GB2112193A - >Circuit for security fences - Google Patents

Abstract

An electrical monitoring system for a security fence includes in a network a number of normally closed vibrating inertia intruder switches 10. Each switch is provided with a shorting resistor 12. When the switches 10 open intermittently monitoring signals passing along the network are influenced by the opening and closing of the switches 10 and suitable alarm signals can be generated. If the resistors 12 are not present and one of the switches 10 remains open due to faulty operation the whole network becomes unserviceable. With the present arrangement this is not the case and in one specific embodiment up to three faulty switches can be tolerated. <IMAGE>

Description

SPECIFICATION Circuit for security fences BACKGROUND OF THE INVENTION The invention relates to security fences and the like.

The invention relates more particularly to electrical circuit means for use in monitoring the condition of mechanically operated switches placed at intervals along a security fence.

Conventionally, various forms of switches are placed along the fence which are operated when the fence is interfered with and operation of such switches is monitored by an electrical circuit connected to the switches to indicate intrusion and/or cause an alarm signal. A number of switches are usually arranged electrically in series so that if one of the switches becomes faulty a zone or section of a security fence must be disabled until the fault is rectified.

It is an object of the invention to overcome or at least reduce this problem.

SUMMARY OF THE INVENTION According to the invention there is provided an electrical monitoring system for a security fence or the like in which a plurality of mechanically intruder operable ON/OFF switches are connected in a network and sensing means are provided to determine by electrical signals passing through the network the state of the switches, in which at least some of the switches are provided with suitable resistors so that when those switches are operated the change in level of signals is modified by the presence of the resistors.

The switches may be normally closed and the resistors arranged in parallel with the switches.

The switches may be normally open and the resistors arranged in series with the switches.

The switches may be vibrating inertia switches.

The frequency of oscillation of the switches may be in the range of 1 00 to 1000 cycles per second.

The electrical circuit may include a monitoring circuit arranged to indicate or provide an alarm signal when variations in voltage in the network are present, caused by operation of one or more of the switches.

The monitoring means may be arranged to indicate whether one, two or more switches have been operated. The electrical circuit may be arranged to ignore the operation of one our a small number of switches if such switch or switches remains in a static operative condition for greater than a predetermined time period so that monitoring of the network can be carried out in the event of one or a small number of switches becoming faulty.

The provision of the resistors in effect provides one or more intermediate levels of signals between an open or closed switch condition which can be monitored by the sensing means to indicate a switch has not fully opened or closed for example as the case may be, or has uncharacteristically opened or closed for a significant period of time. The intermediate levels are of a value and/or a nature (e.g. not intermittent) which can distinguish them from a true alarm condition. However, as the intermediate levels are normally cumulative there is only a finite number of faulty switches that can be tolerated in the network, if such levels are sufficiently distinct to be readily detectable. For general convenience in a preferred arrangement, the intermediate level is chosen so that up to three faulty switches can be accepted in the network.

Further, at any time the number of faulty switches can be determined by sensing their presence and their cumulative effect.

BRIEF DESCRIPTION OF THE DRAWING An electrical circuit arrangement for a security fence according to the invention will now be described by way of example with reference to the accompanying drawing which shows a schematic circuit diagram of the circuit.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawing, the electrical circuit in use extends several metres, say 1 kilometer, along a section of security fence (not shown) and is provided with vibration inertia switches 10 at intervals of 10 metres. Each switch has a shorting resistor 1 2 connected between the switch contacts. An amplifier 14 is provided for each section and supplies signals to a monitoring circuit 1 6 located centrally for receiving signals from a number of like electrical circuits associated with other sections of the security fence. The amplifier 14 receives power via separate electrical cables (not shown) or from an individual battery powerpack if preferred.

if the security fence is disturbed the switches are caused to oscillate at relatively high frequency, their natural frequency being normally in the range 100 to 1 000 cycles per second. Variations of the voltage in the circuit are thus caused by the rapid opening and closing of the switches 10 which is sensed by the monitoring circuit 1 6 and arranged to generate a consequent indication and/or alarm.

The voltage variations are significantly reduced by the presence of the resistors 1 2 (as compared to the conventional arrangement where no such resistors are present) so the amplifier 14 is provided to amplify the voltage developed in the circuit to make the signals transmitted to the monitoring circuit 1 6 easier to sense. To this extent there is a disadvantage of including the resistors 12. However, if the resistors 12 are not present and any one of the inertia switches becomes faulty, such that the switch does not close, the whole section of the fence associated with the circuit becomes inoperative. That section cannot be reactivated until the faulty switch is located and repaired or replaced.With the described arrangement, the resistor 1 2 of any faulty switch allows electrical signals to pass through the faulty switch and the remainder of the switches can be monitored as described.

As the circuit wires which extend between the switches have a finite resistance it is clear that only a certain number of switches can become faulty before it becomes impossible to monitor with accuracy signals generated in the circuit. In the specific embodiment the resistance values of fne resistors 12 are chosen to allow up to three faulty switches to be acceptable before monitoring becomes unreliable.

As each resistor 12 is of known and equal value, a test signal can be applied to each circuit and the resultant voltage indicates the number of faulty switches present. As the number increases, the resistance of the circuit increases because instead of the contacts of a faulty switch being closed such contacts are open and the circuit resistance increased by those resistors 1 2 connecting the open contacts of the faulty switches.

Thus, the described circuit enables sections of a security fence to remain in an activated condition even if up to three switches are faulty. Further, the number of switches that are faulty can be determined when required, so that servicing of the switches can take place before more than three switches become unserviceable.

It will be appreciated that the circuit can be used as described in association with nonvibrating switches. Such switches are normally closed and are opened by an intruder and thereafter remain open at least for a significant period of time. Such switching arrangements may be of the kind which open if the security fence wires are moved apart and remain open for at least several seconds. it can happen that such switches do not close or do not close properly due to distortion of the fence wires caused by an intruder Without a shorting resistor where this happens the circuit becomes thereafter unserviceable.

likewise, some intruder operated switches are arranged to be normally open and closed when the fence is interfered with. In this case the resistors are placed in electrical series with each of the switches. If such a switch fails and remains closed, instead of a direct short caused by the ciosure of the switch remaining in the circuit, the series resistor introduces a specific voltage level. Thus, when monitoring the circuit in this case, direct shorts are not introduced into the circuit when the switches close. Normally, therefore an alarm should be caused if one of the switches closes but it can be assumed that if the resistive connection remains for longer than say a few minutes, the switch has become unserviceable. The alarm is ten cancelled and the monitoring circuit adjusted so as to ignore, in effect, the voltage level caused by one permanent resistive connection in the circuit. Normally, up to three such resistive levels can be tolerated before the particular circuit must be taken out of service and the faulty switches located and rectified.

Claims (9)

1. An electrical monitoring system for a security fence or the like in which a plurality of mechanically intruder operable ON/OFF switches are connected in a network and sensing means are provided to determine by electrical signals passing through the network the state of the switches, in which at least some of the switches are provided with suitable resistors so that when those switches are operated the change in level of signals is modified by the presence of the resistors.

2. An electrical monitoring system according to claim 1 in which the switches are normally closed and the resistors are arranged in parallel with the switches.

3. An electrical system in which the switches are normally open and the resistors are arranged in series with the resistors.

4. An electrical circuit according to any one of claims 1 to 3, in which the switches are vibrating inertia switches.

5. An electrical circuit according to claim 4, in which the frequency of oscillation of the switches is in the range of 100 to 1000 cycles per second.

6. An electrical circuit according to any one of claims 1 to 5, including a monitoring circuit arranged to indicate or provide an alarm signal when variations in voltage in the network are present caused by operation of one or more of the switches.

7. An electrical circuit according to claim 6, in which the monitoring means is arranged to indicate whether one, two or more switches have been operated.

8. An electrical according to any one of claims 1 to 7 arranged to ignore the operation of one or a small number of switches if such switch or switches remains in a static operative condition for greater than a predetermined time period so that monitoring of the network can be carried out in the event of one or a small number of switches becoming faulty.

9. An electrical circuit arrangement for a security fence substantially as herein described with reference to the accompanying drawing.