilometer of conductor and only slightly less than a mile when connected in a loop configuration. So, it is well under the 15 - 30 mile range on most fence chargers.
The main requirement in picking your fence charger is voltage. The higher the voltage the better. Look for a fence charger with a minimum pulse voltage of 8 to 10 thousands volts. There are few chargers that deliver higher voltages than this. Always check the actual voltage delivered when under load.

Example: The below is a 15 acre vegetable farm located in the tropics and harvested 3 crops a year.

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The perimeter was divided into 8 sections. There are 8 fence chargers and 8 voltage monitors. All the monitor contacts were fed back to the bunkhouse, with a common audible alarm and a map of the perimeter. Each fence section had a visual alarm mounted close to the center of the section to guide the response people. The fence sections averaged slightly over 100 yards.

An alarm gave an audible signal, and with a quick glance at the response map, the guard could response directly to the section of fence involved.

The field was contained with a 6 foot chain link fence with barbed wire on the top. Petty theft over the fence was a continuous problem. The total installation costs were more than recovered in the first harvest. The gross volume of the farm has more than doubled.

The old barbed wire on the top of the chain link was stripped away, and new charged conductors were installed as shown below:

All of the top (4) conductors are mounted on insulators, even though the top and alternate conductors (as shown) are connected to earth ground. This is to augment the psychological effect,

Grounding the top conductor helps to minimize "false alarms" from falling foliage.

Alternating the top conductors and bonding the chain link to ground ensures that any trespasser receives the full voltage between ground and the charged conductors.

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Examples of other types of wall and fence extensions:

This wall with a 45 degree extension was already in place, including the (3) strands of barbed wire. People still climbed over the fence. The owner added an additional (2) charged strands of galvanized wire mounted on insulators. This did not simply reduce the trespassing, but eliminated it completely.

This was an old wall with rusted and broken chain link between the cast in posts. The owner removed all the old fencing and installed single strand galvanized wire conductors as shown.

This configuration is probably the most common addition to existing walls. This replaces and is more effective than barbed wire, razor wire, or broken glass (in some countries). Extending over the top of existing decorative walls and fences, this simple design has proven itself over and over again. We recommend using a 10,000 (plus) volt fence charger, installed with (2) or more good ground rods (8 feet long or more) so there is good conductivity through ground, through the wall, and through rubber soled shoes. The addition of grounded conductors does (2) things. 1.) the trespasser gets full voltage between the charged strand and the grounded strand. 2.) light branches and foliage that fall on the top strand will be likely to set off a "false alarm"

This is a private country home with expensive horses pastured in the front of the property. The caretaker's house is at the front and the alarms notify in both locations. The property is almost 1,000 feet deep and about 300 feet across. The fence is divided into 4 sections as shown. The person/people that respond to a break or an alarm, really need to know where to respond to. A single fence charger would easily charge the whole perimeter, however, response was a major design factor.

It is the choice of using a high voltage monitor/alarm that makes an electric fence a secure fence.

Please contact us if you have additional or specific questions.

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