FIELD OF THE INVENTION

This invention relates to fence monitoring. In particular, this invention relates to a fence alarm and to a fence alarm system. BACKGROUND OF THE INVENTION

The inventor is aware of fences that provide physical barriers to entry. The inventor is further aware of fence alarm systems that can be connected to fences, such as electrified fence systems. However, said fence systems often do not provide a physical barrier to entry and is only effective when used in combination with a physical barrier to entry.

The invention aims to address this shortcoming. SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a fence alarm, which includes

a plurality of laterally spaced electrically conductive fence elements, the fence elements electrically being connected in series with each other;

a sensing arrangement electrically connected to the plurality of conductive fence elements, the sensing arrangement being calibrated to a nominal electrical value, the sensing arrangement further being operable to sense a predefined deviation from the nominal electrical value caused by a deviation in an electrical property of the conductive fence elements.

In particular, the nominal electrical value may be a resistive value, the sensing arrangement further being operable to sense a predefined deviation from the nominal resistive value. The sensing arrangement may be operable to trigger an alarm condition.

The conductive fence elements being connected in series with each other may be interspersed with fixed resistive values.

The predefined deviation from the nominal resistive value may be any one of an increased resistance, indicating an open circuit between the fence elements and a decreased resistance, indicating a short between fence elements. The fence alarm may include indication means operable to indicate the trigger status of the sensing arrangement. In particular, the indication means may include audio indication means and visual indication means.

In one embodiment, the plurality of laterally spaced electrically conductive elements may be in the form of elongate profiled metallic bars, such as angled bars, round bars, square bars, rolled metal profiles, or the like.

In another embodiment, the fence elements may be defined by conductors mounted onto non-conductive fence elements.

The fence alarm may include a plurality of cross members extending transversely to the laterally spaced fence elements.

The fence alarm may include isolation means between the laterally spaced fence elements and the cross members, so that the cross members are electrically isolated from the fence elements.

The fence alarm may include support posts, supporting the combination of fence elements and cross members in an upright orientation to create a physical barrier to entry. The support posts may be connected to the cross members.

The fence alarm may include at least one trip wire, electrically connected in series with the plurality of laterally spaced fence elements. The fence alarm, may include a power supply monitor operable to sense anomalies in the power supply to the fence alarm.

The invention extends to a fence alarm system, which includes a plurality of fence alarms as described above, which are connected to a central controller.

The central controller may include a wireless communication interface.

The wireless communication interface may be in the form of a mobile telephone network interface, such as a Global System for Mobile communications (GSM) interface.

The wireless communication interface may be operable to transmit the trigger status of a sensing arrangement of a fence alarm to a remote receiver.

The wireless communication interface may be operable to receive commands from a remote transmitter and the wireless communication interface may include output terminals that are switchable in response to said commands. The central controller may include a fence identification facility, operable upon a sensing arrangement in a fence alarm triggering an alarm condition to identify the fence alarm which triggered the alarm and may be operable to communicate the trigger event via the wireless communication interface. The invention will now be described by way of a non-limiting example only, with reference to the following drawings.

DRAWINGS In the drawings:

Figure 1.1 shows a drawing of a fence alarm system according to one aspect of the invention

Figure 1.2 shows a section of the fence alarm system taken at l-l;

Figure 1.3 shows a detail of a mounting marked B in Figure 1.2; Figure 2 shows a schematic block diagram of a sensing arrangement of one of the fence alarms of Figure 1.1 ;

Figure 3 shows a schematic block diagram of a modular add-on sensing arrangement similar to the sensing arrangement in Figure 2;

Figure 4 shows a schematic block diagram of a central controller, which is connectable to a plurality of sensing arrangements, shown in Figure 3; and

Figure 5 shows a schematic block diagram of a fence alarm system in operation. EMBODIMENT OF THE INVENTION

A side view of a fence alarm system 10 is shown in Figure 1.1.

The fence alarm system 10 includes a plurality of fence alarms 12 to 14 connected to a central controller (not shown in Figure 1 ).

Each of the fence alarms 12 to 14 includes a plurality of laterally spaced electrically conductive fence elements in the form of metal uprights 12.1 - 12. n to 14.1

- 14.n all of which are interspersed with fixed resistors (not shown). The fence elements 12.1 - 12. n to 14.1 - 14.n are electrically connected in series with each other.

Each of the fence alarms 12 to 14 includes two cross members, in the form of angle irons 12.201 , 12.202 to 14.201 , 14.202 extending transversely to the laterally spaced fence elements 12.1 - 12. n to 14.1 - 14. n. The cross members 12.201 , 12.202 to 14.201 , 14.202 are electrically isolated from the fence elements 12.1

- 12. n to 14.1 - 14.n. by means of non conductive isolators 12.301 -12.3n to 14.301 - 14.3n (see Figure 1.3). The fence elements and cross members defines three fence alarm panels 18, 20, 22. The fence alarm panels are supported by support posts 24, 26, 28 in an upright orientation to create a physical barrier to entry.

Each of the fence alarms 12 to 14 further includes a sensing arrangement 12.100 to 14.100 (see Figure 2) electrically connected to the plurality of conductive fence elements 12.1 - 12. n to 14.1 - 14.n. The sensing arrangements 12.100 to 14.100 are each calibrated to a nominal resistive value for its particular series of conductive fence elements 12.1 - 12. n to 14.1 - 14. n. The sensing arrangements 12.100 to 14.100 are then operable to sense a predefined deviation from the nominal resistive value of its particular series of conductive fence elements 12.100 to 14.100.

The fence alarm system includes a trip wire 36 connected in series with the fence elements 12.1 - 12. n to 14.1 - 14.n and the sensing arrangement 12.100 to 14.100, operable, when damaged to cause the sensing arrangement to trigger an alarm condition.

In use, in the event that a particular sensing arrangement 12.100 to 14.100 detects a deviation from the nominal resistive value, the sensing arrangement 12.100 to 14.100 will trigger an alarm condition.

The sensing arrangements 12.100 to 14.100 are each arranged to detect if the deviation from the nominal resistive value is an increased resistance, indicating an open circuit between the fence elements 12.1 - 12. n to 14.1 - 14. n or a decreased resistance, indicating a short between some of the fence elements 12.1 - 12.n to 14.1 - 14.n.

The fence alarms 12 to 14 include sensing arrangement 12.100 to 14.100, of which one example, 12.100, is shown in Figure 2. The sensing arrangement 12.100 of Figure 2 includes a power supply 12.101 , a microprocessor

12.102, which is connected to two LED's 16.1 , 16.2, which are operable to indicate the trigger status of the sensing arrangement and a set of terminals 12.103 for connection to a central controller. Figure 3 shows an add-on sensing arrangement 12.1 10, similar to

12.100 having a microprocessor 12.102, two LED's 16.1 , 16.2, and a set of terminals

12.103, but without the power supply 12.101. Figure 4 shows a schematic block diagram of a central controller 50, which is connectable to a plurality of sensing arrangements, shown in Figure 3. The central controller 50 includes a power supply 50.1 , a microprocessor 50.2 and isolated relay contacts 50.3 for integration onto external alarm systems and fence systems (not shown). The central controller is also connectable to a wireless communication interface in the form of a mobile telephone network interface (not shown in Figure 4).

The wireless communication interface is operable to transmit the trigger status of a sensing arrangement of a fence alarm to a remote receiver 34 via a mobile telephone network 32.

The inventor is of the opinion that the invention described in the specification includes a new a fence alarm, to a fence alarm system and to an alarm installation.