The present application relates to a mechanism for a measurement device, and in particular to a jaw mechanism for a split-core current measurement device. The present application further relates to a method of manufacturing of the same and a method of measuring a current using the current measurement device.

Background

Current sensing equipment may be used to measure the electrical current flowing through a current carrying conductor. It is desirable to sense electrical current without a direct electrical connection to the conductor carrying the current to be sensed, so as to avoid any electrical disturbances to the current to be sensed which may interfere with the current measurement, and also so as to avoid the possibility of the current to be sensed from being transferred to the current sensing equipment and hence to a user, which may present a hazard. In order to allow current measurement of this kind, it is known to use a sensor (or measurement device) operable to measure properties of the magnetic field produced by the current to be measured travelling through a conductor, from which the properties of the current itself can be deduced. It is known to provide such a sensor in the form of a magnetic field sensor such as, for example: a Hall Effect sensor, a MEMS-based magnetic field sensor, or a fluxgate sensor.

Solid core current sensors may be used to sense the current within a current carrying conductor. Current sensors or current transformers of a solid core type typically comprise a single magnetic core of high magnetic permeability material formed as a continuous loop. Because of the non-split nature of the core of this type of current sensor, it usually has to be installed around the conductor when it is first installed or as part of an installation routine in which the conductor is powered down.

Split-core current sensors require that the faces of the two separate cores are aligned, and held in contact with each other by a defined force for consistent performance. Misalignment of the faces when closing a split-core current sensor leads to measurement errors. Presently available split-core current sensors utilise compression or torsion springs to hold the two cores together; however, these introduce stress points on the core and the casing of the core. It is an object of the present disclosure to obviate or mitigate at least one of the above outlined shortcomings of known current sensors and casings for current sensors.

Summary

According to a first aspect of the present disclosure, there is provided a casing for a splitcore current measurement device comprising: a first jaw configured to support a first arm of the measurement device including a first magnetic core portion, wherein the first jaw comprises a retaining portion; a second jaw configured to support a second arm of the measurement device including a second magnetic core portion, wherein the second jaw comprises a clasp, wherein the first and second jaw are pivotally connected to each other, such that the first and second jaws are pivotable between: i) an open configuration corresponding to an open configuration of the measurement device; and ii) a closed configuration corresponding to a closed configuration of the measurement device in which the first and second magnetic core portions cooperate to form a substantially complete magnetic circuit which encircles an axis; an elongate strap having a first end and a second end, wherein the first end is configured to be coupled with the retaining portion and wherein the second end is configured to be coupled with the clasp such that, when the measurement device is in the closed configuration, the strap extends circumferentially around a radially outer surface of the first arm and a radially outer surface of the second arm, and such that when the measurement device is in the closed configuration the strap is under tension.

By supporting the first arm with the first jaw, the second arm with the second jaw, and by coupling the strap with the retaining portion and the clasp, then the tension of the strap provides a radially inwards force to improve contact and alignment between the two magnetic cores. The strap tension can be adjusted when the device is in the open configuration, to obtain the desired tension when the device is in the closed configuration.

The casing and measurement device, provide an improved current measurement device using non-metallic and non-conductive parts. The casing and the split-core measurement device can be moved between an open, closed, and locked configurations such that the measurement device can be easily fitted onto or removed from around a current carrying conductor. The strap may comprise a strap head at the first end of the strap.

The retaining portion may comprise an aperture. The strap and the aperture may be sized such that the strap can pass through the aperture.

The strap head may have a width greater than a width of the aperture, such that, when the strap passes through the aperture, abutment of the strap head with the retaining portion defines a limit of movement of the strap through the aperture.

The casing may further comprise a latch movably coupled with the first end of the strap. The latch may be configured such that, when the measurement device is in the closed configuration, the measurement device is placeable in a locked configuration by coupling the latch with a receiving portion of the second jaw. The latch may be pivotally coupled with the first end of the strap. By coupling the latch with both the first end of the strap and the receiving portion of the second jaw, the latch connects both ends of the strap such that a force is applied radially inwards around a whole circumference of the splitcore measurement device. This improves alignment between the two magnetic cores and prevents unwanted movement of the magnetic cores.

The strap head may comprise a frame movably coupled to the first end of the strap. The latch may be movably attached to the frame. The frame may be pivotally coupled with the first end of the strap, and the latch may be pivotally coupled with the frame.

The first jaw may comprise a first recess configured to receive the frame.

The frame may comprise a first end coupled with the first end of the strap. The frame may further comprise a second end spaced from the first end of the frame and coupled with the latch. The first recess may be configured to receive the first end of the frame. The first jaw may further comprise a second recess configured to receive the second end of the frame, when the device is in the locked configuration.

The second jaw may comprise a second receiving portion. The latch may be configured to be received by the second receiving portion. This allows the latch to be coupled with the second jaw when the measurement device is in the closed and locked configurations. The second receiving portion may be coupled with the clasp. This allows the latch to exert a force on the clasp, such that the coupling force between the clasp and the strap increases when the device is in the locked configuration; this further increases tension in the strap.

One of the first jaw and the second jaw may comprise a pivot and the other of the first jaw and the second jaw may comprise a pivot receiving portion. The pivot receiving portion may comprise a channel in which the pivot is movable in a direction which is generally tangential to the radially outer surface of the first arm or second arm. This allows the device to self-adjust to account for manufacturing tolerances of the arms.

The clasp may comprise a clasp aperture configured to slidably receive the second end of the strap. The clasp may further comprise a pawl configured to engage with one or more teeth on the second end of the strap when the second end of the strap is received by the clasp aperture. The pawl may be configured such that the strap may move in only one direction through the aperture. This can be used to adjust the position of the strap within the clasp, and therefore adjust the tension of the strap. This also ensures that once the device is tightened, the strap does not come undone, thereby ensuring the tension of the strap is maintained.

The strap may be formed of a resilient material such that the strap provides a force biasing the measurement device from the closed configuration towards the open configuration. When the device is closed but not correctly placed into the locked configuration, this provides a self-opening mechanism to indicate to a user that the device is not in the locked configuration.

According to a further aspect of the disclosure, there is provided a split-core current measurement device, the device comprising: a casing as described above; a first arm including a first magnetic core portion, the first arm being supported by the first jaw; a second arm including a second magnetic core portion, the second arm being supported by the second jaw; the first and second magnetic core portions being shaped such that, when the current measurement device is in the closed configuration, the first and second magnetic core portions cooperate to form a substantially complete magnetic circuit which encircles said axis. The first arm may comprise at least one guiding structure, including a guide opening, formed at the radially outer surface of the first arm. The guide opening and the strap may be configured such that the strap can pass through the guide opening of the guiding structure. The guide opening defines the position of the strap along the thickness of the arms, and thereby improves alignment in the direction of the thickness of the magnetic cores. When the strap passes through the guide opening of guiding structure and the aperture, the retaining portion may abut the guiding structure. This further improves alignment between the first jaw and the first arm.

Each of the first arm and the second arm may comprise at least one guiding structure, including a guide opening, formed at the radially outer surface of the respective arm. The guide openings and the strap may be configured such that the strap can pass through the guide opening of the or each at least one guiding structure of the first arm and of the or each at least one guiding structure of the second arm.

The current measurement device may form part of one of: a fluxgate current sensor, a current transformer, an open loop current sensor, or a compensating Hall Effect current sensor.

According to a further aspect of the disclosure, there is provided a method of measuring a current through a current carrying conductor using the measurement device as described above. The method comprises locating the current carrying conductor between the first jaw and the second jaw when the current measurement device is in the open configuration; placing the current measurement device into the closed configuration, such that the first and second magnetic core portions cooperate to form a substantially complete magnetic circuit which encircles the current carrying conductor, the measurement device thereby surrounding the current carrying conductor; coupling the first end of the strap with the retaining portion and the second end of the strap with the clasp such that the strap is under tension.

The method may further comprise coupling the clasp with the retaining portion using the latch, to place the current measurement device in the locked configuration. According to a further aspect of the disclosure, there is provided a method of manufacturing a current measurement device, the method comprising: forming a first arm including a first magnetic core portion; forming a second arm including a second magnetic core portion; the first and second magnetic core portions being shaped such that, when the current measurement device is in a closed configuration, the first and second magnetic core portions cooperate to form a substantially complete magnetic circuit which encircles an axis; forming a first jaw configured to support the first arm, wherein the first jaw com-prises a retaining portion; forming a second jaw configured to support the second arm, wherein the second jaw comprises a clasp, and wherein the first and second jaw are pivotally connected to each other, such that first and second jaws are pivotable between: i) an open configuration corresponding to an open configuration of the measurement device; and ii) a closed configuration corresponding to the closed configuration of the measurement device; forming an elongate strap having a first end and a second end, wherein the first end is configured to be coupled with the retaining portion and wherein the second end is configured to be coupled with the clasp such that, when the measurement device is in the closed configuration, the strap extends circumferentially around a radially outer surface of the first arm and a radially outer surface of the second arm, and such that, when the measurement device is in the closed configuration, the strap is under tension.

Forming the first arm and forming the second arm may comprise forming a closed-loop magnetic core; and dividing the closed-loop magnetic core into a first magnetic core portion and a second magnetic core portion.

The method may comprise: locating the first arm within the first jaw; locating the second arm within the second jaw; coupling the first end of the strap with the retaining portion; passing the strap through a guide opening of each of one or more guiding structures located on the radially outer surface of the first arm and/or the radially outer surface of the second arm; and coupling the second end of the strap with the clasp.

Brief Description of the Figures

Some embodiments of the disclosure will now be described by way of example only, and with reference to the accompanying drawings in which: Figure 1 is an isometric view of a split-core assembly for a current measurement device according to the present disclosure, having four guiding structures which provide assembly alignment;

Figure 2 is an isometric view of an inner portion of an upper jaw of a casing according to the present disclosure;

Figure 3 is an isometric view of a lower jaw of a casing according to the present disclosure;

Figure 4 is an isometric view of an elongate portion of a strap of a casing according to the present disclosure;

Figure 5 is an isometric view of a frame for a strap according to the present disclosure;

Figure 6 is an isometric view of the frame of Figure 5 coupled with the strap of Figure 4;

Figure 7 is an isometric top view of the tensioning strap inserted through two guide openings of a first arm of a measurement device according to the present disclosure;

Figure 8 is an isometric bottom view of a strap inserted through two guide openings of a second arm of a measurement device according to the present disclosure;

Figure 9 is an isometric view of a clasp of a casing according to the present disclosure;

Figure 10 is an enlarged cross section of a strap located through the clasp, and the latch coupled with the clasp of a casing according to the present disclosure;

Figure 11 is an isometric view of a latch of a casing according to the present disclosure;

Figure 12 is an isometric view of a measurement device according to the present disclosure, in a locked configuration;

Figure 13 is an isometric view of a split-core assembly of a measurement device according to the present disclosure; Figure 14 is a cross-section of the split-core assembly of Figure 13 located within a casing according to the present disclosure, in the locked configuration;

Figure 15 shows a side view of a measurement device of the present disclosure, in a locked configuration; and

Figure 16 shows a side view of a measurement device of the present disclosure, in an open configuration.

Detailed Description of the Preferred Embodiments

Figure 1 shows a split-core assembly 100 for a current measurement device, having four guiding structures which provide assembly alignment. The split-core assembly includes first and second arms 102, 104 each including a magnetic core portion.

The split-core assembly comprises first and second substantially identical arcuate arms 102, 104. In other embodiments the first and second arms may have any other appropriate shape - for example they may have a symmetrical or an asymmetrical shape. The first and second arms 102 and 104 are movable relative to one another between an open configuration, in which the arms may be placed around a current carrying conductor (not shown), the current of which is desired to be sensed, and a closed configuration.

In the closed configuration of the arms 102, 104, as shown in Figure 13, the two arms together form a generally toroidal or ring-shaped structure encircling an axis. The fact that the first and second arms (and, more particularly, core portions (discussed in more detail below) within the first and second arms) are movable between open and closed configurations means that the current measurement device may be referred to as a splitcore current measurement device.

The split-core assembly may be manufactured by forming a single toroidal structure and dividing the toroidal structure into a first arm 102 and a second 104 by cutting the toroidal structure across a plane including the axis. Each of the arms 102, 104 thereby includes two planar surface 110 which are configured to abut a respective surface of the other arm in the closed configuration.

The split-core assembly may be formed by placing a ferromagnetic core of a given dimension in a holder. Encapsulation fluid may then be poured into the holder and left to cure. The core assembly can then be cut into two equal halves to form the first and second arms. The range of cutting tolerance typically is 0.5mm to 2.0mm.

Whilst a fluxgate current sensor is herein described, the present disclosure is not limited to a fluxgate current sensor but can be any split-core current sensor such as a split-core current transformer, or split-core open loop or compensating Hall Effect Current sensor.

Each arm of the split-core assembly includes two guide structures 106 located on a radially outer surface 112 of the arm 102, 104. Each guide structure 106 includes a guide opening 108 extending through the guide structure substantially tangentially to the radially outer surface of the arm. In this example, the guide openings 108 are shown as apertures however they may comprise open slots. The guide openings 108 define a position of the strap in the direction of the axis such that the first arm 102 and second arm 104 are aligned in the direction of the axis when the casing and the split-core current assembly are in the closed configuration.

In this example, each arm 102, 104, has two guide structures 106. This allows the arms to be supported at both ends of each arm by one of the jaws. However, the split-core assembly may include more or fewer guide structures.

Figure 2 shows an inner portion 200 of an upper jaw of a casing configured to support the first arm 102 of the split-core assembly shown in Figure 1. The upper jaw may also be referred to a first jaw. The central wall 208 extends between two parallel sidewalls 206 of the inner portion, which each extend radially outwards from the central wall 208.

A retaining portion 202 is located between the two sidewalls 206 and includes an aperture 204 extending through the retaining portion 202 in a direction substantially tangentially to the radially outer surface of the first arm 102 when the first arm is supported within the inner portion 200 of the upper jaw as shown in Figure 7. In this example, the aperture 204 is defined by the space between a lower bar 218, two inner sidewalls 220 and a top bar 222 extending between the two inner sidewalls 220. The inner sidewalls 220 are parallel with the sidewalls 206 of the inner portion 200 of the upper jaw; however, the inner sidewalls 220 are located between the two sidewalls 206. The aperture 204 is configured such that the first arm 102 extends through the aperture 204 when the first arm is supported within the upper jaw.

Each inner sidewall 220 of the retaining portion 202 includes a recess 210 extending radially inwards from a radially outer edge of the respective inner sidewall 220, as shown in Figure 2. The recess is configured to receive the first end 410 of the frame 408, as shown in Figure 5, such that the frame 410 can pivot about the recess 210.

The inner portion 200 of the upper jaw includes a latch recess 216, the latch recess is for receiving the latch 500 when the casing is in the locked configuration as shown in Figures 14 and 15.

Figure 3 shows a lower jaw 300 of a casing configured to support the second arm 104 of the split-core assembly shown in Figure 1. The lower jaw may also be referred to as a second jaw. The central wall 308 extends between two parallel sidewalls 306 of the lower jaw, which each extend radially outwards from the central wall 308.

A retaining portion 318 is located between the two sidewalls 306 and includes an aperture 320 extending through the retaining portion 318. The aperture 320 is configured such that the second arm 104 extends through the aperture 320 when the second arm is supported within the lower jaw, as shown in Figure 3 and Figure 8.

The lower jaw 300 includes a hook 302 configured to receive a pivot 212 located between the sidewalls 206 of the inner portion 200 of the upper jaw, as shown in Figure 14. The hook 302 defines a channel which is generally tangential to the radially outer surface 112 of the second arm 104 when the second arm is located within the lower jaw 300. The channel, defined by the hook 302, is also generally tangential to the radially outer surface of the first arm 102 when the upper jaw 200 and the lower jaw 300 are in a closed configuration. The pivot 212 of the upper jaw is able to move within the channel defined by the hook 302. Figure 6 shows a strap 400. The strap includes a frame 408, as shown in Figure 5, coupled with an elongate portion 402 of the strap as shown in Figure 4.

The elongate portion 402 is formed of a flexibly resilient material. Towards a first end of the strap 404 there is a connector 416 configured to accept the first bar 410 located at a first end of the frame 408 so that the frame 408 is pivotally coupled with the elongate portion 402 at the first end 404 of the strap. The first bar 410 has two flanges 420 configured to maintain the position of the connector 416 on the first bar 410. Towards a second end of the strap 406 there are a series of ridges or teeth 414 configured to engage with a pawl 314 of the clasp 312 shown in Figure 9, as shown in Figure 10. The elongate portion 402 may be tapered at the second end 406 and may be narrower towards the second end 406 than at the first end 404, to facilitate fitting the strap through the guide openings 108 of the split-core assembly, the aperture 204 of the upper jaw, and the clasp 312 of the lower jaw.

The frame 408 is a rectangular frame formed of a first frame bar 410 at a first end of the frame, and a second frame bar 412 at a second end of the frame. The two bars 410, 412 are parallel and joined by two sidebars 418. However, the frame 408 may be any shape or configuration that can be pivotally coupled with both the elongate portion 402 and the latch 500.

Figure 7 shows the strap 400 inserted through two guide openings 108 of a first arm of the split-core assembly, and along the radially outer surface 112 of the first arm. The strap fits first through the aperture 204, and then through a guide opening 108 of the first arm 102 of a split-core assembly. The frame 408 fits into the connector 416, and is held by the inner jaw recess 210. A latch 500 is movably coupled with the frame 408. The latch 500 has a latch connector 504 configured to fit around the second bar 412 at the second end of the frame 408 so that the frame 408 is pivotally coupled with the latch 500.

As can be seen in Figure 7, in some examples, the upper jaw 200 may include a further retaining portion 226 located between the two sidewalls 206 and including an aperture (not visible) extending through the further retaining portion 226. The aperture is configured such that the first arm 102 extends through the aperture when the first arm is supported within the upper jaw. The retaining portion 226 may support one of the guide structures 108 when the first arm is supported within the upper jaw,

Figure 8 shows the upper jaw 200 coupled with the lower jaw 300 and the strap 400 further inserted through two guide openings 108 of guide structure 106 on a second arm of the split-core assembly. The upper jaw 200 and the lower jaw 300 may be pivotally connected by the pivot 212 and hook, as shown in Figure 14.

Figure 10 shows a cross-section of the strap located through the clasp 312 and the latch 500 coupled with the clasp

The clasp 312 is held in place by the pawl 314 engaging with strap 400 teeth 414. The second end 406 of the strap may extend beyond the lower jaw 300.

Once the strap 400 has passed through an aperture 324 of the clasp and tightened, the second end of the strap 406 may be cut down to remove any excess length of strap.

Figure 11 shows the latch 500. The clasp 312, shown in Figure 9, includes a protrusion comprised of a concave surface 316 adjacent to a convex surface 317. The protrusion is located at a second end of the clasp 312. The concave surface 316 is located between a central region of the clasp, where the aperture is located 324, and the convex surface 317. The concave surface 316 is configured to receive a hook 502 on an end of the latch 500, shown in Figure 11 , distal from the latch connector 504, when the casing is in the closed configuration.

As shown in Figure 10, the clasp includes a pawl 314 that is configured to engage with the teeth of the strap 414 when the strap 400 is passed through the clasp aperture 324. The pawl 314 and/or the strap 414 may be configured such that the strap 414 can be passed through the clasp aperture 324 in a first direction only, such that once the strap 400 is tightened around the split-core assembly it does not come undone. The protrusion is monolithic or coupled to the pawl 314 such that, when the latch 500 is received by the concave surface 316, the force provided by the latch 500 on the protrusion cause the pawl 314 to be more tightly coupled with teeth 414 of the strap 400 thereby increasing the tension in the strap. The pawl may have teeth, and the teeth 414 of the strap may be coupled to one or more teeth on the pawl 314. Figure 12 shows the outside of the casing in the locked configuration. In this configuration, the cut surfaces of the opposing arms are in contact with each other, such that the magnetic core portion of the first arm and the magnetic core portion of the second arm together form a magnetic circuit which encircles the axis, A. The second bar 412 of the frame is located within the recess 602 of the outer portion of the upper jaw. This coupling between the latch 500 and the upper jaw provides a force such that the hook 502 of the latch 500 exerts a force on the clasp 312, this provides a further force on the strap 400 to tighten the strap and improve alignment of the split-core assembly.

The upper jaw includes an outer portion 600 that is sized to fit over the inner portion 200 of the upper jaw. The outer portion 600 may be fitted to the inner portion 200 after the strap has passed through the clasp and has been tightened. The outer portion 600 is configured such that only the latch 500 and the frame 408 are visible outside of the upper and lower jaws.

The outer portion 600 may be provided as a separate component to the inner portion 200 of the upper jaw. This improves the ease of fitting the casing to the split-core assembly as the strap can more be easily located within the aperture 204 and the guide openings 108. However, the outer portion 600 and the inner portion 200 may alternatively be a single, monolithic component.

The mechanism of moving the measurement device into the locked position, accommodates the manufacturing tolerances of the magnetic core cutting processes and ensures that the split-core assembly can be held together with a constant force.

In use, the current measurement device utilises the Fluxgate principle to measure current. The split-core assembly may be part of a fluxgate sensor. To increase the consistency of performance of the current measurement device, it is advantageous that the surfaces of the magnetic cores within the two arms align with a defined force.

This clamping force is provided via the strap 400. The sensitivity of the measurement device is dependent on the force holding the two arms together, with a minimum clamping force required for a desired sensitivity. In this example, clamping force is referred to as a radially inward force, and there is a linear relationship between the radially inward force and the tension of the strap. The strap 400 and the jaw portions 200, 300 of the casing, as well as the guide structures 106 on the split-core assembly ensure that all four core cut surfaces 110 are correctly aligned in the X & Y planes when the measurement device is in the closed and locked configurations, and prevents unwanted movement of the arms. Furthermore, the strap applies an equal radial force around the perimeter of the split-core such that an equal force is applied at the core surfaces 110 when the measurement device is in the locked configuration. This prevents stress points on the split-core assembly and the casing. The radially inward force is applied to the split-core assembly, while still allowing the jaw to be easily opened. Closing the jaw reapplies the force.

T o open the casing, the top of the latch (the end of the latch closest to the latch connector 504) may be pushed away from the upper jaw. This releases the tension applied on the strap 400. The latch may then be unclipped from the clasp 312 to allow the casing to fully open. The latch 500 may have a protrusion 506 extending beyond the latch connector 504 and operating as a handle. This aids a user in opening the casing.

In the open configuration, the strap 400 extends around the radially outer surface of the first arm 102 and the second arm 104, between the retaining portion 202 of the upper jaw and the clasp 312 of the lower jaw. The first arm 102 and the second 104 are not in contact with each other, such that there is a gap between the cut surfaces 110 of the opposing arms. The first frame bar 410 is located within the inner jaw recess 210. The second frame bar 412 is located within the recess 602 of the outer portion 600 of the upper jaw.

The split-core assembly may be manufactured by forming a single toroidal structure and then dividing the toroidal structure into a first arm 102 and a second arm 104 by cutting the toroidal structure across a plane shown as B-B’ in Figure 13, the plane including the axis, A as shown in Figure 14. The split-core assembly may also be manufactured as two separate laminated or sintered cores, each forming an arm of the split-core assembly. Each of the arms 102, 104 thereby includes two planar surfaces 110 which are configured to contact a respective surface of the other arm in the closed configuration. The hook 302 of the lower jaw 300 defines a channel which is generally tangential to the radially outer surface 112 of the second arm 104 when the second arm is located within the lower jaw 300. The channel, defined by the hook 302, is also generally tangential to the radially outer surface of the first arm 102 when the upper jaw 200 and the lower jaw 300 are in a closed configuration as shown in Figures 14 and 15. The pivot 212 of the upper jaw is able to move within the channel defined by the hook 302, such that the casing self-adjusts to compensate for the material removed during the cutting process so that the cut surfaces of the first arm and the cut surfaces of the second arm are in contact when the casing is in the closed configuration.

Figure 14 shows a cross-section of the device in a closed configuration. An adjustment to compensate for the minimal amount of material removed during the cutting process is provided by altering the position of the strap within the aperture of the clasp - this adjusts the strap length to fit the relatively larger split-core assembly.

Alternatively, when a larger amount of material is removed during the cutting process, adjustment to compensate for the larger thickness of material removed, is provided by altering the position of the strap within the aperture of the clasp. The strap has several teeth or ridges 414, that are spaced at increments that correspond to the teeth on the pawl 314. The strap may be pushed through the clasp further such that the pawl engages with teeth 414 further from the second end of the strap. This adjusts the length of the strap extending between the retaining portion of the upper jaw and the clasp of the lower jaw to suit the smaller or larger core assembly.

Figure 15 shows a side view of the casing in a locked configuration. In this embodiment the upper jaw and the lower jaw are configured such that the first arm and the second arm are coupled to form a magnetic circuit. The latch hook 502 is coupled with the concave surface of the clasp, and the second bar 412 of the frame is located within the outer jaw recess (as shown in Figure 14) to prevent the casing from opening.

The measurement device may be moved from the locked configuration to this closed but unlocked configuration by pushing the latch protrusion506. The elongate portion of the strap may be formed of a resilient material, such that the strap provides a force biasing the device towards an open configuration. This ensures that the measurement device opens if not placed into the locked configuration correctly. In this configuration, the second frame bar 412 is not located within the recess 602 of the outer portion 600 of the upper jaw, and the latch hook 502 is no longer coupled with the clasp 312. When the measurement device is not in the locked configuration, there is no opposing force to prevent the biasing force from the strap causing the device to move to the open configuration, as shown in Figure 16.

When unlatched and not in the locked configuration, this mechanism means that the jaws will self-open to give a clear indicator that the casing is not locked. This is a built-in self- opening feature that is provided by the strap and its inherent resistance to bending.

It will be appreciated that various modifications to the embodiments of the invention discussed above, without departing from the scope of the invention as defined by the claims, will be apparent to the person skilled in the art.

Reference Numerals

100 Split-core assembly 312 Clasp

102 First arm 314 Pawl

104 Second arm 316 Clasp concave surface

106 Guide structure 30 317 Clasp convex surface

108 Guide opening 318 Retaining surface

110 Cut surface 320 Aperture

112 Radially outer surface 324 Clasp aperture

114 Radially inner surface 400 Strap

200 Inner portion of upper jaw 35 402 Elongate portion

202 Retaining portion 404 First end of strap

204 Aperture 406 Second end of strap

206 Upper jaw sidewalls 408 Frame

208 Upper jaw central wall 410 First frame bar

210 Inner jaw recess 40 412 Second frame bar

212 Pivot 414 Strap teeth

216 Latch recess 416 Connector

218 Lower bar 418 Sidebar

220 Inner sidewall 420 Flange

222 Top bar 45 500 Latch

226 Retaining surface 502 Latch hook

300 Lower jaw 504 Latch connector

302 Hook 506 Latch protrusion

306 Lower jaw sidewalls 600 Outer portion of upper jaw

308 Lower jaw central wall 50 602 Outer jaw hinge recess