Patent Publication Number: US-11031204-B2

Title: Movable contact arm set for switching contactor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application is continuation application of PCT Application No. PCT/CN2017/078718, filed with the Chinese Patent Office on Mar. 30, 2017, which claims priority to British Patent Application No. 1605576.6, filed on Apr. 1, 2016, all of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF INVENTION 
     The present invention relates to a movable contact arm set, in particular but not necessarily exclusively for use with switching contactors. The invention further relates to a switching contact, to a movable arm for a movable contact arm set, and also to a method of providing a lead-lag contact opening arrangement for a switching contactor. 
     BACKGROUND OF THE INVENTION 
     It is often desirable to provide switching contactors current-sharing arm arrangements, wherein a plurality of different movable arms is provided which can be actuated to open and close a switch. By providing a means by which the different movable contacts of the switching contactor can close one after another, a lead-lag contact arrangement can be constructed. This is advantageous in that current can be shared across the various movable contacts, limiting the danger of plasma discharge or electrical arcing, which in turn allows some of the movable contacts to be reduced in size, improving the cost-effectiveness of manufacture of the switching contactors. 
     Typically, lead-lag arrangements have been created by providing a single movable contact arm which is split into individual blades, each blade having one or more movable contacts mounted thereto. This permits current sharing across the blades. The individual blades will then be biased in order that one or more blades open and close their respective movable contacts in advance of the remaining contacts, thereby creating a lead-lag closure arrangement. However, such biased blade arrangements can be both expensive and complicated to manufacture, and alternative means of providing a lead-lag contact arrangement would be beneficial. Biased blades are also reliant on correct actuation of the movable arms in order to ensure a correct opening and closing sequence, and are therefore reliant on more powerful actuator arrangements than might be desirable. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a movable contact arm set and switching contactor so as to obviate or limit the above-mentioned problems. 
     According to a first aspect of the invention, there is provided a movable contact arm set for a switching contactor, the movable contact arm set comprising: first and second movable contact arms which respectively carry first and second movable contacts, the first and second movable contact arms respectively having first and second engagement surfaces which are engagable with an actuation element to permit displacement of the first and second movable contacts, the first and second engagement surfaces being asymmetric to one another to permit asynchronous said displacement. 
     By providing two movable arms in a set which are asymmetric to one another permits a linear actuation force from a movable member of an actuator to ensure that the movable contact arms, and by extension, the movable contacts are opened in the correct sequence. The shaping of the movable contact arms is such that the sequence of opening and closing of the movable contact arms is always executed correctly. 
     Whereas existing blades of movable members may be offset and angle symmetrically to the contact end, there exists a short distance prior to the engagement of a wedge-shaped member with points of engagement of the blades in which highly accurate and consistent manufacturing must be provided in order to achieve consistent pick-up of the movable members. This can lead to lateral displacement or shuffling of the blades when the wedge-shaped member enters engagement. The present invention provides long engagement surfaces which beneficially create a smooth pick-up of the movable members, providing consistent opening timings for contactors. 
     Preferably, each of the first and second movable contact arms may have an elongate body having a stepped portion therein, the respective stepped portions defining the first and second engagement surfaces. Each of the stepped portions of the first and second movable contact arms may then comprise primary and secondary shoulders which are different to each other. 
     In one embodiment, a stepped portion of the first movable contact arm may be larger than the stepped portion of the second movable contact arm. Preferably, the stepped portion of the first movable contact may be larger than the stepped portion of the second movable contact arm in a direction of a longitudinal axis of the respective elongate bodies. Additionally or alternatively, the stepped portion of the first movable contact arm is larger than the stepped portion of the second movable contact arm in a direction perpendicular to a longitudinal axis of the respective elongate bodies. 
     The specifically stepped or shouldered shape of the movable contact arms ensures that a physical contact between a movable member of an actuator produces a reproducible and accurate opening and closing stroke of the switching contactor, without any potential for significant lag between actuator motion and opening or closing of the contacts. This ensures that a steady opening or closing of the contacts can be produced so as to minimise the risk of electrical arcing. 
     Optionally, a leading edge of the first engagement surface may be positioned further from the first movable contact than a leading edge of the second engagement surface is from the second movable contact. Furthermore, an angular orientation of the first engagement surface may be different to that of the second engagement surface. 
     Spacing apart the leading edges of the engagement surfaces, which define a pick-up point for the movable member of the actuator, ensures the asynchronicity of the displacement of the movable contact arms. Modification of the slopes of the engagement surfaces can also advantageously alter the opening and closing characteristics of the switching contactor, which may assist with creating the best opening and closing timing sequences. 
     Preferably, the first movable contact may be formed as a lead contact and the second movable contact is formed as a lag contact. 
     The provision of a lead-lag contact arrangement ensures that the risk of electrical arcing or plasma discharge can be minimised for the lag blades, allowing the manufacturer to minimise the amount of electrically conductive and traditionally expensive material of the movable contacts which is used. This beneficially produces a more cost-effective movable contact arm set. 
     According to a second aspect of the invention, there is provided a switching contactor comprising: first and second terminals; a moveable contact arm set, preferably in accordance with the first aspect of the invention, the first and second movable contact arms being connected to the first terminal; first and second fixed contacts connected to the second terminal; and an actuation element having a movable member engagable with the first and second engagement surfaces to permit asynchronous displacement of the first and second movable contacts relative to the first and second fixed contacts. 
     Preferably, the movable member may be a wedge-shaped member engagable with the first and second engagement surfaces, and the wedge-shaped member may have an stepped actuation surface. 
     A switching contactor having a lead-lag contact arrangement is less likely to become damaged during use, requiring less maintenance, and therefore being more cost-effective to operate over a set period of time. Improvements to the lead-lag arrangement can also reduce the propensity for contact closure bounce, which in turn increases the risk of electrical arcing. 
     According to a third aspect of the invention, there is provided a movable contact arm for use with a movable contact arm set, preferably in accordance with the first aspect of the invention, the movable contact arm comprising: an electrically-conductive elongate body having a stepped portion which includes a contact-clearance shoulder, an engagement shoulder, and a moving-member-engagement body portion which is positioned between the contact-clearance and engagement shoulders, the moving-member-engagement body portion defining an engagement surface which is engagable with a movable member of an actuation element to permit displacement of the movable contact arm. 
     Preferably, the elongate body may be formed from a resiliently flexible electrically-conductive material. 
     According to a fourth aspect of the invention, there is provided a method of providing a lead-lag contact opening arrangement for a switching contactor, the method comprising the steps of providing a pair of movable contact arms of the switching contactor which are asymmetric to one another, and applying a linear actuation force towards the pair of movable contact arms such that the movable contact arms are displaced asynchronously to one another. 
     Asynchronous contacting of a pair of movable contact arms can be achieved by producing asymmetric contact arms, thereby permitting a standard actuator arrangement to be used as part of a switching contactor. This can lead to a greater degree of miniaturisation of the components of the switching contactor, which can allow the use of such a switching contactor in a greater range of applications. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a plan view of one embodiment of a movable contact arm set in accordance with the first aspect of the invention, the movable contact set being in a contacts-closed condition with fixed contacts of a contactor terminal; 
         FIG. 2 a    shows an enlarged plan view of the contact region of the movable contact arm set of  FIG. 1 ; 
         FIG. 2 b    shows an enlarged plan view of the contact region of the movable contact arm set of  FIG. 2 a   , a wedge-shaped engagement member of an actuator having advanced to contact one movable arm of the movable contact arm set to a point of pick-up; 
         FIG. 2 c    shows an enlarged plan view of the contact region of the movable contact arm set of  FIG. 2 b   , the movable contact arm set being intermediate a contacts-closed and contacts-open condition with the fixed contacts following further advancement of the wedge-shaped engagement member to a point of pick-up of the other movable contact arm; 
         FIG. 2 d    shows an enlarged plan view of the contact region of the movable contact arm set of  FIG. 2 c   , the wedge-shaped engagement member having advanced to contact the other movable arm of the movable contact arm past the point of pick-up; 
         FIG. 2 e    shows a plan view of the movable contact arm set of  FIG. 2 d   , the movable contact arm set being in a contacts-open condition with respect to the fixed contacts following further advancement of the wedge-shaped engagement member; 
         FIG. 3  shows a plan view of one embodiment of a two-pole electrical contactor in accordance with the second aspect of the invention; and 
         FIG. 4  shows a qualitative graph of the stroke force applied to the movable contact arms of the electrical contactor of  FIG. 3  with respect to the displacement of the wedge-shaped member of an actuator thereof. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring firstly to  FIG. 1 , there is provided a first embodiment of a movable contact arm set in a contacts-closed condition and indicated globally at  10 , which comprises first and second movable contact arms  12   a ,  12   b  at least in part displaceable relative to one another. 
     Each of the first and second movable contact arms  12   a ,  12   b  has an elongate body  14   a ,  14   b , the elongate bodies  14   a ,  14   b  preferably having identical longitudinal extents. The elongate bodies  14   a ,  14   b  are formed from an electrically conductive material, preferably a resiliently flexible material such as spring-grade copper or copper so as to allow flexion to permit the relative displacement of the first and second movable contact arms  12   a ,  12   b.    
     The first and second movable contact arms  12   a ,  12   b  each have a first, or proximal, end, which is not illustrated in  FIG. 1 . This proximal end of each of the first and second movable contact arms  12   a ,  12   b  is here connected to a terminal through which current may flow. At or adjacent to a second, distal end  16   a ,  16   b  of each of the first and second movable contact arms  12   a ,  12   b  is mounted a first and second movable contact or contacts  18   a ,  18   b  respectively. These first and second movable contacts  18   a ,  18   b  are arranged so as to be contactable with corresponding first and second fixed contact  20   a ,  20   b  of a fixed terminal  22 , for instance, within a switching contactor. Displacement of the first and second movable contact arms  12   a ,  12   b  relative to one another can effect the opening or closing of a contact set formed by the first and second movable contacts  18   a ,  18   b  and the first and second fixed contacts  20   a ,  20   b.    
     The elongate bodies  14   a ,  14   b  of the first and second movable contact arms  12   a ,  12   b  are formed so as to have a major body portion  24   a ,  24   b , which may be attached to a rigidifying member  26  such as that illustrated, and a minor body portion  28   a ,  28   b  to which the first or second movable contacts  18   a ,  18   b  are affixed. The first and second major and minor body portions  24   a ,  24   b ,  28   a ,  28   b  are laterally displaced relative to one another by a stepped portion  30   a ,  30   b  of the elongate bodies  14   a ,  14   b , which forms a kinked, stepped or shouldered region thereof. 
     The stepped portions  30   a ,  30   b  of the first and second movable contact arms  12   a ,  12   b  may have primary and secondary different shoulders  32   a ,  32   b ,  34   a ,  34   b  to create a multi-stage stepped portion  30   a ,  30   b . The primary, or contact-clearance shoulders  32   a ,  32   b  provide the necessary spacing between the first and second minor body portions  28   a ,  28   b  so as to permit the fixed terminal  22  and first and second fixed contacts  20   a ,  20   b  to be positioned between the first and second movable contacts  18   a ,  18   b  in use. 
     The secondary, or engagement shoulders  34   a ,  34   b  act to provide an asymmetry between the first and second movable contact arms  12   a ,  12   b , either in a longitudinal or lateral direction of the elongate bodies  14   a ,  14   b , or by providing a slightly different angular configuration of the stepped portions  30   a ,  30   b . It will be appreciated, however, that a similar effect could feasibly be achieved by forming the primary shoulders  32   a ,  32   b  asymmetrically, and therefore a single step, kink or shoulder in the stepped portion  30   a ,  30   b  could be provided. 
     Between the contact-clearance and engagement shoulders  32   a ,  32   b ,  34   a ,  34   b , there can be a moving-member-engagement body portion  36   a ,  36   b  which defines first and second engagement surfaces  38   a ,  38   b  on the opposed inner faces of the first and second movable contact arms  12   a ,  12   b , which can be contacted by an actuation element, such as the movable member  40 , preferably being a wedge-shaped member as shown, of a switching contactor actuator. In the embodiment shown, the first and second engagement surfaces  38   a ,  38   b  respectively define first and second leading edges  42   a ,  42   b  which are the first points of contact of the engagement surfaces  38   a ,  38   b  with the movable member  40 . By positioning the first and second leading edges  42   a ,  42   b  so as to be spaced apart with respect to one another, an asymmetric actuation force can be created. 
     Whilst the difference in positioning of the first and second leading edges  42   a ,  42   b  is in a longitudinal direction of the elongate bodies  14   a ,  14   b , it will be appreciated that a similar effect could be created by creating different lateral or perpendicular positioning of the first and second leading edges  42   a ,  42   b . Furthermore, it is noted that whilst the leading edges  42   a ,  42   b  are referred to as such, it will be appreciated that leading is used to refer to the first part of the engagement surfaces  38   a ,  38   b  to come into contact with the actuation element, rather than necessary the longitudinally forwardmost portion of the engagement surfaces  38   a ,  38   b , that is, not necessarily the portion closest to the first or second movable contact  18   a ,  18   b.    
     The effect of positioning the second leading edge  42   b  in advance of the first leading edge  42   a  can be achieved by providing a stepped portion  30   b  of the second movable contact arm  12   b  which is larger than the stepped portion  30   a  of the first movable arm  12   a , ensuring that the second leading edge  42   b  is closer to the second movable contact  18   b  than the first leading edge  42   a  is to the first movable contact  18   a.    
     Preferably, the first and second movable contacts  18   a ,  18   b  are respectively formed as lead and lag contacts, that is, the first movable contact or contacts  18   a  being larger than the second movable contactor contacts  18   b  and/or formed from a more resilient or long-lived electrically-conductive material. The first movable contact  18   a  is then arranged to close first and open second during a contact opening or closing action, therefore taking the brunt or majority of the effects of electrical arcing or similar deleterious effects associated with the opening and closing of contacts. 
     A contact opening process is illustrated by  FIGS. 2 a  to 2 e   .  FIG. 2 a    shows the movable contact arm set  10  in a contacts-closed condition with the fixed terminal  22 . The movable member  40  of the actuation element is retracted, and does not contact with the engagement surfaces  38   a ,  38   b  of the first and second movable contact arms  12   a ,  12   b . The first and second movable contacts  18   a ,  18   b  are both respectively in contact with the first and second fixed contacts  20   a ,  20   b , and electrical current can flow through the first and second movable contact arms  12   a ,  12   b  through to the fixed terminal  22 . 
       FIGS. 2 b  and 2 c    show the approach towards and opening of the movable contact arm set  10  into an intermediate condition between a contacts-closed and contacts-open condition following advancement of the movable member  40 . In this intermediate condition, the movable member  40  has been linearly actuated towards the first and second engagement surfaces  38   a ,  38   b . A first lag actuation surface  43   b  of the movable member  40  as shown advances so as to contact with the second leading edge  42   b  of the second engagement surface  38   b , as can be seen in  FIG. 2 b   , which is closer to the second movable contact  18   b  than the first leading edge  42   a  is to the first movable contact  18   a . This first lag actuation surface  43   b  has an angled profile so as to provide a consistently increasing force to the second leading edge  42   b  of the second movable contact arm  12   b . In  FIG. 2 b   , contact has been made by the movable member  40  with the second leading edge  42   b , but the switch remains in a contacts-closed condition, and current is able to flow through both of the first and second movable contact arms  12   a ,  12   b.    
     Upon further advancement of the movable member  40 , the first lag actuation surface  43   b  presses against the second leading edge  42   b  such that the second movable arm  12   b  is flexed or otherwise been displaced such that the second minor body portion  28   b  is urged away from the fixed terminal  22 , and this can be seen in  FIG. 2 c   . This separates the second movable contact  18   a  from the second fixed contact  20   b . The position shown in  FIG. 2 c    shows the movable member  40  having advanced to such a position so as to achieve a point of pick-up on the first movable contact arm  12   a . In  FIG. 2 c   , the second movable contact arm  12   b  has been displaced as the movable member  40  contacts and force outwards the leading edge  42   b . The second movable contact  18   b  is separated from the second fixed contact  20   b ; however, as this is a lag contact set, and the first movable and fixed contacts  18   a ,  20   a  remain closed, current can still flow through the lead contact set. 
     As the first leading edge  42   a  is only just about to come into contact with a first lead actuation surface  43   a  of the movable member  40  of the actuation element, no displacement of the first movable arm  12   a  has occurred. As such, electrical current can still flow through the first movable arm  12   a  and into the fixed terminal  22 , and the switch is not opened. This permits the size of the or each second movable contact  18   b  to be reduced, since the effect of electrical arcing is minimised, since this is associated with the making or breaking of electrical contact in a switch. 
     The complete opening of the movable contact arm set  10  is illustrated in  FIGS. 2 d  and 2 e   , as the first and second movable contact arms  12   a ,  12   b  move into a contacts-open condition. 
     The movable member  40  of the actuations element is driven closer to the proximal ends of the first and second movable contact arms  12   a ,  12   b , and in doing so, the first lead actuation surface  43   a  of the movable member  40  has now applied a force to the first engagement surface  38   a ; this can be seen in  FIG. 2 d   . In doing so, the first minor body portion  28   a  is displaced relative to the fixed terminal  40 , thereby breaking the connection between first movable contact  18   a  and the first fixed contact  20   a , and by extension opening the switch. It is at this point that electrical arcing may occur. The effects of electrical arcing can therefore be mitigated by the increased size and/or resilience of the first movable contact  18   a  and the first fixed contact  20   a . However, in the depicted embodiment, the actuation surface of the movable member  40  engaged with the second movable contact arm  12   b  has plateaued into a second lag actuation surface  44   b , which now is or is substantially parallel to the second engagement surface  38   b  of the second movable contact arm  12   b . This changes the force applied to the second movable contact arm  12   b , providing a gentler, stepped opening and closing action. This in turn reduces a loading on the solenoid actuator with which the movable member  40  is associated. 
     A fully contacts-open condition can then be seen in  FIG. 2 e   , in which the first movable and fixed contacts  18   a ,  20   a  have been separated to such a degree as to mitigate the effects of electrical arcing, the separation being too large to permit electrical discharge between the two contacts  18   a ,  20   a . Notably, at the point of contact, the first lead actuation surface  43   a  of the wedge-shaped member  40  has contacted with the first engagement surfaces  38   a , with a second lead actuation surface  44   a  having been urged into contact with the first engagement surface  38   a , being parallel or substantially parallel therewith. As such, the first and second engagement surfaces  38   a ,  38   b  are parallel or substantially parallel with one another. This geometric arrangement is achieved by careful selection of the angular configurations of the contact-clearance and engagement shoulders  32   a ,  32   b ,  34   a ,  34   b . As such, the loading on the actuator is reduced compared with prior arrangements. 
     The movable member  40  may also formed so as to be asymmetric, having a much longer train to contact the first engagement surface  38   a  than for the second engagement surface  38   b . This may advantageously limit the propensity for or likelihood of contact bounce as the opening or closing force applied to the first engagement surface  38   a  is greatly extended and is much more uniform as a result. For a flexible movable contact arm  12   a , there may be potential for flexion of the distal end  16   a  back towards the fixed contact  20   a  past the point of pick-up, potentially increasing the risk of contact bounce, and therefore the extended train ensures that a displacement force continues to be applied to the first leading edge  42   a.    
     The closure of the movable contact arm set  10  is therefore illustrated by the reverse process, visualised from  FIGS. 2 e  to 2 a   . With contact closure, the risk of electrical arcing occurs at the point of release of the movable member  40  from the first leading edge  42   a , at which point the first movable and fixed contacts  18   a ,  20   a  come into proximity once more. Using such a movable contact arm set  10 , it becomes possible to provide a simple means of providing lead-lag contact opening and closing for a switching contactor, which can limit the damage to the expensive materials of the movable and fixed contacts  18   a ,  18   b ,  20   a ,  20   b  which can occur during use. The careful selection of the stepped profile of the first and second movable contact arms  12   a ,  12   b  in the region of contact with the movable member  40  of the actuator therefore provides for accurate control over a lead-lag opening and closing sequence of the contactor. 
     A pair of a second embodiment of movable contact arm sets is illustrated in the context of a switching contactor, the switching contactor being illustrated globally as  146  in  FIG. 3 , with the movable contact arm sets as  110 . Identical or similar components to those of the first embodiment will be referred to using identical or similar reference numerals respectively, and further detailed description is omitted for brevity. 
     The switching contactor  146  here comprises two movable contact arm sets  110  and an actuator assembly  148 , typically but not necessarily exclusively formed as a magnet-latching solenoid actuator assembly. Here, the actuator assembly  148  has a main drive unit  150  which drives a plunger  152  along a linear axis. The plunger  152  is engaged with a movable bar  154 , to which are engaged two wedge-shaped members  140 , arranged to displace the respective first and second movable contact arms  112   a ,  112   b  of the movable contact arm sets  110  to effect opening and closing of the contact sets. 
     Each of the first and second movable contact arms  112   a ,  112   b  is engaged with a proximal terminal  156  at a first end  158 , having first and second movable contacts  118   a ,  118   b  respectively at a second end  116   a ,  116   b . The first and second movable contacts  118   a ,  118   b  are respectively displaceable with respect to first and second fixed contacts  120   a ,  120   b , engaged with a fixed terminal  122 . 
     Here, the first movable contact arms  112   a  are illustrated having a tang  160  and peg  162  arrangement which serves to aid a contact pressure on the first movable contact  118   a  to limit contact bounce during operation. 
     The stepped portions  130   a ,  130   b  of the first and second movable contact arms  112   a ,  112   b  are different to that of the embodiment depicted in  FIG. 1  and  FIGS. 2 a  to 2 e   ; the asymmetry is not only provided by the longitudinal displacement of the leading edges  142   a ,  142   b  of the engagement surfaces  138   a ,  138   b , but there is also a lateral or perpendicular asymmetry. This is here achieved by difference depths and/or angular positioning of at least one of the primary and secondary shoulders  132   a ,  132   b ,  134   a ,  134   b  between the first and second movable contact arms  112   a ,  112   b.    
     In use, the wedge-shaped member  140  is actuated towards the leading edges  142   a ,  142   b  by the actuator assembly  148 . Notably, the arrangement of the wedge-shaped member shown is such that it may contact the primary shoulders  132   a ,  132   b  of the movable contact arms  112   a ,  112   b  before contacting with the respective leading edges  142   a ,  142   b  and therefore pick-up of the movable contact arms  112   a ,  112   b  may occur prior to contact with the leading edges  142   a ,  142   b , thereby creating a two-stage pick-up. 
     The wedge-shaped member  140  will not only contact the second primary shoulder  132   b  and leading edge  142   b  prior to the first primary shoulder  132   a  and leading edge  142   a  when opening the contacts as a result of the relative longitudinal positioning of the primary shoulders  132   a ,  132   b  and leading edges  142   a ,  142   b , but also as a result of the relative lateral positions of the engagement surfaces  138   a ,  138   b , due to the wedge-shape of the wedge-shaped member  140 . 
     It can also be seen that the angular configuration or slope of the first primary and secondary shoulders  132   a ,  134   a  is different to that of the second primary and secondary shoulders  132   b ,  134   b . These angular configurations or slopes can be formed so as to alter the timing sequence between the lead and lag movable contacts  118   a ,  118   b ; a gentler slope on one of the primary and secondary shoulders  132   a ,  132   b ,  134   a ,  134   b  will result in a gentler opening of the respective contact  118   a ,  118   b . A run of the engagement surface  138   a ,  138   b  can be chosen so as to ensure a stable opening and closure force, minimising the likelihood of contact bounce. 
     As such, the opening and closing of the switching contactor  146  will operate with a lead-lag effect, and, in doing so, achieve the same kind of benefits to those described above. A qualitative graph of the stroke force applied to the first and second movable contact arms  112   a ,  112   b  is illustrated in  FIG. 4  globally at  200 . The lower line F 1  represents the force applied to the first movable contact arm  112   a  and the upper line F 2  represents the force applied to the second movable contact arm  112   b.    
     As the movable member  140  advances toward the movable contact arms  112   a ,  112   b , it will first pick-up the second primary shoulder  132   b , and this is indicated at point D 1  in  FIG. 4 . The force increases as the movable member  140  is pressed further across the angled engagement surface  138   b . At point D 2 , the wedge-shaped member  140  will contact the second leading edge  142   b  of the second movable contact arm  112   b , leading to the stepped force graph shown. 
     As the wedge-shaped member  140  advances further, it will pick-up the first primary shoulder  132   a , at point D 3  illustrated. The first movable contact arm  112   a  therefore begins to be displaced as the wedge-shaped member  140  advances, and is displaced further still as the wedge-shaped member  140  contacts with the leading edge  142   a  of the first movable contact arm  112   a . This leads to the advantageous lead-lag contact opening and closing arrangement as detailed above. The stepped force profile, which ensures that force applied to the movable contact arms  112   a ,  112   b  does not plateau following first engagement by the actuator  148  further limits the ability of the switching contactor  146  to experience contact bounce in normal operation. 
     Such a stepped engagement surface  138   a ,  138   b  ensures that a gentle pick-up of the respective movable contact arms  112   a ,  112   b  occurs, resulting in greater control over the pick-up. This in turn allows for more consistent timing to be applied to the contact opening process, limiting the chances for deleterious opening or closing conditions to be experienced. A fast opening limits the chance of electrical arcing, and this can be provided for by providing relatively steep first and second primary shoulders  132   a ,  132   b . Once the danger of electrical arcing has receded, that is, once the first movable and fixed contacts  118   a ,  120   a  are sufficiently separate, then the gentler slope of the moving-member-engagement body portions  136   a ,  136   b  provides the controlled later urging of the movable contact arms  112   a ,  112   b  which allows for precise chronal control. 
     It is therefore apparent that the above-described arrangements allow for a method of providing a lead-lag contact opening arrangement for a switching contactor  146 , which comprises the steps of providing a pair of movable contact arms  112   a ,  112   b  of the switching contactor  146  which are asymmetric to one another, and then applying a linear actuation force towards the pair of movable contact arms  112   a ,  112   b  such that the movable contact arms  112   a ,  112   b  are displaced relative to one another asynchronously. The asymmetry of the movable contact arms  112   a ,  112   b  ensures that a movable member  140  of the switching contactor  146  is able to convert a linear actuation force into a delayed lead-lag contact opening arrangement by careful selection of the engagement surfaces  138   a ,  138   b  of the first and second movable contact arms  112   a ,  112   b.    
     It will be appreciated that wherever the terms movable or fixed contact have been utilised above, that the plural could apply; multi-bladed movable contact arms having more than one movable contact thereon are known in the art, and the present invention should not be taken to exclude such arrangements. 
     Similarly, whilst flexible electrically-conductive movable contact arms are illustrated, it will be apparent that a pivotable, hinged or similarly displaceable movable arm arrangement could alternatively be provided. Curved or parabolic asymmetric movable arms could also be provided, where the wedge-shaped engagement member contacts a curving engagement surface so as to provide a smooth opening profile. However, such an arrangement is likely to be less resilient to tolerances in the manufacturing of the movable members when compared with the above-described embodiments of the invention, which provides well-defined and achievable pick-up points for the wedge-shaped member, resulting in consistency in the opening gaps. Similarly, the above embodiments, having accurately plateaued engagement surfaces, ensure rapid and decisive opening and closing of the contacts, so as to ensure rapid quenching of any electrical arcing which may have formed. 
     It is therefore possible to provide a movable contact arm set which permits a lead-lag contact opening and closing arrangement, in particular for use with switching contactors. By careful selection of the shape of the movable contact arms, such that they are asymmetric with respect to one another, it is possible to achieve asynchronous opening and/or closing of the respective movable contacts on the first and second movable contact arms, thereby achieving the benefits associated with lead-lag contact arrangements without needing to provide a bespoke or complicated actuator arrangement. The physical form of the first and second movable contact arms ensures that the correct lead-lag contact opening and closing sequences are enforced, since the movable contact arms are physically and outwardly displaced by the actuation of the movable member of the actuator. 
     The words ‘comprises/comprising’ and the words ‘having/including’ when used herein with reference to the present invention are used to specify the presence of stated features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. 
     The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.