This invention relates to a residual current device.

Figures 1 and 2 show a permanent magnet actuator (also commonly referred to as a permanent magnet relay) comprising a generally U-shaped ferromagnetic yoke or frame 10 and a ferromagnetic armature 12 extending across the free (upper) ends of the arms of the frame 10. One end of the armature 12 is pivoted to the free end of one of the arms (the left hand arm in the drawings) , so that the armature can pivot between first and second positions wherein the armature is respectively closed against the other arm of the frame 10 (Figure 1) or spaced from it (Figure Z) . A permanent magnet 14 associated with the frame 10 causes the frame 10 and armature 12 to form a permanent magnetic circuit which is closed when the right hand end of the armature is against both arms of the U-shaped frame 10, Figure 1, and open when the right hand end of the armature is spaced from the right hand arm of the frame, Figure 2. A spring 16 resiliently biases the armature 12 towards the open position, Figure 2, and a plunger 18 is movable by the armature 12.

The principle of operation is as follows . The permanent magnet 14 produces a magnetic flux which permeates ferromagnetic material in contact with or in close proximity to it. Since the frame 10 and armature 12 are both made of ferromagnetic material, the magnetic flux is induced into these two elements to form a permanent magnetic circuit as described (instead of having a separate magnet 14, the frame and/or armature may itself be made as a permanent magnet) . When the armature is open, Figure 2, an external force applied to the plunger 18 can be used to move the free end of the armature 12 into contact with or into very close proximity to the right hand end of the frame 10 such that the magnetic flux

within the armature and frame will attract and hold the free end of the armature in the closed position substantially in contact with the right hand arm of the frame, Figure 1. The frame 10 and armature 12 now form a closed magnetic circuit with minimal air gaps, under which condition the flux density in the magnetic circuit and hence the holding force on the armature 12 will be maximised such that the armature will be held firmly against the right hand end of the frame ^' 10. against the bias of the spring 16, even after removal of the external force on the plunger 18. This is the latched or closed position of the actuator. An external force could be applied to the tip of the armature to prize it away from the frame. Measurement of the minimum external force required to separate the armature and frame is referred to as the net holding force in that any force exceeding this level would overcome the magnetic holding force and prize the free end of the armature away from the frame.

A solenoid bobbin and coil 20 surrounds the right hand arm of the frame 10, and any current flowing in the coil will generate an electromagnetic field within the coil. ^• This field will also induce a magnetic flux into the frame 10 and armature 12. If the polarity of the current is in a certain direction, the resultant electromagnetic flux will add to the permanent magnet flux such that the net holding force on the armature will be increased. Conversely, if the polarity of the current is reversed, the electromagnetic flux will oppose the permanent magnet flux, and the net holding force will be reduced. It follows that a current of a certain magnitude and polarity can be used to automatically release the armature 12 and force the plunger 18 away from the frame 10. When the armature 12 is in the closed position, the opening spring 16 is tensioned such that it exerts a stronger force on the armature than when the armature is in the open position. In effect, the tension on the spring when the armature is in the

closed position can be considered to be stored energy, with the actuator primed for actuation. When the net holding force is sufficiently reduced to allow the armature to be released, the stored energy in the spring 16 will move the armature from the closed position to the open position and in turn provide the plunger with mechanical energy to achieve a desired objective, for example displacement of a latch plate or trip lever in a circuit breaker to cause automatic opening of the circuit breaker. This is the actuated or tripped position of the actuator. The current through the coil to achieve automatic opening could be derived from a variety of sources, such as an AC or DC source, discharge of a capacitor voltage though the coil, etc.

In practice, actuators based on the above principle of operation are commonly used to achieve automatic opening of the contacts of a residual current device (RCD) and the majority of a classification of RCDs known as voltage independent RCDs (VI RCDs) are operated on this principle. The RCD has a current transformer (CT) comprising at least two primary windings and one secondary winding, and a current is induced into the secondary in response to a differential or residual current in the primary windings. This current is fed to the relay coil 20 to achieve automatic actuation of the relay when the residual current exceeds a predetermined level. A key characteristic of the VI RCD is that the residual current provides the only source of energy to cause automatic actuation of the relay.

Figures 3a and 3b show an example of a typical VI RCD circuit. The CT comprises a toroidal core with at least two supply conductors L, N (live and neutral) passing through the aperture of the core to form a primary winding, and a coil 22 wound around the core several times to form the secondary winding of the transformer. The supply conductors L, N are

connected across a load (not shown) . A permanent magnet actuator 24 is similar to that of Figures 1 and 2. Under normal conditions, the magnitude of the current flowing in each conductor L, N is the same, but because they are flowing in opposite directions through the CT, the two currents cancel with the result that there is no output produced in the secondary winding 22. In the event of an earth fault, some current will flow to earth, thereby resulting in a difference in the magnitude of the currents flowing in the load conductors L, N. The two currents will no longer cancel and there will be a net current flow in the primary winding which will produce a current in the secondary winding 22. The magnitude of the current flow in the secondary winding will depend on the magnitude of the differential current, the CT core size and material, and the number of primary and secondary turns. The primary winding can be passed round the core several times in a similar manner to the secondary winding so as to increase the ampere turns in the primary circuit for a given differential current. The output from the secondary winding is fed directly or indirectly to the coil 20 of the actuator 24 so as to cause automatic opening of the armature 12 and opening of the RCD contacts 26 in the event of the residual or fault current exceeding a certain magnitude for a certain period of time (Figure 3b) .

The electronic circuit 28 is optional and may be used for surge suppression or to delay the response of the circuit to a residual current and minimise problems of nuisance tripping, etc .

Because of the toroidal shape of the current transformer CT and the fact that it has primary and secondary windings, placing the windings on the CT is a very specialised process which does not lend itself to automation or high volume production. In addition, the relatively large diameter of the

supply conductors severely limits the numbers of primary winding turns that can be accommodated on the core, which in turn limits the primary ampere turns. To maximise the output energy from the secondary winding, the core material needs to have an extremely high relative permeability. All of these factors contribute towards the relatively high cost of a conventional toroidal CT as used in VI RCDs.

It is a key object of the invention to avoid or mitigate this disadvantage.

Accordingly, the present invention provides a residual current device comprising an electromagnet having a plurality of windings connected in series with respective supply conductors and a ferromagnetic element operably associated with the electromagnet such that upon the occurrence of a differential current in a given direction exceeding a predetermined level the resultant magnetic field produced by the windings causes movement of the element from a first position to a second position relative to the electromagnet to perform a predetermined function..

As used herein, the term "electromagnet" is intended to include any device in which an electromagnetic force of sufficient magnitude generated by the flow of current through one or more coil windings results in a mechanical displacement .

The advantage of the invention is that no separate current transformer CT is necessary. In the preferred embodiment the pre-determined function is the opening of contacts in the supply conductors, but it could be another function, such as raising an alarm.

The actuator shown in Figures 1, 2 and 3 is typical of the actuator widely used to trip VI RCDs. When actuated, the plunger is used to strike a trip lever or latch plate 30 and cause automatic opening of the RCD contacts 26. The actuator and trip lever must be mechanically coupled or aligned so that the plunger can displace the trip lever and cause reliable and effective tripping of the RCD under earth fault conditions. However, after tripping, the RCD must be reset to enable power to be supplied to the load again on removal of the fault that caused tripping in the first place. This not only requires re-closing of the RCD contacts, but also re-closing of the armature to reset the actuator.

The RCD reset switch is normally the only means to achieve re- closing of the actuator and re-closing of the contacts. If it were possible to re-close the contacts before re-closing the actuator, it would be possible for an earth fault current to flow without operation of the actuator, which would be an extremely dangerous and unacceptable situation. In addition, it is a general requirement of RCDs that it should not be possible to hold the contacts closed by means of the reset switch under an earth fault condition. This therefore requires that the actuator must be closed and primed to automatically open in the event of an earth fault and it must not be possible to prevent automatic opening of the contacts regardless of the condition of the reset button. In effect, the reset button must not be used to directly close the contacts, and the entire tripping mechanism must be free to trip under fault conditions.

Meeting all of these requirement in a conventional RCD requires the use of very sophisticated mechanical designs and precise coupling of the various elements of the actuator, trip lever or latch plate, contacts and reset button, etc. Thus the latching and de-latching mechanism in a conventional VI

RCD is very complex, and manufacturers have to go to considerable trouble to design, manufacture and assemble these parts so as to ensure reliable operation throughout the expected life of the RCD. In the conventional RCD using a permanent magnet actuator, the actuator causes automatic opening of the RCD contacts, but the actuator has to be re- closed independently of re-closing of the contacts. In effect, the actuator plays no part in the re-closing of the contacts, its role being confined to opening of the contacts.

It is an object of the preferred embodiment to provide a simpler arrangement of equal or superior reliability.

According to the preferred embodiment, therefore, the electromagnet comprises a second ferromagnetic element surrounded by the plurality of windings, wherein the first and second ferromagnetic elements form a substantially closed permanent magnetic circuit when the first ferromagnetic element is in the first position, and wherein the magnetic circuit is opened by movement of the first ferromagnetic element to the second position. In such embodiment the first ferromagnetic element is resiliently biased towards the second position but is held in the first position against the resilient bias by the second ferromagnetic element in the absence of the said differential current exceeding a predetermined level. The second ferromagnetic elements is movable against a further resilient bias towards the first ferromagnetic element when the latter is in the second position to re-close the magnetic circuit, release of the second ferromagnetic element entraining the first ferromagnetic element to draw the latter to the first position under the action of the further resilient bias.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 to 3 (previously described) are schematic diagrams of a prior art actuator-based RCD.

Figures 4 to 6 are schematic diagrams illustrating the principle of operation of a first embodiment of the invention.

Figure 7 is a schematic diagram of a first embodiment of the invention .

Figure 8 is a schematic diagram of a second embodiment of the invention.

Figure 9 is a schematic diagram of a third embodiment of the invention .

Figure 10 is a schematic diagram of a fourth embodiment of the invention.

Figure 11 is a schematic diagram of a fifth embodiment of the invention .

In the various figures the same reference numerals have been used for the same or equivalent components.

Figure 4 shows an arrangement of a permanent magnet actuator similar to that of Figure 1. In the arrangement of Figure 4, the solenoid bobbin and coil have been replaced by two windings, Wl and W2, having an equal number of turns, which surround respective arms of the U-shaped frame 10. The permanent magnet 14 induces a flux into the ferromagnetic frame and armature as before, enabling the armature to be held

in a closed position on the frame against the bias of the opening spring 16. The windings can be arranged such that a current of equal magnitude flowing in each winding will result in no net current flow through the magnetic circuit because of cancellation of the electromagnetic field in each winding. However, any difference in the current flow in the two windings will result in a net electromagnetic field being induced into the magnetic circuit. When that differential current exceeds a certain threshold, the holding force of the permanent magnet will be sufficiently weakened as to cause automatic opening of the armature.

By placing the windings Wl and W2 in series with the respective supply conductors L, N connected to the load, a differential or residual current of sufficient magnitude in the load circuit will cause automatic opening of the armature. This is the principle of operation of the embodiments of the present invention.

Windings Wl and W2 may be pre-wound and simply placed on to each arm of the frame 10 prior to placement of the armature 12. Because the windings are axially wound, it is possible for the windings to have a relatively large number of turns so as to maximise the ampere turns arising from a residual current .

The armature 12 can be coupled indirectly or, as shown in Figures 5 and 6, directly to one or more movable electrical contacts. In these figures a movable contact 32 carried by the armature 12 engages and bridges a pair of fixed contacts 34 in series with the winding W2 when the armature is in the closed position, as shown in Figure 5. However, when the armature moves to the open position, it moves the contact 32 away from the fixed contacts 34, as shown in Figure 6. A stop 36 can be placed above the movable contact 32 to limit the

open gap between the armature 12 and frame 10. It follows that the arrangement of Figures 5 and 6 can be used to cause the contacts 32, 34 to open in response to a residual current flow in windings Wl and W2.

It is necessary to re-close the contacts 32, 34 in order to reset the RCD after removal of the fault current. A means to do this is shown in the embodiment of Figure 7, in which the frame 10 is shown rotatable about a fixed pivot point 38 at one end of the frame (in the context of the present specification, "fixed" means fixed relative to the housing, circuit board, or other structure supporting the RCD actuator, such structure not being shown but exemplified in Figure 7 by stops 36 and 40 and spring anchor point 42) . A reset button 44 is mechanically coupled to the other end of the frame 10, and a reset spring 46 biases the reset button in a direction opposite to that of the armature movement. When the reset button 44 is pushed upwardly against the bias of the spring 46, the end of the frame 10 coupled to the reset button rotates counter-clockwise, as seen in Figure 7, until the upper end of its right hand arm engages or moves close to the armature 12. At this point the armature 12 becomes magnetically entrained to the frame 10, so that when the reset button is released, the frame 10 draws the armature 12 in a downward direction against the bias of the spring 16 until the moving contact 32 rests once more on the fixed contacts 34, thereby re-closing the circuit breaker. The reset spring 46 provides the contact pressure.

A possible disadvantage of the arrangement of Figure 7 is that one or both windings Wl, W2 may need to be movable so as to facilitate the movement of the frame 10, in which case the wires may act as a constraint or drag on the movement of the frame and impact to some extent on the forces and behaviour of

the actuator. This problem can be resolved by the arrangement of Figure 8.

In this embodiment the reset button 44 and reset spring 46 have been repositioned and mechanically coupled to the centre of the base of the U-shaped frame 10. Windings Wl and W2 are fixed in position, but with clearance above and below the windings, and between the windings and the frame, so as to allow the frame 10 to be moved bodily in an upward or downwards direction without interference from the windings which remain stationary relative to the supporting structure for the actuator. When the reset button is pressed upwards it lifts the entire actuator in the same direction, the clearance between the bottom of the windings and the frame allowing sufficient upward movement of the frame to close the frame 10 onto the armature 12. When the armature has been magnetically entrained, the reset button is released and the entire frame, with actuator entrained, moves in a downward direction independently of the windings until the contacts 32, 34 close. When the armature 12 is subsequently released due to a residual current flow in Wl and W2, the armature 12 reverts to the open position and the frame 10 is free to move downward towards the lower stop 40.

Figure 9 shows another embodiment wherein movable contacts 32 are carried at each end of the armature 12 and corresponding fixed contacts 34 placed in series with each winding. In this arrangement, the armature is designed as a horizontal beam across the top of the frame 10. The opening spring 16 is a compression spring positioned centrally within the actuator so as to bias the entire armature 12 as a whole away from and out of contact with the frame 10 in the open condition of the actuator. The permanent magnet 14 is in this case located on the armature so as to provide the permanent magnetic circuit and consequent magnetic holding force as in the previous

embodiments. When the reset button 44 is pressed upwards, the armature 12 is magnetically entrained to the frame 10, closing the magnetic circuit. When the reset button is released, the entire frame together with the entrained actuator moves in a downwards direction until the moving contacts 32 come to engage the fixed contacts 34 under the force of the ₍ reset spring 46. When a residual current of sufficient magnitude, polarity and duration flows through windings Wl and W2, the magnetic holding force is sufficiently reduced so as to allow the armature 12 to be moved to the open position under the force of the opening spring 16, the frame and reset button being free to move to a lower position against the stop 40 to indicate that the contacts have opened. The permanent magnet 14 can be positioned at any convenient place on the armature or frame, or could even be the armature and/or all or part of the frame, so as to achieve the desired effect.

The embodiments shown in Figures 7 to 9 will detect an AC residual current. However, when power control equipment is used in a circuit, the AC supply may be rectified with the result that a residual current could comprise of pulsating DC currents of only one polarity, either positive or negative. For one such polarity, the residual current will generate a magnetic field which will oppose that of the permanent magnet and cause automatic opening in response to the residual current. However, for a pulsating DC residual current of the opposite polarity, the resultant electromagnetic field will actually reinforce the magnetic field of the permanent magnet, and tripping of the RCD will not occur. It can be desirable in some installations to have protection against pulsating DC residual currents of either polarity. Detection of rectified AC can be achieved by various known means, but Figure 10 shows one example of how it can be achieved within the actuator- based RCD of the present embodiments.

In the embodiment of Figure 10, two windings W3 and W4 have been added to the frame 10. For residual currents of a certain polarity, the actuator will open automatically as described before. For residual currents of either polarity, but more particularly those of the opposite polarity, a current will be induced into W3, the output of which is fed to an electronic circuit 50 which comprises circuitry to charge a capacitor Cl. When the voltage on Cl exceeds a certain threshold an SCR will be turned on, effectively placing winding W4 directly across the capacitor such that the voltage on Cl will be discharged into W4. It can be arranged that for a residual current of certain polarity, magnitude and duration, the current discharged into W4 will cause automatic opening of the actuator.

Some RCDs also incorporate means to detect excessive load currents. The present embodiments also lend themselves readily to incorporating such capability, as demonstrated in the embodiment of Figure 11. The voltage across winding Wl will be proportional to the current flow through it, and by monitoring this voltage, it is possible to determine the current flow through the circuit. An electronic circuit 52 placed across Wl monitors the voltage across it, and when the load current exceeds a certain level, the voltage across Wl will also exceed a certain level. This voltage can be used to charge a capacitor Cl which in turn can be discharged via the SCR through winding W4 if the current in Wl exceeds a certain level for a certain period of time (i.e. the voltage on Cl exceeds a certain threshold) . When the voltage across Cl is discharged through W4, the armature will open automatically, disconnecting the supply from the load.

In the various embodiments shown, the armature is used to open and re-close the contacts. Such opening and closing may be achieved directly or indirectly by suitable means. In all

embodiments the forces of the springs and the strength of the permanent magnet can be calibrated by known means to ensure that the armature is automatically opened for a certain level of residual current under the various conditions described above .

The following summarises the advantages of the actuator-based RCDs of the present embodiments compared to the cnventional RCD.

Conventional RCD:

- Requires a separate CT. To ensure that the CT produces sufficient output to release the actuator, the CT tends to be bulky and expensive. The CT has to be coupled directly or indirectly to the actuator coil.

- Difficult to get multiple primary turns on to CT so as to maximise functional ampere turns.

- The actuator requires a separate coil for actuation.

- Mechanical coupling between the actuator and the circuit breaker elements requires a high degree of precision and complexity .

- Actuator cannot be used to reclose contacts.

- High cost due to need for CT, PM actuator, and mechanism complexity.

Actuator-based RCD:

- Does not require a CT, so CT problems obviated.

- Easy to provide multiple load conductor turns on armature so as to maximise functional ampere turns.

- Actuator uses load conductors for actuation.

- Mechanical coupling is simple, effective and inherently more reliable .

- Actuator used to reclose contacts.

- Low cost because no need for CT, and actuator design is simple.

The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.