Patent Publication Number: US-9837229-B2

Title: Method and apparatus for controlling circuit breaker operation

Description:
FIELD OF THE INVENTION 
     The present invention relates to the operation of electrical switches, especially circuit breakers. 
     BACKGROUND TO THE INVENTION 
     Circuit breakers, including reclosers, typically comprise an electromagnetic actuator for moving an electrical contact between open and closed states. Closing the actuator usually involves energising one or more electromagnetic coils to move the contact against a mechanical bias such as a spring. In order to preserve the mechanical life of the circuit breaker, the speed at which the contact moves should be restricted. This adversely affects the efficiency of the actuator, resulting in increased weight size and power consumption for the circuit breaker. 
     It would be desirable to provide an improved method for controlling the operation of circuit breakers that mitigates the problem outlined above. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a method of controlling an electrical switch, the electrical switch comprising a movable contact and an electromagnetic actuator for causing said movable contact to move between an open position and a closed position, said method comprising: 
     with said movable contact in said open position, applying a voltage to said actuator to cause a motive force to be applied to said movable contact to cause said movable contact to move towards said closed position, wherein said voltage is applied for a first time period ending before said movable contact reaches said closed position, and 
     at the end of said first time period, adjusting said voltage to reduce said motive force. 
     In typical embodiments, said method further includes, after said voltage is adjusted to reduce said motive force, further adjusting said voltage to increase said motive force. Said further adjusting of said voltage is preferably performed before said movable contact reaches said closed position, especially immediately before said movable contact reaches said closed position. In particular, it is preferred that said further adjusting of said voltage is performed sufficiently close to the moment when said movable contact reaches said closed position that said further voltage adjusting does not appreciably affect the speed of said movable contact. For example, said further adjusting of said voltage may be performed up to 2 ms, preferably up to 1 ms, and more preferably up to 0.5 ms, before said movable contact reaches said closed position. Said further adjusting of said voltage may be performed substantially at the same time as said movable contact reaches said closed position. 
     Optionally, said adjusting said voltage to reduce said motive force involves reducing said voltage to a non-zero level. Said adjusting said voltage to reduce said motive force may involve reducing said voltage by at least approximately 50% to a non-zero level. 
     Alternatively, said adjusting said voltage to reduce said motive force involves reducing said voltage to zero. 
     Alternatively still, said adjusting said voltage to reduce said motive force involves reversing the polarity of said voltage. 
     Alternatively, said adjusting said voltage to reduce said motive force involves modulating said voltage. Said adjusting said voltage to reduce said motive force may involve pulse width modulating said voltage. Said pulse width modulation may be arranged to cause zero volts to be applied to said actuator between pulses. 
     In typical embodiments, said switch includes a control circuit, said control circuit including at least one capacitor for storing said voltage, and wherein said applying a voltage to said actuator to cause a motive force to be applied to said movable contact involves applying said voltage from said at least one capacitor to said actuator. Adjusting said voltage to reduce said motive force may therefore involve adjusting said voltage applied from said at least one capacitor to said actuator. 
     In preferred embodiments, said actuator comprises at least one electromagnetic coil, and wherein said applying a voltage to said actuator to cause a motive force to be applied to said movable contact involves applying said voltage to said at least one coil. Typically adjusting said voltage to reduce said motive force involves adjusting said voltage applied to said at least one coil. 
     From a second aspect the invention provides an electrical switch comprising a movable contact and an electromagnetic actuator for causing said movable contact to move between an open position and a closed position, said switch further comprising 
     a voltage source, 
     a controller for selectably applying voltage from said voltage source to said actuator, 
     wherein said controller is arranged to, with said movable contact in said open position, cause a voltage to be applied to said actuator from said voltage source to cause a motive force to be applied to said movable contact to cause said movable contact to move towards said closed position, 
     and wherein said controller is arranged to apply said voltage for a first time period ending before said movable contact reaches said closed position, 
     and wherein said controller is further arranged to, at the end of said first time period, adjust said voltage to reduce said motive force. 
     Preferably, said voltage source comprises at least one capacitor. 
     Typically, said actuator comprises at least one electromagnetic coil, said controller being arranged to selectably apply voltage to said at least one electromagnetic coil. 
     Said actuator may include a movable part movable into and out of a closed position in response to changes in the energization of said at least one electromagnetic coil. Preferably, said actuator includes a non-movable part, and wherein said movable and non-movable parts are configured to latch magnetically with one another in a closed position as a result of residual magnetism of said movable and non-movable parts (said residual magnetism resulting from the prior effect of said at least one coil when energised (i.e. by the flow of current) on said movable and non-movable parts). 
     Said electrical switch may comprise a circuit breaker or a vacuum interrupter. 
     Further advantageous aspects of the invention will become apparent to those ordinarily skilled in the art upon review of the following description of a specific embodiment and with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1  is a sectioned side view of a circuit breaker suitable for use with the present invention; 
         FIG. 2  is a sectioned side view of an actuator suitable for use in the circuit breaker of  FIG. 1 , the actuator being shown in a closed state; 
         FIG. 3  is a side sectioned view of the actuator of  FIG. 2 , the actuator being shown in an open state; 
         FIG. 4  is a schematic view of a control circuit suitable for use in controlling the operation of the circuit breaker of  FIG. 1 ; 
         FIG. 5A  is a graph showing actuator coil voltage against time for a simple control method; 
         FIG. 5B  is a graph showing contact speed against time for the simple control method; 
         FIG. 6A  is a graph showing actuator coil voltage against time for a first control method embodying the invention; 
         FIG. 6B  is a graph showing contact speed against time for said first embodiment; 
         FIG. 7A  is a graph showing actuator coil voltage against time for a second control method embodying the invention; 
         FIG. 7B  is a graph showing contact speed against time for said second embodiment; 
         FIG. 8A  is a graph showing actuator coil voltage against time for a third control method embodying the invention; 
         FIG. 8B  is a graph showing contact speed against time for said third embodiment; 
         FIG. 9A  is a graph showing actuator coil voltage against time for a fourth control method embodying the invention; and 
         FIG. 9B  is a graph showing contact speed against time for said fourth embodiment; 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring now in particular to  FIG. 1  of the drawings, there is shown, generally indicated as  10  an electrical switch device of a type commonly referred to as a circuit breaker or interrupter. The switch  10  is configured to operate automatically in a fault condition, e.g. a current overload or short circuit, to protect the circuit (not shown) into which it is incorporated during use. It achieves this by breaking the electrical circuit in response to detecting a fault, thereby interrupting current flow. In some embodiments, the switch  10  can be reset manually (e.g. mechanically or electro-mechanically by manual activation of a user control (not shown)) or automatically (typically electro-mechanically in response to the switch  10  detecting that the fault has gone, and/or after a threshold period of time has expired since activation). Circuit breakers that reset automatically are commonly known as reclosers. 
     The switch  10 , which is hereinafter referred to as a circuit breaker, comprises first and second electrical contacts  12 ,  14 . The first contact  12  is movable between an open position (as shown in  FIG. 1 ) and a closed position (not illustrated) in which it makes electrical contact with the second contact  14 . The open position of the contact  12  corresponds to the open, or breaking, state of the circuit breaker  10  in which it interrupts current flow. The closed position of the contact  12  corresponds to the closed, or making, state of the circuit breaker  10  in which current is able to flow between the contacts  12 ,  14 . 
     In the illustrated embodiment, the contacts  12 ,  14  are located in a vacuum chamber  16  and the circuit breaker  10  may be referred to as a vacuum circuit breaker. 
     Movement of the contact  12  between its open and closed positions is effected by an electromagnetic actuator  18 , which is described in further detail hereinafter with reference to  FIGS. 2 and 3 . To this end, the actuator  18  is mechanically coupled to the movable contact  12 . In the illustrated embodiment, a mechanical coupling device  20  is provided between the actuator  18  and the contact  12  and is configured to translate movement of the actuator  18  into a corresponding movement of the contact  12 . In particular, the coupling device  20  translates substantially linear movement of the actuator  18  into substantially linear movement of the contact  12 . Preferably, the coupling device  20  comprises a coupling member  22  formed from an electrically insulating material. 
     Referring now to  FIGS. 2 and 3 , the preferred actuator  18  is described. The actuator  18  comprises a body  24  having a first part  24 A and a second part  24 B. The first part  24 A is movable with respect to the second part  24 B between a closed position ( FIG. 2 ) and an open position ( FIG. 3 ), the second part  24 B typically being fixed with respect to the circuit breaker  10  during use. Resilient biasing means are provided to urge the first part  24 A towards and preferably into the open position. In typical embodiments, the resilient biasing means is arranged to urge the first part  24 A into the open position, and may comprise any suitable resilient biasing device, e.g. one or more compression springs  26 . 
     The actuator  18  comprises a stem  28  which conveniently carries the spring  26 . In the illustrated embodiment, the free end  30  of the stem  28  is coupled to the coupling member  22 . In use, as part  24 A moves towards part  24 B, it causes rod  30  to move upwardly (as viewed in the drawings). Corresponding movement is imparted to a second stem  29  via the coupling member  22 , the second stem  29  being coupled between the coupling member  22  and the movable contact  12 . This movement of the second stem  29  causes the contact  12  to move towards and ultimately into the closed position. Resilient biasing means, for example comprising one or more compression springs  27 , may be coupled between the movable part  24 A and the stem  28 . The preferred arrangement is such that, when the part  24 A is in its closed position, spring  27  is compressed and so imparts force to the stem  28  to help maintain contact  12  in its closed position. 
     Hence, movement of the part  24 A towards its closed position causes movement of the contact  12  towards its closed position. It is noted that the part  24 A and contact  12  may not reach their respective closed positions at the same time. For example, in the illustrated embodiment, contact  12  reaches its closed position before part  24 A does. The preferred arrangement is such that the movement of the part  24 A that occurs after contact  12  is closed serves to compress spring  27 . 
     The actuator  18  includes an electromagnetic operating device  32  comprising one or more electromagnetic coil  36  (which may comprising one or more windings), and typically a coil holder. The coil  36  is typically annular and is shown in  FIGS. 2 and 3  in cross section. The coil  36  is typically configured to form a solenoid. The coil  36  is energised by applying a voltage to it causing current to flow through the coil, the current creating an electromagnetic field around the coil. Conversely, the coil  36  is de-energised by reducing the current flowing through the coil  36 . The arrangement is such that, when energised, the coil  36  acts as an electromagnet that urges the movable part  24 A towards the closed position and also, in preferred embodiments, magnetises the parts  24 A,  24 B to create latching residual magnetism between them. 
     In the preferred embodiment, a solid core is not present within the coil  36 . However, movable part  24 A may be regarded as an electromagnetic core for the coil  36 , while non-movable part  24 B may be regarded as a yoke. Typically, parts  24 A,  24 B are formed at least partly from magnetisable, or ferromagnetic, material that is non-permanently magnetised but is susceptible of being magnetised by the electromagnetic field generated in use by the coil  36 . Alternatively, one or both of parts  24 A,  24 B may be formed at least partly from permanently magnetised material. 
     The coil  36  is carried by, typically fixed to, one of the parts  24 A,  24 B, in this example the second part  24 B. The preferred arrangement is that the coil  36  projects from the second part  24 B and the first part  24 B is shaped to receive the projecting portion of the coil  36  when the parts  24 A,  24 B are together. The first part  24 A may be held in the closed position by one or more of a variety of ways depending on the embodiment. For example, where one or both of the first or second parts  24 A,  24 B comprises a permanent magnet, or is otherwise formed at least partly from magnetisable material, the first part  24 A may be held closed by residual magnetism (indicated by magnetic flux lines RM in  FIG. 2 ) in the first and/or second parts  24 A,  24 B. Alternatively, or in addition, the coil  36  may remain energised to hold the first part  24 A in the closed position by electromagnetic force created by the electromagnetic field around the coil. In the illustrated embodiment, the coil  36  creates residual magnetism in the first and second parts  24 A,  24 B such that, when the coil  36  is subsequently de-energised, the first and second parts  24 A,  24 B are held together. 
     The coil  36  may be operated to release the first part  24 A by controlling the voltage applied to the coil  36 , and in particular by controlling the current flowing in the coil. For example, in embodiments where the coil  36  is energised to maintain the latching state by electromagnetism, the coil  36  may be released by de-energising the coil  36  (i.e. reducing the current flowing in the coil). In preferred embodiments, a suitable voltage may be applied to the coil  36  resulting in an electromagnetic field that has the effect of overcoming or cancelling any residual magnetism (including permanent magnetism) that is maintaining the latched state. Conveniently, this is achieved by applying a voltage to the coil with opposite polarity to the voltage used to close the actuator  18 . 
     When the coil  36  is operated as described above (i.e. when the first and second parts  24 A,  24 B are de-magnetised), the spring  26  actuates the first part  24 A of the body into its open position ( FIG. 3 ). Returning the first part  24 A to the closed position can be achieved by energising the coil  36  with a voltage suitable for creating an electromagnetic field around the coil  36  that has the effect of drawing the first part  24 A into its closed position (and such that the bias of spring  26  is overcome). Movement of the first part  24 A towards its open position causes movement of the contact  12  towards its open position. In the illustrated embodiment, an initial movement of the part  24 A out of its closed position causes decompression of spring  27  and no movement of contact  12 . Subsequently, contact  12  moves towards its open position as the part  24  continues to move towards its open position. 
     Referring now to  FIG. 4 , there is shown a control circuit  40  for controlling the operation of the actuator  18 , and so controlling operation of the circuit breaker  10 . The circuit  40  is electrically connected to the, or each, electromagnetic coil  36  and is configured to control the energisation of the coil  36 , i.e. by controlling the voltage across the coil and thus the current though the coil. The circuit  40  includes a controller  42  arranged to detect a fault condition and to energise or de-energise the coil  36  accordingly. The controller  42  may take any suitable form, e.g. comprising logic circuitry, and PLC (programmable logic controller) and/or a suitably programmed microprocessor or microcontroller. The controller  42  may be coupled to any suitable fault detection device, e.g. a current monitor. 
     In a simple embodiment (not illustrated), the control circuit may be arranged to apply an energising voltage to the coil  36  when it is desired to close the actuator  18  or keep it closed (i.e. keep the parts  24 A,  24 B magnetised), and to de-energise the coil  36 , e.g. cut or reduce the voltage, when it is desired to open the actuator  18  (wherein the parts  24 A,  24 B are such that residual magnetism does not continue to hold them together). 
     In preferred embodiments, however, where the coil  36  is held in its latching state by residual magnetism, the control circuit  40  is configured to respectively apply a voltage to the coil  36  to open the actuator  18  and to close the actuator  18 . When opening the actuator  18 , the applied voltage is selected such that it has the effect of de-magnetising the first and second parts  24 A,  24 B of the actuator as described above. When closing the actuator, the applied voltage is selected such that the coil  36  creates an electromagnetic field causing the first part  24 A to be drawn to the closed position (overcoming the bias of the spring  26 ), i.e. the energised coil  36  creates a motive force acting on the movable part  24 A of the actuator, causing the movable part  24 A to move towards the closed position, which in turn creates a motive force on the movable contact  12 , causing the contact  12  to move towards the closed position. 
     Typically, the circuit  40  includes one or more storage capacitors  44 ,  46  for energising the coil  36 . In particular, the coil  36  is energised by discharging the capacitor voltage across the coil, thereby causing current to flow through the coil to energise the coil. To this end, the circuit  40  includes one or more switches for selectably applying the or each capacitor voltage to the coil  36 . In preferred embodiments, a respective one or more capacitors are provided for opening the actuator  18  and for closing the actuator  18 . In  FIG. 1 , the voltage stored by capacitor  44  is used to close the actuator  18 , while the voltage stored by capacitor  46  is used to open the actuator  18  (and therefore to trip the circuit breaker  10 ). A respective switching device  48 ,  50  is provided for selectably applying the respective capacitor voltage to the coil  36 , the switching devices being controlled by controller  42 . The switching devices  48 ,  50  may take any suitable form but conveniently comprise one or more transistors. In the preferred embodiment, each switching device  48 ,  50  comprises a respective two transistors arranged as a transistor bridge. Typically, the circuit  40  is arranged such that the respective voltages of the capacitors  44 ,  46  are applied to the coil  36  with opposite polarity (to create respective currents in the coil with opposite polarity). The voltages applied to the coil  36  by discharging the respective capacitors  44 ,  46  are transient and have a respective profile (over time) that is determined by the respective capacitance, and typically also on the associated resistance of the circuitry by which the voltage is discharged. 
     Closing the actuator  18  consumes much more energy than opening the actuator  18  especially where the bias of the spring  26  must be overcome. One way of controlling the closing process involves direct connection of the respective capacitor  44 ,  46  to the actuator coil  36  for a limited duration (i.e. application of a transient voltage). A disadvantage of this method is the substantial energy required for actuator closing. This energy could be reduced if there were no limitation on the speed at which the actuator closes, since with increasing closing speed actuator efficiency increases. However, closing velocity should be limited in order to preserve the mechanical life of the circuit breaker  10 . For example, the closing velocity of the movable contact  12  should typically not exceed 1-1.5 m/s. Therefore, the parameters of the actuator are selected in such a way that the closing velocity does not exceed the acceptable limit. However, in this case the actuator operates with relatively low efficiency, resulting in increased weight, size and power consumption. 
     For example,  FIG. 5A  illustrates the control method described above where the capacitor voltage is applied to the coil  36  via switch  48  in the relatively uncontrolled manner described above. It will be seen that the voltage applied to the coil  36  takes an initial value V 1  and is present for a limited period ending at time T 2 , during which the applied voltage level decays.  FIG. 5B  is a graph showing how the speed of the movable contact  12  varies over the same period in response to the applied capacitor voltage. It can be seen that the contact speed grows roughly exponentially from zero during the closing process until closure occurs at time T 1 &lt;T 2 . To prevent the contact speed from exceeding an acceptable level (assumed to be approximately 1 m/s in this example), the capacitor  44  is selected such that V 1  is relatively reduced at approximately 200V. The required capacitor value is relatively high at 2.5 mF in this example, the contact closing time is relatively long (approximately 24 ms in this example) and the total duration of the closing process (including magnetization time) is relatively long at approximately 50 ms in this example. 
     In preferred embodiments, the controller  42  is configured to control the application of voltage to the coil  36  during the closing process as is now described with reference to  FIGS. 6 to 9 . In an initial stage where the movable part  24 A of the actuator  18  is in its open position (and the contact  12  is in its open position), a voltage V 1  is applied to the coil  36  from capacitor  44  for a time period P 1  ending at time T 3 , which is before the contact  12  reaches its closed position. Voltage V 1  tends to decrease relatively slowly as the capacitor  44  discharges. During P 1 , the coil  36  is energised to create a motive force on the movable part  24 A of the actuator  18  causing it to move towards its closed position, which in turn creates a motive force on the movable contact  12  causing it to move towards its closed position. Hence, during period P 1 , the movable contact  12  is accelerated to an initial speed (which may alternatively be referred to as an initial velocity since the contact  12  typically moves substantially linearly towards contact  14 ). Normally, the movable part  24 A and the movable contact  12  are stationary at the beginning of the period P 1 , i.e. at time T=0. 
     At the end of time period P 1 , the controller  42  is configured to adjust the voltage applied to the coil  36 , preferably for a second time period P 2  ending at time T 4 , where T 4  is before or substantially at the same time as the contact  12  reaches its closed position. The adjustment of the voltage is such that it reduces the motive force exerted on, and therefore the acceleration of, the movable part  24 A (by de-energisation of the coil  36 ) and correspondingly on the movable contact  12 . 
     In one embodiment, as exemplified by  FIG. 6A , the voltage applied to the coil  36  is reduced at the end of P 1  to a non-zero level that is lower than the available capacitor voltage, preferably between zero volts and, for example, approximately 50% of V 1  or of the available capacitor voltage at that time. This may be achieved by any suitable means, for example providing control circuit  40  with voltage dividing circuitry (not shown) controllable by controller  42  so that it may selectably cause all or part of the capacitor voltage to the coil  36 , or by the provision of pulse width modulation circuitry (not shown). 
     In another embodiment, as exemplified by  FIG. 7A , the voltage applied to the coil  36  is reduced at the end of P 1  to zero. Conveniently, the controller  42  may effect this by operating switch  48  to isolate the coil from the voltage across capacitor  44 . 
     In a further embodiment, as exemplified by  FIG. 8A , the voltage applied to the coil  36  at the end of P 1  has a reversed polarity, i.e. a negative voltage value, with respect to the capacitor voltage. This may be achieved by any convenient means. For example, the controller  42  may operate switch  50  to apply a voltage across the coil  36  from capacitor  46 , which in preferred embodiments has a polarity opposite that of the capacitor  44  (advantageously, the controller  42  operates switch  48  to isolate capacitor  44  in this case). 
     In a still further embodiment, as exemplified by  FIG. 9A , the voltage applied to the coil  36  at the end of P 1  is modulated, preferably pulse width modulated, and more preferably modulated between zero and the maximum available capacitor voltage. This may be achieved by any suitable means, for example providing control circuit  40  with voltage modulation circuitry (not shown) controllable by controller  42  so that it may selectably cause modulation of the capacitor voltage to the coil  36 . 
     Advantageously, at the end of time period P 2 , the controller  42  is configured to increase the voltage (including the option of increasing the effective voltage, e.g. by adjusting the modulation) applied to the coil  36 , preferably to the maximum level attainable by the control circuit  40  (which in the present embodiment is determined by the voltage across capacitor  44  and is typically less than the voltage V 1 ), for a time period P 3  ending at time T 5 , where T 5  typically ends after contact  12  has reached the closed position. This has the effect of re-energising the coil  36  to create sufficient residual magnetism in parts  24 A,  24 B to hold the actuator  18  in its closed state after the capacitor voltage has gone. In the illustrated embodiment, the voltage is increased during P 3  to increase the current in coil  36  in order to increase the magnetic flux in parts  24 A,  24 B to such a level that the parts  24 A,  24 B are held closed by residual magnetism (magnetic latching). In embodiments where residual magnetism is not required to hold the latch in its closing state, increasing the voltage during P 3  is not necessary. 
     Period P 3  may begin before (preferably just before, e.g. up to 2 ms, preferably up to 1 ms, and more preferably up to 0.5 ms before), at substantially the same moment as, or after the movable contact  12  reaches its closed position. As a result, increasing the voltage at this time does not appreciably increase the speed of the contact  12 . 
     In preferred embodiments, the desired initial speed of the contact  12  at time T 3  is determined by the desired maximum speed of the contact  12  when it engages with the fixed contact  14 . The desired maximum speed depends on the physical characteristics of the circuit breaker  10  but in general is selected so as not to cause undue damage to the contacts  12 ,  14 . Once the initial speed is known, the duration of period P 1  can be determined. This will depend not only on the physical characteristics of the circuit breaker  10  (e.g. respective masses of the movable parts  24 A,  12 , strength of the spring  26  etc.) but also on the voltage available from the capacitor  44 . It is preferred to accelerate the contact  12  to the initial speed as quickly as possible since this reduces the energy required to do so. Therefore, it is preferred to use a capacitor  44  that allows the highest practicable voltage to be provided to the coil  36 . In practice, the control circuit  40  has current limitations and so the capacitor  44  is chosen to provide the highest voltage possible without exceeding the current limitations. For example, in the circuit  40  of  FIG. 4 , the switching transistors have a current limit that determines the maximum voltage that can be provided to the coil  36  by capacitor  44 . Once the capacitor voltage is known, T 3  can be calculated. Alternatively, it can be determined empirically. 
     It will be seen therefore that in the preferred embodiment, the entire available capacitor voltage is applied to the coil  36  during the initial stage P 1  to begin to close the actuator  18  and to accelerate the movable contact  12  to the desired initial velocity. Then, the voltage (or effective voltage) is decreased deliberately (as opposed to decreasing as a result of capacitor voltage decay) by the controller  42  to suppress acceleration of the contact  12 . When the movable contact  12  approaches the closed position (and there is no time left to accelerate the respective movable parts beyond the desired maximum speed), or afterwards, the voltage is increased again, providing growth of coil current to a level sufficient for effective magnetization of the actuator&#39;s components to allow magnetic latching in the closed position. 
     In the example of  FIG. 6 , an initial voltage of 385V is applied to the coil  36 , then at T 3 =7 ms the voltage is reduced by approximately 50%. Subsequently, at time T 4 =16.5 ms the voltage is increased again. As a result, for the same circuit breaker  10 , in comparison with the method of  FIG. 5 , actuator closing time is reduced from 24 ms to 17 ms, total closing time (including latch magnetization time) is reduced from 50 ms to 27 ms and stored energy required for closing is reduced from 50 J to 22 J. Even so, it is noted that the respective closing speeds of the contacts in the examples shown in  FIG. 5  and  FIG. 6  are substantially the same (approximately 1 m/s). 
     In practice, the speed of moving contact  12  is important as it affects the mechanical life of the vacuum interrupter or other device. Typically, the respective speeds of movable contact  12  and part  24 A of the actuator  18  are substantially equal until movable contact  12  hits the fixed contact  14  (due to the fact that part  24 AB during upward movement pushes stem  28  of the insulator  22  with the aid of additional contact pressure spring  27 ). At the moment when contacts  12 ,  14  close together, there is a gap, e.g. of approximately 2 mm, between the parts  24 A,  24 B of the actuator  18 . After this moment movable contact  12  does not move but part  24 A keeps moving until the gap is closed. 
     The invention is not limited to the embodiment described herein, which may be modified or varied without departing from the scope of the invention.