IE20030211U1 - Residual current device - Google Patents

Abstract

ABSTRACT A residual current device comprises a solenoid having a plurality of windings (WLW2) connected in series with respective supply conductors L, N and a ferromagnetic element (12) operably associated with the solenoid. Upon the occurrence of a differential current exceeding a predetermined level a magnetic field is produced by the windings (WLW2) of sufficient strength to cause movement of the element (12)out of a first position. Movement of the element (12) may open contacts, (22.24) to disconnect the supply and/or raise an alarm.

Description

This invention relates to residual current devices.

A residual current device (RCD) is a device which can detect the flow of a differential, or residual, current. In a normal electrical installation, current flows from the supply to a load via one or more conductors and returns to the supply via one or more conductors. Such installations include but may not be limited to the following examples: — Single phase (phase and neutral) where the current flows from the supply to the load via the live phase conductor and returns to the supply via the neutral conductor.

— Two phase (phase and phase) where the current flows from the supply to the load via one phase conductor and returns to the supply via another phase conductor.

Three phase (phase 1, phase 2 and phase 3) where the current flows from the supply to the load and back to the supply via two or three of the phase conductors.

Four phase (3 phase and neutral) where the current flows from the supply to the load via one or more of the phase conductors and returns to the supply via the neutral Conductor.

Under normal conditions, the current flowing from the supply to the load will be of equal magnitude to the current flowing back to the supply. However, as these currents will be flowing in opposite directions, their vector sum will be zero. This zero vector sum value and relationship can be verified by passing all of the conductors through a current transformer and monitoring the output, which under normal supply conditions will be zero.

Under a fault condition, an additional current path will be established, for example through an impedance to earth. Such currents are known as differential or residual currents. Under such conditions, the vector sum of the currents in the load conductors will no longer be zero, and will have a value equal to the value of the residu uc currents ca e hazardous, and RCDs are used to detect such currents and usually to disconnect the supply to the load in the event of the residual current exceeding a predetermined value. The part of the RCD which effects disconnection of the supply is known as a circuit breaker. A similar residual current detecting device is known as a residual current monitor (RCM) which operates on the same basic principle, but is usually used to activate an alarm rather than cause disconnection of the supply. The term residual current device, or RCD, is used in the present specification to refer to both disconnection and monitoring type devices.

Detection of residual currents can be achieved in several ways, such as by the use of a current transformer (CT) or Hall effect sensors. In the case of the CT, the resultant output current tends to be used in one of two ways to activate the circuit breaker, as follows: (a) The CT output current is caused to flow through the coil of a permanent magnet relay which holds a movable armature in a first position against a bias spring such that the resultant magnetic flux induced into the relay coil by the CT current opposes the magnetic flux of the permanent magnet. When the net magnetic flux is sufficiently reduced due to the magnitude of the CT current, the movable armature moves from its initial position to a second position and this resultant mechanical displacement causes the circuit breaker to open. (b) The CT output current is fed to an electronic circuit which compares the magnitude of the output to a reference value and when the CT output exceeds the reference value the electronic circuit causes activation or deactivation of a solenoid or relay to achieve mechanical displacement so as to cause the circuit breaker to open.

The Hall effect sensor output is fed to an electronic circuit which compares its magnitude to a reference value and when the Hall effect sensor output exceeds the reference value the electronic circuit causes activation or deactivation of a solenoid or relay to achieve mechanical displacement so as to cause the circuit breaker to open.

RCDs which can detect AC current only are usually referred to as Type AC devices.

RCDs which can detect AC current and rectified AC current are usually referred to as Type A devices. RCDs which can detect AC current, rectified AC and DC currents are usually referred to as Type B devices.

Regardless of whether the RCD uses a CT or a Hall effect sensor to detect the residual current, dependency on the use of such components places a considerable financial burden on the RCD, making it substantially more expensive to produce than for example a conventional circuit breaker. It also gives rise to other problems such as complexity and reliability.

It is an object of the present invention to provide a residual current device by which the residual current in an electrical installation may be detected without the use of a dedicated current transformer, Hall effect sensor or other conventional means for detecting such currents.

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 exceeding a predetermined level the resultant magnetic field produced by the windings causes movement of the element out of a first position to perform a particular function.

There is also provided, as a further and separate invention, an electromechanical device comprising an electromagnet having at least one winding, a first ferromagnetic element operably associated with the electromagnet such that the occurrence of a net magnetic field produced by the winding(s) above a predetermined level causes movement of the first element out of a first position, a second ferromagnetic element, and means for holding the second element in a first position spaced from the first element, at least one of the first and second elements comprising a permanent magnet, wherein movement of the first element out of its first position reduces the spacing between the two elements so as to increase the magnetic attraction between them and thereby overcome the holding force on the second element so that the latter moves out of its first position.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a diagram of a conventional solenoid; Fig. 2 is a diagram of a solenoid with two windings; Fig. 3 is a circuit diagram of a typical single phase mains showing an earth fault; Fig. 4 is a circuit diagram of a first embodiment of the invention; Fig. 5 is a circuit diagram of a second embodiment of the invention; Fig. 6 is a circuit diagram of a third embodiment of the invention; Fig. 7 is a circuit diagram of a fourth embodiment of the invention in its set or latched state; Fig. 8 is a circuit diagram of the fourth embodiment in its tripped state; and Fig. 9 is a circuit diagram of a fifth embodiment of the invention.

Electromagnets normally operate on the principle of passing a current through a coil winding to generate an electromagnetic force which attracts and moves a ferromagnetic element from a first position to a second position in opposition to a biasing force which attempts to keep the ferromagnetic element in the first position. Examples of such electromagnets are solenoids, relays and contactors and similar devices which operate on the same basic principle.

An alternative form of electromagnet is an arrangement where a ferromagnetic element is held in a first position by the magnetic force of a permanent magnet acting against a biasing force which is trying to move the ferromagnetic element to a second position upon sufficient weakening of the net magnetic force acting on the element, such weakening of the permanent magnet force being achieved by passing a current through a coil winding on the electromagnet which generates an electromagnetic force which opposes the magnetic holding force of the permanent magnet sufficient to release the ferromagnetic element and allow it to be moved to the second position.

As used herein, the term "electromagnet" is intended to include both types referred to above, as well as any other 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.

It is widely accepted that the term solenoid means an electromechanical device used to achieve a mechanical displacement in response to the flow of an electric current through a coil winding, whereas the terms relay and contactor are generally applied to electromechanical devices intended to close or open electrical contacts in response to the flow of an electric current through a coil. However, in the absence of electrical contacts on the relay or contactor, the action of all three devices is essentially the same. For the purpose of this invention, therefore, no distinction between the terms solenoid, relay or contactor is intended, and each refers to an electromagnet as defined above.

Fig. 1 shows a typical solenoid. It comprises a coil winding W1 on a non—magnetic bobbin and a ferromagnetic plunger 12 slidably accommodated in the bobbin.

The plunger 12 is normally positioned partly outside the bobbin 10 and held in this first position by a biasing means such as a spring 14. When a current of sufficient magnitude is passed through the winding W the resultant electromagnetic force will cause the plunger 12 to be drawn into the bobbin to a second position (shown by the dashed line) against the force of the biasing means. This mechanical displacement of the plunger 12 can be used to perform a specific function. For example, it may be used to open or close a pair of contacts. lE030211 Fig. 2 shows the same solenoid with a second winding W2 on the same bobbin 10. The second winding W2 can be wound on top of the winding W1 as shown, or the two windings W1 and W2 can be wound end-to—end on the bobbin. When a current of sufficient magnitude is passed through either of the windings W1 or W2, the resultant electromagnetic force will cause the plunger 12 to be drawn into the solenoid body to the second position (shown in dashed lines) against the force of the biasing means 14. If the number of turns of each winding is made the same, each winding will produce an electromagnetic force of equal magnitude for equal current flow through each winding. If equal currents are caused to flow simultaneously through each winding, the direction of the current flow through each winding can be arranged such that the resultant electromagnetic fields combine to produce a net attracting force on the plunger. Conversely, if the direction of current flow in one winding is arranged to be in the opposite direction to that of the other winding, the two electromagnetic fields will oppose each other, resulting in zero net electromagnetic force acting on the plunger. It follows that under this condition any difference in the level of currents flowing through the two windings will produce a net electromagnetic force which will attract the plunger. If this differential current is of sufficient magnitude, the plunger will be displaced out of its first position.

A typical single phase electrical installation is shown in Fig 3. A mains supply has live L, neutral N and earth E conductors and is connected to a load 16 having a metal casin g 18.

In this arrangement the current IL flowing from the supply to the load in the live conductor L returns to the supply through the neutral conductor N as the current IN. If there is no earth fault current, IL = IN. However, if there is an earth fault condition 20, an earth fault (residual) current IF flows back to the supply via the earth return path, i.e. the earth conductor E, with the result that IL > IN.

If the live L and neutral N conductors are connected in series with the solenoid windings W1 and W2 respectively of Fig. 2, the result will be as shown in Fig. 4 (the biasing spring is present but not shown).

Under normal conditions with no earth fault (IL = IN) there will be no net magnetic flux acting on the plunger and the latter will remain in its first position under the biasing action of the spring 14. In the event of an earth fault (IL > IN) there will be a net magnetic flux acting on the plunger 12. If this net flux is above a certain threshold, corresponding to 11: being greater than some predetermined level, the plunger will be attracted by the magnetic field and displaced out of its first position against the bias of the spring 14. The resultant displacement of the plunger 12 can be used to activate a tripping mechanism to cause opening of circuit breaker contacts 22, 24 in the live and neutral conductors and perform the protective function of an RCD. This action will be readily understood by those familiar in the art of RCDs and circuit breakers, etc. The plunger movement could additionally or alternatively be used to activate an alarm and perform the warning function of an RCM. In both cases, the mechanical displacement of the plunger is used to enable the device to perform its end function, which in the case of the RCD is disconnection of the supply to the load, and in the case of the RCM it is actuation of an alarm. The mechanical displacement can be used directly or indirectly to achieve actuation of the device, and such actuation may be achieved through a pulling, pushing or rotary action.

Fig. 5 shows another embodiment of the invention which uses a solenoid fitted with a permanent magnet. In this arrangement the plunger 12 is held in a first position within the bobbin body by a permanent magnet 26 which has a holding force greater than the spring 14 (not shown) which is biasing the plunger in the direction out of the bobbin towards a second position indicated by the dashed lines. In the event of an earth fault current the resultant differential current IF will generate an electromagnetic force which will oppose the magnetic force of the permanent magnet, and when the differential current exceeds a certain level the plunger will be released by the permanent magnet. The resultant displacement of the plunger 12 towards the second position by the spring 14 can be used to trip circuit breaker contacts 22, 24 and/or activate an alarm as previously described.

The same functionality of mechanical displacement can also be achieved by using the configuration of a relay and moving armature as shown in Fig. 6. In this arrangement, the elcctromagnet is configured as a relay with a fixed pole piece 28, fixed ferromagnetic yoke or frame 30 and movable armature 32 which is biased by a spring 14 to a first (open) position and can move to a second (closed) position, shown in dashed lines, under the force of the electromagnet in response to a differential current I}: of sufficient level.

Alternatively, not shown, but analogous to fig. 5, the use of a permanent magnet with the pole piece 28 would enable the armature 32 to be held initially in the closed position against the biasing force of the spring 14, and when the magnetic holding force was sufficiently reduced by an electromagnetic force caused by a differential current Ip above a certain level, the armature would move to the open position.

In either arrangement, the mechanical displacement of the armature can be used to cause activation of a tripping mechanism in an RCD, and/or activation of an alarm in an RCM, both in known manner.

In the implementation of the present invention constraints may be placed on the design of the electromagnet due to limitations of space or the number of windings or cross sectional area of ferromagnetic parts, etc. Such constraints will result in a limited available magnetic force from the electromagnet for a given differential current with the result that the force exerted on the movable ferromagnetic element may not be sufficient to achieve the desired tripping action at a specified differential current. Whilst additional force can usually be provided by increasing the number of turns in the coil or by increasing the cross sectional area of the ferromagnetic element, the use of these and other conventional measures may not possible under certain circumstances. Figs. 7 and 8 show an embodiment of the invention having means for achieving the desired tripping action with limited magnetising force from the electromagnetic. In Figs. 7 and 8 the winding W2 has been omitted for clarity, although it is present.

In this embodiment a ferromagnetic plunger 12 is slidably accommodated for axial movement in a non—ferromagnetic housing 36 supported by a yoke or frame 38. The plunger 12 has a non—ferromagnetic pin 40 extending from one end, the free end of the pin 40 carrying a permanent magnet 42. A compression spring 44 normally biases the plunger 12 to the end of the housing 36 opposite the pin 40, as seen in Fig. 7, which is the set or latched state of the device. The device further includes a ferromagnetic latch plate 30 /E flgfigii pivotable about an axis 48. The top of the latch plate 46 is biased anticlockwise by a leaf spring 50 such that in the latched state of the device (Fig. 7) the bottom of the latch plate bears against a stop 52. The device also includes a trip lever 54. This is biased for rotation in an anticlockwise direction by a biasing spring (not shown), but in the latched state of the device the trip lever bears against the latch plate 46 so that such rotation is prevented.

Although in the latched state of the device the permanent magnet 42 exerts a magnetic attracting force on the latch plate, due to the size of the air gap between the magnet 42 and latch plate 46 this force is insufficient to de—latch the latch plate.

Under normal conditions with no earth fault (IL = IN) there will be no net magnetic flux acting on the plunger 12 and the latter will remain against the end of the housing 36 remote from the pin 40 under the biasing action of the spring 44. In the event of an earth fault (IL > IN) there will be a net magnetic flux acting on the plunger 12. If this net flux is above a certain threshold, corresponding to IF being greater than some predetermined level, the plunger will be attracted by the magnetic field and drawn into the electromagnet against the bias of the spring 44. The resultant displacement of the plunger 12 reduces the air gap between the permanent magnet 42 and the latch plate 46. When this gap is sufficiently reduced, the bottom of the latch plate 46 is drawn towards the permanent magnet 42 thereby releasing the trip lever 54 and tripping the device (Fig. 8). Rotation of the trip lever 54 disconnects the supply contacts 22, 24 in known manner. Alternatively or additionally, the displacement of the latch plate could be used to raise an alarm. Upon resetting, the plunger, plunger pin, latch plate and trip lever are restored to their initial positions as shown in Fig. 7.

With this arrangement, a relatively small magnetic force generated by the electromagnet can be used to provide a substantially greater force to achieve the tripping action. An additional advantage of this technique is that once the plunger starts its movement into the coil body, the reducing air gap between the permanent magnet and the latch plate increases the attracting force between these two parts, and this increased force reduces the effort required by the electromagnet to draw the plunger further into the coil.

[E0302], The plunger pin 40 may be indirectly coupled to the plunger 12 where this could prove advantageous. Indirect coupling of these two parts could be used to provide damping or to control the speed of movement of the permanent magnet towards the latch plate, etc. For example, the plunger pin could be operated by the plunger via a pivoted lever so as to magnify the movement of the plunger into a greater movement of the permanent magnet.

It will be understood that although shown attached to the plunger pin 40, the permanent magnet 42 may instead be provided on the bottom of the latch plate 46, or alternatively the entire latch plate may be a permanent magnet. In either case the element 42 on the plunger pin 40 can then be simply a piece of unmagnetised ferromagnetic material, although it can remain 21$ 8. p€I‘I’I1E1I1€I'1t magnet.

Any of the embodiments described above can be modified to operate according to the principle of Figs. 7 and 8 of using a reducing air gap between two ferromagnetic elements, of which at least one comprises a permanent magnet, to enhance the action of a relatively weak electromagnet. In fact, the principle is applicable to electromagnets generally, and is not limited to those responsive to differential currents in an RCD.

The above embodiments are representative only and the actual solenoid/relay design can be configured to maximise or optimise certain performance characteristics of the device without deviating from the basic principle of operation. Such design refinements could include provision of a frame to completely enclose the magnetic field and minimise stray flux, the use of two or more air gaps, positioning of the movable armature within the coil to produce benefits of compactness or improved efficiency, etc. These and other refinements could be accommodated provided that the mechanical displacement can be harnessed to achieve the desired actuating functions.

It is commonly known that the magnetic response of a relay or solenoid to a given DC current is different to that of an AC current of equal RMS value due to the bipolar nature of the AC current which takes the magnetic flux through a magnetic cycle during each cycle of the mains supply. This magnetic cycle is commonly represented by use of a hysteresis curve. The changing polarity of the AC current can give rise to problems of reduced IE 0302 efficiency and chatter, etc. These problems have been addressed by various techniques used in relay design over many years and will be familiar to those acquainted in the art.

For example, a copper shading ring or segment is often added to the pole piece of a relay to reduce or prevent chatter.

It is well known that the flow of Eddy currents in AC solenoids and relays reduces the efficiency of these devices in comparison to DC devices. Where reduced efficiency with AC currents could be problematic, this problem can be mitigated to a considerable extent by the use of laminations instead of a solid frame, pole piece or plunger.

A fail safe capability can be provided whereby failure of the movable armature or plunger to return to its first position for any reason during resetting of the RCD will make reclosing of the RCD impossible. Such reasons may include problems of stiction, magnetic The residual or differential current level at which an RCD is required to trip is known as its rated residual operating current, IAn, and in most RCD product standards there is a requirement that the RCD shall not trip at any level up to 0.5 mm, which is referred to as the rated residual non operating current. To comply with these requirements, it will be necessary to adjust or calibrate the RCD to ensure that the trip current level falls within the specified limits.

The RCD can be used for a range of load currents. It follows that as the intended load current requirement increases, the cross sectional area of the conductors will need to increase so as to avoid problems of over heating, etc. The RCD can also be used for single phase or multiphase applications, and it follows that for increasing load currents and for increasing phases, the bulk of the wire required for a given number of ampere turns will increase. This may not present a problem in applications where the additional wire capacity can be accommodated, but it may on occasions be desirable to use less wire whilst maintaining an overall level of performance of the RCD to differential currents. lE@3@2i‘i Calibration of the residual current operating level or reduction in the amount of wire used in the windings may be achieved in this RCD in a number of ways including but not limited to the following: — Alteration of the number of turns in the windings — Adjustment of the air gap(s) between the moving and fixed parts of the relay.

— Alteration of the magnetic circuit length.

- Alteration of the coil length.

— Alternation of the cross sectional area of any of the metal parts within the magnetic path.

- Alteration of the shape, form, or size of any of the metal parts within the magnetic path.

- Alteration of the material content or composition of any of the metal parts within the magnetic path.

— Removal of material from any of the metal parts within the magnetic path.

- Alteration of the magnetic properties of any of the component parts within or comprising the magnetic circuit.

Use of one or more of the above means or other suitable means can accommodate adjustment or calibration of the residual operating current or optimisation of the amount of wire that needs to be used in the RCD windings.

It will be understood that, in the foregoing embodiments, the differential current has to exceed the trip threshold for a sufficient period of time to actually trip the relevant device.

Although this period of time is very short (and will vary with the amount by which the differential current exceeds the trip threshold), it is not zero since inertia in the moving parts will mitigate against significant movement of the plunger or armature in the case of differential currents which only exceed the trip threshold for periods of extremely short duration.

Even so, it may still be desirable to control the response time of the electromagnet to differential currents, for example to avoid a fast response to a momentary differential current arising from an inrush current to a load, or to provide an inverse time current response where the RCD responds faster to larger magnitude differential currents and slower to lower value currents. Various technique are used to achieve this type of response and these may be applied to the device described herein. An example of such a technique, applied to the relay of Fig. 6, is shown in Fig. 9. In Fig. 9, the winding W2 has been omitted for clarity.

The ferromagnetic metal plunger 12 is in this case fitted into a non—ferromagnetic sealed tube 34 which is fixed in position inside the bobbin l0 and contains a viscous liquid such as oil. The tube 34 also contains a spring (not shown) which biases the plunger 12 towards the end of the tube 34 remote from the armature 32. When a net current of sufficient magnitude is passed through the windings, the plunger is drawn into the solenoid body towards the moving armature 32, thus reducing the air gap between these two ferrous metal parts. When the air gap is sufficiently reduced the armature will move from its initial open position towards the closed position. The liquid in the tube 34 provides a damping effect such that the movement of the plunger 12 will be delayed by a finite time for a given current through the coil, the delay being longer for low current levels and shorter for higher current levels. This arrangement can be used to provide a degree of immunity to nuisance tripping of the RCD in response to transient differential currents, or to provide a specific response characteristic to differential currents of various magnitudes.

Another technique that can be used to control the response characteristic of the device is to use magnetic material with a particular magnetic saturation characteristic. Below saturation, increasing current will cause a resultant increase in magnetising force.

However, once saturation occurs there will be little or no increase in magnetising force for increased current. Thus the response of the device to currents above a certain level can be made relatively uniform.

In the case of those configurations which do not use a permanent magnet, it follows that any electromagnetic force of sufficient magnitude will cause the desired mechanical displacement, and such electromagnetic forces can be derived from AC, rectified AC or IEOSOZ DC differential currents. The ability to detect this wide range of current types is an inherent feature of the design.

In the case of those designs using a permanent magnet, the differential current will only cause displacement of the armature when the resultant electromagnetic force opposes the magnetic force of the permanent magnet. In the case of an AC differential current, one half cycle will produce an opposing magnetic field whereas the other half cycle will produce a reinforcing field, therefore mechanical displacement will only occur in response to those half cycles producing an opposing magnetic field. In the case of rectified AC and DC currents, a unidirectional electromagnetic field will be produced. If this field is in opposition to the permanent magnet field, mechanical displacement will occur and if the field is a reinforcing field, displacement will not occur. Thus, the simple design shown is confined to AC Type operation only. However, it is known that relays and solenoids using permanent magnets can be configured in such a way as to produce two magnetic paths or circuits so as to enable them to respond to currents of either polarity. This can be achieved by use of two permanent magnets or by the use of a single permanent magnet providing two magnetic paths. These techniques can be applied to the permanent magnet embodiments of the present invention.

An increasingly common problem in electrical installations is that associated with noise being superimposed on the supply and its impact on appliances and equipment. Such noise can be generated by any equipment where control of power is applied, for example hair dryers, electric knives, power tools, etc. These appliances tend to generate radio frequency interference (RF 1) which can adversely effect equipment such as radios and TVs, etc.

Suppression of such RFI is highly desirable, and RFI filters are used to achieve this.

A known solution is to wind the supply conductors round a magnetic element such as ferrous metal, carbon or ferrite material, etc. so as to create an inductance in each leg of the supply. This inductance will have a negligible impedance at normal supply frequency but will have a significant impedance at RFI frequencies, which are typically in the kHz and MHZ range. The high impedance of the inductance has the effect of attenuating any RFI lEo302 that may be generated by the load and fed back to the supply, and attenuating supply borne RFI that could otherwise effect the load.

An examination of the above embodiments of the present invention shows that the windings W1 and W2 also constitute an inductance in each leg of the supply which will behave in a similar manner to RFI filters. Thus, an inherent feature of the present design is that it incorporates inductance in each leg of the supply which will attenuate RFI components originating on either the supply side or load side of the device. The value of this inductance can be adjusted by one or more of the means set out above for altering the magnetic properties of the device, with a view to optimising the RFI suppressing qualities of the device.

Conversely, the inherent high impedance of the inductance in each leg of the supply may preclude the use of the device in circuits where the mains wiring is used as a communications medium, for example mains signalling applications. This problem can be overcome by placing a capacitance in parallel with part or all of each winding such that the capacitor presents a low impedance to high frequency components, thereby allowing the passage of such signals.

Although the above embodiments have been described for the case of a single phase installation, the invention is equally applicable to other types. This is achieved by connecting the supply conductors in series with a like number of solenoid windings such that the net current flow in the windings for an earth fault current of a given level is sufficient to move the plunger or armature out of its first position sufficiently to disconnect the contacts and/or raise an alarm.

Also, in the foregoing embodiments, it has been assumed that movement of the plunger, armature or other ferromagnetic element out of the first position activates a tripping mechanism which disconnects the contacts 22, 24. It will be understood, however, that the invention applies in addition to embodiments wherein the ferromagnetic element, when in its first position, directly or indirectly holds the contacts closed against some bias tending lE0302 to open them, so that when the element is sufficiently displaced out of its first position the contacts are released and automatically open.

As mentioned above, problems of surge currents or surge voltages can give rise to nuisance tripping of RCDS. Various conventional means are used to provide RCDS with a degree of immunity to such disturbances, such as damping, suppression, choking, etc. Such methods can also be applied to the present invention. For example, in the ls‘, 2", 3rd and 5"‘ embodiments, the inertia of the moving member will provide a degree of immunity to such problems. Damping means as described above will also mitigate against such problems in these embodiments. Inductive chokes can be added on the supply or load side of the electromagnet to provide a degree of suppression in all embodiments. Capacitors can be connected in parallel with the electromagnet so as to shunt voltage or current spikes around the electromagnet again in all embodiments. Surge suppression devices, such as varistors or zeners or similar voltage clamping devices can be connected anywhere in the circuit so as to enhance the immunity of the devices according to the invention to nuisance tripping again in all embodiments.

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

Claims (12)

Claims

1.l. 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 exceeding a predetermined level the resultant magnetic field produced by the windings causes movement of the element out of a first position to perform a particular function.

2. A residual current device as claimed in claim 1, comprising a second ferromagnetic element and means for holding the second element in a first position spaced from the first ferromagnetic element, at least one of the first and second elements comprising a permanent magnet, wherein movement of the first element out of its first position reduces the spacing between the two elements so as to increase the magnetic attraction between them and thereby overcome the holding force on the second element so that the latter moves out of its first position, the movement of the second element performing the said function.

3. A residual current device as claimed in claim 1 or 2 wherein the ferromagnetic element is normally biased into the first position but is moved against the bias out of the first position towards a second position upon the occurrence of the said magnetic field.

4. A residual current device as claimed in claim 1 or 2 wherein the ferromagnetic element is normally held in the first position by a permanent magnet against a bias tending to move the element to a second position, the said magnetic field acting in opposition to the permanent magnet to allow the bias to move the ferromagnetic element out of the first position.

5. A residual current device as claimed in claim 3 or 4 wherein the ferromagnetic element is a plunger slidable in the electromagnet.

6. A residual current device as claimed in claim 3 or 4 wherein the ferromagnetic element is an armature closable against one end of the electromagnet.

7. A residual current device as claimed in claim 3 or 4 wherein the ferromagnetic element is an armature closable against a ferromagnetic yoke or frame.

8. A residual current device as claimed in any preceding claim, wherein the movement of one or more ferromagnetic elements of the electromagnet is damped.

9. A residual current device as claimed in any preceding claim, wherein magnetic saturation of any ferromagnetic element of the electromagnet is used to control the response of the electromagnet to a differential current.

10. A residual current device as claimed in any preceding claim, wherein the said function includes the disconnection of the supply.

11. A residual current device as claimed in any preceding claim, wherein the said function includes the raising of an alarm.

12. An electromechanical device comprising an electromagnet having at least one winding, a first ferromagnetic element operably associated with the electromagnet such that the occurrence of a net magnetic field produced by the winding(s) above a predetermined level causes movement of the first element out of a first position, a second ferromagnetic element, and means for holding the second element in a first position spaced from the first element, at least one of the first and second elements comprising a permanent magnet, wherein movement of the first element out of its first position reduces the spacing between the two elements so as to increase the magnetic attraction between them and thereby overcome the holding force on the second element so that the latter moves out of its first position.