| WO/2006/118041 | MICROWAVE SENSOR |
| JP03289581 | MULTIELEMENT TYPE ULTRASONIC SWITCH |
| JP2010054410 | MOVING OBJECT DETECTION DEVICE AND MOVING OBJECT DETECTION METHOD |
SKOCZYLAS, Romauld (Becatech Limited, Becatech HouseSharpham Road,Cheddar, Somerset BS27 3DR, GB)
CAMP, Warwick (Becatech Limited, Becatech HouseSharpham Road,Cheddar, Somerset BS27 3DR, GB)
SKOCZYLAS, Romauld (Becatech Limited, Becatech HouseSharpham Road,Cheddar, Somerset BS27 3DR, GB)
| Claims 1. Apparatus for the protection of a secure structure, comprising : a sensor for attachment to the structure which sensor outputs a signal; a first analyser which receives said signal from the sensor; a second analyser which receives said signal from the sensor; wherein the first and second analysers use different analysis methods to analyse the said signal and wherein an alarm signal is output if both analysers detect an attack event within a predetermined time window. 2. Apparatus according to claim 1 wherein the sensor comprises an acoustic and seismic sensor. 3. Apparatus according to any of claims 1 or 2 wherein the sensor is attachable directly to the secure structure. 4. Apparatus according to any one of the preceding claims wherein the sensor comprises a piezoelectric cable. 5. Apparatus according to any one of the preceding claims wherein the signal output by the sensor is continuous. 6. Apparatus according to any one of the preceding claims, wherein the first and second analysers interpret the first and second signals independently within different frequency spectrums . 7. Apparatus according to any one of the preceding claims, wherein one of the first and second analysers is digital whilst the other of the first and second analysers is analogue. 8. Apparatus according to any one of the preceding claims wherein each of the first and second analysers generate an attack event when the analysed signal crosses a predefined level for the frequency and algorithm in use. 9. Apparatus according to any one of the preceding claims wherein the predetermined time window is not more than 20 seconds . 10. Apparatus according to any of claims 2 to 8 wherein the sensor comprises an acoustic element that also acts as a microphone, able to provide audio verification of an alarm signal . 11. Apparatus according to any one of the preceding claims wherein the secure structure is a water tank. 12. Apparatus according to any one of the preceding claims wherein the sensor comprises a first and a second sensor, that provide a similar signal to the first and second analysers respectively. 13. A method of detecting an attack on a secure structure, comprising the steps of: monitoring a signal output from a sensor attached to the structure; analysing said signal at a first analyser and analysing said signal at a second analyser, wherein the first and second analysers use different analysis methods to process the said signal and indicate an attack event; and outputting an alarm signal if both first and second analysers indicate an attack event within a predetermined time window . 14. A method according to claim 13, wherein the sensor detects acoustic and seismic signals and the first and second analysers interpret the resultant signal independently within different frequency spectrums . 15. A method according to either one of Claims 13 or 14 wherein one of the first and second analysers uses an analogue analysis method to process said signal whilst the other of the first and second analysers uses a digital analysis method to process said signal . 16. A method according to any one of Claims 13 to 15 wherein the sensor is attached directly to the structure. 17. A method according to any one of claims 13 to 16 wherein the sensor comprises a first sensor and a second sensor that detect similar signals and pass these to the first and second analysers respectively. 18. A method according to any one of claims 13 to 17 wherein the secure structure is a water tank. |
The present invention relates to the protection of secure structures or enclosures, for example secure containers such as those found in treated water installations, such as above ground fluids storage tanks. In particular, the invention relates to a means of protection of such structures or enclosures from terrorist attack, for example by contamination of treated drinking water.
Secure structures are used in many instances to prevent unauthorised access to the contents. For instance, treated water installations are prime targets for terrorist attack, not only by weapons such as missiles but also by more subversive attacks, such as contamination of the water supply. The present invention relates to apparatus and method for detecting attempts at gaining access to such structures or installations, in particular those associated with the water supply.
Treated water storage tanks come in a variety of shapes, sizes and materials and a number of solutions to their protection have been put forward by the water industry. These range from removing above ground storage and building more underground tanks, installing various types of fencing including an electronic perimeter protection system and building an Λ agricultural' type secure shed or barn completely over the tank.
These solutions all have their own problems, in that it is costly to put a tank underground and ordinary fencing can be easily scaled or removed. In addition, monitored fencing is open to false alarms; alongside this is the high cost of installation and maintenance. The over-building of a tank is very expensive and often requires detailed planning consent (as does fencing over 2 metres) as well as requiring an electronic alarm system. The applicants have recognised that a solution to the protection of treated water installations must be cost effective, quick and easy to install, able to detect an attack on any part of the tank to the required Home Office standard for SEAP (Security Equipment Assessment Panel) approval and able to operate with minimal false alarms linked to weather and in particular inclement weather, or any other natural site phenomena.
The inventive system may be used to protect any secure structure and in particular external secure structures, such as perimeter fencing enclosing land or buildings, cages or buildings, or containers such as tanks of any kind, such as those holding foodstuffs or a fluids material such as liquid fuels or natural gas. The skilled person will understand that the features of the system can be adapted for application and use with all such installations, with water tank installations being described herein as a particular example.
A first aspect of the present invention provides apparatus for the protection of a secure structure comprising: a sensor for attachment to the structure, which sensor outputs a signal; a first analyser which receives said signal from the sensor; a second analyser which also receives said signal from the sensor; wherein the first and second analysers use different analysis methods to analyse the signal and wherein an alarm signal is generated only if both analysers indicate a perceived attack event within a predetermined time window of each other.
This arrangement enables the better discrimination between attack signals and 'generic' signals. Thus, the occurrence of false alarms is significantly reduced. In one embodiment, the said sensor may comprise a pair of sensors, specifically a first and a second sensor, each of which receives a signal and passes it to the first and second analyser respectively. Thus in this embodiment, the present invention provides apparatus for the protection of a secure enclosure, comprising : a first sensor for attachment to the enclosure, which sensor outputs a first signal; a second sensor for attachment to the enclosure which sensor outputs a second signal; a first analyser which receives said first signal from the first sensor; a second analyser which receives said second signal from the second sensor; wherein the first and second analysers use different analysis methods to analyse the first and second signals and wherein an alarm signal is generated only if both analysers indicate a perceived attack event within a predetermined time window of each other.
Preferably however, a single sensor is used and this feeds the same signal to each of the first and second analysers, suitably simultaneously.
In particular, the sensor is directly attached to the material forming the external boundary of the structure to be protected. This material may be in the form of a fence, wall and/or roof surrounding an enclosed space that may, as mentioned above, be an area of land, or it may suitably be a wall or roof of a building or a container such as a fluids storage tank. In a particular embodiment, the sensor is attached to a wall of a container such as a fluids storage tank.
By using different analysis method in the first and second analysers, if one of the analysers produces a false trigger, the second analyser will exclude it. Thus false triggers are reduced or even avoided.
For example, one of the first and second analysers may be digital whilst the other of the first and second analysers may be analogue. The analogue system may use a system of filters to analyse the signal, for example, by counting pulses, whilst the digital system will utilise a predetermined algorithm to determine the nature of the disturbance giving rise to the signal. Alternatively, they may both be digital or analogue, but be programmed to identify different signal patterns as being indicative of an attack profile.
An advantage of using an analogue and a digital analyser respectively is that these characteristically have sensitivity in different regions of the spectrum. A typical front end of a digital analyser responds to higher frequencies than the typical front end of an analogue processor. Using an analyser of each type then provides a broader frequency spectrum analysis than could be provided by either type of analyser alone.
The sensor is able to detect signals such as acoustic signals, seismic signals or radiation signals including heat or light signals. The precise selection will depend upon factors such as the nature of the enclosure being protected, the likely generic signals and the nature of possible attack threats. In a particular embodiment, the sensor comprises an acoustic or a seismic sensor element, or it may comprise a single sensor capable of detecting both acoustic and seismic events. Where a first and a second sensor are provided, these detect the same signal. In a particular embodiment comprising two sensors, both the first and second sensors are each capable of detecting both acoustic and seismic events.
Suitable sensors are piezoelectric cable sensors, that convert stress, strain, vibration, impact, sound or pressure change into minute electrical signals. These sensors generate an electrical output signal that increases as the level of mechanical disturbance increases. The associated monitor records signals that range in frequency from a few mHz to hundreds of kHz or more .
The signal output by the sensor (s) may be continuous, although in some instances, it may be taken at predetermined intervals, or may be activated by a trigger event such as a seismic or acoustic disturbance.
The first and second analysers interpret the signal independently within different signal frequency spectrums . This enables differentiation of different aspects of the same attack signal by independent measurements. In the case of acoustic and seismic sensors, suitable frequency ranges may be, for example, between about IHz and about 1OkHz or any sub-range encompassed therein.
The first and second analysers may each generate an indication of an attack event depending upon the analysis method by which they individually operate. Thus, for example, in some cases, an analyser will generate an indication of an attack event when the analysed signal crosses a predefined level for the amplitude and/or frequency of acoustic and/or seismic waves. However, the other analyser will operate differently, for example using a particular signal algorithm to indicate an attack event. Attack indications from the analysers are suitably provided to a comparator unit or combiner processor which conducts a further analysis of the signals from the first and second analysers and provides an alarm signal only if both analysers are generating an attack indication as described above, within a predetermined time window.
The predetermined time window may have a duration of less than about 60 seconds, more preferably not more than about 20 seconds. For example, the predetermined time window may be about 10 seconds. In most attack circumstances however, both sensors will output signals substantially simultaneously.
The system is suitably designed such that where both analysers generate an attack indication as described above, within the predetermined time window, the comparator unit or combiner processor will generate a confirmed alarm output rapidly, in particular within 20 seconds or less from detection of the combined attack indications.
Suitably, where the sensor is able to detect acoustic signals, for example as a result of the presence of an acoustic element, said acoustic element is arranged to also act as a microphone. The microphone may provide audio verification of an alarm signal by the controller.
Where the secure enclosure is a container such as a water treatment container, it suitably comprises a steel tank or a plastics container.
A second aspect of the invention provides a method of detecting an attack on a secure structure, comprising the steps of: monitoring a signal output from a sensor attached to the structure; analysing said signal at a first analyser and analysing said signal at a second analyser, wherein the first and second analysers use different analysis methods to process the signal and indicate an attack event; and outputting an alarm signal if both first and second analysers indicate an attack event within a predetermined time window .
As described above, the first and second analysers interpret the signal independently for example within different frequency spectrums . In one embodiment, the sensor comprises a pair of sensors, specifically a first and second sensor that detect the same signal but feed it to the first and second analysers respectively. Thus this embodiment provides a method of detecting an attack on a secure enclosure, comprising the steps of: monitoring a first signal output from a first sensor attached to the enclosure; monitoring a second signal output from a second sensor attached to the enclosure; analysing said first signal at a first analyser and analysing said second signal at a second analyser, wherein the first and second analysers use different analysis methods to process the first and second signals and indicate an attack event; and outputting an alarm signal if both first and second analysers indicate an attack event within a predetermined time window .
When the method according to the second aspect of the invention is in continuous use (except when deliberately de-activated to allow legitimate access to the structure) , the number of false alarms may be reduced. In particular, the method may be operated so that a false alarm signal is generated less than about 10 times per annum, preferably less than about 5 times per annum, more preferably 2, 1 or 0 times per annum.
The invention will now be particularly described by way of example with reference to the accompanying diagrammatic drawing in which:
Figure 1 is a schematic diagram showing the apparatus of the invention on a secure structure; Figure 2 is a schematic diagram showing an alternative embodiment of the apparatus of the invention on a secure tank;
Firgure 3 shows a set of examples of typical attack profiles obtained by digital spectrum analysis of data obtained by apparatus according to either of Figures 1 or 2; and
Figure 4 shows a schematic diagram of the apparatus of Figure 1 in more detail .
The illustrated apparatus comprises a sensor that is suitably a combined seismic and acoustic sensor that reports its status through a security system.
At least one sensor is attached around the structure or container to be protected such as, by way of example, a water tank. The sensor may be in the form of a cable. The sensor may in some cases comprise two individual sensors that are attached suitably at the same or different positions on the tank. The or each sensor supplies signals from the tank continuously. Thus, acoustic sensors will detect sounds of water entering/leaving the tank; sounds of the tank expanding/contracting; sounds of items dropping on the tank (branches etc) ; sounds of personnel/intruders climbing on the tank; as well as sounds of attack, for example sawing, drilling, etc. Each type of sound will generate a particular "profile", for example in terms of the frequency and loudness of the acoustic signal and the frequency of the seismic signal. It is necessary that the apparatus is able to distinguish between these signals and generate an alarm only in the case of an attack event.
There are two individual analysers, that receive signals from the or each sensor and these analysers interpret the signals independently and within a different frequency spectrum. They in turn alarm at a predefined level depending upon the frequencies of the signals received and the analysis method in use in the analyser. However, since each analyser operates a different analysis method, for example they operate under a different algorithm, or are different in nature (analogue vs digital), they will produce independent outputs regarding possible attack situations .
If, as a result of the respective sensor activation, both analysers indicate an attack scenario within a predetermined time window of each other, this is processed so as to produce a confirmed alarm output. Such confirmed alarm output is suitably generated as quickly as possible, and preferably within 20 seconds of the commencement of an attack.
The alarm output is suitably directed to for example, a site security system control panel to indicate a confirmed intrusion. This may be passed on to the appropriate security service or control such as a water company control room.
The acoustic element of the tank sensor system may also act as a microphone and will provide audio verification if required, allowing the person receiving the alarm signal to listen to the intrusion event.
As previously mentioned, the protection system is designed to detect physical attack on the large above ground water storage tanks used by the water utilities. These tanks are typically of welded steel construction, with approximate dimensions of 20 feet x 40 feet x 12 feet high (6 metres x 12 metres x 4 metres high) . The steel plate used in the construction is between 5 to 6 mm thick. Work is continuing to include plastic (in the broadest sense of the word) tanks in the range.
The Centre for the Protection of National Infrastructure (CPNI) has decided that the attack methods and levels that will act as a test ^yardstick' for each installation shall be: drilling anywhere on the tank surface with a 25mm diameter hole cutter; sawing any part of the tank structure that would give access to water; and hammering on the tank surface. Each of the above tests has an individual time limit (20 seconds) set by the CPNI by which the alarm must activate.
The main threat to these tanks is perceived to be one of penetrating the wall above the water line, through the roof or incorporated hatches to enable the water contained within it to be polluted.
Specifically, the sensor system must detect the activity of using the tools mentioned above mounted in a portable electric drill, (running in non impact mode) , or a range of standard hand tools within a period of twenty seconds.
Clearly, such activity will produce high levels of acoustic noise and vibration within the fabric of the tank; this is monitored using a seismic/acoustic sensor such as piezoelectric sensor cable (for example, Vibetek ® ) cable, followed up with a signal processor system to discriminate between attack signals and "tank generic" signals by means of an algorithm which analyses the frequency ranges. These "tank generic" signals are generated by water flow, pump operation and environmental noise such as wind or rain etc. within or around the tank structure. It is important that the detection system possesses a very high probability of detecting an attack and a very low false alarm or nuisance alarm rate.
Referring to Figue 4, this has been achieved by using a dual analyser system that, in a particular embodiment, utilises a piezoelectric sensor cable or cables 50, which is connected to independent signal analysers 52, 54. Suitable analysers include those which were originally designed for use with fence intrusion detection systems (FIDS) . One such analogue signal analyser 54 suitably utilises the well- tried and tested "pulse count" method for detecting an intrusion event. In summary, this may operate in the following manner.
The analogue electrical signal from a single Vibetek 10 sensing cable 50 is amplified for example using a variable gain amplifier 42. An intrusion event produces an analogue electrical signal from the cable which has certain characteristics such as frequency, amplitude, duration and repetition rate. The amplified analogue signal generated from the cable is passed through a band pass filter 44 whose upper and lower limits are set in the region of 300 Hz to 3.3 KHz in order to eliminate signals unlikely to be due to intrusion events. The output of this band pass filter 44 is then passed to an adjustable threshold detector 46.
When the preset threshold of the detector 46 is exceeded, the detector 46 generates a single pulse then goes inactive for a short time period. After the short time period, it activates again and will emit another pulse if the preset threshold is again exceeded. Each pulse increments a digital counter 48 from zero towards a preset number. If the counter 48 reaches the preset number, then an alarm signal is given, for example of three seconds duration.
However, the counter 48 resets to zero if there is a given time period in which no pulse is received from the detector 46. This means that a potential intrusion event has to produce detector pulses which are repeatedly produced for long enough for the counter 48 to reach the preset number in order to generate the alarm signal.
The inactivation period of the detector 46, the preset number for the counter 48 and the reset to zero time period of the counter 48 are all adjustable and can be defined during the setting up procedure of the system. For example, the inactivation period might be set from 3 up to 9 seconds and the reset to zero time period might be set from 8 up to 128 seconds.
Such a sensor 50 as the piezoelectric Vibetek cable may also be used to provide a buffered audio monitoring signal 64, that may be passed to a control system and for possible onward transmission to a monitoring station or desk.
Still referring to Figure 4, another known type of signal analyser uses a sophisticated analysis algorithm running within a digital signal processor 52. For example, such a digital analyser 52 may operate as follows. Again, the signal from a sensor 50 such as a single Vibetek 10 sensing cable is amplified by an amount that may be set using a variable gain amplifier 42. The amplified output signal is then passed through a band pass filter 44 whose upper and lower limits are set to IHz to 20 kHz. The output of this filter is then passed via an analogue to digital converter 41 to a Digital Signal Processor (DSP) .
The DSP performs a Fast Fourier Transform 43 of the signal in order to analyse it in the frequency domain and uses a comparator 45 to compare the result against a stored library of "Attack Profiles" 40. If a comparison is found, then a pulse is generated and passed to a microprocessor 47. Once the microprocessor 47 receives the pulse it generates an alarm.
Referring to Figure 3, the digital spectrum analysis of four different types of attack is shown, these being a 6mm drill 300, 20mm hole cutting 305, random hitting 310 and a 12mm drill 315. In this case, Fourier analysis was performed on an audio output of the Vibetek cable, after amplification and filtering. All four of the attacks differ in the level of vibration that they cause. The highest audio output was caused by randomly hitting the tank surface with steel hammer, shown as curve 310, at a frequency of IkHz. The second highest output was caused by a 20mm hole cutter, shown as curve 305, at a frequency between IkHz to 2kHz. The 12mm and 6mm drills had a significantly lower vibration output than other forms of attack.
It is preferred to use an analyser that is most sensitive to signals ranging from about IkHz to 3kHz as this is a frequency range relevant to most attack profiles, thus allowing such attacks to be detected.
Apparatus including an analogue analyser 54 as a first analyser and a digital analyser 52 as a second analyser, as described above, provides a particular embodiment of the invention.
In the present invention, output pulses from both analysers 52, 54 are combined in a custom designed, microprocessor circuit 56. An alarm signal is only presented to the security system input when both analysers 52, 54 have simultaneously detected, for example within a twenty second time window, an attack event.
During a number of different site trials this has proven to be a very effective detection system that exhibits the desired extremely low rate of false alarms, whilst achieving a 100% detection rate in all of the attack trials conducted.
In the schematic illustration of Figure 1, sensor element 50 is directly attached to a secure structure. A suitable sensor could be composed of cable, such as Vibetek ® 10 Piezoelectric Sensor Cable Part Number OT046 from Ormal Electronics and Engineering Limited, 77 Woodland View, Wroughton, Swindon, SN4 9AA. This cable may have a 1 Meg Ohm Termination resistor, factory installed to the end of each Vibetek ® cable and low noise coaxial cable part number OT021, both supplied by Ormal Electronics and Engineering Limited.
This sensor is able to acquire in particular a seismic signature from the structure which is preferably continuous. The signal is fed both to a digital signal analyser 52 such as the FiberSensys Copperhead analyser and an analogue signal analyser 54 such as the Stealthflex AN-1000 Single zone analyser. These analysers are further detailed below.
Both analysers are intended to detect the same type of alarm situation although the alarm profiles will have different characteristics appropriate to the analysers. Unless both analysers produce an alarm output, it is considered a false alarm.
The digital signal analyser 52 is set to produce an alarm output if a particular attack profile is generated, such as one of those shown in Figure 3, for example indicative of drilling, sawing, hole cutting or hammering. It is also set to discard incidental signals indicative of rain, wind, wildlife or birds as these will not show the same characteristics as an attack profile, such as frequency at peak signal strength or indeed the signal strength itself.
The analogue signal analyser 54 is set to produce an alarm signal if it detects a signal which produces repetitive pulses from the detector sufficient to reach the preset number in the counter before it resets to zero, this being indicative of a relevant attack profile for the analogue analyser. Similarly, it will discount the signal if it is not sufficiently sustained, or is not powerful enough to trigger the threshold detector.
It will be noted that the analogue analyser 54 as described above will respond to any sustained signal above a threshold amplitude, within the frequency range of its band pass filter 44 which is set as described to cover the frequency range 300 Hz to 3.3 KHz. The digital analyser 52 on the other hand is used to detect a "signature" alarm condition which is a combination of signal strength and frequency, but not necessarily duration. This alarm condition is not necessarily sustained. Thus the digital analyser 52 is detecting alarm conditions in a significantly different way. Indeed, it is possible to limit the digital analyser 52 to analysis in a very limited part of the spectrum as the most distinctive nature of the attack profiles can be seen by looking for a spike in the IkHz to 2kHz region. The rest of the spectrum can be treated as noise for the purposes of the digital analyser 52.
Referring also to Figure 4, outputs from the two analysers 52 and 54 are combined and processed in a combining processor 56. The combining unit 56 also contains an audio signal processor 60 for processing an audio output 64 of the sensor element 50. As mentioned above, the sensor element 50 can provide audio verification 49 of an intrusion event which has generated a verified alarm. This audio signal processor 60 comprises an audio amplifier 61 to change the audio levels to those required by the alarm system. It also provides power supply noise and GSM noise filtration 62.
The combining processor 56 compares the signals from both analysers 52, 54 within the predetermined time period, for instance simultaneously. If it detects that both are indicating the presence of a valid attack scenario, the processor will generate a verified alarm 58. If not, then no alarm is generated and the signal continues to be monitored.
In more detail, both analysers 52, 54 have relay outputs that give a 5V pulse, two seconds long. These are converted to logic states and used in the combining processor 56 to detect if a logic state indicating an alarm has been received from both analysers 52, 54 within a 20 second window. The 20 second window has been found particularly appropriate in avoiding false alarms being given in the environments under consideration, such as outdoor installations such as water tanks, and given likely attack scenarios discussed above, such as drilling, sawing and hammering on a tank surface. If it responds without delay, this window also allows the combining processor 56 to produce a verified alarm within a maximum of 20 seconds of the onset of an attack, in accordance with the CPNI guidelines. However the 20 second window in which alarms have to be received from both processors 52, 54 can be reduced, for example to something of the order of 10 seconds. This allows the combining processor 56 to respond well within the CPNI guidelines or indeed to perform additional processing, perhaps of the audio output 64 of the sensor element 50.
It might be noted that the functionality of the various processors 52, 54, 56 might be combined in different ways than as shown in Figure 4. For example, the filtration function 62 can also be made available to the analogue signal processor 54.
Figure 2 shows a schematic illustration of the tank sensor apparatus. Figure 2 shows a treated water tank 10 that is typically a welded steel tank. Attached to the tank 10 are two acoustic / seismic sensors 12, 14. Again a suitable sensor could be composed of cable, such as Vibetek ® 10 Piezoelectric Sensor Cable Part Number OT046 as described above. Each cable has a 1 Meg Ohm Termination resistor, factory installed to the end of each Vibetek ® cable and low noise coaxial cable part number OT021, both supplied by Ormal Electronics and Engineering Limited. The Vibetek ® 10 cable is sufficiently long to wrap once around the perimeter of the tank. The non-sensitive coaxial cable is sufficiently long to reach from the Vibetek ® 10 cable at the top of the tank to the signal analyser input at ground level .
Each sensor 12, 14 outputs a continuous signal 16,18 to its associated analyser 20,22. The analogue signal analyser 20 was a Stealthflex AN-1000 Single Zone Analyser, with modification for drilling detection, from Ormal Electronics and Engineering Limited (UK) . The digital signal analyser 22 was a FiberSensys Copperhead Analyser, supplied from Ormal Electronics and Engineering Limited (UK) . Each of the analysers 20,22 analyse the input signals 16,18 from sensors 12, 14. The first and second analysers use different algorithms to analyse the input signals and also analyse different frequency ranges of their respective input signals. Each analyser 20,22 has a pre-set threshold, which will trigger an attack signal if the analysed signal crosses the threshold.
The analysers 20,22 outputs processed signals 24,26 to a combiner processor 28. The combiner processor analyses inputs from both analysers 20,22 and in the event of an attack signal from both analysers within a predetermined time window, it will produce an 'attack signal' 30 to the security system 32.
On the system described above, when fitted to a welded steel tank (approximately 12 x 6 x 4 metres high) and with the sensitivity levels set correctly, it was noted that drilling with a 6mm HSS bit would activate an alarm and could be heard on the audio system.
Depending on the method of access, the system will alarm when personnel ascend to the top of the tank, walk about and open access hatches. Legitimate access would normally only occur after the security system has been 'unset' .
Although initially the system has been installed on all welded steel tanks, it is known that it will work on larger/smaller tanks of different construction. However, the methods of sensor attachment and the analyser settings will vary according to the size and construction of each tank. This means that a survey is undertaken of each tank to establish the exact requirements for achieving an installation that will operate in accordance with the CPNI guidelines.
Individual access hatches situated on top of a tank would normally not require separate protection unless mounted on thick rubber gasket material -separate tests are carried out in this instance and conventional hatch protection sensors fitted if required.
A tank may be situated on a site where:
(i) It is Λ stand alone' with power and telemetry. This requires a tank sensor system and a security system
(ii) It is a Λ stand alone' without power or telemetry. This requires a solar- or wind-powered tank sensor system.
(iii) If it is on a site where a security system already exists to protect another facility. This requires a tank sensor/analyser system that would then be connected to the existing security system.
(iv) It is a site that will also require other facilities to be protected in addition to the tank. This requires that a new security system but allows for expansion of the security system.
(v) It is a site where there is a suitable building or kiosk or similar within an acceptable distance from the tank to securely house the sensor analyser equipment. This requires a tank system analyser and the security system to be installed in a suitable existing building or kiosk.
(vi) It is on a site where no buildings or kiosks suitable for housing the necessary equipment exist. This requires a weather proof kiosk to be erected to house the control equipment at additional cost.
It has been stated that the tank sensor system, in common with the security system to which it will be connected, must not generate false alarms which exceed two per annum. (Technically false alarms are those that are generated by a component failure, workmanship or extraneous noise caused by weather, animals or similar sources - not by the client's operatives inadvertently setting off the alarm) .
During a test period no technical false alarms were generated with the apparatus described above.
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