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Patent Searching and Data


Title:
CONTACTORS
Document Type and Number:
WIPO Patent Application WO/2003/049129
Kind Code:
A1
Abstract:
A two-pole contactor includes a contact assembly (24) for making and breaking contact between a pair of fixed contacts (8, 10) at each pole. The contact assembly is a plastics moulding carrying a resiliently supported shorting bar and is coupled to a plunger of a solenoid. In order to make a contact, the plunger accelerates the contact assembly towards the fixed contacts, until the contact bar touches the fixed contacts. The plunger and the remainder of the contact assembly then continues to move through a predetermined distance, loading the resiliently supported shorting bar against the fixed contacts. To break contact, the procedure is reversed. The solenoid has two plungers slidable within a common bore and separated by a compression spring. Permanent magnets situated at each end of the solenoid core induce an attractive force between the plungers. When the separation between the plungers is smaller than a threshold separation, the magnetic attraction overcomes the spring force and the plungers latch together. When the separation between the plungers is greater than the threshold separation, the force exerted by the spring is greater than the magnetic attraction and so the plungers de-latch in a separated position, in which the contacts at both poles of the contactor are open.

Inventors:
CONNELL RICHARD ANTHONY (GB)
Application Number:
PCT/GB2002/005332
Publication Date:
June 12, 2003
Filing Date:
November 27, 2002
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
BLP COMPONENTS LTD (GB)
CONNELL RICHARD ANTHONY (GB)
International Classes:
H01F7/16; H01H50/54; H01H51/22; H01F7/122; H01F7/124; H01H1/22; H01H1/54; (IPC1-7): H01H51/22; H01F7/16; H01H50/54
Domestic Patent References:
WO1990005374A11990-05-17
WO1998040898A21998-09-17
Foreign References:
US5652558A1997-07-29
EP0373271A11990-06-20
US4430579A1984-02-07
US3272949A1966-09-13
DE2733800A11979-02-15
GB816636A1959-07-15
DE966034C1957-07-04
US4388535A1983-06-14
Attorney, Agent or Firm:
Goodman, Simon John Nye (Reddie & Grose 16 Theobalds Road London WC1X 8PL, GB)
Download PDF:
Claims:
Claims
1. An electromagnetic actuator comprising; two plungers which are movable relative to one another to vary the spacing between them; a biassing means for urging the plungers away from each other; and a magnetic latch means for causing a continuous magnetic attraction between the plungers., at least one of the magnetic attraction force and the force exerted by the biassing means varying with the separation between the plungers such that when the separation is less than a threshold separation the magnetic attraction force is greater than the force exerted by the biassing means, and when the separation is greater than the threshold separation the force exerted by the biassing means is greater than the magnetic attraction force.
2. An actuator according to claim 1, in which the magnetic latch means acting in combination with the biassing means is effective to latch the plungers at either a separation greater than the threshold separation or at a spacing less than the threshold separation.
3. An actuator according to claim 1 or 2, in which the magnetic latch means comprises a permanent magnet.
4. An actuator according to any of claims 1 to 3, further comprising; a first magnetic switching means for temporarily causing an increased magnetic attraction between the plungers so as to move them from a separation greater than the threshold separation to a separation less than the threshold separation.
5. An actuator according to any of claims 1 to 4, further comprising; a second magnetic switching means for temporarily causing a decreased magnetic attraction between the plungers so as to move them, or allow the force exerted by the biassing means to move them, from a separation less than the threshold separation to a separation greater than the threshold separation.
6. An actuator according to claim 4, in which the first magnetic switching means comprises a temporarily energisable coil.
7. An actuator according to claim 5, in which the second magnetic switching means comprises a temporarily energisable coil.
8. An actuator according to claim 7, in which the first and second magnetic switching means comprise the same temporarily energisable coil, drivable with opposite current directions in each case.
9. An actuator according to any of claims 1 to 8, implemented as a solenoid in which the plungers are slidable in a common bore.
10. An actuator according to claim 9, in which the biassing means comprises a compression spring located within the bore between the plungers.
11. An electrical contactor comprising; a pair of fixed contacts spaced from one another; and a contact assembly drivable between a contactopen position and a contactclosed position; in which the contact assembly comprises; a shorting bar for bridging the pair of fixed contacts when the contact assembly is in the contact closed position; and a biassing means for urging the shorting bar into contact with the fixed contacts when the contact assembly is in the contactclosed position.
12. A contactor according to claim 11, comprising an actuator coupled to the contact assembly for driving the contact assembly between the contactopen position and the contactclosed position.
13. A contactor according to claim 12, in which the actuator is a solenoid.
14. A contactor according to any preceding claim, in which the shorting bar is shaped to provide a"blowon" geometry.
15. A contactor according to claim 11, in which the contact assembly comprises a housing, the shorting bar being movably retained by the housing for motion in a direction substantially parallel to a direction of motion of the contact assembly towards the contactclosed position.
16. A contactor according to claim 15, in which the biassing means acts between the housing and the shorting bar and optionally comprises a spring.
17. A contactor according to any preceding claim, in which the shorting bar is one of a plurality of shorting bars.
18. A contactor according to any preceding claim, in which the pair of fixed contacts is one of a plurality of pairs of fixed contacts which are bridged by the contact assembly when in its contactclosed position.
19. A contactor according to any of claims 11 to 18, in which when the contact assembly travels from the contact open position to the contactclosed position it is freely movable for a first portion of its travel until the shorting bar contacts the pair of fixed contacts, and then during a last portion of its travel the shorting bar is impeded by the pair of fixed contacts, the resulting motion of the remainder of the contact assembly relative to the shorting bar causing the biassing means to urge the shorting bar against the pair of fixed contacts.
20. A contactor according to any of claims 11 to 19, in which the contact assembly is coupled to a drive element of an actuator, and in which when the drive element moves so as to drive the contact assembly from the contactopen position to the contactclosed position the drive element is freely movable for a first portion of its travel until the shorting bar contacts the pair of fixed contacts, and then during a last portion of its travel the shorting bar is impeded by the pair of fixed contacts, the further motion of the drive element relative to the shorting bar causing the biassing means to urge the shorting bar against the pair of fixed contacts.
21. A contactor according to any of claims 11 to 20, comprising two or more contact assemblies for bridging a corresponding number of different pairs of fixed contacts.
22. A contactor according to claim 21, in which two contact assemblies are driven by a common actuator.
23. A contactor according to claim 22, in which the common actuator is a solenoid having two plungers, one coupled to each contact assembly.
24. A contactor according to any of claims 11 to 23, actuated by an actuator as defined in any of claims 1 to 10.
25. A method for openably closing a contact between a pair of fixed contacts in a contactor, comprising the step of; moving a contact assembly towards the fixed contacts, the contact assembly comprising resiliently biassed shorting bar; such that the contact assembly moves freely during a first portion of its motion, until the shorting bar contacts and bridges the fixed contacts, and such that during a subsequent second portion of the motion of the contact assembly the shorting bar is restrained by its contact with the fixed contacts while the remainder of the contact assembly advances relative to it, increasing a biassing force between the shorting bar and the fixed contacts.
26. A method for switchably latching an electromagnetic actuator or a solenoid comprising two plungers which are relatively movable to vary the separation between them, the method comprising the steps of; resiliently biassing the plungers to produce a biassing force acting to increase the separation between them; and causing a magnetic attraction between the plungers, at least one of the magnetic attraction force and the force exerted by the biassing means varying with the separation between the plungers such that when the separation is less than a threshold separation the magnetic attraction force is greater than the force exerted by the biassing means, and when the separation is greater than the threshold separation the force exerted by the biassing means is greater than the magnetic attraction force.
27. A method according to claim 26, further comprising the step of switching the actuator or solenoid between its latched positions by temporarily causing an additional magnetic force, or temporarily varying the magnetic force, between the plungers to move the plungers from a separation less than the threshold separation to a separation greater than the threshold separation, or to move the plungers from a separation greater than the threshold separation to a separation less than the threshold separation.
Description:
Contactors The invention relates in its various aspects to a contactor and a method for making a contact, to an electromagnetic actuator such as a solenoid, and to a method for operating an electromagnetic actuator.

Introduction and prior-art : existing, metering (load- disconnect) contactor designs.

In recent years, there has been a growing demand in numerous domestic and industrial applications for compact, modular, medium-to-high-current contactors, especially for switching electric loads at 240 V AC. The drives for these are mainly derived from local or remote control circuits, with the option of manual re-connection, for safety reasons. Various means of actuation have been used for performing the switching function; from permanently energised solenoids (with the convenience of automatic power supply failure, safety drop-out) and magnet-latching solenoids, to elaborate motor drives and mechanisms of varying complexity and cost.

GB 2 287 831 (Ampy Automation Ltd) is a good example of a simple metering load-disconnect contactor, using a relatively cheap DC motor, a single flexi-blade and heavy- duty terminals for load switching. The motor is stall- driven for a half rotation between two stops, this motion being translated to the switching blade. GB 2 154 371 (Bonar Bray Ltd) illustrates a different type of rotary- magnetic actuator, in which a flexi-blade pivots (see- saws) in a vertical plane for the switching function.

Another example of a rotary-magnetic actuator is GB 2 280 063 (W Gruner GmbH Relaisfabrik), in which a vertical-axis rotary (see-saw) motion is translated

linearly sideways to actuate a flexi-blade. EP 0 532 586 (BLP Ltd) illustrates a contactor which employs a conventional magnet-latching solenoid to actuate a single flexi-blade via a pivoted translating arm, for performing the switching function.

One large application area for these types of high-current contactors is in domestic electric metering, for load- disconnection in pre-payment systems and for load-shedding or maximum-demand energy-control (zoning) systems. They most commonly use pulsed magnet-latching solenoids or rotary actuators for performing the switching function, both of which advantageously consume no quiescent coil power, and hence run cooler. Coil energisation is usually at low voltage DC for a short duration (typically 50-100 mSec), or at 240 V AC mains half-wave pulsing of both polarities. Metered current is commonly supplied at 60,80 or 100 Amp for single-phase, or 200 Amp maximum for 2-phase North American metering applications, as detailed later.

The solenoid/actuators used tend to be of conventional design, comprising a ferrous U-frame with a pair of strong permanent magnets, fixed ferrous stop and moving plunger, linked via an insulated lever-arm to a switching blade system. Actuation is effected by driving a single or dual coil in the solenoid bobbin, which produces the necessary latching and closure force for translation to the switch.

Single coil types may be driven via a reversing H-bridge circuit at low voltage, whereas dual-coil versions may be pulsed separately with the same polarity DC voltage via transistors conducting to the supply OV common line, to perform the necessary Closing and Opening load-switching function.

Existing single-phase (non shortina-bar) metering contactors.

In single-phase meters, the built-in load switching function is typically performed by the use of heavy-duty contactors, comprising robust terminals and copper flexi- blades fitted with suitable silver-alloy contacts. These are rated not only for the nominal load switching function but also have to withstand large overload fault currents.

Generally, the low-voltage drive side is safely isolated from the high-voltage load-switching side by suitable plastic barriers integrated within the contactor casing.

At a current of 100 Amps for example, in the event of a fault a contactor within a meter must safely satisfy a particular meter Specification requirement, such as carrying 30 x nominal current, ie. 3000 Amps rms, or 4250 Amp peak. This requirement is derived from conventional metering Specifications (pre-dating the introduction of built-in load-disconnect switches) and is related to a requirement for the statutory measuring accuracy of the meter not being made worse, especially after a short-circuit current fault. All meter components, including the built-in contactor, must comply with this requirement and other safety-related aspects.

The contactor in a meter generally has to be designed in such a way that, at large fault currents, the solenoid and switch remain closed while a protective fuse blows or a circuit-breaker opens safely. Under high-current short- circuit fault conditions, the large magnetic fields generated by the switch parts must not influence the latched state of the solenoid. If the fault field influence links adversely with the latched solenoid, the switch might open spuriously at the worst possible time,

creating welded contacts or, worse still, destroy itself by the excessive opening-arc energy created at 240 V AC.

Short-circuit fault currents can be very large.

A good example of an enhanced contactor blade layout is illustrated in GB 2 295 726 (BLP Ltd). This uses to good advantage the electro-dynamic forces generated between a fixed flexi-blade and feed bus-bar structure (due to the opposing currents) providing a so-called"blow-on"TM force. This counteracts the contact repulsion force, thus giving a more consistent contact pressure, especially at high fault-current conditions. It also provides reliable, consistent switching and consequently longer contact life.

Existing 2-phase metering contactors (for North America): a rationale Existing meters, as used in North America, are fed from a 2-phase Utility transformer, each phase being 115 V with- respect to a centre-tap Neutral connection. Heavy loads like air-conditioners, heaters and dryers etc. are fed directly across both supply phases at 230 V AC, light loads being fed from fused sockets wired from either phase at 115 V. Thus, the power load-disconnect function requires two normally-open (N/0) ganged switches, each rated at 200 Amps. At a minimum level, the typical meter specification requires that each switch must survive 6,000 Amp rms or 8,500 Amp peak for 6 cycles, with no contact welding, and normal operation afterwards.

In some metering applications, the Specification requirements are even more demanding; in such cases, for example, contactors may be required to withstand faults up to 12,000 Amp rms, or 17,000 Amp peak. In this context,

welds may be permitted but the entire structure of the switches and actuators must be intact, un-damaged and, most importantly, fail"safe"from the point of view of the customer/end-user, or adjacent circuits and systems.

US 4 388 534 and US 4 430 579 (Automatic Switch Company) are good examples of early 2-pole, load disconnect contactors as used in metering applications in North America. These use a powerful solenoid with a 2 kWatt coil and robust mechanical latching (as opposed to magnetic latching) for actuating a simple N/O twin (bifurcated) shorting-bar switch on each pole. Strong compression springs on each switch provide adequate contact pressure, overcoming the large repulsion forces generated during short-circuit faults, thus preventing opening, arcing or welding. Open actuation is against these strong compression springs, which puts considerable burden on the solenoid drive.

US 6 292 075 (BLP Components Ltd) is an improvement on the two US patents mentioned above. It employs two mirror- image, feed bus-bar/flexi-blade systems, which use to good effect the electro-dynamic blow-on forces referred to in GB 2 295 726 (BLP Ltd) previously. It also employs an improved form of U-frame magnet-latching solenoid, which actuates the two N/0 twin (bifurcated) blade sets via strong leaf springs for developing the necessary contact pressure. Coil drive requirements are considerably lower, only in the order of 30-40 Watts (being magnet-latching) compared with 2 kWatt in the cited US examples (being mechanical-latching). Figure 1 herein, and versions A and B illustrate the basic"blow-on"enhanced, twin (bifurcated) flexi-blade layout arrangement.

As might be expected, the existing 2-pole layout and flexi contact-blade design is much more complex and critical than that for the single-phase, single-bladed versions.

At higher withstand fault levels, the solenoid/actuator and switches must remain closed and the much larger interfering magnetic fields and contact repulsion forces generated must not influence the latched state of the solenoid. The solenoid must. have adequate resultant hold/retention under the worst load-fault condition.

For these situations, the solenoid in US 6292075 has been carefully shrouded, so as to prevent the large interfering magnetic fields from coupling with the drive coil (s) or plunger; if not, the plunger, being spring-loaded, might de-latch at the most critical and undesirable time.

Hence, 4-sided (shrouded) U-frames are required as an alternative to simple U-frames as used in less critical applications, as are the strongest rare-earth magnets, to provide the highest possible plunger hold-retention.

Blade design and layout is really critical for reliable operation. As mentioned, in existing BLP patented designs innovative, fixed folded-back"blow-on"blades have been optimised so as to withstand the large fault currents concerned. Enhanced electro-dynamic blade forces are developed which are proportional to the square of the load through-current, which are arranged to cancel out the similarly large repulsion forces generated by the switch contacts. Bifurcated blades and contacts reduce these forces to a more manageable level, putting considerably less burden on the solenoid drive capability and thus maximising the potential hold/retention required for reliable latching.

Problems have arisen, however, with the present designs which use bifurcated, folded-back"blow-on"flexi-blades and shrouded-frame solenoid constructions. Their layout occupies considerable space within the meter envelope (i. e. within the space available for a domestic or utility electricity meter) and it is desirable to reduce contactor size in order to reduce meter size. In addition, the layout of these contactors results in a relatively high switch resistance because of the long"blow-on"copper flexi-blade runs involved (see Figure 1). This generates undesirable self-heating even at a nominal rated current of 200 Amp, the heating being exacerbated by the confinement of the contactor within the restricted casing of a meter. In many cases, as a result of this problem the inclusion of the contactor within the meter envelope has resulted in the maximum current deliverable to be undesirably down-rated to only 160 Amp, instead of the intended Utility supply"badge"rating of 200 Amps.

The limitations in all the present designs referred to here illustrate problems in designing contactors, and particularly in designing a 2-pole load-disconnect contactor. This latter type of contactor not only applies to 2-phase metering applications as in North America but can also be for more general use, for example in a single- phase"safety"isolator switch for disconnecting both the Line (live) and Neutral connections; both configurations are identical as far as the two separate isolation- switches are concerned.

Summary of the Invention The invention provides a contactor and a method for making a contact, and an electromagnetic actuator and a method for operating an electromagnetic actuator, as defined in

the appended independent claims. Preferred or advantageous features of the invention are set out in dependent subclaims.

In a first aspect, the invention may thus advantageously provide a contactor comprising a contact assembly which is movable between a contact-closed, or make, position and a contact-open, or break, position. The contact assembly includes a resilient or resiliently supported shorting bar which bridges a pair of fixed contacts of the contactor when the contact assembly is in the make position.

In a preferred embodiment, the switching action of the contactor is as follows. The contact assembly is driven by a drive element of an actuator, such as a solenoid plunger. As the contact assembly starts to move away from its break position, it initially moves freely, until the shorting bar comes into contact with the pair of fixed contacts. Because the shorting bar is resiliently supported, the other portions of the contact assembly can then move further until reaching a predetermined make position in which the shorting bar is not only in contact with the fixed contacts but is loaded against them with a predetermined force by the resilient support. Thus, as the contact assembly move through the latter part of its travel it progressively loads the resilient support until the predetermined contact force between the shorting bar and the fixed contacts is achieved. The contact assembly is then retained in this make position until it is required to break or open the contact. The contact assembly is then driven in an opposite direction, initially reducing the contact force as the resilient support is progressively unloaded, and then carrying the shorting bar away from the fixed contacts until the contact assembly reaches its break position.

In alternative embodiments, the resilient support of the shorting bar may be implemented in various ways. For example the contact assembly may be rigidly coupled to a drive element of an actuator and incorporate a housing within which the shorting bar movably retained, being for example slidably retained. A spring or other bias means may then act between the housing and the shorting bar to provide the resilient support. In an alternative structure, the housing may again be rigidly coupled to a drive element but a portion, such as a central portion, of the shorting bar may be rigidly retained by the housing.

The shorting bar in this embodiment may then be flexible in order to provide the resilient support of the portion (s) of the shorting bar which contact the fixed contacts. In a further alternative the housing retaining or supporting the shorting bar may itself be resiliently coupled to the drive element. In such a structure the shorting bar may be rigidly supported within or by the housing. Further alternative embodiments of the invention may be envisaged, as long as the steps are enabled of providing initial free movement of the shorting bar towards and into contact with the fixed contacts followed by continued motion of the actuator drive element, which is resiliently coupled to the shorting bar to load the shorting bar against the fixed contacts.

The switching action of the contactor in the first aspect of the invention may thus achieve a number of advantages including the following.

It may achieve an advantageously high switching speed, particularly in terms of the speed at which the shorting bar approaches or leaves the fixed contacts during the make or break operations. When the contact is made, the initial free movement of the contact assembly allows it to

accelerate to an advantageously high speed before the shorting bar touches the fixed contacts. During this free movement, the contact assembly including the shorting bar can advantageously be directly driven by an actuator and it can therefore be driven at any arbitrarily high speed depending only on the actuator design. By contrast, in prior art contactors using shorting bars, a shorting bar is typically accelerated into contact with fixed contacts only by a spring acting between the shorting bar and a fixed support, and is not driven into contact by an actuator. The force exerted by the spring on the prior art shorting bar is limited by the required contact load when the contact is ultimately made, and therefore the acceleration and maximum speed of the shorting bar towards the fixed contacts is limited by the spring force and the mass of the shorting bar. In the contactor embodying the invention described above, higher shorting bar speeds may be achieved because the motion of the shorting bar towards the fixed contacts need not be determined by the contact force which is ultimately required.

A further advantage is that the contact load when the contact is made is determined by the deflection of the resilient support between the actuator drive element and the shorting bar, and preferably between the housing (rigidly coupled to the drive element) and the shorting bar. The contact load can therefore be determined by the final position of the actuator drive element, and the spring rate (or equivalent parameter if the shorting bar is not supported by a spring) of the resilient support.

In a preferred embodiment, the actuator may comprise a solenoid and the contact assembly can be coupled to a plunger of the solenoid. This may advantageously achieve

a very simple contactor geometry and high-speed controllable switching.

In further embodiments, various shorting bar geometries may be implemented within the contact assembly. In a simple structure, a shorting bar carrying contacts facing the fixed contacts may be resiliently supported by a spring, such as a leaf spring, and may be slidably movable within guides formed in the contact assembly or housing.

There are advantages in designing a contactor to use two or more shorting bars to connect in parallel between either large fixed contacts common to all of the shorting bars or between multiple contacts, such as one pair of fixed contacts for each shorting bar. In a preferred embodiment of the invention, such geometries can easily be implemented by locating two or more shorting bars within a single housing. Each may be individually resiliently supported or a common support may be used for some or all of the shorting bars.

In a preferred embodiment, as many as six parallel shorting bars may be used. In addition, one or more of such a set of multiple shorting bars may be arranged to lead other bar (s) in the set, so as to make and break contact with the corresponding fixed contacts respectively earlier and later than the other shorting bar (s) in the set and to provide"sacrificial"contacts to bear the brunt of any arcing on making or breaking contact.

There may also be advantage in using the"blow-on" geometry described in GB 2295726, which is incorporated herein by reference in its entirety. The"blow-on" geometry may be implemented in a further embodiment of the invention by designing the shape of the or each shorting

bar within the housing into a generally C shape. Such a shorting bar would carry its contacts on its outer face at each end of the C-shape, the curved portions at each end of the C shape providing a"blow-on"force to increase the contact load in the event of a high current flow through the contactor, such as in the event of a fault.

In a second aspect, the invention may advantageously provide an electromagnetic actuator which, as well as being suited to a wide range of other applications, may find particular application as an actuator in combination with the contactor of the first aspect described above.

In its second aspect, the invention may thus advantageously provide an actuator having two plungers which are moveable relative to one another to vary the spacing between them. In a particularly preferred embodiment, the actuator may be a solenoid and the two plungers slidable within a common bore within a core of the solenoid. The actuator further comprises a biassing means for urging the plungers away from each other, to increase the separation between them, and a magnetic latch means for causing a magnetic attraction between the plungers, tending to pull them towards each other. The forces exerted on the plungers by the biassing means and the magnetic latch means vary with the separation between the plungers such that when the plungers are close together, the magnetic attraction is greater than the force exerted by the biassing means and when the plungers are further apart, such as beyond a threshold separation, the magnetic attraction is less than the force exerted by the biassing means. This result may be achieved, depending on the design of the actuator, if either or both of the forces exerted by the biassing means and the

magnetic latch means varies with the separation between the plungers in an appropriate manner.

The effect of this structure is to provide an actuator which is latchable in either of two positions, either with the separation between the plungers being less than the threshold separation or the separation between the plungers being greater than the threshold separation.

In a preferred embodiment, the magnetic latch means may comprise a permanent magnet, or an arrangement of permanent magnets, either mounted at fixed positions or carried by one or both of the plungers. Alternatively, the magnetic latch means may comprise a coil for generating a field electromagnetically.

The plungers may be moved between the latching positions in any appropriate manner, including being moved mechanically, but in a preferred embodiment they may be moved electromagnetically. To achieve this, the actuator can be designed such that a temporary magnetic field can be applied to cause an increased magnetic attraction between the plungers, so as to move them from a separation greater than the threshold separation to a smaller separation. When the temporary magnetic field is removed, the plungers latch in the new closed position.

In addition, provision may be made to apply a temporary magnetic field to reduce the magnetic attraction between the plungers, or to cause a magnetic repulsion between the plungers. The force exerted by the biassing means and, if present, a magnetic repulsion, can then drive the plungers from a separation less than the threshold separation to a larger separation, where they will latch when the temporary field is removed.

Although it would generally be preferred to generate the continuous magnetic attraction between the plungers using permanent magnets and to apply the temporary switching fields electromagnetically, other arrangements are possible. For example, the magnetic latch means could comprise a coil for electromagnetically generating the continuous magnetic attraction and the switching fields could be generated by varying the current within that coil.

In a preferred embodiment, the plungers may be slidable within a common bore and the biassing means implemented as a compression spring, such as a helical spring, located within the bore between the plungers. In this embodiment, a recess in the inner end of either or both plungers may advantageously be provided to contain the spring when compressed. This would allow the end faces of the plungers to touch when the spring is compressed, advantageously increasing the attractive force between the plungers when they are latched at their minimum separation.

As noted above, in a preferred embodiment involving both aspects of the invention, a contactor is provided in which the actuator is a solenoid comprising two plungers as described above. In this embodiment, a 2-pole contactor may be provided in which a contact assembly is coupled to the outer end of each plunger. Depending on its design, the greatest force exerted by the plungers is typically when they are latched at a separation less than the threshold separation, particularly since the design permits them to come into contact with each other. Using such a solenoid as the actuator, the 2-pole contactor would preferably be arranged so that the contacts are made when the plungers draw the contact assemblies inwards,

towards the solenoid. To open, or break, the contacts, a switching coil of the solenoid may be temporarily energised to reduce the magnetic attraction between the plungers and thus allow the spring to drive the plungers apart, carrying both contact assemblies away from their corresponding fixed contacts.

This contactor embodiment may thus provide a symmetrical, balanced layout using a shorting-bar design. Preferably, it may also be able to withstand large short-circuit fault current disturbances without de-latching or opening, because of the large attraction force provided by the actuator when the plungers are in contact, and the structure of the contact assemblies.

In a preferred embodiment, the magnetic latch means may comprise permanent magnets mounted in one or more fixed positions and the plungers may be free to slide within the bore of the solenoid, the permanent magnets being positioned such that the plungers tend towards a stable, central position within the bore when latched at their minimum separation. This structure may advantageously exert a self-centring force on the latched plungers, but without any positive latching or fixing of the plungers in a central position.

In summary, the contactor of the invention, optionally incorporating the actuator of the invention, may advantageously be used for a two-phase metering load- disconnect contactor and may then provide a number of innovative features, as follows. a) Create a very short path for each phase switch, minimising switch resistance and self-heating,

advantageously enabling full 200 Amp rating within a meter or meter-base socket or adapter, b) Utilise simple shorting-bar switches, or ones with efficient"blow-on"closure capability, c) Enhance the blade (bar) design for bifurcation, tri- blade, quad-blade or other multi-blade or multi-bar shorting-bar sets, d) Very low contact closure forces required, thus reducing or minimising the solenoid drive burden, and increasing or maximising its latching/retention performance, e) Using a unique, novel, dual"pull-pull"plunger arrangement, with automatic self-balancing and self- centring actuation translated to both switches, compensating for any slight irregularities in the blade sets (shorting-bar sets), f) Utilise a simple solenoid construction, with dual sets of rare-earth magnets at each end of basic, flat, frame plates, tightly integrated within a bobbin and coils, g) The balanced nature of the two pole switches, dual "pull-pull"plungers and magnet sets, giving auto- compensation, interference-free, fault-field withstand, at any elevated current level experienced, h) The layout and modular construction to satisfy meter- incorporated and meter-base socket/adapter/"collar" implementations, with sufficient adjacent space within meter housings for a range of drive and interface circuit boards, and i) Where implemented as described below, to provide a simple, indirect, integrated status-switch (reed switch) for indicating the operational condition of the contactor (whether open or closed).

Description of specific embodiments and best mode of the invention Specific embodiments of the invention will now be described by way of example, with reference to the drawings, in which: Figure 1 illustrates three views of a bifurcated"blow-on" contact blade as used in the prior art contactor of GB 2295726; Figure 2 is a perspective view of a contactor according to a first embodiment of the invention; Figure 3 is a cut-away view showing the solenoid within the contactor of Figure 2; Figure 4 is a vertical section of the contactor of Figure 2; Figure 5 is an annotated vertical section of the contactor of Figure 2; Figure 6 is an exploded view of a plunger and a contact assembly of the contactor of Figure 2; Figure 7 is a perspective view of the contactor of Figure 2 within its casing ; Figure 8 is an exploded view of a contact assembly of a second embodiment, incorporating rigid shorting bars ; Figure 9 is an exploded view of a contact assembly according to a third embodiment, incorporating brazed strip contacts;

Figure 10 is a cut-away perspective view of the contactor of Figure 2 housed within an electricity meter base adaptor ; Figure 11 is a perspective view of the contactor of Figure 2 housed in a second orientation within an electricity meter base; Figure 12 is a perspective view of the contactor of Figure 2 housed within an electricity meter base incorporating current monitors; Figure 13 is a cut-away perspective view of a contactor incorporating the contact assembly of Figure 9; Figure 14 is an exploded view of the contactor of Figure 13 ; Figure 15 is a further exploded view of the contactor of Figure 13; Figure 16 is a perspective view of a plunger and part of the contact assembly housing of the embodiment of Figure 9; and Figure 17 is a cut-away view of the embodiment of Figure 13.

Figures 2 to 7 illustrate a contactor according to a first embodiment of the invention, suitable for switching a US mains electricity supply. Figures 10,11 and 12 illustrate this contactor housed in various ways for use with an electricity meter.

The contactor 2 is a two-pole contactor comprising two pairs of fixed contacts 4,6, 8,10, a solenoid 12 positioned centrally between the pairs of fixed contacts and a contact assembly 14,16 for making and breaking contact across each pair of fixed contacts. Each contact assembly is coupled to a respective plunger 18,20 of the solenoid by means of a respective contact-assembly lifter 22,24, typically moulded from an electrically-insulating plastics material.

As shown in Figure 7, when in use the contactor is contained in a contactor housing 26 which is typically of plastics materials. Each pair of fixed contacts is located near a respective end of the contactor housing and comprises a pair of copper fixed terminals which extend from opposite sides of the housing. For compatibility with a standard US utility meter, the copper fixed terminals are typically 19 millimetres wide and 2.4 millimetres thick. Advantageously, the copper terminals of each contact pair are designed to be as short as possible and in line with each other, as far as compatible with the connections of the meter in which the contactor is to be housed, in order to minimise their resistance.

As shown in the embodiment, the terminals are therefore aligned with each other and separated by a small gap through which the lifter extends. This provides electrical isolation between the terminal ends. At the end of each terminal, on its surface facing the respective contact assembly, the terminal carries heavy-duty, fixed, silver-alloy contacts 28, arranged to contact multi- shorting-bar switch sets in each contact assembly.

Each lifter 22,24 also comprises suitable barriers on both sides of each fixed contact pair to give the necessary creapage and clearance isolation distance

between the low-voltage drive solenoid and high-voltage switch parts, and to set a defined open stroke for each contact assembly, as described below. Advantageously, the closed switch resistance may be less than 0.1 mOhm, representing a self-heating of less than 4 Watts for each contact pair at full 200 Amp load current. For comparison, the prior art bifurcated, flexi-blade"blow- on"design (US 6292075, BLP Components Limited) as illustrated in Figure 1 generates about 10 Watts for each contact pair, switch set.

The structure of the contact assemblies and lifters will now be described in more detail. For simplicity, reference will be made only to one contact assembly and lifter. These components are substantially identical on each pole of the contactor.

The structure and motion of the contact assembly housing 16 and lifter 24 is shown in figures 3 to 6. The plunger 20 of the solenoid is bonded to an end of the lifter 24.

A central portion 30 of the lifter extends between the fixed contact pair 8,10, and a flange 32 extending outwardly from the central portion limits the travel of the lifter moulding as the plunger moves outwardly from the solenoid by abutting the inner surfaces of the fixed contact pair. The central portion 30 of the lifter moulding is of cruciform cross section, fitting both within the gap between the fixed contact pair and within perpendicular slots extending centrally through each fixed contact in order to locate the lifter both vertically and horizontally (as illustrated in Figure 4 for example) At its outer end, furthest from the plunger, the lifter is coupled to the contact assembly 16. In the embodiment, the contact assembly comprises four parallel shorting bars

34, each being located between the central portion 30 of the lifter and a contact-assembly backing plate 36. The contact assembly carries four parallel contact bars, each of which is curved to form a C-shape, to provide a blow-on geometry. The continuous back portion of the C-shape is supported by the back plate 36 and silver contacts are provided, facing outwards from the outer face of each end of the C shape, so as to face corresponding silver contacts on the fixed contacts 8,10. The portion of each shorting bar supported by the backing plate is substantially rigidly supported but each end of the shorting bar is flexibly supported by the curved portion 38 which joins it to the rear portion. The shorting bar is advantageously fabricated from a material such as copper, of very high electrical conductivity, but which does not necessarily provide sufficient resilience to support the free ends of the contact bar. A multi-looped leaf spring 40 fabricated from a more resilient material may therefore advantageously be inserted within the C shape of each shorting bar in order to support the free ends of each shorting bars which carry the contacts. The leaf spring is advantageously fabricated in a multi-looped form so that a single spring element can simultaneously support all of the contact bars.

In operation, the lifter extends between the ends of the fixed contact pair such that the fixed contacts are positioned between the flange 32 of the lifter and the contacts 42 carried by the set of shorting bars. When the plunger of the solenoid is driven outwards, the lifter carries the shorting bars away from the fixed contacts, breaking the electrical connection, until the flange contacts the rear faces of the fixed contacts. When the plunger is drawn into the solenoid, the plunger, the lifter and the contact assembly, which are rigidly

connected together, accelerate freely such that the shorting bar contacts accelerate towards the fixed contacts. The solenoid is fabricated such that when the shorting bar contacts first touch the fixed contacts, the plunger has not reached the limit of its travel, so that the plunger, the lifter and the backing plate of the contact assembly continue to move. This continued motion deflects the C-shaped shorting bars and compresses the leaf springs within them, loading the shorting bar contacts against the fixed contacts. The solenoid is designed such that the maximum travel of the plunger corresponds to a predetermined deflection of the leaf springs and therefore to a desired contact force.

Figure 4 illustrates in cross section the contactor with the contacts in the open, or break, position while Figure 5 illustrates the contacts in a closed, or make, position.

As can be seen in Figure 5, the limit of the motion of the plunger 20 into the solenoid is determined by its contact with the second plunger 18.

Figure 8 illustrates a second embodiment of the contact assembly. In this embodiment, the contact assembly comprises an outer portion 50 of the lifter and a backing plate 52, between which four shorting bars 54 are housed.

The outer portion of the lifter comprises upper and lower supports 56,58 in which slots 60 are formed, aligned parallel to the direction of motion of the plunger, to receive corresponding tabs 62 on each end of each shorting bar. Each slot terminates at an end stop 64 which retains the shorting bars. A leaf spring 66 is mounted behind the shorting bars, located on pegs 68 extending from the end surface of the lifter. Finally, the backing plate, in which holes are formed to receive the pegs, is mounted on

the end surface of the lifter and secured, for example by melting or gluing the pegs. It may be clipped together.

Each shorting bar 54 carries two silver contacts 70 on its surface facing, in use, the fixed contacts. In Figure 8, one of the shorting bars is shown separated from the contact assembly in order to show the silver contacts more clearly.

When the contact assembly is assembled, the leaf spring urges each shorting bar along its guiding slots and against the stop 64, at a pre-loaded force.

In operation, the contact assembly of the second embodiment operates in a similar way to that of the first embodiment. Starting from the contact open position, in order to make the contact the solenoid plunger is drawn into the solenoid and accelerates the contact assembly towards the fixed contacts. When the silver contacts 70 on the shorting bars contact the corresponding fixed contacts, the plunger, lifter and contact assembly housing continue to move, compressing the leaf springs and further urging the shorting bars against the fixed contacts.

Figure 9 illustrates a contact assembly according to a third embodiment. In this embodiment, the outer end of the lifter 80 is formed into a cage structure shaped to encircle four rigid shorting bars 82. Each shorting bar is of rectangular outline and carries two brazed strip contacts 84 on its front surface. One shorting bar is shown separately in Figure 9 for clarity. Across the central portion of the end of the lifter, a stop bar 86 abuts a central portion of the front of each shorting bar to limit its forward motion. The cage 80 formed at the end of the lifter is of a thickness greater than the

thickness of the shorting bars, to allow movement of the shorting bars parallel to the direction of motion of the contact assembly. Locating pins 88 extend from the end surface of the cage and locate a leaf spring assembly 90 and a backing plate 92, which is secured by melting or gluing the pins within corresponding holes in the backing plate. Again, it may be clipped together.

The leaf spring assembly 90 comprises four individual leaves positioned to press against a rear surface of each shorting bar, near the corners of the rear surface. This arrangement prevents the shorting bars from tilting within the moulded cage 80.

The contact assembly of the third embodiment operates in the same way as for the first and second embodiments, in that to close the switch, the plunger is drawn into the solenoid and accelerates the contact. assembly towards the fixed contacts. When the contacts 84 on the shorting bars strike the fixed contacts, the shorting bars are prevented from moving further but the contact assembly housing continues to move through a predetermined distance, loading the shorting bars against the fixed contacts with a predetermined contact load.

This third embodiment uses welded (or brazed), profiled, tri-metal strip contacts rather than conventional rivetted contacts. Using four shorting bars, if rivetted contacts are used as in the first and second embodiments, the two- pole contactor would require 32 individual rivetted contacts, which may be disadvantageously costly. The tri- metal strip. contact principle may be preferable for high volume, strip-based production methods and in-situ welding techniques, and may advantageously reduce costs.

Most of the drawings show conventional circular, flat and domed contacts, which are rivetted onto individual shorting bars. Rectangular rivets may be used; these are less common than circular rivets but provide better thermal sinking. For robustness and reliability at elevated current levels, special silver alloy contacts are preferably used. These may be advantageous despite the use of multiple shorting bars to reduce the current carried by individual contacts. The silver-alloy contacts are advantageously better able to withstand closing and opening arc energy and to prevent welding.

The three embodiments described above fall into two broad categories, namely a simple multi shorting-bar (shorting- bar set) of copper bars comprising heavy-duty silver-alloy contact pairs which align with the contact set of the fixed terminals, and a looped-back"blow-on"shorting-bar set, which has a theoretical advantage over the simple shorting-bar type regarding contact repulsion forces as described below.

Folded-back"blow-on"shorting-bar sets may provide an advantage in that the opposing currents and magnetic fields in the blade loop-backs provide an additional force to counter the contact repulsion forces, particularly at high currents during short-circuits or other faults. The blow-on effect can give a consistent contact pressure, always keeping the contacts closed, derived from the solenoid plunger's high hold retention force capabilities, even during normal operation, which should improve contact life. Suitable non-magnetic leaf springs on or within the blow-on shorting bars provide the necessary uniform contact-closure force on each individual contact (or contact head), which may again provide clean switching and long contact life.

In both cases, the contacts generate a mean repulsion force when current flows, effectively pulling against the springs in the contact assembly and against the solenoid plunger. This force is proportionately reduced (inverse- square) with each additional shorting-bar pair used in parallel, as the total current is divided between the shorting bars of the set. There is a distinct technical advantage to using tri-blade or quad-blade sets, which must be balanced against fabrication costs. In practice, however, it has been discovered that as many as six parallel shorting bars in a set is advantageous.

In each embodiment described above, the shorting-bar sets are contained within a contact assembly, or contact assembly housing, which may include part of the lifter, and are pre-loaded by bias means against suitable faces or stops within it when the contacts are open. The bias means may comprise part of the contact bars or be provided as a separate component. When the switch is actuated, the contact assembly accelerates towards the fixed contacts and the shorting bars are picked up by, or contact, the fixed contacts at the end of the free stroke or free movement of the contact assembly (i. e. at closure). From this point, over-travel of the contact assembly develops spring forces uniformly on each individual contact.

As shown in Figures 2 to 5, the contactor of the embodiment is actuated by a twin-plunger solenoid, each plunger driving an independent contact assembly. When both poles of the contactor are closed, the plungers contact each other within the solenoid. When the poles of the contactor are open, the combined open contact gap path needs to be adequate for isolation. In the two-pole shorting-bar contactor this can be realised by a free stroke distance for each contact assembly of at least 1.75

mm, since there are in effect two contact gaps in series on each side.

The Advantages of Multiple Shorting Bars With conventional blow-on flexi-blades, as shown in Figure 1, bifurcation divides the through current equally between two blades, approximately halving the total contact repulsion force for a given total current. Tri-blade sets reduce this total repulsion force to approximately one third and quad-blade sets reduce it further still, to approximately one quarter that of a single contact.

Advantageously, contact bar (or blade) and contact-set design should aim to give a balanced and consistent contact force over the life of the contactor, preferably independent of the load current magnitude (or short- circuit fault current magnitude) and well within the drive and hold/retention capability of the drive solenoid or actuator.

Using multiple shorting bars in shorting-bar contact sets only provides half of the advantageous contact force reduction (for each shorting bar) achieved by the prior art flexi-blades shown in figure 1, as each shorting bar comprises two contacts, generating twice the nominal repulsion force generated by a conventional single-contact flexi-blade. Thus, a pair of shorting bars (a bifurcated shorting-bar set) may provide only the same performance in terms of contact forces as a single-contact flexi-blade.

Triple shorting bar arrangements offer a marginal improvement on a single flexi-blade and so four-bar (quad- blade shorting-bar sets) may advantageously be used, as illustrated in the three embodiments described above. As noted above, as many as six or more shorting bars in parallel may advantageously be used.

The optional addition of the blow-on structure of the shorting bars of the first embodiment may advantageously improve on the present bifurcated flexi-blade construction and its fault-withstand capability. Importantly, however, the shorting bar switches may advantageously provide significantly lower switch resistance than the prior art flexi-blade arrangements.

Reducing Contact Closure Forces and Increasing Solenoid Hold/Retention Capability Simple shorting bar sets as in the second and third embodiments may be fabricated from copper strip occupying a total area (across all of the shorting bars), of approximately 20 mm by 30 mm, by 1.0 mm or more thickness for each shorting bar to reduce switch resistance. This total area may be suitably subdivided into multiple shorting bars, such as the four shorting bars shown in the drawings in each embodiment. For the blow-on shorting bars in the first embodiment, the looped-back blade portions are thinned (or skived) to provide sufficient flexibility during actuation, commensurate with controlling self-heating and to maintain a consistent drive, contact pressure and hold-retention capability of the solenoid.

In the blow-on shorting bar embodiment, it should be noted that during the inward switch closure action, and the over-travel of the contact assembly to load the shorting bar contacts against the fixed contacts, the looped-back blow-on gap between the shorting bar portions is reduced, as shown in Figure 5, rather than increased as is the case with all known flexi-bladed contactors, such as in Figure 1. This gives an advantageous counter-force to the contact repulsion force at each contact on closure. The

presence of a magnetic looped leaf spring within each blow-on loop back enhances the intervening repulsion field strength and hence further increases the blow-on counter- force.

When blow-on shorting bars are used, it is important that the shorting-bar design and contact geometry is arranged so that the contact repulsion forces and the blow-on forces more or less counteract each other, especially during fault conditions. This advantageously provides a consistent average force loading on the solenoid plungers, which is preferably well within their drive and hold- retention capability.

Solenoid Structure The actuator for the contactors of each of the illustrated embodiments is a solenoid in which two coaxial plungers 18,20 are slidable within a common bore within a solenoid bobbin 110. As shown most clearly in figures 13 to 15 and 17, near each end of the bobbin a pair of longitudinally- spaced flanges 111 extends outwards, defining a compartment at each end of the bobbin for housing a set of permanent magnets. A pair-set of strong rare-earth magnets 112,114, separated by iron pole pieces 116 is contained in repulsion within each compartment. Two flat frame plates 118,120 span the magnets within the compartments at the opposite ends of the bobbin. The flux loops generated by the permanent magnets are illustrated in figures 4 and 5.

A solenoid activating coil 123 is wound onto the central portion 121 of the bobbin between the end compartments and coupled to contacts 122 on the external housing of the

contactor (see Figure 7) for the application of drive currents.

Within the bore, a push-off spring 124 acts between the inner ends of the plungers 8,10. The inner end of each plunger is advantageously formed with a central blind bore to receive an end of the helical push-off spring so that the end faces of the plungers can touch each other when the force exerted by the push-off spring is overcome.

It should be noted that the solenoid construction incorporates no U-frame and no fixed stop or stops to limit the motion of the two ferrous plungers. The plungers are free to slide within the bore.

The rare earth magnet pair sets are magnetised in-situ in opposite repulsion polarity sense at each end of the bobbin, so that when the plungers are introduced into the solenoid bore, two strong longitudinal magnetic-flux loops are set up via the flat frame-plates. These flux loops are illustrated in figures 4 and 5. The tendency for the plungers within these flux loops is to move towards each other to close the open-stroke gap between them and to latch together. Their natural balanced position within the flux loops is a central position on the axis of the solenoid bore and between the frame plates and the magnet pair sets.

In the following description, we assume that the magnet polarity is, say, south-dominant at one end of the bobbin and north-dominant at the other.

Although the plungers are attracted together within the flux loops, they are urged apart by the push-off spring.

The plungers thus have two stable positions. When their

separation is greater than a predetermined threshold separation, the force exerted by the push-off spring is greater than the magnetic attraction, and so the plungers remain separated by a small distance, termed the open- stroke gap of the solenoid. If the plungers are pressed or urged together to a separation smaller than the threshold separation, the magnetic attraction overcomes the force exerted by the push-off spring and the plungers latch together. These two stable positions are illustrated in Figures 4 and 5 respectively.

Whether open or latched together, the two plungers tend to seek a central, axial position within the solenoid, urged by the flux loops. Within the contactor, this advantageously enables the actuation force exerted on each contact set to normalise in a balanced manner.

In the actuator, the plungers can be switched between the open and latched positions electromagnetically, by operation of the single or dual energisation coils surrounding the central portion of the bobbin. When the plungers are fully open (typically at a 5.0 mm stroke gap in the contactor embodiments) an electrical pulse through an operate energisation coil strongly enhances the leakage flux on both magnet sets. This increases the magnetic attraction between the plungers and accelerates the plungers towards each other. In the contactor, this action rapidly accelerates the contact assemblies towards the fixed contacts at both poles. The plungers accelerate the contact assemblies not only through their free travel distance but also through the over-travel phase during which contact pressure increases on each contact set.

Finally, the plungers latch together. At, or before, this point the operate coil pulse may be removed.

The rapid free-travel plunger acceleration provides clean and consistent contact closure, and virtually no contact bounce (or arcing). Closure time is typically less than 100 microseconds, compared with between one and two milliseconds for a conventional flexi-blade contactor. In the embodiments, each contact typically travels through a 1.75 mm minimum free stroke before contact closure, and each plunger then moves through a further 0.75 mm over- travel to increase contact pressure to a predetermined level. These distances add to 2.5 mm travel for each contact, equating to the 5 mm open-stroke gap of the actuator described above. It should be noted, however, that because the latched plungers of the actuator are not rigidly centred within the solenoid core but are urged towards the centre by the permanent magnetic field, the over-travel on the two switch sets is automatically balanced so that comparable contact forces (the contact forces being much greater than the force exerted by the permanent magnetic field to centralise the latched plungers) are applied on both contact sets despite any dimensional variation between the contact sets due to manufacturing tolerances.

To open the contacts, a release coil is actuated, briefly neutralising both permanent magnet sets. The push-off spring and the leaf springs within each contact assembly then act together to cleanly and rapidly open the contacts, breaking any whisker welds at the same time.

This rapid opening may advantageously keep contact-open arcing to a minimum.

It has been noted above that the actuator may comprise single (as shown in figure 17) or dual energisation coils.

If dual energisation coils are provided, one may act as the operate energisation coil and the other as the release

coil. If only one energisation coil is provided, then it acts as the operate coil when current is applied to it in one direction and as the release coil when it is reverse driven.

Fault Withstand Capability In the embodiments described above, the copper fixed terminals, the switch sets and the actuator assembly are all perfectly symmetrical (within manufacturing tolerances) and balanced. Hence, as shown in figure 5, a load current flowing through one pole switch of the contactor, via an external load 130 and returning through the other pole switch is always in the same phase relationship. Thus, if the external load suffers a short- circuit fault condition and the load current increases, the large interfering magnetic fields generated at each pole switch are also in the same phase relationship.

If we assume that the magnetic field polarity in the first pole switch (142 in figure 5) at a particular time is such that the flux linkage between the interfering magnetic field generated by the current and the permanent magnets of the actuator weakens the south-dominant magnet set of the actuator, denoted by a small"s"in figure 5, then at the same time the field polarity in the return path pole switch 144 links more strongly with the north-dominant magnet set of the actuator, denoted by a large"N".

Within the actuator, the magnetic latching force between the plungers depends on the difference between the south and north fields at each end of the solenoid, which is maintained despite the high fault current. The interfering field generated at each pole switch will oscillate with the alternating current passing through the pole switch, and consequently the flux linkage to the

permanent magnetic field at each end of the solenoid also oscillates, but if the interfering fields at each pole switch are in phase, the nett differential between the field experienced by the plungers at each end of the solenoid (that is, the difference between"s"and"N"in figure 5) is effectively constant, preserving the solenoid latching hold force.

In other words, although the nominal flux strength of each individual permanent magnet pair set changes with time under the influence of the interfering field generated by the neighbouring pole switch, one pair being weakened while the other set is strengthened, the nominal flux strengths of both pair sets change together as a double pair set, maintaining the relative magnetic field strength. Since both pair sets comprise very high strength rare-earth magnets, their magnetic saturation limits are not generally exceeded even during fault conditions.

Since the magnetic hold or retention force between the plungers is not adversely affected during fault conditions, the contacts at each pole switch are securely held closed even at high fault currents, advantageously reducing the risk of contact opening and the formation of arcs or welds.

Embodiments for Meter Base Applications A US electricity utility meter is typically housed in a cylindrical plastics casing of about 20cm diameter. The meter is removably couplable to utility connections at a standard electricity supply utility connection box. It is known to house a contactor in a matching socket or adaptor, couplable between the meter and the connection

box, but prior art contactors are disadvantageously large, meaning that the socket or adaptor housing (which must be of the same diameter as the meter) has to be disadvantageously high. Prior art contactors may also generate excessive heat due to their electrical resistance.

In recent years, meter manufacturers have been actively redesigning their meters, aiming to reduce meter height and to incorporate additional features in order to make meters more commercially saleable and compatible with the latest requirements of the utility companies.

Additionally, more options are requested by utility companies and customers for meter monitoring, remote meter reading using various communications means and, most importantly, for remote shut-off of the metered supply, for example if a customer does not pay a utility company bill. Pager and power-line carrier means of communications and control are common Consequently, meter envelope volumes have been reduced dramatically and conventional load-disconnect contactors now occupy a larger proportion of the available space.

This means that there is a significant advantage in reducing contactor size.

Existing contactor designs also generate undesirable self- heating at their full rating (typically 200 Amps) which puts considerable stresses on the materials of the contactors and meters, virtually demanding the use of military-specification drive electronics. Alternatively, systems have to be down-rated to only 160 Amps, for example, which has limited commercial appeal to utility companies because, for example, the same contactor or meter design cannot be used in all applications.

A further problem arises as meter package size and volume is reduced, because the high current switches of the contactor must be placed in closer proximity to the sensitive electronics within the meter, putting extra demands on circuit design and resident control software integrity, especially regarding gaining full EMC compliance. A fully-compensated, modular, robust, fault- withstand design which can be universally applicable is therefore highly desirable.

Reduced size and cooler-running operation appear to be difficult to achieve using conventional flexi-blade, "blow-on"design principles, because of the large size and long current paths of such designs. There is therefore a real demand for new, compact, modular designs that better fit the new meter envelopes, generate reduced self-heating and are completely insensitive to or substantially unaffected by large short-circuit fault conditions.

The embodiments of the invention described herein may advantageously satisfy all of these criteria. Such embodiments can then be installed within a meter base or socket/adaptor as follows.

The Meter-base Version In an embodiment of the present invention, as shown in figure 7, the contactor casing size is selected to be compatible with the available head-space within a utility meter casing, this being typically 30mm in height. Figure 11 illustrates an installation of such a contactor in a meter base 110. The contactor is positioned centrally within the meter base and its fixed terminals extend to right-angled extensions 150, which extend through the meter base for coupling to utility connections. The upper

surface of the contactor casing carries pin-connectors 152 for supply and coil drives and a status switch 174 as described below. Positioning these on the upper surface of the contactor casing enables them to be connected from within the meter. This meter-base installation of the contactor plugs straight into the basic meter-base socket, which is a standard part of a US wall-mounted utility connection box, typically wired for the two-phase 200 Amp supply and the external load.

Figure 12 illustrates a further embodiment in which a contactor is mounted towards one side of a meter base.

The contactor terminals on one side of the contactor casing are coupled to extension pieces 156 which each pass through one of two current transformers 113 for monitoring the load energy usage in each phase. It can be seen that this embodiment occupies only a portion of the space within the meter base so that there is still sufficient space alongside the contactor casing for relevant drive and communication circuits; for example pager carrier means of control, are common.

The Adaptor Embodiment This embodiment of the invention is illustrated in figure 10 and uses a contactor as illustrated in figure 7. In this embodiment (the so-called"up/down"version) the contactor is integrated within an adaptor (or base socket or collar) which is separate from and interposed between a conventional electricity meter and the wall-mounted utility connection box. Implementation is thus very simple; the conventional meter is first removed from the connection box, the new adaptor containing the load- disconnect switch is coupled to the connection box and the standard meter coupled to the adaptor. This embodiment

thus allows a conventional meter to be upgraded to provide many more communications control, drive and remote load- disconnect options.

The adaptor comprises a base 102 which is functionally compatible with a conventional connection box. The contactor is mounted on the base, with one pair of terminals 6,10 extending through the base for connection to the connection box and the second pair of terminals 4, 8 extending upwardly for connection to the utility meter.

The upwardly-extending terminals are provided with sprung jaws 160 for receiving two terminal blades of the meter.

The two poles of the contactor are each arranged to switch one of the two phases of the electricity supply. The return path for each phase from the utility meter to the connection box is carried by a second terminal 162,164 which extends from a sprung jaw 166 for connection to the utility meter to a blade end 168 for connection to the connection box. A cylindrical collar 100 (cut away for clarity) extends from the base of the adaptor, surrounding the contactor and terminals. The upper edge 104 of the collar provides a seat for the base of the utility meter.

This embodiment may provide significant advantages in that it allows an existing utility meter to be flexibly upgraded to provide various meter-reading and remote shut- off options, as described herein, without the requiring modification of the conventional utility protocols or connections. Communications means, remote meter reading facilities and load-disconnect can all be implemented using this embodiment. It therefore provides a means of implementing these facilities with minimal delay and inconvenience and, more importantly, with minimal cost increase incurred by the utility company concerned.

It should be noted that this embodiment may be enabled by the compact, modular, shorting-bar designs proposed in this patent application, and then may advantageously enable a physically compact adaptor in which the full, intended 200 Amp load rating can be achieved with less self-heating and better short-circuit fault withstand than in conventional systems.

The"Status"Reed Switch In the embodiment illustrated in figure 7, the contactor casing incorporates a pocket 170, in which a reed switch 172 is mounted for indicating the status of the contactor, that is, whether the contactor is in the open or closed condition. Connections 174 to the reed switch extend outwardly from the pocket.

The reed switch is carefully positioned parallel to the solenoid axis, centrally along the length of the solenoid and laterally offset, alongside the edge of one of the flat frame-plates, so that it links with the solenoid flux in a predetermined way. Thus, when the plungers of the solenoid are separated, as illustrated in figure 4, the relatively weak leakage flux along the frame-plate does not link through the offset reed switch, and the reed switch stays open. When, however, the plungers are latched together as illustrated in figure 5, a small proportion of the very strong holding flux links through the reed switch, closing it. This provides the basis for the reed switch to act as a contactor status switch for integrating with control electronics, either housed in the meter base (as in the embodiment of figure 11 or 12) or in the adaptor (as shown in the embodiment of figure 10).