GB2077471A

GB2077471A - Cable for use in a security system

Info

Publication number: GB2077471A
Application number: GB8018622A
Authority: GB
Original assignee: FIBUN BV
Current assignee: FIBUN BV
Priority date: 1980-06-06
Filing date: 1980-06-06
Publication date: 1981-12-16

Abstract

A cable (15) is for use in a security system, the cable having two electrical conductors (20,22) whose relative resistance is affected on application of pressure to the cable. A fibre optic member (11) traverses the centre of the cable. <IMAGE>

Description

SPECIFICATION Cable for use in a security system This invention concerns a cable for use in a security system.

According to the present invention there is provided a cable for use in a security system, said cable having two conductors whose relative resistance is differently affected on application of pressure to the cable.

Preferably, the resistance of one of the conductors is relatively high when no pressure is applied to the cable and is relatively low on the application of such pressure.

The said one conductor may be made of electrically conductive rubber.

The other conductor may be a metallic conductor.

Thus the metallic conductor may be constituted by a metallic coating on a fibre optic conductor.

The invention also comprises a security system comprising a cable as set forth above, there being means responsive to the said relative resistance for indicating whether pressure is being applied to the cable.

The security system may also be provided, when a fibre optic conductor is used, with transmitting means for transmitting radiation into one end of the fibre optic conductor, receiving means for receiving radiation from the other end of the fibre, and means for indicating a predetermined change in the radiation received by the receiving means.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a diagrammatic view of a security system incorporating a cable according to the present invention, Figure 2 is a broken-away plan view of the said cable, and Figure 3 is a cross-sectional view of the cable of Figure 2.

In Figure 1 there is shown a diagrammatic view of a security system comprising an optical transmitter 10, a fibre optic member 11, an optical receiver 13, and a detector 14 which provides an alarm indication whenever the amount of light or other radiation received by the optical receiver 13 falls below a predetermined value. The optical transmitter 10 is arranged to transmit coded pulses of light or other radiation to the optic fibre 11. If the fibre optic member 11 is cut or damaged by an intruder the light or other radiation received by the optical receiver 13 will be reduced and an alarm will be given by the detector 14.

Although Figure 1 shows a single transmitter and receiver, the system would in practice be provided with a series of sections each having a respective transmitter and receiver. It may be arranged that each section receives respective coded pulses of light or other radiation, in which case a single detector 14 may be provided which both indicates whether a break-in has occurred and also indicates, by identification of the respective code, in which particular section the break-in has occurred.

As best shown in Figure 3, the fibre optic member 11 forms part of a cable 15, the fibre optic member 11 comprising a glass central core 16 of a first refractive index and a glass sheath 17 which surrounds the central core 16 and which ensures total internal reflection of the light passing through the fibre optic member 11.

The sheath 17 is coated with a metal layer 20 which forms an inner conductor. The inner conductor 20 is sheathed by a perforated annular layer 21 of insulating material about which there is mounted an outer conductor made of electrically conductive rubber 22. The latter is in turn surrounded by a perforated internally insulated metal tube or coating 23, e.g. of copper.

The conductivity or resistivity between the two conductors 20, 22 can be arranged to be detected for each section of the system as illustrated in Figure 1.

Thus, as shown in Figure 1, the conductors 20, 22 are connected across a bridge detector 24, the output from the bridge detector 24 being amplified at an amplifier 25 and passed to a level detector 26. Thus if an intruder steps on the cable 15 so as to increase the pressure on the outer conductor 22, the conductive rubber of the latter will be compressed and this changes the loop resistivity of the cable so as to cause an alarm signal to be generated. The alarm signal may be such as to indicate in which particular section of the system the break-in has occurred.

Alternatively, the layer 20 could be constituted by a plastics layer, and the layer 21 could be constituted by a metal coating which forms the inner conductor.

In this case, the inner and outer conductors 21,22 would be adjacent to each other, but this would not matter because electrically conductive rubber 22 is normally of very high resistance. The electrical resistance of the electrically conductive rubber is fairly high when no pressure is applied but decreases rapidly with the application of pressure.

Once the system has been appropriately calibrated it can detect (a) the static resistance of the system when the cable 15 is neither cut nor subjected to pressure, (b) the increased static resistance when the cable 15 is cut, and (c) the reduced static resistance when local pressure is applied to the cable 15.

The cable 15 may be incorporated in a fence surrounding an area to be protected. It may, moreover, be covered by a plastics sheath (not shown), in which case it could easily be mistaken for an ordinary electric cable.

1. A cable for use in a security system, said cable having two conductors whose relative resistance is differentially affected on application of pressure to the cable.

2. A cable as claimed in claim 1 in which the resistance of one of the conductors is relatively high when no pressure is applied to the cable, and is relatively low on the application of such pressure.

3. A cable as claimed in claim 2 in which the said one conductor is made of electrically conductive rubber.

4. A cable as claimed in claim 2 or 3 in which the other conductor is a metallic conductor.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Cable for use in a security system This invention concerns a cable for use in a security system. According to the present invention there is provided a cable for use in a security system, said cable having two conductors whose relative resistance is differently affected on application of pressure to the cable. Preferably, the resistance of one of the conductors is relatively high when no pressure is applied to the cable and is relatively low on the application of such pressure. The said one conductor may be made of electrically conductive rubber. The other conductor may be a metallic conductor. Thus the metallic conductor may be constituted by a metallic coating on a fibre optic conductor. The invention also comprises a security system comprising a cable as set forth above, there being means responsive to the said relative resistance for indicating whether pressure is being applied to the cable. The security system may also be provided, when a fibre optic conductor is used, with transmitting means for transmitting radiation into one end of the fibre optic conductor, receiving means for receiving radiation from the other end of the fibre, and means for indicating a predetermined change in the radiation received by the receiving means. The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a diagrammatic view of a security system incorporating a cable according to the present invention, Figure 2 is a broken-away plan view of the said cable, and Figure 3 is a cross-sectional view of the cable of Figure 2. In Figure 1 there is shown a diagrammatic view of a security system comprising an optical transmitter 10, a fibre optic member 11, an optical receiver 13, and a detector 14 which provides an alarm indication whenever the amount of light or other radiation received by the optical receiver 13 falls below a predetermined value. The optical transmitter 10 is arranged to transmit coded pulses of light or other radiation to the optic fibre 11. If the fibre optic member 11 is cut or damaged by an intruder the light or other radiation received by the optical receiver 13 will be reduced and an alarm will be given by the detector 14. Although Figure 1 shows a single transmitter and receiver, the system would in practice be provided with a series of sections each having a respective transmitter and receiver. It may be arranged that each section receives respective coded pulses of light or other radiation, in which case a single detector 14 may be provided which both indicates whether a break-in has occurred and also indicates, by identification of the respective code, in which particular section the break-in has occurred. As best shown in Figure 3, the fibre optic member 11 forms part of a cable 15, the fibre optic member 11 comprising a glass central core 16 of a first refractive index and a glass sheath 17 which surrounds the central core 16 and which ensures total internal reflection of the light passing through the fibre optic member 11. The sheath 17 is coated with a metal layer 20 which forms an inner conductor. The inner conductor 20 is sheathed by a perforated annular layer 21 of insulating material about which there is mounted an outer conductor made of electrically conductive rubber 22. The latter is in turn surrounded by a perforated internally insulated metal tube or coating 23, e.g. of copper. The conductivity or resistivity between the two conductors 20, 22 can be arranged to be detected for each section of the system as illustrated in Figure 1. Thus, as shown in Figure 1, the conductors 20, 22 are connected across a bridge detector 24, the output from the bridge detector 24 being amplified at an amplifier 25 and passed to a level detector 26. Thus if an intruder steps on the cable 15 so as to increase the pressure on the outer conductor 22, the conductive rubber of the latter will be compressed and this changes the loop resistivity of the cable so as to cause an alarm signal to be generated. The alarm signal may be such as to indicate in which particular section of the system the break-in has occurred. Alternatively, the layer 20 could be constituted by a plastics layer, and the layer 21 could be constituted by a metal coating which forms the inner conductor. In this case, the inner and outer conductors 21,22 would be adjacent to each other, but this would not matter because electrically conductive rubber 22 is normally of very high resistance. The electrical resistance of the electrically conductive rubber is fairly high when no pressure is applied but decreases rapidly with the application of pressure. Once the system has been appropriately calibrated it can detect (a) the static resistance of the system when the cable 15 is neither cut nor subjected to pressure, (b) the increased static resistance when the cable 15 is cut, and (c) the reduced static resistance when local pressure is applied to the cable 15. The cable 15 may be incorporated in a fence surrounding an area to be protected. It may, moreover, be covered by a plastics sheath (not shown), in which case it could easily be mistaken for an ordinary electric cable. CLAIMS

5. A cable as claimed in claim 4 in which the metallic conductor is constituted by a metallic coating on a fibre optic conductor.

6. A cable for use in a security system substantially as hereinbefore described with reference to the accompanying drawings.

7. A security system comprising a cable as claimed in any preceding claim, and means responsive to the said relative resistance for indicating whether pressure is being applied to the cable.

8. A security system as claimed in claim 7 and comprising a cable as claimed in claim 5, there being provided transmitting means for transmitting radiation into one end of the fibre optic conductor, receiving means for receiving radiation from the other end of the fibre, and means for indicating a predetermined change in the radiation received by the receiving means.

9. A security system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.