Shielded enclosure – shield monitoring

Abstract

There is discloses a testing system for detecting compromise of a shielded enclosure, where the shielded enclosure forms an electromagnetic barrier between a first side and second side of the shielded enclosure. The system comprises: an injection probe for electrically supplying a first surface on the first side (e.g. exterior) of the shielded enclosure with an RF signal; and a monitoring probe for electrically detecting surface signals on a second surface on the second side (e.g. interior) of the shielded enclosure. A processor monitors the detected surface signals and determines whether or not a shielding compromise exists based on whether the RF signal supplied on the first surface is detected on the second surface.

Description

[0001] SHIELDED ENCLOSURE - SHIELD MONITORING

[0002] Field This specification is concerned with electromagnetic (EM) shielded enclosures, in particular a system and method for monitoring or testing shielding performance and detecting compromises of EM shielded enclosures.

[0003] Background

[0004] It is often desirable to shield electronic equipment from electromagnetic interference, which can have undesirable effects on their operation. Electromagnetic interference may be naturally occurring or deliberate. Shielded enclosures are used for this purpose. Protection is obtained using the shielded topology concept, which relies on storing equipment within a completely enclosed and continuous conductive housing analogous to a Faraday cage. The topologically closed surface of the housing forms an EM barrier which prevents or limits EM fields from entering the protected volume. IEEE 299, and IEC 61000-4- 23 are known standards which set-out the technical and design objectives and validation test methods for such shielded enclosures. The use of shielded enclosures to provide protection from external High Power Electromagnetic (HPEM) transient disturbances such as High Altitude Electromagnetic Pulses (HEMP) and Intentional Electromagnetic Interference (IEMI) is of particular interest.

[0005] Shielded enclosures can have intentional apertures such as doors, hatches and windows to allow for interaction with the electronics inside the shielded enclosure protected volume and are also likely to have conductive penetrations in the form of electronic cables etc. to penetrate the EM barrier into and out of the protected volume. However, such points of entry (PoE) are often used with additional protective devices, e.g. glands, at the PoE interface which act to form an EM seal between the penetrating element(s) and the aperture. Other common PoE protective devices include waveguides, electromagnetic closure plates and filters together with electric surge arresters on penetrating conductors. Given the often critical purpose of a shielded enclosure, e.g. to allow safe, secure and ongoing operation of the protected equipment, the performance or shielding effectiveness of the shielded enclosure is crucial. However, the shielding performance evaluation requires a fairly laborious test usually in accordance with the standards specified above or a derivative there-of. The standardised test methods require placement of a transmit antenna on the inside of the shielded enclosure, and a receive antenna on the exterior of the shielded enclosure. The transmit antenna radiates an EM signal on the inside of the shielded enclosure and the receive antenna detects any radiated EM signals on the outside of the shielded enclosure. The difference between the receive signal and the transmit signal is indicative of the shielding effectiveness.

[0006] The test method may require the equipment within the protected volume to be shut-down, taken off-line or removed and it may be necessary to obtain a radio transmission license or other regulatory approval to enable the radiated testing. Therefore, given the difficult or impractical nature of this test method the testing is seldom done and perhaps the shielding effectiveness performance of the enclosure is evaluated on annual basis at best.

[0007] While such methods adequately measure shielding effectiveness, they cannot offer real-time monitoring throughout the year when the shielded enclosure is deployed and used for its intended purpose. It is very possible that the shielding performance of a critical shielded enclosure could be compromised between test cycles leaving significant periods of time with the critical electronics housed within the enclosure at risk from disturbance.

[0008] Aspects of the present invention are conceived to address the foregoing. Summary

[0009] According to an aspect of the present invention, there is provided a testing system for detecting compromise of a shielded enclosure, where the shielded enclosure forms an electromagnetic barrier between a first side and second side of the shielded enclosure; wherein the system comprises: an injection probe for electrically supplying a first surface on the first side (e.g. exterior) of the shielded enclosure with an RF signal; a monitoring probe for electrically detecting surface signals on a second surface on the second side (e.g. interior) of the shielded enclosure; and a processor configured to monitor the detected surface signals and to determine whether or not a shielding compromise exists based on whether the RF signal supplied on the first surface is detected on the second surface. The injection probe may comprise an (e.g. toroidal) inductor configured to receive an RF signal from a signal generator and to supply the RF signal to the first surface via an electrical conductor.

[0010] The injection probe may further comprise the electrical conductor (e.g. a coaxial cable) which is in operative proximity to the inductor so as to inductively couple the RF signal into the electrical conductor.

[0011] The electrical conductor may comprise a fixing (e.g. a coaxial connector) for connecting to a bulkhead connector (e.g. bulkhead RF cable connector) on the first surface.

[0012] The electrical conductor may be an extension of the first surface of the shielded enclosure and the inductor may be movable to a position which is in operative proximity to the inductor so as to inductively couple the RF signal into the electrical conductor.

[0013] The monitoring probe may comprise an (e.g. toroidal) inductor configured to receive the surface signals from the second surface of the shielded enclosure via an electrical conductor. The monitoring probe may further comprise the electrical conductor, which is in operative proximity to the inductor so as to inductively couple the surface signals from the conductor to the inductor and in turn a processor.

[0014] The injection probe and the monitoring probe may be terminated on the first side and the second side of the shielded enclosure, respectively. The processor may be configured to determine that a shielding compromise exists if the RF signal supplied on the first surface is detected on the second surface at an amplitude above a reference level.

[0015] The testing system may comprise a human-machine-interface which is configured to output an indication of whether or not a shielding compromise has been determined to exist. The testing system may comprise a human-machine-interface which is configured to output an indication of the extent of shielding compromise based on an amplitude difference between the detected signals and a reference level.

[0016] The testing system may further comprise one or more receive antennas to monitor ambient EM fields and the processor may be configured to detect an electromagnetic interference event has occurred if an amplitude of the detected EM fields is above an ambient amplitude level.

[0017] According to another aspect of the present invention, there is provided a testing method for detecting compromise of a shielded enclosure, where the shielded enclosure forms an electromagnetic barrier between a first side and second side of the shielded enclosure; the method comprising: electrically coupling an injection probe to a first surface on the first side of the shielded enclosure and supplying the first surface with an RF signal; electrically coupling a monitoring probe to a second surface on the second side of the shielded enclosure and detecting surface signals on the second surface; and a processor determining whether or not a shielding compromise exists based on whether the RF signal is detected on the second surface.

[0018] The method may be repeated at periodic intervals, e.g. daily, set by a timer module. The processor may determine that a shielding compromise exists if the RF signal supplied on the first surface is detected on the second surface above a reference level. The reference level may be indicative of local noise within the shielded enclosure, as measured by detecting signals present on the second surface when no RF signal is supplied to the first surface by the injection probe.

[0019] The processor may determine that a shielding compromise does not exist if: the RF signal supplied to the first surface is not detected on the second surface; or the RF signal is detected on the second surface in an amount equal to or below a reference level.

[0020] The RF signal may be a swept frequency continuous wave signal, and the frequency may be swept over a preset RF range. The preset RF range may be 75-150MHz.

[0021] The injection probe may supply the RF signal to an injection point on the first surface which is distal to an Earth point of the shielded enclosure.

[0022] According to another aspect of the present invention, there is provided a kit comprising a testing system as described herein and a shielded enclosure configured to form an electromagnetic barrier between a first side and a second side of the shielded enclosure.

[0023] The shielded enclosure may be a substantially completely enclosed and continuous conductive housing.

[0024] Any processor(s) and controller(s) (and various associated elements) described herein may comprise any suitable processing circuitry to cause performance of the methods described herein and as illustrated in the Figures. The processor or controller may comprise: at least one application specific integrated circuit (ASIC); and / or at least one field programmable gate array (FPGA); and / or single or multiprocessor architectures; and / or sequential (Von Neumann) / parallel architectures; and / or at least one programmable logic controllers (PLCs); and / or at least one microprocessor; and / or at least one microcontroller; and / or a central processing unit (CPU), to perform the methods.

[0025] The processor or controller may include at least one microprocessor and may comprise a single core processor, may comprise multiple processor cores (such as a dual core processor or a quad core processor), or may comprise a plurality of processors (at least one of which may comprise multiple processor cores).

[0026] The processor or controller may be part of a system that includes a human- machine-interface such as or including an electronic display, which may be any suitable device for conveying information, e.g. shielding performance, to a user.

[0027] The processor or controller may comprise and / or be in communication with one or more memories that store the data described herein, and / or that store software for performing the processes described herein.

[0028] The memory may be any suitable non-transitory computer readable storage medium, data storage device or devices, and may comprise a hard disk and / or solid state memory (such as flash memory). The memory may be permanent non- removable memory, or may be removable memory (such as a universal serial bus (USB) flash drive).

[0029] The memory may store a computer program comprising computer readable instructions that, when read by a processor or controller, causes performance of the methods described herein, and as illustrated in the Figures. The computer program may be software or firmware, or may be a combination of software and firmware.

[0030] The computer readable storage medium may be, for example, a USB flash drive, a compact disc (CD), a digital versatile disc (DVD) or a Blu-ray disc. In some examples, the computer readable instructions may be transferred to the memory via a wireless signal or via a wired signal.

[0031] The skilled person will appreciate that except where mutually exclusive, a feature or parameter described in relation to any one of the above aspects may be applied to any other aspect. Furthermore, except where mutually exclusive, any feature or parameter described herein may be applied to any aspect and / or combined with any other feature or parameter described herein. Brief Description of the Drawings

[0032] Embodiments of the invention will now be described by way of non-limiting example with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram generally showing a testing system in accordance with an embodiment of the present invention;

[0033] Figure 2 is a block diagram illustrating a method for determining whether the shielded enclosure has been compromised;

[0034] Figure 3 is a schematic diagram which illustrates a further embodiment of the system;

[0035] Figure 4 is a schematic diagram which shows the testing system of the present invention in a wider, multi-functional system; and

[0036] Figure 5 is a schematic diagram of the system in embodiments where an alternative type of injection probe and monitoring probe are used. Like reference numerals will be used throughout the detailed description to denote like features of the invention.

[0037] Detailed Description With reference to Figure 1 , there is generally shown a testing system 10 in accordance with an embodiment of the present invention. The system 10 is to be used to test the integrity of a shielded enclosure 11 , and correspondingly whether its EM shielding performance has been compromised. The shielded enclosure 11 is in the form of a completely enclosed and continuous conductive housing. It may, for example, be of a known type that is certified to the shielding requirements of IEEE 299, and / or IEC 61000-4-23 standards. The shielded enclosure 11 may be an electrically conductive, e.g. metal, casing having an inner side 12 which is shaped to define a protected volume 13, and an exterior side 14 which is exposed to ambient surroundings. Sensitive electronic equipment 15 may be housed within the protected volume 13 and the shielded enclosure 11 forms an EM barrier which prevents or limits EM fields in the ambient surroundings from entering the protected volume 13, and vice versa.

[0038] While the shielded enclosure 11 is said to completely enclose the protected volume 13, it may comprise one or more deliberate PoEs 16 including apertures 16a or conducted penetrations in the form of cables or conductors 16b. However, in such arrangements the shielded enclosure 11 comprises one or more PoE interfaces 17 which are configured to form a seal between the enclosure 11 and the PoE 16, such that the shielded enclosure 11 will still form an EM barrier to shield the protected volume 13 from EM interferences. PoE interfaces 17 can be in the form of a door 17a, hatch, or removable panel, or protective device 17b.

[0039] During normal use, and assuming anticipated performance, the shielded enclosure 11 is expected to attenuate incident Electromagnetic EM energy (electric and magnetic fields) by at least 80 decibels (dB), and in some cases over 110dB over a certain range of frequencies / wavelengths. However, in cases where the shielded enclosure 11 (i.e. its EM shielding performance) is compromised, shielding performance will be noticeably degraded. For example, a poorly fitted PoE interface 17, could act as a shield violation and may reduce the shielding performance by 20-30 dB. Other violations could include corrosion, doors, panels and hatches not sealing correctly or being removed entirely, damage to some important part and incorrect installation at a PoE. Additional, violations are likely to cause the most significant degradation, in the extreme causing the shielding performance to degrade to near zero.

[0040] The shielded enclosure 11 is configured to attenuate EM fields incident on one side of the enclosure 11 such that they do not reach the other side of the enclosure 11 . The Applicant has recognised, through an understanding of the Faraday effect and Maxwell’s equations, that the same can also be said for surface signals. The Applicant has also recognized that, in cases where the shielded enclosure 11 has been compromised, electric currents, for example, on its exterior surface will be conducted and ported through the compromised region to its interior surfaces, and vice versa. This can be detected and measured to provide an indication that the shielded enclosure has been compromised.

[0041] Accordingly, to determine whether the shielded enclosure 11 has been compromised, the system 10 comprises an injection probe 18, a monitoring probe 19, and a processor 20. The injection probe 18 is movable or attachable to a position as shown in Figure 1 , at which it abuts or is otherwise electrically (e.g. conductively or inductively) coupled to a first surface 21 on the exterior side 14 of the shielded enclosure 11. The monitoring probe 19 is movable or attachable to a position at which it abuts or is otherwise electrically (e.g. conductively or inductively) coupled to a second surface 22 on the interior side 12 of the shielded enclosure 11. In the present embodiment, both the injection probe 18 and the monitoring probe 19 may comprise electrical conductors which, in use, directly contact and are terminated on the first side 21 and the second side 22 of the shielded enclosure 11 , respectively.

[0042] The injection probe 18 is electrically connected to an amplifier 23 and in turn a signal generator 24, where the signal generator 24 is operable via control signalling received by a controller 25. The amplifier may be integrated with the signal generator 24. The signal generator 24 generates a radio-frequency (RF) signal which is amplified by the amplifier 23 and supplied by the injection probe 18 to the first surface 21 via the electrical coupling.

[0043] The RF signal is a continuous wave unmodulated signal that is swept over a preset frequency range from a start frequency to a stop frequency. The duration of the sweep, the frequency step and the dwell time at each frequency step are controllable. In this embodiment the frequency sweep range is set between 75 MHz to 150 MHz selected based on the geometry of a large shielded cabinet type shielded enclosure. In another embodiment the frequency sweep range can be adjusted to suit the application or a specific perceived threat. In other embodiments the RF signal can be a pulsed RF signal a pulse modulated RF signal, or a modulated RF signal.

[0044] While the RF signal is applied, the monitoring probe 19 detects signals on the second surface 22 on the interior side 12 of the shielded enclosure 11 via the electrical coupling and the received signals are recorded by the processor 20.

[0045] The processor 20 is configured to monitor the detected signals and to determine whether or not a shielding compromise exists based on the signals detected on the second surface 22. The RF signal level is set such that, with the shielded enclosure in good order and achieving the required level of shielding, the RF signal is not detectable at the monitoring point. Accordingly, the absence of the RF signal at the monitoring point is indicative that a shielding compromise is not present (shielding effectiveness is preserved) and the presence of an RF signal at the monitoring point is indicative of a shield compromise. This can be verified manually by a user purposefully introducing a shield violation, e.g. by opening a door or panel, and checking that an RF signal is detectable at the monitoring point in that instance. The shielded enclosure can then be made good again, in which case the absence of a signal at the monitoring point will be observed.

[0046] It will be appreciated that, by using electrically coupled probes to test the integrity of the shielded enclosure with an RF signal, the present invention provides a testing system which does not use or rely on radiated EM fields. Accordingly, the system and method of the present invention can be used to monitor the health of the shielded enclosure while electronic equipment are housed and actively used (e.g. turned ON) inside the protected volume. That is, the system can offer realtime monitoring without having to shut down or remove sensitive equipment from the protected volume. It can also be used without having to seek regulatory approval before performing testing.

[0047] It will also be appreciated that while the processor 20 and controller 25 are shown to be separate units in the block diagram of Figure 1 , they may be combined to form part of the same unit, inside or outside of the shielded enclosure 11 . Further, it will be appreciated that many different types of shielded enclosures are commercially available, with different structures to that described above with respect to Figure 1. The present invention is suitable for use with any type of shielded enclosure, where the enclosure is configured during normal use to prevent or substantially reduce the amount of EM fields or RF currents that pass from one surface thereof to another.

[0048] Further still, while the system has been described above with respect to the injection probe 18 and the monitoring probe 19 being located on the exterior side 14 and interior side 12 of the shielded enclosure 11 , respectively, the opposite arrangement is equally possible.

[0049] The method by which the system is configured to determine whether a shielding compromise exists will now be described in further detail with respect to Figure 2.

[0050] Figure 2 is a block diagram showing an embodiment of the method for determining whether the shielded enclosure 11 of Figure 1 has been compromised.

[0051] The method begins under the direction of the controller 25. Specifically, the controller 25 issues a trigger signal to switch the system 10 from an idle mode to a testing mode of operation. This may be initiated on-demand by a user, i.e. in response to a user input, or set to begin at pre-defined periodic intervals, e.g. hourly, daily, fortnightly, monthly etc., as controlled by a timer module. Indeed, an advantage of the present invention is that it can be used without having to shut down and disrupt sensitive equipment. This ease of use enables more frequent testing than what is convenient or practical for conventional testing methods / systems.

[0052] The testing mode of operation begins at block 27, at which reference measurements are made. Specifically, the monitoring probe 19 is electrically coupled to the second surface 22 of the shielded enclosure 11 and an ambient measurement of surface RF signals is taken while no signal is supplied to the first surface 21 by the injection probe 18. The surface signals on the second surface 22 is detected and recorded at each test frequency. It will be appreciated that there will often be some ambient EM fields or current transients within the protected volume 13, which will be detected and recorded as surface signals by the monitoring probe 18. This is especially the case in circumstances where sensitive electronic equipment 15 are concurrently being operated. Thus the detected, e.g. averaged, surface signal measurements recorded at block 27 may be regarded as the reference level which is indicative of local noise within the protected volume 13.

[0053] At block 28, the injection probe 18 is electrically coupled to the first surface 21 of the shielded enclosure 11 and the RF signal is amplified and supplied to the first surface 21. As described above, the RF signal is a continuous wave unmodulated signal that is swept over a preset frequency range from a start frequency to a stop frequency.

[0054] The preset frequency range may be any set of frequencies, as desired. However, in preferred embodiments it is set to cover frequencies which correspond to expected violations or compromises of the shielded enclosure. For example, a violation in the form of a slot aperture will only let through a narrow range of frequencies having a wavelength comparable to the size of the aperture and the frequency range can be set to include that (e.g. above 100 MHz (broken door seal) to a few GHz (small slot)). Further, a compromised PoE cable will be resonant at frequencies related to the cable length but also allow conducted interference at higher frequencies to be ported through into the shielded enclosure.

[0055] Further still, a primary requirement for the shielded enclosure (as set by various standards) is to protect against high-altitude electromagnetic pulse (HEMP) E1 where >98% of the energy in the HEMP E1 pulse has a frequency of less than 100 MHz. Aperture / slot type violations which lead to compromise at higher frequencies than 150 MHz are of little interest / importance for HEMP E1 protection. Thus, a preset frequency range of 75-150MHz provides good coverage of violations that lead to shielding compromises within the frequency range to be protected by the shielded enclosure for HEMP E1.

[0056] While the RF signal is being applied, the monitoring probe 19 detects signals on the second surface 22 on the interior side 12 of the shielded enclosure 11. At each test frequency, the difference between the detected surface signal and the reference level is recorded.

[0057] At block 29, the processor 20 determines whether a shielding compromise exists based on a comparison of the detected surface signal and the reference level.

[0058] The processor 20 will determine that a shielding compromise exists if the amplitude of the detected signal is above that of the reference level. Otherwise, if the amplitude of the detected signal is at or below that of the reference level, the processor will determine that a shielding compromise does not exist.

[0059] In other embodiments, however, the processor 20 determines that a shielding compromise exists if the difference between the detected signal and the reference level meets or exceeds a predetermined threshold value for one or more of the test frequencies. For example, a shielding compromise may be detected if the detected signal is greater than a predefined acceptable level above the reference level for one or more test frequencies. However the predetermined threshold value may be set as desired, e.g. depending on how tolerant the sensitive equipment is to EM fields. At block 30, the system 10 of the present invention reports the outcome of the determination at block 29 to a user. For example, the controller 25 described above with respect to Figure 1 may comprise technical means, e.g. a human- machine-interface (HMI), for presenting information in a way that will allow a human user to monitor the state of shielded enclosure. The HMI may comprise a computer monitor which outputs the result of the determination in real-time. In other embodiments, the HMI may report to or alert the user only on the condition that the processor 20 determines that a shielding compromise exists, e.g. where the detected signal is above the reference level or above some acceptable level beyond the reference level.

[0060] The HMI may provide the user with a binary indication as to whether or not a shielding compromise was detected during testing. However, in embodiments, the HMI may indicate the extent by which the performance of the shielded enclosure has degraded. That is, the HMI may display different indicators depending on the difference between the detected signal and the reference level (referred to hereafter as the “differential value”). In embodiments, the dynamic range of the testing system 10 above the reference level may be divided into subranges, where each sub-range covers a respective set of differential values. Each sub-range is associated (in memory) with a different indicator. The controller 25 or processor 20 will then identify the maximum differential value recorded during testing, and correspondingly which sub-range the maximum differential value falls within, before outputting to the user the indicator associated to the identified subrange. In embodiments, the indicators are colour indicators displayed using a light- emitting-diode or as a graphical representation on a computer monitor, as set out in Table 1 below. Table 1 - Indicators of the extent of shielding compromise The indicators may assist a user to perform a guided human-machine interaction. Specifically, the indicators will prompt the user to perform technical tasks such as to inspect the shielded enclosure, to turn OFF and protect electronic equipment housed therein, or to perform immediate repairs as may be necessary.

[0061] Following block 30, the testing mode of operation ends and the system 10 switches back to the idle mode until another trigger signal is issued by the controller 25 to restart the method at block 27. The system can be operated such that the steps of Figure 2 are repeated regularly. The system can perform the testing more than once per year, thereby providing better monitoring than what is typical for conventional testing procedures. In embodiments, the testing mode of operation is repeated once per day to ensure that the shielded enclosure is tested often enough to detect compromises relatively soon after a degradation event, while minimising power consumption.

[0062] It will also be appreciated that, while the testing mode of operation has been described above with respect to performing reference measurements at block 27 on each occasion, this is not required. The reference level may be measured ahead of time such that the testing mode of operation comprises only the steps set out in blocks 28-30 described above with respect to Figure 2. Alternatively, the reference level may be predetermined or predicted without physically measuring the signals present on the second surface 22 of the shielded enclosure 11. The reference level may be assumed, i.e. set, to be zero across the preset frequency range. It will, however, be appreciated that physically performing reference measurements will increase the accuracy of the determination and minimise false alarms.

[0063] Figure 3 is a schematic diagram which illustrates a further embodiment of the system 10. Specifically, it shows a part of the shielded enclosure 11 , the injection probe 18 and the monitoring probe 19 in more detail.

[0064] The injection probe 18 is in the form of a coaxial bulk current injection probe which effectively comprises one half of a transformer in the form of a toroidal inductor 31 . The transformer core is shaped to have a substantially cylindrical outer profile and an aperture 32 extending axially therethrough. The electrical conductor, which in this embodiment is a Coaxial cable 33, is arranged so as to extend through the aperture 32 of the toroidal inductor 31 , such that a part of the Coaxial cable 33 is held in operative proximity to the inductor 31 . In use, the inductor 31 will receive the RF signal from the signal generator 24 via signal cable 34, which is connected to the primary winding of the inductor 31. The inductor 31 will inductively couple the RF signal into the Coaxial cable 33 without direct electrical contact. The bulk current injection probe is able to supply large RF signals across a large operating frequency range of 1 MHz to 500 MHz.

[0065] The monitoring probe 19 has the same structure as the injection probe 18 in that it effectively comprises one half of a transformer in the form of a toroidal inductor 35. The transformer core is shaped to have a substantially cylindrical outer profile and an aperture 36 extending axially therethrough. The electrical conductor is a Coaxial cable 37 arranged so as to extend through the aperture 36 of the toroidal inductor 35, such that a part of the Coaxial cable 37 is held in operative proximity to the inductor 35. In use, the RF cable 37 will receive surface signals from the second surface 22 of the enclosure 11 and those signals will be inductively coupled to monitoring cable 38 via the transformer (without direct electrical contact), before being received by the processor 20.

[0066] The use of transformers and associated coaxial cables is advantageous in that it is a more practical and easy-to-use way of coupling the probes to the surfaces of the shielded enclosure 11 . Further, the use of coaxial cables lends itself to easier connection to existing fixtures and fittings which are typically already present on the shielded enclosure surfaces. For example, the coaxial cables may be removably attached to the shielding surfaces via one or more coaxial connectors or fixings. Specifically, in the embodiment shown in Figure 3, the first surface 21 of the shielded enclosure 11 comprises a coaxial bulkhead connector 40. Correspondingly, the Coaxial cable 33 of the injection probe 18 has a fixing 41 in the form of a coaxial connector for electrically connecting the Coaxial cable 33 to the bulkhead RF cable connector 40. The coaxial bulkhead connector 40 is electrically coupled to the first surface 21 and, in this embodiment the PoE interface 17, so as to supply the RF signal to the first surface 21.

[0067] The second surface 22 of the shielded enclosure 11 has a coaxial bulkhead connector 42 and the Coaxial cable 37 of the monitoring probe 19 has a coaxial connector fixing 43 for electrically connecting the Coaxial cable 37 to the coaxial bulkhead connector 42.

[0068] While Figure 3 shows the coaxial bulkhead connectors 40, 42 being aligned on opposite sides of the enclosure 11 , it will be appreciated that they may instead be off-set.

[0069] While the present invention has been described above with respect to a system and method of testing the shielded enclosure for compromises that affect its shielding performance, the present invention may form part of a wider system that offers additional functions.

[0070] Figure 4 is a schematic diagram which illustrated a multi-functional system which not only detects shielding compromises, but can also detect electromagnetic interference (EMI).

[0071] As can be seen, the system 10 of Figure 4 corresponds substantially to that described with respect to Figure 1 , and like reference numerals are used to denote like features. However, the system 10 of Figure 4 differs from that of Figure 1 only in that the system 10 further comprises one or more receive antennas. In this embodiment there is a first receive antenna 44 on the exterior of the shielded enclosure 11 , and a second receive antenna 45 within the protected volume 13 on the interior of the shielded enclosure 11. More or less receive antennas is possible.

[0072] The receive antennas 44, 45 are each connected to the processor 20 and are directed, under control signalling from the controller 25, to operate in a receive mode, where any radiated EMI incident on the antennas 44, 45 are transduced into a voltage suitable for processing by the processor 20. In conditions where no EMI is present, the receive antennas 44, 45 and the processor 20 will output an approximately flat voltage signal over a predefined frequency range, e.g. 30 MHz to 6 GHz. However, in conditions where EMI is present, each receive antenna 44, 45 and the processor 25 is configured to detect such incidents in the form of an amplitude spike in the voltage signal outputted by the processor. For example, an EMI event may be detected if the voltage signal exceeds a predetermined threshold amplitude at a given frequency.

[0073] The threshold amplitude may be set as desired. However, the threshold amplitude may be based on the average voltage signal detected by the receive antenna(s) during a survey time period set or triggered by the user. For example, the threshold amplitude may be equal to or above the averaged voltage signal amplitude.

[0074] The controller 25 may be configured to store information indicative of common, routine or ambient EMI events detected during survey time periods. Any subsequently detected EMI events may then be compared to the stored information to determine whether the newly detected EMI event corresponds to a common, routine or ambient EMI event (i.e. in terms of their voltage signal profiles). The controller 25 may be configured to issue an alarm via the HMI to warn that an unexpected or abnormal EMI event has occurred. For example, it can comprise visual indications via light emitting diodes, or audible indications using a speaker or buzzer. Further, the system 10 of the present invention may further comprise means of powering-down electronic equipment housed within the protected volume 13 in response to the controller 25 detecting an EMI event.

[0075] In embodiments where the controller 25 stores information indicating the amplitude of common, routine or ambient EMI events, the threshold amplitude value may be set based on that information. For example, the threshold amplitude value may be set to be above the averaged or maximum amplitude of a pre- determined common, routine or ambient EMI event or events. This may minimize the occurrences of false alarms.

[0076] It will be appreciated that by providing two receive antennas 44, 45, one on each side of the shielded enclosure 11 , the present invention is able to not only detect EMI events on the exterior of the enclosure 11 but also detect whether EM fields penetrated the protected volume 13 during the EMI event. This may provide a useful indication of whether or not sensitive equipment housed within the protected volume 13 is likely to have been damaged by the EMI event. The controller 25 may be configured to log EMI events, shielding compromises, and associated data in memory, or to report such events to a computer, server, or cloud infrastructure which is located off-site.

[0077] While the present invention has been described above with respect to the probes comprising electrical conductors which are separate to the shielded enclosure but are brought into abutment and terminate at the shielded enclosure surfaces, this is not required. It will be appreciated that the probes can have any structure, form or components which are suitable for electrically coupling an RF signal to and from the shielded enclosure surfaces. In some embodiments, for example, the electrical coupling may be achieved ‘galvanically’, i.e. where a first wire from the signal generator 24 and a second wire from the processor 20 are directly connected to respective outer and inner surfaces of the shielded enclosure. In other embodiments, the probes may be surface current probes which do not require an electrical conductor. The electrical coupling may even be provided by capacitive coupling, where the probes are in the form of capacitive coupling clamps.

[0078] Figure 5 is a schematic diagram of the system 10 in embodiments where an alternative type of injection probe and monitoring probe are used.

[0079] The system corresponds substantially to that of Figure 3 except that, in the present embodiment, the injection probe 18 and monitoring probe 19 do not themselves comprise electrical conductors provided in apertures extending through the toroidal inductors 31 , 35. Instead, the surfaces of the shielded enclosure 11 comprises protrusions that act as the electrical conductors. The toroidal inductors 31 , 35 may therefore be brought into inductive proximity with the protrusions so as to electrically couple the inductors 31 , 35 to the protrusions.

[0080] Specifically, the first surface 21 on the exterior side 14 of the shielded enclosure 11 is fitted with a first electrically conductive protrusion 50. Similarly the second surface 22 on the interior side 12 of the shielded enclosure 11 is fitted with a second electrically conductive protrusion 51. Each one of the first and second protrusions 50, 51 , is in the form of an elongate stub cable, wire, metal stud or a rod that extends away from the shielded enclosure. The protrusion may be formed of any conductive material including but not limited to copper, steel or brass. It may have any outer diameter suitable for being received in the aperture of a respective one of the toroidal inductors 31 , 35 of the injection or monitoring probes. Further, a section of each protrusion 50, 51 has a longitudinal extent that is sufficient to pass throughout the aperture of the toroidal inductor 31 , 35. Further, although not shown, the protrusion 50, 51 may be bent or shaped as desired to accommodate the physical space available within or around the shielded enclosure 11.

[0081] The protrusions 50, 51 are welded to the first and second surfaces 21 , 22 respectively to ensure good electrical contact between them. For example, the weld or braise is applied in a continuous manner around the periphery (circumference) of the protrusion 50, 51 at the point that it meets or contacts the first and second surfaces 21 , 22. The protrusions 50, 51 should not penetrate the first or second surfaces 21 , 22 of the shielded enclosure 11 to ensure that it does not violate the shielded enclosure 11. In this way, the protrusions 50, 51 effectively represent extensions of the first and second surfaces 21 , 22 of the shield, to enable surface RF signals to be induced to and from the surfaces for monitoring purposes. Figure 5 shows the protrusions 50, 51 being offset from each other on opposite sides of the enclosure 11 , however it will be appreciated that they may instead be aligned.

[0082] As mentioned above, in use the toroidal inductor 31 of the injection probe 18 inductively couples the RF signal to the first protrusion 50 and the toroidal inductor 35 of the monitoring probe 19 is inductively coupled to the second protrusion 51 to monitor and detect surface signals.

[0083] It will be appreciated that, while determining whether the shielded enclosure has been compromised, the location of the injection probe and the monitoring probe on the surfaces of the shielded enclosure can be different to that shown in the Figures. Indeed it is a practical advantage of the system shown in Figure 1 that the probes are movable to different positions on the surfaces of the shielded enclosure. However, in embodiments there may be an optimum location for the RF signal injection. In that regard, many shielded enclosures that are the basis for the present system are Earthed, where the Earth point is typically at the bottom of the enclosure 11 (e.g. below item (17b) at the bottom right hand side in Figure 1). In embodiments, the RF signal is injected or supplied to the first surface 21 at a distal, e.g. furthest, point away from the Earth point as possible. This may be achieved by bringing the injection probe 18 into abutment with the first surface 21 at the distal point, or by providing a conducting protrusion 50 at the distal point. The RF signal will want to flow to the Earth point, i.e. to complete a circuit with the signal generator (which is effectively Earthed via its power supply), so injecting the RF signal at a distal point from the Earth point will ensure that most if not all of the shielded enclosure surface will be subjected to the RF signal. Furthermore, there may be an optimum location for the monitoring probe. In embodiments, therefore, the monitoring probe 18 may be placed at a location proximate an expected violation where signal loss is expected to be highest. This will be different from installation to installation. It will be appreciated that whilst various aspects and embodiments of the present invention have heretofore been described, the scope of the present invention is not limited to the embodiments set out herein and instead extends to encompass all methods and arrangements, and modifications and alterations thereto, which fall within the scope of the appended claims.

Claims

Claims1. A testing system for detecting compromise of a shielded enclosure, where the shielded enclosure forms an electromagnetic barrier between a first side and second side of the shielded enclosure; wherein the system comprises: an injection probe for electrically supplying a first surface on the first side of the shielded enclosure with an RF signal; a monitoring probe for electrically detecting surface signals on a second surface on the second side of the shielded enclosure; and a processor configured to monitor the detected surface signals and to determine whether or not a shielding compromise exists based on whether the RF signal supplied on the first surface is detected on the second surface.

2. The testing system of claim 1 , wherein the injection probe comprises an inductor configured to receive an RF signal from a signal generator and to supply the RF signal to the first surface via an electrical conductor.

3. The testing system of claim 2, wherein the injection probe further comprises the electrical conductor which is in operative proximity to the inductor so as to inductively couple the RF signal into the electrical conductor.

4. The testing system of claim 3, wherein the electrical conductor comprises a fixing for connecting to a bulkhead connector on the first surface.

5. The testing system of claim 2, wherein the electrical conductor is an extension of the first surface of the shielded enclosure and the inductor is movable to a position which is in operative proximity to the inductor so as to inductively couple the RF signal into the electrical conductor.

6. The testing system of any preceding claim, wherein the monitoring probe comprises an inductor configured to receive the surface signals from the second surface of the shielded enclosure via an electrical conductor; wherein the monitoring probe optionally further comprises the electrical conductor, which is in operative proximity to the inductor so as to inductivelycouple the surface signals from the conductor to a the inductor and in turn a processor.

7. The testing system of any preceding claim, wherein the injection probe and the monitoring probe are terminated on the first side and the second side of the shielded enclosure, respectively.

8. The testing system of any preceding claim, wherein the processor is configured to determine that a shielding compromise exists if the RF signal supplied on the first surface is detected on the second surface at an amplitude above a reference level.

9. The testing system of any preceding claim, further comprising a human- machine-interface which is configured to output an indication of whether or not a shielding compromise has been determined to exist.

10. The testing system of any preceding claim, further comprising a human- machine-interface which is configured to output an indication of the extent of shielding compromise based on an amplitude difference between the detected signals and a reference level.11 . The testing system of any preceding claim, further comprising one or more receive antennas to monitor ambient EM fields and the processor is configured to detect an electromagnetic interference event has occurred if an amplitude of the detected EM fields is above an ambient amplitude level.

12. A testing method for detecting compromise of a shielded enclosure, where the shielded enclosure forms an electromagnetic barrier between a first side and second side of the shielded enclosure; the method comprising: electrically coupling an injection probe to a first surface on the first side of the shielded enclosure and supplying the first surface with an RF signal; electrically coupling a monitoring probe to a second surface on the second side of the shielded enclosure and detecting surface signals on the second surface; anda processor determining whether or not a shielding compromise exists based on whether the RF signal is detected on the second surface.

13. The testing method of claim 12, wherein the method is repeated at periodic intervals set by a timer module.

14. The testing method of claim 12 or 13, wherein the processor determines that a shielding compromise exists if the RF signal supplied on the first surface is detected on the second surface above a reference level.

15. The testing method of claim 13 or 14, wherein the reference level is indicative of local noise within the shielded enclosure, as measured by detecting signals present on the second surface when no RF signal is supplied to the first surface by the injection probe.

16. The testing method of any one of claims 12-15, wherein the processor determines that a shielding compromise does not exist if: the RF signal supplied to the first surface is not detected on the second surface; or the RF signal is detected on the second surface in an amount equal to or below a reference level.

17. The testing method of any one of claims 12-16, wherein the RF signal is a swept frequency continuous wave signal, and the frequency is swept over a preset RF range.

18. The testing method of claim 17, wherein the preset RF range is 75-150MHz.

19. The testing method of any one of claims 12-17, wherein the injection probe supplies the RF signal to an injection point on the first surface which is distal to an Earth point of the shielded enclosure.

20. A kit comprising a testing system as claimed in any one of claims 1-11 and a shielded enclosure configured to form an electromagnetic barrier between a first side and a second side of the shielded enclosure.

21. A kit as claimed in claim 20, wherein the shielded enclosure is a substantially completely enclosed and continuous conductive housing.

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