Perimeter security - Electrified fence

About AuthorKiron Kunte has over 35 years of design and solutions experience, in Security, Telephony and Networking infrastructure. He is a graduate of IIT Bombay, a Fellow of the Institute of Engineers and has a post-graduate diploma, from Bombay University, in Systems Management. He is professionally certified in the design and engineering of Data Centres, IP Telephony, Video surveillance and Structured Cabling solutions. He heads Norik Konsult, a practice that offers advisory, design and system planning help to upgrade or build a new Telephony, Security and Networking infrastructure.

What is an electrified fence? An electric fence is a barrier that uses electrical shocks to deter intruders from crossing a boundary. The voltage of the shock may have effects ranging from uncomfortable, to painful or even lethal. Most electric fencing is used today for sensitive locations and agriculture fencing.

Components of an electric fence

• Physical Fence.
• Fence wire that carries the electric pulse.
• Electric pulsing device - a charge device that pushes power through the fence.
• Earthing - a series of metal rods sunk into the earth and connected to the charge device

Metal post with insulators to which
the wire is attached.

Fence wire Smooth steel wire is the material most often used for electric fence, ranging from a fine thin wire used as a single line to thicker, high-tensile (HT) wire. Synthetic webbing and rope-like fencing materials woven with fine conducting wires are particularly useful for areas requiring additional visibility or as temporary fencing.

The electrified fence itself is insulated from the earth and from any materials that will conduct electricity. Typically, wooden or metal posts are driven into the ground and plastic or porcelain insulators are attached to them. These insulators are generally UV stabilized. The conducting material (wire) is then attached to these insulators.

Electric fence wires are connected in series arrangement. Live and earth wires are placed alternately.

Electric pulsing device

The pulsing device sends impulses along the fence line at regular intervals, generally every 1 sec. An Impulse is a high voltage pulse, like a voltage spike. The duration of the pulse is generally around 0.3ms. Apart from powering up the fence it also monitors the voltage of the fence. Thus, due to any reason, shorting of wires or cutting, if the return voltage becomes zero or goes below an adjustable voltage level, the device will trigger an alarm.

Earthing

Earthing is critical to the functioning of the electric fences. Galvanized earth rods are installed at regular intervals.

How an electric fence works Electricity will only travel through a closed circuit. The fence wire, the charge device and ground rods are three parts of a circuit waiting to be closed. When an intruder makes contact with the wire, the circuit is closed and a surge of current will travel through the intruder. The intruder will feel a shock that is likely to discourage him from touching the fence again.

The effects of the shock depend upon the voltage, the energy of the pulse, the degree of contact between the recipient and the fence and ground and the route of the current through the body. It can range from barely noticeable to uncomfortable, painful and in some cases even lethal.

The strength of the shock depends on the voltage and amperage. Voltage, measured in kilovolts (kV), is the force or pressure with which a current flows through the circuit. The higher the voltage, the stronger is the “startle” from the shock. Amperage (amps) measures the magnitude or strength of the current. The higher the amperage, the greater the sensation the current will cause when it enters a body.

The level of unpleasantness produced by a shock depends more on the amperage than on the voltage. A large charge travelling at a very slow rate can actually produce a fairly mild shock. For instance, when you walk across a carpet and touch a doorknob, the shock you feel may be as much as 5,000 volts, but it has very low amperage. At the opposite extreme, an electric chair operates on only about 2,000 volts, but the high amperage is enough to kill.

As the nature of electric current is pulsating DC, an intruder does not get stuck to the wire and gets thrown back. The shock is high enough to create a strong psychological barrier so that the intruder will not dare to touch the fence again, but not high enough to cause any permanent damage to mind or body, not even temporary unconsciousness. With an electric fence, the goal is to sting or startle the intruder without causing harm, so electric fences operate with low amperage and higher voltage.

While intruders are the intended targets of electric fences, anything else that comes in contact with both fence and ground will also complete the circuit. The fear of electricity is intimidating and this will keep almost all of the criminals at bay. Criminals are more likely to drive by and keep going, looking for easier targets.

Multiple Zones Zones can be created using multiple pulsing devices and controllers. Most manufacturers offer centralized controllers that can alert the security personnel to the location / zone where the intrusion is taking place.

Regulatory requirementsPulse synchronization

When multiple pulsing devices are used their pulse must be synchronized. Not doing so may result in the intruder, who touches parts of two separate adjacent fences at the same time, getting a big shock which is the addition of magnitudes of each pulse, or the intruder may get two shocks within an interval of 1 sec. This can be fatal. Without synchronization the system could be ‘illegal’. The pulsing devices across the fence are generally connected to each other and each of them fires pulses at exactly the same instant.

Certification

The pulsing device must comply with IEC 60335-2-76 standard.

Warning signs

Warning signs have to be displayed. These must be prominent and located all along the perimeter.

Advantages of an electrified fence.Very effective: Provides round-the-clock vigilance. The sharp but safe electric shock when touched or when tampered with can trigger an alarm and can also indicate the zone in which the intrusion is taking place.

Reduction in Man Power: Existing security personnel deployed to man the perimeter can be reduced and redeployed for better assignments.

Power consumption: As the ‘pulsing device’ consumes very little power, Solar Power could also be effectively used to power the fence.

Centralized control: Any length of perimeter can be secured. Most manufacturers offer centralized monitoring of several controllers.

CaveatsPower lines running close to the fence could impair normal functioning of an ‘electric fence’. Performance could be affected in areas prone to ‘lightning’.

Grass and other vegetation growing near the foot of the fence must be trimmed regularly in order to avoid this from coming into contact with the live wire and causing a ‘false alarm’.

Intrinsic safety may be an issue when deployed in areas with flammable gasses or fuels.

Integration and Multi-layer protectionPerimeter security is about deterrence, detection, assessment and delaying of the intrusion so that a response could be initiated. Its role is to act as the first level of protection. Every application is unique in the type of facility to be protected, operating environment, perimeter fence construction, history of intrusion and the threat perception.

The type of technology deployed to prevent intrusion is to be selected based on the unique requirements of the site. Site layout, sensitivity of the location, surrounding environment, activities surrounding the site, local weather conditions and topography must be considered when planning a perimeter intrusion detection system.

Last month we discussed Perimeter security deploying fibre optic cable (refer pages 80, 82, 84 of the September issue). This article explains the use of ‘electrical shock’ to deter intrusion. In an earlier article (July-pages 64-66) we discussed the use of video surveillance combined with video analytics.

Often the intrusion detection system will comprise of different but complementary technologies to form ‘Layers of protection’ and it must be a part of an integrated site security plan.