Safety test switch with actuation lever

An interface test device for testing a circuit, the interface test device including a module assembly including a plurality of modules, wherein a test block assembly is formed from individual test blocks that are arranged at one another in parallel and fixated at one another, wherein a test plug assembly is formed from individual test plugs that are arranged at one another other in parallel and fixated at one another, wherein a movement of a lever arm out of a plane of an insertion direction of the test plug assembly into the test block assembly is transferred by strut elements to pinions of the test plug assembly and inserts test fingers of the test plug assembly into openings of the test block assembly, wherein one of the test fingers of the test plug assembly opens a medium to high voltage monitoring circuit.

FIELD OF INVENTION

The present invention relates generally to an interface test device and method that opens a medium to high voltage circuit, and more specifically to an interface test device that opens a medium to high voltage monitoring circuit where the interface test device is configured to prevent accidental damage to the medium to high voltage monitoring circuit during maintenance and/or allows for maintenance of certain components without taking the medium to high voltage monitoring circuit off line.

BACKGROUND OF THE INVENTION

Most of the components of power system generation, transmission or distribution facilities, such as transmission lines, step-up and step-down transformers, power breakers and generators are monitored and controlled. The control and monitoring is usually performed by electromechanical or electronic equipment that are able to measure electrical quantities, perform calculations based on pre-defined algorithms and thresholds and actuate the system when necessary. Due to the high voltage, current and power flowing through the high-power components, current transformers, potential transformers and breakers are employed as an interface between the high-power components and the low-power control and monitoring devices such as a medium to high voltage monitoring circuit. This medium to high voltage monitoring circuit and its associated circuitry are tested by technicians. For example, a technician might test the operation of a medium to high voltage monitoring circuit or its associated circuitry by inserting a disconnect plug into an interface test device and performing various tests. Unfortunately, it is inevitable that mistakes happen during such testing which results in damage to the equipment or harm to the technician. During such testing, the technician might also adjust the medium to high voltage monitoring circuit by changing the parameters of the medium to high voltage monitoring circuit based upon the testing or based upon other factors. Unfortunately, such testing and adjustments take substantial amounts of the technician's time which is expensive. Furthermore, it is typical to perform period maintenance on the circuitry of the medium to high voltage monitoring circuits. In order to perform maintenance on medium to high voltage monitoring circuits, the associated power circuits must be powered down to allow the technician to perform the maintenance since the interface or other components in the medium to high voltage monitoring circuit might otherwise be damaged. These interruptions in operation of the medium to high voltage monitoring circuit and in the power circuit increase the cost of operation. For example, there are costs associated with switching to another power circuit and there are costs associated with the lost usage of the equipment powered by the power circuit. Accordingly, there is a strong need in the art to improve medium to high voltage monitoring circuits and their associated circuitries to reduce or eliminate the aforementioned drawbacks. Several different types of test interfaces are known in the power industry.

Test interfaces to be used for testing of substation devices can be inconvenient or cumbersome to activate due to problems of manually disconnecting the monitoring circuits from the system lines to the devices. One version of said interface works with a test plug to be inserted into a test block. Said test plug can be particularly difficult to insert, if it consists of many modules, i.e. covers many poles, which need to be inserted simultaneously. To aid in the insertion process of the plug, the current invention provides a leverage mechanism which is incorporated into the plug and renders possible a particularly smooth insertion of easy motion. In addition, a fastening mechanism is provided which guarantees that the plug fingers are precisely aligned with the block openings and the lever can be turned to accurately push the plug fingers into the block. The construction of the test plug is such that the plug is as light as possible and as solid and robust as possible to endure a high number of plug-in mounts.

Electromechanical or electronic devices such as relays and reclosers are typically installed in substation facilities and connect to medium to high voltage power lines via so-called monitoring circuits. The purpose of said devices is to monitor the operational power grid, e.g. protect grid sections from faults or record in-situ values at specific grid locations. The proper operation of said devices needs to be verified frequently over their entire lifetime. To this end, functional tests are undertaken at regular time intervals which indicate whether there is a defectiveness in any of their built-in functions. To facilitate an efficient testing process, it is common practice to use permanently installed test interfaces which are integrated into the monitoring circuits. Said test interfaces both provide a simple and safe disconnect functionality for any connected device.

The invention provides multiple improvements over the inventions described in U.S. Pat. Nos. 8,031,487 and 8,461,856 co-owned by Applicant, both of which are incorporated in their entirety by this reference.

BRIEF SUMMARY OF THE INVENTION

The invention relates to an interface test device for testing a circuit, the interface test device including a module assembly including a plurality of modules, each module configured to open and close at least one medium to high voltage monitoring circuit and the module assembly including a test block assembly and a test plug assembly, each module having at least one pair of contacts biased towards each other that are electrically connected and in line with the medium to high voltage monitoring circuit; and a test circuit, wherein the test circuit is connected to the medium to high voltage monitoring circuit before or substantially simultaneously with the medium to high voltage monitoring circuit being opened, wherein, each module is configured to provide at least one output based upon at least one parameter of the medium to high voltage monitoring circuit to the test circuit in order to measure the at least one parameter by an external tester connected to the test circuit, and wherein the test block assembly is formed from individual test blocks that are arranged at one another in parallel and fixated at one another, wherein the test plug assembly is formed from individual test plugs that are arranged at each other in parallel and fixated at each other, wherein the test plug assembly is movably supported between two side pieces attached at the test block assembly and respectively including guide slots for pinions of the test plug assembly, wherein the side pieces are removably attached at a front face of the test block assembly, wherein a lever arm is pivotably attached at the two side pieces, wherein two strut elements are pivotably attached at the lever arm and pivotably attached at the pinions of the test plug assembly at opposite sides of the test plug assembly, wherein a movement of the lever arm out of a plane of an insertion direction of the test plug assembly into the test block assembly is transferred by the strut elements to the to the pinions of the test plug assembly and inserts test fingers of the test plug assembly into openings of the test block assembly, wherein one of the test fingers of the test plug assembly opens the medium to high voltage monitoring circuit.

One embodiment of the test interface includes an assembly of a test block and a test plug with an identical electrical configuration. Upon insertion, the test plug opens up the monitoring circuits within the test block. Depending on the functionality of the connected device, such an interface can have many poles. A high number of poles can make it inherently challenging to manually insert the plug due to the aggregate weight of the plug and the aggregate friction during the insertion process. To alleviate such problems, the current invention provides a leverage mechanism which is incorporated into the plug and renders possible a particularly smooth insertion of easy motion.

The invention relates in particular to a test interface built into the monitoring circuits for medium to high voltage power lines. The test interface includes a test plug which can be inserted into a test block when put into operation. Both the test plug and the test block are made up of multiple modules which correspond to individual poles in the monitoring circuits. After the insertion of the plug, the monitoring circuits are interrupted between the connected device and the system lines. Then a test set can be wired to the device and testing can begin.

To facilitate a particularly smooth insertion process and ease of motion, the test plug assembly includes a lever mechanism. A lever is pivotably arranged at side elements of the plug assembly to that pivoting the lever pushes the plug fingers back and forth. Thus, the lever principle is applied to reduce a manual force needed for inserting the test plug assembly into the test plug assembly.

To ensure an accurate fit of the plug fingers into the designated openings of the block, the test plug is hooked into a fastener on the block, before the lever is turned for the insertion. When the plug is properly hooked into the block, it is guaranteed that the plug fingers are precisely aligned with the block openings and the lever can be turned to accurately push the plug fingers into the block.

The leverage mechanism facilitates inserting the test plug into the test block with as little force as possible. Additionally, the plug is designed to be as light as possible in order to make it preferably easy to carry it around or hold it in front of the test block.

DETAILED DESCRIPTION OF THE INVENTION

Monitoring of interface test devices for medium to high voltage circuits and systems according to an exemplary embodiment of the invention may be implemented in an automated manner to provide for more continuous and comprehensive monitoring, greater efficiency and safety, reduced costs associated with the monitoring, as well as other advantages. Furthermore, the circuitry used in monitoring and control of an interface test device also may be configured such that maintenance on the medium to high voltage monitoring circuit is able to be performed safely and efficiently without taking the medium to high voltage monitoring circuit off line. With such monitoring circuitry incorporated into the medium to high voltage monitoring circuit, disruptive maintenance may be avoided because the medium to high voltage monitoring circuit does not need to be taken off line during testing and servicing of the medium to high voltage monitoring circuitry which means the servicing is performed without interrupting the medium to high voltage monitoring circuit. This improves efficiency and eliminates the problems that would otherwise be caused by these service interruptions.

The interface test device according to an embodiment of the invention also may be implemented such that when a test plug (either one that is inserted into an aperture of the interface module or one that is integrally formed with the interface module) opens the medium to high voltage monitoring circuit, the medium to high voltage monitoring circuit is protected. For example, when a medium to high voltage monitoring circuit is coupled to a power circuit through a transformer with one coil in the power circuit and the other coil in the medium to high voltage monitoring circuit, the medium to high voltage monitoring circuit cannot be opened without the risk of damaging the coil disposed therein. In order to open the medium to high voltage monitoring circuit for maintenance, the power circuit would have to be shut down because otherwise the primary transformer coil in the power circuit will attempt to continue driving current across the effectively infinite impedance of the secondary transformer coil and will produce high voltage across the open secondary transformer coil that can damage components and endanger operators. To avoid such problems, the test plug may be configured to make another circuit before the medium to high voltage circuit is opened. Such a system may be implemented with an automated monitoring system for the interface module or may be implemented with a monitoring system for the interface module that is not automated. Similarly, other elements including potential transformers and breakers also are protected.

FIG. 1shows a block diagram of an exemplary interface test device1according to an embodiment of the invention. The interface test device1includes a power circuit6monitored by a medium to high voltage monitoring circuit3, an interface module2to connect the medium to high voltage monitoring circuit3to a test circuit7. The test circuit7is also known as an external tester.

FIG. 2illustrates an embodiment of the interface test device1including an interface module2with two test plugs15(also known as test paddles) and two test blocks5(also known as test switches or disconnect devices) where the test plugs15are not inserted into the test blocks5. The interface test device1ofFIG. 2includes a medium to high voltage monitoring circuit3, a monitoring component4, a power circuit6, a test circuit7, an aperture10, two test plug B-side contacts16, two test plug A-side contacts17(test plug B-side contact16and test plug A-side contact17are collectively referred to as a pair of test plug contacts16,17), two shorting bars18, two fingers20, two insulators21, two keying features22, two test block B-side biased contacts26, two test block A-side biased contacts27(test block B-side biased contact26and test block A-side biased contact27are collectively referred to as a pair of biased contacts26,27and may be formed from a high-quality silver-plated copper contacts, high-quality gold plated copper contacts or any other suitable material or materials), biasing springs29, terminals30, and a piece of equipment62(e.g. a relay to be tested). The two test blocks are used in series. The second test block, which is only partially shown on the right side ofFIG. 2is configured identical to the fully shown test block.

The first and the second test plugs, which are only partially shown on the right side ofFIG. 2and which are identical to the fully illustrated test plug, can be used to isolate and test the piece of equipment62. The test plugs15may be shaped such that only suitable test plugs15will mate with the test blocks5via apertures10with an optional keying feature22on fingers21. This keying feature22prevents inadvertent insertion of unsuitable test plugs that can damage the interface module2or other devices and harm the person inserting the unsuitable test plug. Suitable test plugs15break the medium to high voltage monitoring circuit3and connect the test circuit7with the medium to high voltage monitoring circuit3substantially simultaneously. This prevents the medium to high voltage monitoring circuit3from ever being interrupted and thus prevents any of the problems that would otherwise result from such an interruption. The test plugs15can be inserted into the test blocks5for testing potential, current, and signal disconnect links, thereby providing electrical access to all poles on both sides of the test block5. The simple, safe, and efficient design of the interface test device provides access to in-service currents without interrupting the current path prior or during test plug insertion.

Additionally, the keying feature22assures the various contacts are properly matched such that the test block B-side biased contact26is connected to the test plug B-side contact16and the test block A-side biased contact27is connected to the test plug A-side contact17. The insulator21is disposed between the test plug B-side contact16and the test plug A-side contact17. In other words, the finger20includes a keying feature22that engages the aperture10of the test block5such that the finger20can only be inserted into the aperture10in one orientation and the test plug B-side contact16of the test plug15connects to the test block B-side biased contact26of the test block5and the test plug A-side contact17of the test plug15connects to the test block A-side biased contact27of the test block5such that a connection with the correct polarity is assured.

The medium to high voltage monitoring circuit3is coupled to the power circuit6through a monitoring component4. The pairs of biased contacts26,27are connected to the medium to high voltage monitoring circuit3through terminals30. The test plug15includes a finger21supporting the pair of test plug contacts16,17configured to connect to the pair of biased contacts26,27of the medium to high voltage monitoring circuit3. The pair of test plug contacts16,17are connected to the test circuit7, for testing the medium to high voltage monitoring circuit3including the monitoring component4and the piece of equipment62. The test block5and the test plug15including the finger21may be formed from impact resistant insulator material, such as a plastic (e.g. polypropylene or polyethylene) or any other suitable material that will mechanically support and insulate components of the medium to high voltage monitoring circuit3and of the test circuit7. The materials of the test block5may be clear so as to assist in maintenance, detection or sabotage or the like or may be opaque.

The medium to high voltage monitoring circuit3operates a monitoring component4, such as a secondary coil of a transformer, which is used for monitoring a power circuit6with the primary coil disposed in the power circuit6and the secondary coil disposed in the medium to high voltage monitoring circuit3and couples the medium to high voltage monitoring circuit3to the power circuit6. This protects the monitoring and control components4from damage because the higher voltages and/or currents in the power circuit6would damage or destroy the monitoring and control components4in the medium to high voltage monitoring circuit3if directly applied. For example, a current transformer may be used to monitor the power circuit6when the current and/or voltage in the power circuit6is too high to directly apply to measuring instruments in the medium to high voltage monitoring circuit3or in the test circuit7. A current transformer and/or other elements may be used to produce a reduced current that is accurately proportional to the current in the power circuit6that can be conveniently connected to measuring and recording instruments in the medium to high voltage monitoring circuit3and in the test circuit7. For example, the secondary winding of a current transformer should not be disconnected from its load while current is flowing in the primary winding in the power circuit6, as the current transformer will attempt to continue driving current across the effectively infinite impedance and produce a very high voltage (into the range of several kilovolts in some cases) in the secondary current transformer coil that can permanently damage the current transformer such that either the current transformer no longer functions or is no longer an accurate indicator for the power circuit6, and the very high voltage can compromise operator and equipment safety.

The test block5includes an aperture10configured to receive a finger20of the test plug15. The test block5also houses a pair of biased contacts26,27that act as disconnect links that normally connect the medium to high voltage monitoring circuit3to external terminals30.

The terminals30may be made of conductive metal material such as brass, copper or any other suitable material. The terminals30may be configured to receive standard connectors or other connectors. The finger20may be made of impact resistant insulator material such as polypropylene, polyethylene or any other suitable material, and the finger may be configured to insulate against the voltages of the medium to high voltage monitoring circuit3. As illustrated inFIG. 2, the pair of biased contacts26,27in the test block5are in the closed position. In the closed position, the pair of biased contacts26,27are securely pressed together by their own tension and may be additionally pressed together by one or two biasing springs29acting substantially against the opening direction of the pair of biased contacts26,27and exerting force from one or both sides to create a constant contact pressure that minimizes internal resistance. The pair of biased contacts26,27may be spread apart and disconnected from one another by insertion of the finger20of the test plug15between the pair of biased contacts26,27.

FIG. 3illustrates an embodiment of the interface test device1where the test plugs15are partially inserted into the test blocks5. Specifically, the test plugs15have been inserted into apertures10of the test blocks5where the pair of test plug contacts16,17contact the pair of biased contacts26,27but do not cause the pair of biased contacts26,27to separate. The pair of test plug contacts16,17being in contact with the pair of biased contacts26,27ground the medium to high voltage monitoring circuit3through the test plug A-side contacts17of the test plugs15and the shorting bars18, which act as a safety precaution to protect the monitoring circuit3and the test circuit7and helps to prevent an electric arc from forming when the pair of biased contacts26,27are opened.

FIG. 4illustrates the interface module2ofFIG. 1with the test plugs15fully inserted into the test blocks5. The test plug A-side contact16connects to the test block A-side biased contact26and the test plug B-side contact17connects to the test block B-side biased contact27of the medium to high voltage circuit3and the pair of biased contacts26,27are separated. This means that the test block B-side biased contacts27are connected to the test plug B-side contact17and thus are grounded by the shorting bar18and thus may be used for testing.

Insertion of the test plug15farther into the test block5as illustrated inFIG. 4pushes the finger20between the pair of biased contacts26,27and separates the pair of biased contacts26,27from each other causing the opening of the medium to high voltage monitoring circuit3and thereby connecting to the test circuit7and simultaneously isolating the device to be tested in the same motion. The insertion of the finger20between the pair of biased contacts26,27occurs against the natural direction of the electric arc opening between the pair of biased contacts26,27and inserts an insulator21between the two poles of the pair of biased contacts26,27which guarantees that no electric arc occurs while the pair of biased contacts26,27is being opened. The interface test device1is designed to perform a “make-before-break” function, where make means shorting the current transformer ends. This “make-before-break” function provides superior protection for current transformers and other circuit elements. For example, upon insertion of the test plug15, the pair of biased contacts26,27is automatically short-circuited by the shorting bar18along pre-assigned poles, in a single step. The simple, safe, and efficient design of the test plug15and the test block5provides access to in-service medium to high voltage monitoring and control components4and the equipment62without interrupting the current path prior or during test plug15insertion. The interface test device1utilizes “make-before-break” function to maintain electrical system continuity and automatically short circuit medium to high voltage component current channels before opening the medium to high voltage monitoring and control circuit3. Potential and signal links are disconnected by the test plug15with high quality electrical insulation. The single movement of test plug15insertion both “makes” and “breaks” the medium to high voltage circuit3in a fail-safe sequence that achieves proper isolation and restoration every time. With the test plug5inserted as illustrated inFIG. 4, testing and replacement of a defective medium to high voltage monitoring component4and of the equipment62can be safely performed.

The pair of biased contacts26,27automatically closes upon removal of the test plug15. For example, the biasing springs29that press the pair of biased contacts26,27towards each other guarantee that the medium to high voltage monitoring circuit3is closed when the testing procedures are finished.

The use of multiple test plugs15allows for the testing of portions of the test circuit7. Alternatively, if the entire test circuit is to be tested, a single test plug may be used.

FIG. 5illustrates a test plug assembly11and an insertion lever101,102,103for the test plug assembly tilted down and test plug fingers20of test plugs15pushed down. The test plug assembly11includes multiple test plugs15arranged and attached at each other in parallel and enclosed by two frame rails108. The lever includes two arm pieces103on each side which are rotatable about pivot points109which define a pivot axis of the lever. A handle101-102of the lever includes multiple centerpieces101,102aligned on a through bolt110. Load transfer from the lever arm pieces103onto outermost test plugs15of the test plug assembly11is facilitated by two lateral strut elements104(designated inFIG. 7) which are pivotably connected at the lever arm pieces103and pivotably connected at pinions115of the outermost test plugs15of the plug assembly.

The pinions115provide sliding support for the plug assembly11in guide rails106provided in both terminal plates105. In the illustrated position, the test plug assembly11is pushed down in a direction towards a test block assembly14and plug fingers20of the test plug assembly11stick out of the plug assembly frame formed by the terminal plates105and the frame rails108. This would correspond to an inserted condition of the plug assembly, if the test plug assembly were fastened to the test block assembly. On each, outside flank of the terminal plates105there are two protrusions111that are used for locking the terminal plates105of the plug assembly to side pieces201that are bolted to the test block assembly14.

FIG. 6illustrates a face view of the test block assembly, e.g., it shows its front onto which the test plug assembly will be connected. The test blocks5are enclosed by two side pieces201. The side pieces201respectively include a screw hole207for bolting the test plug assembly to the test block assembly and small guide rails205for the protrusions111of the terminal plates105of the test plug assembly. Each of the guide rails205receives a protrusion111of a terminal plate105. The guide rails205include snap locks210to lock the terminal plates105in place in the side pieces201. After the protrusions111are inserted into the guide rails205and snap locked at a bottom of the guide rail205the test plug assembly11is in a position suitable for the insertion of the test plug fingers20into the test blocks5. All the test blocks5have openings10in their centers to receive the test plug fingers20.

FIG. 7illustrates the test interface with the test plug assembly11fastened to the test block assembly14with the test plug fingers20not yet inserted into the test blocks5. In this position, the lateral strut element104is visible which provides a force transfer from the lever arm pieces103of the lever onto the pinions115(designated inFIG. 5) of the test plug assembly11. The test block assembly14includes multiple test blocks5which extend from one end with the openings10for the plug fingers20(designated inFIG. 5) to the other end with the wire connections204. The test blocks5are pulled together by parallel bolts203which penetrate the test blocks5from the first test block5to the last test block5. The front side of the test block with the openings10for the plug fingers20is enclosed by two side pieces201. The side pieces201include the guide rails205including the snap locks210(designated inFIG. 6) for attaching the terminal plates105at the test block assembly11. Each of the guide rails205supports a protrusion111of the terminal plates105that include the guide rails106for guiding an insertion movement of the test plug assembly11into the test block assembly14.

FIG. 8illustrates the test interface according to the invention with the test plug assembly11fastened to the test block assembly14and the test plug fingers20inserted into the test blocks5. Unlike from the condition illustrated inFIG. 3, the handle is no longer aligned with the longitudinal axis of the test blocks and an insertion direction of the test plug fingers20into the test blocks but arranged at an angle relative to the longitudinal axis of the test blocks and the insertion direction of the test plug fingers. The test plug assembly has been pushed to a bottom end of the guide rails106in the terminal plates105and the test plug fingers20are pushed into the openings10of the test blocks5and inserted into the test blocks5so that the contacts26,27are opened. The angled position of the test plug lever arms103corresponds to the position shown inFIG. 3.

The present invention relates to a test interface built into the monitoring circuits for medium to high voltage power lines. It includes a test plug which can be inserted into a test block when put into operation. Both the test plug assembly11and the test block assembly14are made up of multiple test plugs15and test blocks5which correspond to individual poles in the monitoring circuits. After insertion of the test plugs15into the test blocks5, the monitoring circuits are interrupted between the connected device and the system lines. The test plug fingers20then separate a pair of converging contact springs26,27which are arranged below each opening10in the test blocks. Then simulated currents and voltages can be infected on the device side via banana jacks on the plug and testing of the device can be initiated.

To facilitate particularly smooth insertion and ease of motion, the test plug assembly11is moved relative to the test block assembly14by a lever mechanism. The lever handle is101-102is assembled by a through bolt110and can be and can be pivoted around the pivot axis defined by the pivot pints109to move the plug fingers20back and forth. Thus, a lever principle is applied to reduce a manual force required for inserting the plug fingers20in the test blocks5. The lever arms103move two lateral strut elements104which are pivotably connected to both pinions115(designated inFIG. 5) of the test plug assembly and both lever arms103. The pinions115are guided in the guide rails106provided in the terminal plates105and provide the load transfer to move the plug fingers20back and forth. Since the guide106rails are centered relative to a plane that includes the centers of the plug fingers20and that is aligned parallel to a vertical extension of the test block arrangement11and an orientation of the plug fingers20, an activation of the lever arms103pushes the plug fingers20straight into the test blocks5and does not misalign the plug fingers20relative to the test blocks5. A force applied to push the test plug fingers20into the test blocks5is evenly distributed over all individual plug fingers, since the force transfer from the handle101-102is split up into two equal portions at the two pinions115. The configuration of the plug assembly promotes straight insertion of the plug fingers20into the test blocks5and avoids potential friction originating from a misalignment of the plug fingers20relative to the test blocks5.

To ensure an accurate fit of the plug fingers20into designated openings10of the test block assembly, the test plug assembly is attached to two side pieces201bolted to the test block assembly before the lever arms103are turned for the insertion of the test plugs15in the test blocks5. The two side pieces201each include short guide rails205on both sides of the test block assembly11. Each guide rail205is configured to receive a protrusion111which is laterally provided at the side plates105. The protrusions111are insertable through a rectangular cavity into each guide rail205. The surface of each guide rail205is smooth enough for the inserted protrusion to slide inward until it reaches a designated stop and is locked in place by the snap locking arrester210(designated inFIG. 6). In this position, it is guaranteed that the plug fingers20are correctly aligned with the openings10of the test block5and the lever arms103can be rotated to accurately push the plug fingers20into the test blocks5.

The leverage mechanism aids in manually inserting the test plug into the test block with as little force as possible. The test plug assembly11is designed to be as light as possible in order to make it easy to carry or hold in front of the test block assembly14. This way the plug becomes particularly handy which additionally promotes an easy insertion process. A light weight of the plug is achieved by its construction type with strutted pieces where possible.

The test plug arrangement includes three joints: one in the lever's swivel points109and two at each end of the lateral strut elements104. These joints are linked by custom-made rivets which are specifically designed to be sufficiently robust with respect to mechanical impacts and to endure a high number of load cycles.

Although several embodiments of the present invention and its advantages have been described in detail, it should be understood that changes, substitutions, transformations, modifications, variations, permutations and alterations may be made therein without departing from the teachings of the present invention, the spirit and the scope of the invention being set forth by the appended claims.

REFERENCE NUMERALS AND DESIGNATIONS