The invention relates to electrical contactors. The invention further relates to a contact set suitable for use in such contactors.

Many switching systems exist for connecting or disconnecting a power load from a voltage source, the source being either mains power, as used in domestic heating, usually in the range of 110 to 250V AC or low voltage DC sources for example vehicle batteries.

For domestic electricity disconnection as employed in prepayment metering, tariff switching or load shedding, built in power contactors are usually one pole for single phase AC loads or two pole for two phase electricity such as is supplied in the United States of America from street side utility owned power transformers.

Existing AC (or low voltage DC) power disconnect contactors have a very basic modular construction comprising heavy duty terminals, a fixed electrical contact usually attached internally to one of the terminals, a flexible conductive blade with a moving contact and an actuating means for closing and opening the contacts. Drive is usually achieved by a solenoid actuator, motor drive, or may be by any other suitable means.

Nominal contactor ratings are usually in the range 50 to 200 Amps requiring suitable blade and contact combinations in order to achieve a low resistance switch path when closed in order to minimise internal self heating when connected to large electrical loads. In some critical applications, multiple arrangements of simple flexible blades and contacts are employed in parallel to share the load current and reduce self heating even further. These blade stacks tend to be actuated in a conventional way.

Solenoid actuators may be continuously energised for switch closure, thus

generating undesirable coil self heating, or magnet latching types which require short duration drive pulses which contribute virtually no additional self heating once the contacts have closed and the drive pulse is removed.

Within the power disconnect switch and meter housing it is desirable that temperature rise due to load current flowing in the switch blades is kept to a minimum in the given system ensuring that all mechanical and electronic component stresses in the assembly are minimised in order to enable reliable operational performance throughout its designed lifetime. This is particularly important in two and three phase metering systems where two or three contactors are used, especially at high nominal currents.

In domestic electricity metering systems power disconnect contactors are employed within the metering systems for prepayment load shedding or whole house disconnect. Metering systems have very stringent requirements with respect to nominal current rating, and in particular, surviving excessive overload faults on the switched load side. These demands stem from a metering requirement related to the return accuracy of power measurement within the meter following brief short circuit surges, which may be in the order of thousands of Amps on the switched load side. These short circuit surges are reflected through the current sensing measuring transducer coils housed within the meter.

Many metering specifications contain the requirement that any components within the meter subjected to excessive overload current excursions, including power disconnect contactors housed within, interfacing with the switch domestic loads must be capable of surviving very demanding overload criteria, especially when subjected to a range of potentially damaging short circuit fault conditions. These faults can occur for a variety of reasons.

According to IEC (International Electrochemical Commission) metering specifications, the meter and other related components housed within it, including power disconnect contactors, must survive an overload condition

30 times their nominal current rating. Thus contactors designed to carry a nominal current of 100 Amps will be expected to survive 30 times these nominal current values for six full supply cycles (approximately 10OmS at 60Hz) and still perform satisfactorily afterwards. This represents overload level of 3000 Amps rms (or a peak value of almost of 4500 Amps). Clearly for nominal current of 200 Amps, commonly found in the USA, these figures will be doubled.

A typical contactor for switching AC or low voltage DC currents is disclosed in US Patent No. 5227750. This basic switch design uses a relatively simple modular construction involving heavy duty terminations incorporating fixed contacts, single copper or copper alloy moving blades with contacts, and permanently energised solenoid actuation for achieving the desired switching function.

Metering contactors more commonly use magnetic latching solenoids, since being pulsed in operation they introduce virtually no self heating. Contact pressure is provided via the solenoid actuator and a compression or leaf spring impinging on the single blade. For 100 Amp nominal current load switching a contact pressure of 250 to 300 gF is required for obtaining moderately low switch resistance, minimal contact erosion and reliable switching performance throughout its life.

Domestic metering power disconnect contactors have to survive the arduous overload current conditions as specified earlier. These require much greater contact pressure to be derived from the solenoid actuator than for the simple case outlined above. For a simple single bladed contactor, the contact pressure required in metering power disconnect contactor applications will need to be greater than 1 KgF for a 100 Amp nominal current in order to withstand 3000 Amps RMS overload currents without spurious opening of the contacts.

A contactor in which the arduous contactor pressure demands are reduced is

shown in UK Patent Application No. 2295726. This document discloses a contactor where the moving contact blade is connected to an input terminal via a feed bus-bar which runs parallel and adjacent to the moving contact blade. When high currents are passed through the contact blade a repulsive force is generated between the feed bus-bar and the adjacent blade, urging the flexible blade towards the fixed contact thereby increasing the contact pressure. Consequently the repulsive force experienced at the contacts on overload currents is overcome, hence the static contact pressure does not need to be as high and may be designed merely for the nominal current capacity.

For example contact pressure loading for nominal 100 Amp carrying current derived from the magnet latching solenoid actuator and compression or leaf spring is of the order of 250 to 300 gF for reliable long life operation. During an overload or short circuit fault the counter magnetic forces generated between the blade and feed bus-bar increases this nominal force to more than 700 gF at 3000 Amps rms thus keeping the switch closed and preventing a catastrophic destruction condition, which would be caused if it opened, or even worse partially opened, spuriously; causing welded (and thus permanently closed) contacts.

This arrangement has the disadvantage that the resistance of the feed bus-bar is added to the resistance of the moving blade and consequently the heating effect is increased especially at high current. In addition, because the force generated on overload currents is repulsive, the distance between the bus-bar and flexible moving blade tends to increase which has the effect of reducing the nominal force available.

It is an object of the invention to enable the provision of a contactor which mitigates at least some of the disadvantages of the prior art contactors.

In a first aspect the invention provides an electrical contactor comprising a first electrically conductive cantilevered arm carrying a first contact adjacent its fixed end and a second contact adjacent its free end, a second electrically

conductive cantilevered arm carrying a third contact adjacent its fixed end and a fourth contact adjacent its free end, the first and second arms being arranged in opposed alignment such that the fixed end of one arm is opposite the free end of the other arm, the first contact being aligned with the fourth contact and the second contact being aligned with the third contact, a first terminal connected to the fixed end of the first arm, and the second terminal connected to the fixed end of the second arm; the arrangement of the first and second arms being such that when the first and fourth contacts and second and third contacts are closed, current flowing between the first and second terminals through the first and second arms produces an attractive force between the first and second arms.

In a preferred embodiment the first and second arms carry substantially equal currents, since they are virtually identical.

Since the first and second arms are electrically connected in parallel their effective resistance is halved. This is in contrast to the series connection of the feed bus-bar and moving blade in the arrangement disclosed in UK Patent Application No. 2295726. Thus heating caused by the current flowing through the blades and contacts is reduced. Additionally, since in a contactor according to the present invention the current flowing in the two arms are flowing in the same direction, an attractive force is produced between them, causing the flexible arms to move closer together, which enhances the attractive effect with increasing currents.

In an embodiment suitable for carrying larger currents each arm comprises two longitudinally separated portions each portion extending over a major portion of the length of the arm and carrying a first contact adjacent fixed end and the second contact adjacent to the free end, wherein current flowing in each arm is divided between the longitudinally separated portions. Preferably the current is divided substantially equally between the portions.

In a particular embodiment the first and second arms may be preformed and

preloaded so as to bias them towards each other such that the contacts at the free ends engage with the contacts at the respective fixed ends with a preset contact pressure in the absence of a force separating the movable arms.

In a second aspect the invention provides a contact set for a contactor comprising a first cantilevered arm carrying a first fixed contact at or adjacent its fixed end and a second movable contact at or adjacent its free end and a second cantilevered arm carrying a second fixed contact at or adjacent its fixed end and a second movable contact at or adjacent its free end, the first and second arms being in parallel aligned opposition with the first fixed contact opposite the second movable contact and the second fixed contact opposite the first movable contact.

The two arms may be bifurcated, the free ends of each limb being provided with an individual movable contact and the fixed end of each arm being provided with a corresponding number of fixed contacts.

The arms may be preformed and preloaded such that in the absence of any other force the contacts are closed with a predetermined contact pressure.

In an alternative embodiment of the contact set one arm is preformed and preloaded such that in the absence of any other force acting on that arm the contact at its free end is urged against the fixed contact of the other arm, and the other arm is preformed and preloaded such that in the absence of any other force acting on that arm the contact at its free end is urged away from the contact at the fixed end of the one arm.

The above and other features and advantages of the invention will be apparent from the following description, by way of example, of embodiments of the invention with reference to the accompanying drawings, in which:

Figure 1 shows a first example of a contact set according to the invention, in plan and side elevation views when in its open state suitable for use in a contactor according to the invention,

Figure 2 shows the contact set of Figure 1 in the closed state.

Figure 3 shows in plan and side elevation views, a further contact set according to the invention suitable for use in a contactor according to the invention, the contact set being designed for higher currents,

Figure 4 shows the contact set of Figure 3 in the closed position.

Figure 5 shows current distributions and forces exerted in the contact set of Figure 1 during normal operation,

Figure 6 shows the current distribution and forces acting on the contact set of Figure 1 when carrying overload currents,

Figure 7 shows generalised forces generated on the contact set of Figure 1.

Figure 8 shows the current distribution and contact forces generated on the contact sets of Figure 3 when carrying normal rated currents.

Figure 9 shows the current distribution and contact forces generated on the contact set of Figure 3 when carrying overload currents.

Figure 10 shows in schematic form an actuator and contact set for a contactor according to the invention,

Figure 11 shows a plan view of a contactor according to the invention, and

Figure 12 shows an alternative contact set according to the invention, suitable for use in a contactor according to the invention.

Figures 1 and 2 illustrate in open and closed positions, respectively a contact set for a contactor incorporating the invention. The contact set comprises a first cantilevered arm or blade 1 , which is fixed at one end 2 and carries a contact 3 at or close to the fixed end and a second contact 4 at or close to its free end. A second cantilevered arm or blade 5 is fixed at an end 6 and carries a contact 7 at or near to its fixed end and a second contact 8 at or near to its free end. The fixed end 2 of the first arm 1 is connected to a first terminal (not shown) for application of the total current while the fixed end 6 of the blade 5 is connected to a second terminal (not shown). Thus, as can be seen from Figure 2, when the contacts are closed the arms 1 and 5 are electrically connected in parallel between the terminals connected to the fixed ends 2 and 6. The gap between the arms 1 and 5 is defined in the closed position by the dimensions of the contacts 3, 4, 7 and 8.

Figures 3 and 4 show an alternative, bifurcated, contact set in plan and side elevation, Figure 3 showing the contacts open and Figure 4 showing the contacts closed. The contact set shown in Figures 3 and 4 comprises a first arm or blade 11 fixed at one end 12 carrying a pair of contacts 13 at or close to the fixed end and a further pair of contacts 14 at or close to the free end.

Similarly a contact blade 15 is fixed at one end 16 and carries a pair of contacts 17 at or close to the fixed end and a further pair of contacts 18 at or close to the free end. Each blade 11 and 15 comprises two portions longitudinally separated over a major portion of their length, the arm 11 having a first portion 19 and a second portion 20. Each portion 19 and 20 carries a contact at or close to its free end. When the contacts are closed as shown in Figure 4 the height dimensions of the contacts determine the spacing between the arms 11 and 15. Each blade carries a contact at the end of the two portions and also a pair of contacts at its fixed end, the contacts at the fixed end being aligned with the contacts at the free ends of the portions of the other blade. This arrangement is particularly designed for higher current carrying capacity as the number of contacts is increased. In effect there are four switches connected in parallel reducing the contact resistance.

Figure 5 illustrates the current flow under normal current conditions in the contacts set of Figure 1 and 2. In this example the contact blades 1 and 5 are preformed and preloaded to give a contact pressure in the region of 300 gF. If a 100 Amp current is passed through the contactor then each of the blades 1 and 5 carries 50 Amps and the current flow in each of the blades is in the same direction.

Figure 6 shows the contact set of Figure 2 when carrying a high overload current for example 3000 Amps. In this case each arm 1 and 5 will carry 1500 Amps and the currents will be flowing in the same direction in each arm. This provides an enhanced attractive force between the two blades to increase the nominal contact pressure between the contacts and prevent repulsive forces generated in the contacts due to the high current causing the contacts to partially open creating large heating effects which could weld the contacts together leaving them permanently closed.

Figure 7 illustrates the generalised forces on the contacts set of Figure 2. The blades are preformed and preloaded to give a nominal contact force C _F which for a contactor designed for a nominal current of 100 Amps may be of the order of 300 gF. A repulsion force R _F is produced at the contacts and is related to the current passing through them and their geometry. This relationship is given by the Equation 1 set out below.

Where D is the contact head diameter, d is the contact touch diameter, and l _S c is the total short circuit current.

An attractive force is produced between the blades 1 and 5 by the divided current flowing through them. This attractive force B _F is given by Equation 2 set out below.

g

Where L is the active length of each arm, that is the distance between the two contacts of the arm, W is the active width of each arm, g is the spacing between the arms, and lsc is the total short circuit current.

By appropriate choice of contact and blade dimensions the attractive force caused by the currents flowing through the two blades may be made greater than the repulsion force caused by currents flowing through the contacts. This enables the initial preload force C _F to be lower than would be required if the attractive force caused by the currents flowing through the two parallel blades was not present.

Figures 8 and 9 show a similar analysis of the contact set shown in Figures 3 and 4. In this case the repulsive force at each contact is given by Equation 3 set out below.

As there are four contacts in this case the repulsive force at each contact is proportional to (% Isc) ² -

The attractive force between each adjacent blade in this case is given by Equation 4 set out below.

It can be seen that the attractive force on each adjacent blade is also proportional to (% l _sc ) ² as there are now four conductive blades in parallel. In this case the active width W is the width of each longitudinal portion rather than the total width of the blade.

The principle of the present invention is the provision of opposed cantilevered flexible blades having a small parallel gap between them. As the two blades, when the contacts are closed, are electrically in parallel the arrangement

removes the requirement for the heavy duty adjacent bus-bar or feed blade of UK Patent Application No. 2295726. As a result the total resistance between the inlet and outlet terminals is reduced (to approximately one quarter of that of the prior art construction if the bus-bar (or feed blade) resistance is equal to the contact blade resistance). In addition since the blade currents are flowing in the same direction the blades are attracted towards each other by their respective magnetic fields so that with strong overload currents the blades have a strong attractive force towards each other increasing the contact pressure.

At any sensible overload current level the strong magnetic attractive forces between the cantilevered parallel flexible face to face blades causes some inward bowing in both blades which decreases the gap slightly and thus enhances any attractive forces between the blades. This contrasts with the prior art construction in which the current in the bus-bar feed blade is flowing in the opposite direction to that in the flexible contact blade and consequently the forces generated between them are repulsive and as a result large currents will tend to increase the gap between the feed blade and flexing contact blade, which causes a reduction in the repulsive force generated.

Figure 10 shows schematically an H armature rotary actuator drive for the blades as shown a drive coil 100 is provided with pins 101 , 102 and 103 for receiving electrical inputs to the coil 100. The coil 100 and pins 101 , 102 and 103 are mounted on a bobbin 104. Two field pieces 105 and 106 extend through the core of the drive coil 100 and interact with a pivoted moulding 107. The moulding 107 is pivoted about an axis 108 and carries two pole pieces 109 and 110. Preformed preloaded contact blades 111 and 115 are fixed at respective ends 112 and 116. The blade 111 has a contact 113 adjacent its fixed end 112 and a contact 114 adjacent its free end. Similarly the blade 115 has a contact 117 adjacent its fixed end and a contact 118 adjacent its free end. The moulding 107 further carries magnets 120 and 121 and arms 122 and 123 that engage with the free ends of the blades 111 and 115. Magnets 120 and 121 may be formed as a single magnet located centrally between the pole pieces 109 and 110.

The moulding 107 is pivoted about the axis 108 which is arranged parallel to

the plane containing the blades 111 and 115 and at right angles to the longitudinal direction of the blades 111 and 115. Thus rotation of the moulding 107 in a clockwise direction will cause the arm 122 to lift the free end of the blade 111 and the arm 123 to push down the free end of the arm 115 separating the contacts 113 and 114 from the contacts 117 and 118, thus opening the switch.

In this embodiment the magnet latching H armature rotary actuator functionality is reversed from the normal functionality used in the prior art. Thus, for closing the contacts the actuator is pulsed so that the arms 122 and 123 are clear of the blade ends allowing the preloaded blade contacts to close freely onto the fixed contacts at the fixed ends of the cantilevered blades. To open the switch the rotary actuator is pulsed to open the contacts. In this case the moulding 107 rotates counter clockwise causing the arms 122 and 123 carried by the moulding 107 to be brought into contact with the blade ends picking them up rapidly after a pre-travel free stroke. The contacts are then forced open by a known gap relating to the final rotary actuator stroke limit at the same time breaking any whisker welds that may have been created by bounce at the last contact closure. A typical stroke for the rotary actuator is approximately 1 mm controlled by the integral fixed pole pieces 109 and 110 within moulding 107, but it is suitably amplified by the arms 122 and 123 to give a stroke of about 2mm. A free travel of about 0.5mm allows free unhindered closure thus giving an assembly which needs minimal adjustment.

Contactors of this construction having a reverse H armature rotary actuator functionality have typical operate and release times that are relatively fast, for example 6-7mSec with minimal bounce well suited for 50 or 60Hz half wave AC drives. The rotary actuation is symmetric for each latching state. This allows for AC coil drives using zero crossing triggering in the electronics or capacitor discharge DC drives to close the contacts after a waveform peak on rapidly declining load current as near to the next mains zero crossing as possible to minimise open arcing wear and maximise contact life.

For contactors using half wave AC drives the drive signal is applied across the outer pins 101 and 103 (a single coil being used). Usually a +ve half cycle is

used for switch closure and the -ve half cycle for opening the switch contacts.

For DC drives a split coil is used with the centre pin 102 as a common connection and one outer pin used for switch (contact) closure and the other outer pin for switch (contact) opening.

Advantages of the H armature rotary actuator reverse functionality can be summarised as set out below.

1. The relatively large nominal magnetic fields produced by the large currents flowing through the blades under overload conditions being symmetrical with respect to the magnet pole pieces and field pieces within the latched rotary actuator do not influence the latched condition. The pivoting lever mechanism will not attempt to rotate and thus open the contact sets spuriously during overload currents.

2. The rotary actuator release latching stroke opens the preloaded contacts. Consequently, there can be no high current influence or interference attempting to delatch the latched state and reclose the switch contacts erroneously.

Figure 11 shows in plan view a contactor according to the invention. The contactor is housed within a plastics base moulding 200, an appropriate lid (not shown) will be clipped over the base moulding when the contactor is fully assembled to protect and prevent access to the working parts within the moulding. A first blade 201 is firmly fixed to a terminal 206 at one end and carries a fixed contact 203 at the end fixed to the terminal 206 and moving contact 204 adjacent its other end. The other end is unsupported so that the blade 201 is a cantilevered construction. A second blade 205 is firmly connected to a terminal 202 at one end and carries a fixed contact 207 adjacent that end. The other end is unsupported so that the blade 205 is a cantilevered construction and carries a moving contact 208 adjacent its free end, the contact 208 being located behind and below the terminal 206 and the free end of the blade 205 being clear of the terminal 206. Thus the blades 201 and 205 are arranged in parallel opposed fashion and are both cantilevered,

the fixed end of blade 201 being aligned with the free end of blade 205 and vice-versa. A small parallel gap is produced between the blades 201 and 205 when the contacts are closed, this gap being defined by the height of the contacts 203 and 208 and the contacts 204 and 207, in this embodiment all four contacts have identical dimensions.

An actuator for the contacts comprises a bobbin 220 carrying a drive coil 221. Pins 222, 223 and 224 carry electrical signals to drive the coil 221. A plastics moulding 227 carries two pole pieces with ends 228, 229, 230 and 231. Two field pieces 225 and 226 pass through the middle of the drive coil 221. The field piece 225 has an extension solidly located between the pole piece ends 228 and 229, while the field piece 226 has an extension solidly located between the pole piece ends 230 and 231. The moulding 227 is mounted on a pivot 233 and pivots about an axis which is parallel to the plane of the blade 205 and perpendicular to the longitudinal axis of the blades 201 and 205. The moulding 227 further carries an integral extension lever 234 which acts on sliding lifters 235 and 236. Optionally the lifter 235 has an internal compression spring 244 which allows complete latching of the rotating actuators on closure, the pressure in the compression spring providing the contact pressure for the contacts 204 and 207.

As shown in Figure 11 the contactor is in its closed state, that is a current will flow between terminals 202 and 206 through both blades 201 and 205 in parallel. When the drive coil is activated, the moulding 227 rotates about the pivot 233 in a clockwise direction since the magnetic fields are such that the pole piece ends 229 and 230 are attracted to the field pieces 225 and 226. This causes the lifter 235 to move in a direction towards the drive coil 221 and the lifter 236 to move in a direction away from the drive coil 221 thus separating the contacts 203 and 208, and also the contacts 204 and 207 thus opening the switch. The actuator requires only a momentary pulse in order to rotate the moulding 227, the moulding 227 being held in the two extreme positions by the latching magnetic fields produced by the field piece ends 225 and 226 and the pole piece ends 228, 229, 230 and 231 respectively.

Clearly the precise form of actuation of the blades 201 and 205 may be varied

in any convenient way. The essential features of the invention being the provision of the cantilevered blades in parallel opposed positions so that when the contacts are made and an overload current arises there is an increased force of attraction between the two cantilevered blades to ensure that the contacts do not open under excessive overload conditions.

For applications where the temperature rise of the contactor is required to be as small as possible then a pulsed actuator has advantages since a continuous current in the conventional solenoid drive coil with the consequential heating effect that it would produce is not present in the present embodiment.

The basic switch concept can be operated by any means of actuation, that is manual, mechanical, electrical, or magnetic and still give the advantage of the attractive force produced on excessive overload current conditions and minimal contact and contact blade resistance at nominal currents.

It would, of course, be possible to provide the same switching action without using preformed preloaded blades. This then requires the actuator to perform a positive action to both open and close the contacts. As described above there is advantage in having the preformed preloaded contact blades and using the actuator to open the contacts and then allowing the contacts to become closed under the forces produced by preforming and preloading the blades without requiring any positive actuation.

Figure 12 illustrates a modification of the contact set shown in Figures 1 to 4 and its incorporation into a contactor. As shown in Figure 12a the contact set comprises first and second preformed and preloaded arms (or blades) 301 and 305. The blade 301 is arranged to be clamped at one end 302 and carries a (fixed) contact 303 adjacent the end 302. A (moving) contact 304 is carried adjacent to a free end of the arm 301. The arm 305 is arranged to be clamped at one end 306 and carries a (fixed) contact 307 adjacent that end and a (moving) contact 308 adjacent a free end. When assembled into a contactor the arms are so arranged that the fixed contact on one arm is aligned with the moving contact on the other arm and vice versa. The contact 308 in the absence of any other forces is urged against the contact 303 with a contact

pressure dependent on the preforming and preloading of the blade 305. In contrast, in the absence of any applied forces the contacts 304 and 307 are separated by a distance dependent on the spacing of the arms and the preforming of the arm 301. Thus the contact set shown in Figure 12a comprises two cantilevered arms 301 and 305, the contact at the free end of each arm being aligned with the contact at the fixed end of the other arm. In this embodiment, in the absence of any actuating force, the contacts 303 and 308 will be closed and the contacts 304 and 307 open.

Figures 12b and 12c show the contact set of Figure 12a in the open and closed positions, respectively, together with a schematic rotary actuator. The actuator comprises a moulding 327 mounted on a pivot 333 so that the axis of rotation is perpendicular to the plane of the drawing. The moulding 327 carries two extension arms 335 and 336 that act directly on the free ends of the arms 301 and 305. A compression spring 334 is mounted between the extension arm 335 and the arm 301. When the moulding 327 is rotated in a clockwise direction (Figure 12b) the extension arm 336 bears down on the free end of the blade 305 separating the contacts 303 and 308. At the same time the extension arm 335 acts on the underside of the free end of the blade 301 to lift the contact 304 clear of the contact 307.

In a particular embodiment the arm (or blade) 301 has a preloading force in the region of 150 gF and the arm 305 has the required contact pressure of 300 gF. These forces are chosen for a particular application and the invention is not limited to these particular values. The appropriate values will depend, inter alia, on the current carrying capacity of the blades, the overload requirements, and the composition of the blades and contacts.

When the moulding 327 is rotated in an anticlockwise direction the extension arm 336 will move upwards allowing the contacts 303 and 308 to close under the preloading force of the blade 305, that is about 450 gF. At the same time the extension arm 335 rotates so that the compression spring 334 urges the free end of arm 301 downwards to close the contacts 304 and 307.

During rotation of the moulding 327 in an anticlockwise direction a force

generated by the preloading of arm 305, which may be between 400-500 gF, acts upwardly on the arm 336 assisting the rotation initiated by a magnetic actuation as described with reference to Figures 10 and 11. The compression spring 334 located between the extension arm 335 and the free end of blade 301 has to exert a force of approximately 150 gF to overcome the preloading of arm 301 to just close the contacts 304 and 307 plus a force of 300 gF being the desired contact pressure. Thus the actuator has to generate a force greater than 450 gF to close the contacts 304 and 307 with the desired contact pressure. This force is well within the hold capability of a magnet latching actuator as shown in Figure 11.

An advantage of this embodiment is that the extra forces build up at both blade ends during overtravel helps to promote a stronger translation from one latched state to the other. The release of pressure built up in one latched position within the preloaded blades aids the drive through its neutral mid region and towards the opposite latched position in a more efficient and decisive manner. That is the release of energy in the appropriate preloaded blade aids the rotation of the member 327 when magnetic forces acting on it are at their minimum.

When latched as shown in Figure 12c, that is with the contacts closed, a considerable pressure has been built up between the extension arm 335, the compression spring 334 and the arm 301, the actuator having deflected the upwardly preformed blade 301 downward to close the gap, for example 1.0mm, between the contacts 304 and 307 and developed further overtravel contact pressure of, for example, 300 gF. The total built up pressure is in this example between 400-500 gF. This can be considered as stored energy on that side. It will be seen from Figure 12c that the extension arm 336 is clear of the blade 305 and consequently there are no stored forces on that side.

When the coil (equivalent to coil 221 in Figure 11) is driven in the opposite direction to delatch the actuator magnetically and cause the moulding 327 to rotate clockwise to the position shown in Figure 12b and hence open the contacts 304 and 307 and the contacts 303 and 308 the pressure built up in the left hand side, 400-500 gF, is suddenly released rapidly opening the contacts

304 and 307. The energy released aids the rotation of the moulding 327 and drives the extension arm 336 downwardly onto the free end of the blade 305 causing the contacts 303 and 304 to open. Thus during the rotation of the moulding 327 there is an additional force to the magnetic actuating force enabling rapid opening of the contacts. The magnetic actuator is then latched in its opposite state and a pressure of 400-500 gF is stored in the open blade 305. This again can be considered as stored energy that can be used to assist rotation of the moulding 327 when the magnetic field in the coil is reversed to close the contacts.

This arrangement of the two cantilevered arms provides an energy release drive advantage aiding the rotation of the moulding 327 through its neutral mid region where the magnetic forces on the moulding are at their weakest. The final contact pressure is determined mainly by the preloading of the blade as described and the magnet hold strength at each extreme position.

This loaded and energy release method is more drive efficient than the embodiment shown in Figures 10 and 11 in that it uses the different preformed and preloaded blades to develop, at each latched state, forces that aid the rotation of the moulding 327 once the initial magnetic actuation is initiated.

Clearly, the contact set as described in Figure 12 can take the form of single contact blades as shown in Figures 1 and 2 or bifurcated contact blades as shown in Figures 3 and 4. In principle, it could be applied to multiple contact blades of any number depending on the number of contacts required and the design of the extension arms (or lifters) 335 and 336.