GB2399828A - Electrified fence - Google Patents

Abstract

An electrified fence 101 comprises a number offence posts P-1, P-6, with insulator supports 102-1, 102-3 disposed at vertically spaced intervals along the length of each fence post, the insulator support to carry the fence wires 103, 104. At least two fence wires extend in a plurality of runs between the fence posts, these runs being interwoven with one another, without making electrical contact between the wires. Preferably the fence may be combined with circuitry (Fig 3) to electrify the wires to provide an intrusion detection function by detecting the touching together or severing of the wires, and / or an electric shock deterrent function.

Description

Electrified Fence Barrier Configuration The present invention relates to

the barrier configuration of an electrified fence.

Perimeter security solutions utilising electrified fences are being used more widely in the UK as such systems are now able to offer intrusion detection capability combined with the significant deterrent effect of an electric shock.

Electrified fence systems designed for security applications comprise two main elements, namely: An energiser unit which provides the high voltage impulses and the monitoring electronics to deter and detect attempted intrusions.

A physical barrier comprising a number of electrified wires configured to provide the best system performance. In this regard, system performance is usually a function of the probability of detection of the system measured against the false alarm rate of the system.

Energisers may operate in a number of modes. The usual mode of operation is where a high voltage pulse is delivered to the electrified wires at a repetition rate limited by applicable regulations. To comply with current standards, repetition rates do not exceed I pulse per second. The pulse current may be monitored to determine the loading on the wires and an alarm declared if the pulse current exceeds a predetermined value. Some energisers provide both positive and negative high voltage pulse outputs which allows greater flexibility in barrier configurations. Other simpler configurations use a single high voltage referenced to earth.

Electrified fence systems are available either as standalone, full height systems where the electrified wires form the entire barrier, or alternatively as an addition to ordinary fence structures where the electrified wires may be used to extend the height of a standard fence by addition of a number of electrified wires running along the top of the standard fence. Such systems are referred to as fence topping systems and are probably more common than full height systems.

Figure I of the accompanying drawings depicts a typical arrangement of an electrified fence 1.

The support posts P-l, P-2, P-3, P-4 are usually metal with moulded plastic insulators 2 fixed to the post to provide the required spacing between the electrified wires 3,4.

An end view of the barrier shows that the electrified wires are spaced vertically within the same plane. i.e. each wire vertically above or below its immediate neighbour.

The electrical arrangement of the electrified wires is usually that contact between adjacent wires will cause an alarm. Adjacent wires may be alternately high voltage/earth or Thigh voltage/-high voltage depending on the system.

Systems will also provide alarm indication if any of the electrified wires are cut. This functionality is provided by the termination resistors at the end of the fence furthest from the energizer.

Currently, most electrified fence security systems also use solid metal wires as the electrified elements. The wires may be stainless steel, aluminium, or hard drawn copper.

Solid wires have the disadvantage that they require to be placed under reasonable tension (typically 5kgf or greater) in order to maintain a straight line between each support insulator. In a typical fence topping configuration using 12 wires, the stress placed on the end posts therefore becomes significant and such posts require to be of substantial construction with braces to prevent deflection of the posts under the static load imposed by the wire tension.

Furthermore, the requirement for longitudinal wire tensions of the order described limits the degree of directional change in the electrified barrier. To illustrate this, the following comparison is described.

If the electrified barrier is fined to an existing fence that follows a straight line, the stress imposed by the longitudinal tension in the wires need only to be absorbed at the end posts. Intermediate posts will be subject to very liKle lateral stress as the wires simply pass through the support insulators on each posts, the function of these supports being simply to maintain the appropriate spacing between adjacent wires.

If however the fence line does not follow a straight line, and perhaps includes a right-angled corner for example, it becomes necessary to add additional bracing posts at such points to counteract the stresses imposed by the wire tension. Friction at the insulators on a right angled corner would also prevent the wire tension being evenly distributed along the wire. Such limitations are significant and result in increased installation costs.

Use of solid wires therefore imposes a requirement that the physical strength of the barrier structure needs to be quite high - much higher for example than would be required simply to withstand normal environmental and weather conditions. A fundamental drawback of having a physically strong barrier is that it becomes much easier to climb over, thereby reducing the probability of detection of an intruder.

A knowledgeable intruder could defeat the system by placing an insulated ladder (wooden or plastic wrapped metal) against the support posts or even the electrified wires and carefully climbing up the ladder and jumping over the barrier from the top of the ladder.

The inherent strength of the barrier would be capable of supporting the ladder, particularly if the ladder is placed at an acute angle to the barrier, thereby assisting an intruder to defeat the system.

While this attack style may deflect the electrified wires, unless an electrical load is placed on the wires, either by the intruder touching the wires, or by the ladder causing the electrified wires to touch each other or a ground connection, a conventional system would not detect this mode of intrusion.

According to the present invention there is provided an electrified fence barrier comprising: a number of fence posts disposed in side-by-side configuration; the fence posts each having a series of insulator supports for supporting fence wires, the insulators being disposed at vertically spaced intervals up their heights; at least two fence wires forming, with the posts, a fence barrier and extending in a multiplicity of runs between the fence posts, the runs of the respective wires cries-crossing with one another both in the vertical direction and in the front to back direction of the fence, without electrical contact between the respective wires in the absence of intruder activities; consecutive runs of the same wire between successive pairs of fence posts being oppositely inclined relative to the horizontal and relative to the plane of adjacent fence posts.

Preferably between adjacent fence posts, there are: a) a plurality of vertically spaced runs of the first wire, which are inclined in one direction relative to the horizontal; b) a plurality of vertically spaced runs of the second wire which are inclined relative to the horizontal in the other direction with the rugs of the two wires cries-crossing one another in the plane of those fence posts.

Preferably between adjacent fence posts, runs of at least one of the wires lie in a plane which is inclined relative to the plane of those fence posts in an arrangement such that runs of the wires cries-cross one another in the thickness direction of the fence.

The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which: Figure I is a somewhat schematic front and side view of a prior art fence; Figure 2 is a somewhat schematic front (and side) view of an embodiment of the present invention; Figure 3 is a block diagram of detection and electrification circuitry for use with the fence of Figure 2; Figure 4 shows waveforms of the circuit of Figure 3; and Figure 5 shows the front and side elevation of a fence post of a variant of the embodiment of Figure 2.

Figure 2 shows a fence 101 according to one embodiment of the invention.

It has a number of equally, spaced, parallel vertical posts P-1 P-6. For simplicity of illustration, the fence is a simple linear one, rather than one which forms a closed contour and the central vertical axes of the posts are assumed to be arranged in a plane parallel to that of the figure.

Whereas existing systems employ parallel runs of electrified wires that occupy the same vertical plane, the illustrated embodiment of fence 101 employs electrified wires 103,104 in a 3 dimensional configuration.

Fig.2 depicts a 2D representation of one configuration of electrified wires according to the invention. In this configuration, two layers of wires are used one of which is connected to a positive high voltage of an energiser circuit 105 while the second layer is connected to a negative high voltage of that circuit.

The wires are arranged so that they cross each other in both the horizontal and vertical planes while maintaining the appropriate spacing at the crossing points to prevent arcing between the wires. The wires are effectively interwoven and the tension in the wires maintains the horizontal and vertical spacing between the wires. The insulators used to route the wires therefore have the facility to accommodate multiple runs of wires.

Each of the fence posts has a number of insulator supports 2 distributed at defined vertical levels up its height. These supports may be generally cylindrical posts made from plastics material firmly attached to and extending horizontally from the associated fence post, i.e. perpendicularly to the plane of Fig.2 and have along their lengths a number of parallel, spaced, through-holes for the passage of the fence wires which the insulators mechanically support.

The fence in Fig.2 has two electrification/sensing wires though, as will become apparent from the following, other embodiments with more than two wires are within the scope of the invention as defined in the appended claims.

In order to simplify the following explanation it will be assumed that each of the insulator supports 102 has a pair of through-holes at defined positions along their length. These through-holes will be referred to as F and B (F=Front and B--Back).

Figure 5, described below shows a variant in which each insulator support has three through-holes F. C and B (C=Centre).

Each wire 103,104 traverses the length of the fence a number of times, each time at a different height. In each traverse, runs of the wire between successive posts alternate between insulators at two different vertical heights. Thus it will be seen that the first run of the first wire 103 starts at the insulating support 102-3 of post P- 1 and passes through its B through-hole, from which the run extends to the F throughhole of the lowermost insulator support 102 of the second post P-4, this latter insulator support 102 being two rows below the one containing the insulator support 102-3. The wire 103 then alternates between the corresponding insulators of alternate posts and insulators which are offset two insulator intervals below those of the posts in between. When it reaches the right most end of the fence, the first wire is led up one insulator interval and then alternates between pairs of insulators which are again two intervals apart until it reaches the leftmost end, and so on until it reaches the topmost insulator of the leftmost post. After that, it proceeds horizontally to the corresponding insulator of the rightmost post, where it is connected to earth via a terminating resistor 106. It will be seen that the first run of the second wire 104 begins at the lowermost insulator 102-1 of the leftmost post, passing through its F through-hole and the wire 104 then alternates between the B and F through holes of insulators which are one interval apart on successive posts, moves up one interval at the righthand end and so on until it, too, reaches the uppermost insulator of the rightmost fence post and is connected to earth via a termination resistor 107.

The overall effect of arranging the two wires in the above described way is that they cries-cross in the vertical direction repeatedly up the space between adjacent posts and define between them cellular apertures which are parallelograms with each side inclined relative to the horizontal.

However, the arrangement includes a further measure which significantly enhances the effectiveness of the fence, namely that the wires 103,104 are arranged in a three-dimensional structure, by virtue of the wires alternating between the F and B through-holes.

Specifically, considering an individual run of one of the wires, at its ends it passes through through-holes of the insulators which are at different point along the lengths of the insulators, i.e. are at different unit distances from the plane of Fig.2. This is done in a way such that the various runs of the same wire between the same two fence posts are coplanar, in a plane which is inclined relative the plane of the fence. The runs of the second wire similarly alternate between through-holes so that they lie in a plane which is also inclined relative to the plane of the fence, but oppositely so compared with the plane of the first wires. The net result of this is that in the thickness (front to back) direction of the fence the runs of the two wires also cries- cross one another.

In the absence of intruder activity, of course, the two wires must not touch one another and must be sufficiently spaced apart at their closest points of approach as to avoid injury if the fence is being provided with an electric shock-based deterrent capability. The necessary separation between the cries-crossing runs of the two wires is achieved in this illustrated embodiment by the different vertical spacings of the insulators between which runs of the first wire (two unit spacings) and the second wire (one unit spacing) extend, in the combination with the opposite inclinations of cries-crossed runs of the two wires in the thickness direction of the fence.

It will be seen from Fig.2 that except at the edges each run of the second wire passes over one run of the first wire and then under the next run of the first wire and vice versa for the first wire ("over" and "under" referring to the thickness direction of the fence, perpendicular to the plane of the Figure).

This configuration has the following advantages over the single plane of wires currently used in other systems.

Horizontal loading of the wires by, for example, placement of an insulated ladder against the wires, will cause a horizontal deflection of the wires resulting in contact between the outer and inner planes of wires causing an alarm to be generated. Because the composite wires are of lightweight construction and inherently elastic, the loading required to cause sufficient deflection to make contact between the inner and outer planes of wires is much lower than would be the case for solid wires.

Attempts to spread the wires in a vertical direction to allow a man to squeeze through the resulting gap will also result in contact between adjacent runs of electrified wires because they are arranged to cross over each other in the vertical plane.

An alarm is therefore generated for this attack scenario also.

Electrified fence systems have been historically easy to defeat by introducing bridging wires to bypass sections of the electrified wires. With existing simple horizontal runs of electrified wires, it is a simple matter to identify the electrical circuitry and bypass the appropriate wires with jumpers. The visual effect of the interwoven wires of the proposed configuration makes it very difficult to interpret the electrical circuit formed by the wires thus increasing the difficulty in defeating the system by introducing bridging links across sections of the barrier.

While Figure 2 depicts an arrangement showing two layers of wires, additional benefits may be had by adding a third layer behind the two layers carrying the high voltage. The third layer could be connected to earth to provide additional detection capability for systems that operate by detecting leakage currents from the high voltage lines to earth. This third plane of wires simply forms a ground plane behind the electrified wires and detects contact between either of the high voltage wires and the ground plane.

This can be thought of as an extension of the metallic fence structure upwards and behind the electrified wires. This ground plane is not interwoven with the electrified wires and forms a single vertical plane of wires.

The third plane of wires makes it even more difficult to interpret the wiring arrangement as it becomes very difficult to focus on the same plane of wires while visually tracing the circuitry with the intention of defeating the system with bridging wires.

The provision of this third plane of wires is one use of the version of the fence post shown in Figure 5 where the insulator supports have F. C and B through-holes.

An additional variant shown in Figure 5 is the fact that the fence post is formed of galvanised steel strip material.

In order to utilise the third plane of wire using post as shown in Figure 5, the wire forming this plane could pass through the B through-holes exclusively, with the wires 103 and 104 alternating between the F and C through-holes. In an alternative use of posts as shown in Figure 5, using just two wire planes, a run of each wire could pass through F. C and B through-holes in succession. A future variant using two wires would have one of them using the C holes exclusively and the other alternating between the F and B through holes.

The electrified wires used in the embodiment are of lightweight composite construction comprising stainless steel strands laid together with polypropylene strands.

The typical diameter of the composite construction is 2.5mm.

This composite material is both flexible and strong and, as a result of it's inherent flexibility, the tension needed to ensure a straight run is very low - typically Ikg.

Furthermore, because the composite construction utilises a plastic strength member, the composite wire is inherently much more elastic than a single strand of solid wire. The inherent elasticity allows the wire tension to be maintained throughout temperature and wind load variations while exerting a relatively low 'dead load' on the support posts at the end of the zone.

The polypropylene material can be made resistant to the effects of ultra violet radiation by the addition of carbon black powder to the polypropylene. The overall colour of such material is therefore black. This confers a further benefit in so far as it reduces the visual impact of the barrier making it more aesthetically acceptable.

The energiser 105 for the fence of Fig.2 is shown in Fig.3 and comprises: an electronic timing unit 111; a high voltage impulse generator 112; a low voltage DC source 113; a window comparator 114; and an electronic change-over switch 115.

The high voltage impulse generator 112 produces, under the control of timing unit 111, positive and negative pulses of amplitude +/- V2, with duration time t2, and a typical repetition rate of 1 second as indicated in the waveform chart of Fig.4. The actual voltage amplitude and pulse duration determines the energy level delivered to the electrified wires and is generally limited to in terms of joules to comply with current legislation. The pulse width t2 is much shorter than the pulse interval period tl.

The energiser 105 operates by applying a low voltage DC (typically 24v) from the source 113 to the electrified wires 103,104 for the duration of the pulse interval periods tl. During this period, the resistors 116 & 106, and 117 & 107 form a potential divider which determines the voltage applied to the input of the window comparator circuit 114. For a correctly terminated line, the window comparator produces a logic 'LOW' alarm output.

If either of the electrified wires are broken, short-circuited to ground, or to each other, the input to the window comparator circuitry changes and a logic 'HIGH' alarm output is generated.

The voltage applied to the electrified wires is controlled by the electronic timing unit 111 which switches the applied voltage between the low voltage monitoring system, and the high voltage impulse deterrent system. The timing unit 111 controls the high voltage pulse widths t2, the pulse interval durations tl, as well as the 'dead' interval where the system is switched between monitoring mode and high voltage mode.

The timing unit also disables the alarm output during the period when the high voltage deterrent pulses are applied.

Claims (8)

1. An electrified fence barrier comprising: a number of fence posts disposed in side-by-side configuration; the fence posts each having a series of insulator supports for supporting fence wires, the insulators being disposed at vertically spaced intervals up their heights; at least two fence wires forming, with the posts, a fence barrier and extending in a multiplicity of runs between the fence posts, the runs of the respective wires cries-crossing with one another both in the vertical direction and in the front to back direction of the fence, without electrical contact between the respective wires in the absence of intruder activities; consecutive runs of the same wire between successive pairs of fence posts being oppositely inclined relative to the horizontal and relative to the plane of adjacent fence posts.

2. A fence according to claim I wherein, between adjacent fence posts, there are: a) a plurality of vertically spaced runs of the first wire, which are inclined in one direction relative to the horizontal; b) a plurality of vertically spaced runs of the second wire which are inclined relative to the horizontal in the other direction with the runs of the two wires cries-crossing one another in the plane of those fence posts.

3. A fence according to claim 1 or 2 wherein, between adjacent fence posts, runs of at least one of the wires lie in a plane which is inclined relative to the plane of those fence posts in an arrangement such that runs of the wires cries-cross one another in the thickness direction of the fence.

4. A fence according to any one of the preceding claims wherein the wires are attached to the insulators at points which are different distances from the axis of the fence post which that insulator is on.

5. A fence according to any one of the preceding claims and including an additional wire, which in use is maintained at ground potential, which extends in a number of vertically spaced horizontal runs between the fence posts and which underlie, in the thickness direction of the fence, the first and second wires.

6. A fence according to any one of the preceding claims wherein each wire comprises a mechanical fleer support element which is made of elastic plastics material, carrying a thinner electrical conductor, whereby the force required to tension the wire is determined by the support element.

7. A fence constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in Figures 2 to 4 of the accompanying drawings.

8. An electrified fence according to any one of the preceding claims in combination with circuitry to electrify the wires to provide a) an intrusion detection function by detecting touching together or severing of the wires andlor b) an electric shock intruder-deterrent function.