Patent Publication Number: US-11657994-B2

Title: Protected switch

Description:
This application is the U.S. national phase of International Application No. PCT/EP2020/061302 filed Apr. 23, 2020 which designated the U.S. and claims priority to FR Patent Application No. 1904314 filed Apr. 24, 2019, the entire contents of each of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a protected switch and, more generally, to the field of electrical switching devices. 
     BACKGROUND 
     Safety electrical switching devices are known, such as relay-based switches, which are used to alternately enable or disable the flow of an electrical current in an electrical circuit. 
     Particularly known are switching devices containing one or more electromechanical relays, the contacts of which are connected together in series to form an electrical interrupting circuit, known as safety chain, which serves to electrically connect an electrical load to an electrical source, for example. 
     Depending on the relay state, the safety chain is switchable between an “off” state, in which at least one of the contacts is open, to prevent the flow of an electric current, and an “on” state, in which all the contacts are closed, to allow the flow of the current. 
     Such devices are typically used in control systems, for controlling railway facilities or equipment for example, and must meet high safety and reliability requirements. 
     In the absence of a control signal, such a device must be able to ensure that the safety chain is switched to an open state and thus the electrical load cannot be supplied. In particular, such a device must ensure that the safety chain cannot remain in an “on” state in the event of a failure, as a result of one of the contacts being kept in the closed state accidentally, for example. 
     For example, so-called intrinsic safety relays are known, in which the electrical contacts of the safety chain open under the effect of gravity when the relay is no longer energized, such as the NS1 relays defined in standard NF 70-030. However, these relays have the disadvantage of being heavy and bulky. They must also be installed with a particular orientation, according to the direction of the earth&#39;s gravity. Their use is therefore complicated. These relays are also difficult to miniaturize, which can be an obstacle to their use in certain applications. 
     On the other hand, devices containing two electromechanical relays are known, with guided contacts controlled by an electronic control unit that permanently measures the state of each of the two contacts. If one of these contacts remains closed while the corresponding relay is not controlled, then the control unit detects this and prevents the other relay from being energized, in order to maintain the safety chain in its “off” state. 
     However, such a device has the drawback of requiring a dedicated electronic control unit to measure the relay state, which requires a permanent power supply, in addition to being costly and complicating the facility and operation of the device. 
     Finally, DE 44 41 171 C1 describes a switching apparatus containing interconnected electromechanical relays. However, the operation of this device is unsatisfactory in certain circumstances, particularly with respect to the relay switching order upon a change of state. 
     The invention intends to remedy these drawbacks more particularly by proposing a protected switch with a simplified design for supplying electrical appliances and which, in the event of a failure ensures the opening of an electrical circuit in a safe manner. 
     SUMMARY 
     To this end, one aspect of the invention relates to a protected switch comprising:
         a first electromechanical relay with guided contacts comprising a first electromagnet and a plurality of electrical contacts;   a second electromechanical relay with guided contacts comprising a second electromagnet and a plurality of electrical contacts, a first electrical contact of the first relay and a first electrical contact of the second relay being electrically connected in series between switch terminals;   a rechargeable power supply;   an interconnection circuit that connects at least a portion of the other electrical contacts of the first and second relays and wherein:   the first electromagnet is connected to control electrodes of the switch, via second electrical contacts of the second relay and second electrical contacts of the first relay, to make the first electromagnet connection to the control electrodes conditional on the state of the second relay;   the second electromagnet is connected to the control electrodes, via third and fourth electrical contacts of the first relay and said second electrical contacts, to alternately connect or disconnect the second electromagnet to the control electrodes depending on the state of the first relay, the third contact being a normally closed contact connected between the power supply and the first electromagnet, the fourth contact being a normally open contact connected between the power supply and the second electromagnet.       

     Thanks to the invention, the interconnection circuit makes the power supply to the electromagnet of each relay conditional, according to the state occupied by the other relay, which intrinsically ensures control over the state of the contacts of the interruption circuit, without the need for an electronic control unit. 
     Thus, if one of the two electrical contacts of the interrupt circuit fails and the relay to which it belongs is in an abnormal state, the other relay cannot be energized, thus keeping the other electrical contact of the interrupt circuit in the open state. 
     This intrinsic safety is achieved here without the use of the earth&#39;s gravity, thus reducing the mechanical complexity and size of the switch compared to known intrinsic relays. In addition, the switch is not dependent on the earth&#39;s gravity and can therefore be installed without orientation constraints. 
     In addition, the configuration of the interconnection circuit ensures that the opening or closing of the relays is done with a specific predefined sequencing, in particular to avoid the safety chain being in the “on” state when it should not be. 
     According to advantageous but non-mandatory aspects of the invention, such a switch may incorporate one or more of the following features, taken alone or in any technically permissible combination:
         The control voltage of the first electromagnet is different from the control voltage of the second electromagnet.   The control voltage of the first electromagnet is greater than the control voltage of the second electromagnet, preferably greater than twice the control voltage of the second electromagnet.   The second contact of the first relay and the second contact of the second relay are connected in parallel with each other, the second contact of the first relay being a normally open contact, the second contact of the second relay being a normally closed contact.   The second electromagnet is further connected to one of the control electrodes via a fourth contact of the second relay, this fourth contact being a normally open contact.   The switch includes an electrical resistor, connected to the second electromagnet and configured to lower the electrical voltage across the energy reserve when the energy reserve is in a charging configuration.   The resistor is connected in series between the second electromagnet and the fourth contact of the second relay.   The energy reserve is a capacitor.   The amount of energy storable by the energy reserve is greater than or equal to the amount of energy required to power the second electromagnet in order to switch the second relay to an energized state.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other advantages thereof will become clearer in the light of the following description of an embodiment of a switch, given by way of example only and made with reference to the appended drawings, in which: 
         FIG.  1    shows, schematically, a switch in accordance with embodiments of the invention; 
         FIG.  2    shows, schematically, the equivalent electrical diagram of the switch of  FIG.  1   , in a first state during its operation; 
         FIG.  3    shows, schematically, the equivalent electrical diagram of the switch of  FIG.  1   , in a second state during its operation; 
         FIG.  4    shows, schematically, the equivalent electrical diagram of the switch of  FIG.  1   , in a third state during its operation; 
         FIG.  5    shows, schematically, the equivalent electrical diagram of the switch of  FIG.  1   , in a fourth state during its operation. 
     
    
    
     DETAILED DESCRIPTION OF SOME EMBODIMENTS 
       FIG.  1    shows a protected switch  1 , which includes an interrupt circuit  2 , also called a safety chain. 
     For example, the circuit  2  is intended to be connected to an electrical circuit, such as an electrical appliance to an electrical power source. For this purpose, the circuit  2  is provided with connection terminals  22 . 
     The circuit  2  is selectively and reversibly switchable between a “off” state, which prevents the flow of an electric current through the circuit  2 , and an “on” state, which allows the flow of an electric current through the circuit  2 . 
     This switching is controlled here by supplying a control signal to control electrodes of the switch  1 , which are designated here as  131  and  132 . 
     In the absence of a control signal, the switch  1  remains in the “off” state and, in the presence of a control signal, the switch  1  switches to the “on” state. 
     In this example, the control signal is an electrical voltage, noted Vcc, applied between the electrodes  131  and  132 . 
     As an illustrative example, the electrical voltage Vcc is a DC voltage, with an amplitude greater than or equal to 24 V and less than or equal to 110 V. The switch  1  is configured to ensure safe switching of the circuit  2  between its off and on states, in particular to prevent the circuit  2  from remaining in the “on” state while no control signal is applied to the switch  1 . 
     Preferably, the switch  1  has a high safety level, such as “SIL  4 ” on the safety integrity level scale as defined by the IEC 61508 standard of the International Electrotechnical Commission or by the EN 50129 standard. 
     Preferably, the switch  1  is intended for use in a control system, such as in the railway field. In variants, the switch  1  may also be used in a power circuit to control the power supply of an electrical device. 
     As an illustrative and not necessarily limiting example, the circuit  2  is adapted to receive a DC electrical signal between its terminals  22 , having an electrical voltage less than or equal to 110 volts and an electrical current less than or equal to 3.5 A. 
     As illustrated in the example of  FIG.  1   , the switch  1  comprises a first electromechanical relay  10 , a second electromechanical relay  11  and an interconnection circuit  13  that connects the relays  10  and  11  to each other, as explained in the following. Advantageously, the switch  1  also comprises an outer casing, not shown, made of plastic for example, inside which the components of the switch  1  are housed. By way of illustrative example, the casing may have the shape of a block with dimensions of 12 cm×9 cm×2 cm, for example. 
     The relay  10  includes an electromagnet  101  and movable electrical contacts  102 ,  103 ,  104  and  105  coupled with the electromagnet  101 . Each of the contacts  102 ,  103 ,  104  and  105  is switchable between an open and a closed state. 
     In this example, contact  102  is of the “normally closed” type, while contacts  103 ,  104  and  105  are of the “normally open” type. 
     Switching is accomplished by means of electromagnet  101 , also referred to as coil  101  in the following, which exerts an electromagnetic force on the contacts  102 ,  103 ,  104  and  105  when electrically energized. 
     When the electromagnet  101  is not energized, the relay  10  remains in an inactive state, also known as the quiescent state, and the contacts  102 ,  103 ,  104  and  105  remain in a corresponding quiescent state. Here, in the quiescent state, the “normally closed” contact  102  remains closed, while the contacts  103 ,  104  and  105  remain open. In  FIG.  1   , the relay  10  is shown in its inactive state. 
     When the electromagnet  101  is electrically energized, here by the control signal, then the contacts  102 ,  103 ,  104  and  105  switch to their opposite states. Here, the contact  102  opens, while the contacts  103 ,  104  and  105  close. The relay  10  is said to be activated or energized. As long as the electromagnet  101  is energized, the contacts  102 ,  103 ,  104  and  105  are kept in that state and the relay  10  remains energized. 
     The relay  10  here is an electromechanical guided contact relay, i.e., the contacts  102 ,  103 ,  104 , and  105  are mechanically coupled together. Such a relay with guided contacts is described by the NF EN 50205 standard, for example. 
     Thus, if one of the contacts  102 ,  103 ,  104  and  105  accidentally remains locked in a given state, regardless of the state of the electromagnet  101 , then the other contacts  102 ,  103 ,  104  and  105  are kept locked in a corresponding state. For example, if the contact  102  remains locked in the open state even in the absence of energization of electromagnet  101 , then contacts  103 ,  104  and  105  remain in the closed state. The relay  10  then remains locked in the energized state. In other words, the contacts of such a relay cannot switch between their open and closed states independently of each other. 
     Similarly, the relay  11  includes an electromagnet  111  and movable electrical contacts  112 ,  113  and  114  coupled to the electromagnet  111 . Each of the contacts  112 ,  113 , and  114  is switchable between an open state and a closed state by means of the electromagnet  111 . In this example, contact  112  is of the “normally closed” type, while contacts  113  and  114  are of the “normally open” type. In  FIG.  1   , the relay  11  is shown in its inactive state. The relay  11  is also an electromechanical relay with guided contacts. 
     The contacts  105  and  114  are electrically connected in series with each other to form the interrupt circuit  2 . Thus, the circuit  2  is in the “off” state when at least one of the contacts  105  and  114  is open, and is in the “on” state only when both contacts  105  and  114  are closed. 
     Advantageously, the relays  10  and  11  belong to different manufacturing series and/or come from different manufacturers. This considerably reduces the risk that the relays  10  and  11  are both affected simultaneously by the same manufacturing defect that could compromise their operation. 
     Preferably, the relay  10  comprises a housing inside which the electromagnet  101  and the contacts  102 ,  103 ,  104  and  105  are housed. Similarly, the relay  11  comprises a housing inside which the electromagnet  111  and the contacts  112 ,  113  and  114  are housed. 
     In a variant, the switch  1  may further include one or more additional interrupt circuits, similar to the interrupt circuit  2 . For example, the relays  10  and  11  may include additional movable, “normally open” type contacts that are mechanically coupled with contacts  102 ,  103 ,  104 ,  105  or  112 ,  113  and  114 , respectively. Each additional interrupt circuit may include an additional contact of the first relay  10  and an additional contact of the second relay  11 , electrically connected in series. What is described with reference to the interrupt circuit  2  therefore also applies to these additional interrupt circuits. 
     According to another embodiment, the relays  10  and  11  may include additional contacts that are not connected to the interconnection circuit  13  or to the interrupt circuit  2 . 
     Advantageously, the switch  1  further comprises a resistor  14  connected in series between the electromagnet  111  and the contact  113  of the second relay  11 . According to examples, the resistor  14  is a wound resistor, although in a variant other embodiments are possible. 
     For example, the resistor  14  forms a voltage divider bridge that allows the electrical voltage present across the terminals of the energy reserve  12  to be lowered when the energy reserve  12  is in a charging configuration, such as when the contacts  104  and  113  are closed and the control voltage Vcc is applied across the terminals  131  and  132 . 
     Advantageously, the switch  1  includes a rechargeable energy reserve  12 , the role of which is described in more detail in the following. For example, the energy reserve  12  is a capacitor. 
     Preferably, the electromagnet  101  of the first relay  10  has a different control voltage than the control voltage of the electromagnet  111  of the second relay  11 . 
     The term “control voltage” here refers to the electrical voltage that must be applied across the electromagnet terminals to energize the relay. In other words, the relay is not energized if a voltage less than the control voltage is applied across the electromagnet terminals. 
     Preferably, the control voltage of the electromagnet  101  of the first relay  10  is greater than the control voltage of the electromagnet  111  of the second relay  11 , preferably still greater than twice the control voltage of the electromagnet  111 . 
     For example, the control voltage of the electromagnet  101  of the first relay  10  is equal to 24 volts. The control voltage of the electromagnet  111  of the second relay  11  is equal to 6 volts. 
     Advantageously, the energy reserve  12  is dimensioned so that the electric voltage it delivers when discharging, once the relays  10  and  11  are energized, is strictly lower than the control voltage of the electromagnet  101  of the first relay  10  while being higher than the control voltage of the electromagnet  111  of the second relay  11 . 
     Preferably, the amount of energy storable by the energy reserve, noted E, is greater than or equal to the amount of energy, noted Emin, that is required to power the second electromagnet  111  so as to switch the second relay  11  from the inactive state to the energized state. For example, the amount of energy E is greater than or equal to the amount of energy Em in and is less than or equal to 1.5×Emin, or less than or equal to 1.2×Emin. 
     As an illustrative example, the energy reserve  12  is a capacitor with a capacity equal to 47 μF. The electromagnet  111  here has a resistance equal to 500Ω The interconnection circuit  13  connects the relays  10  and  11  to each other and, more specifically, connects the electromagnets  101 ,  111  and the contacts  102 ,  103 ,  104 ,  112 ,  113  to each other, as described below. The interconnection circuit  13  further connects the power supply  12  to the relays  10  and  11 . 
     Preferably, the circuit  13  is electrically isolated from the interrupt circuit  2 . 
     For example, the circuit  13  comprises a substrate on which electrically conductive tracks are formed. The relays  10  and  11  are mounted on this substrate and electrodes corresponding to the electromagnets  101 ,  111  and corresponding contacts are connected to these electrically conductive tracks. 
     In a variant, the circuit  13  may be implemented using cables to connect the relays  10  and  11 . 
     In this example, the circuit  13  includes the control electrodes  131  and  132 . In a variant, the circuit  13  may include other control electrodes, such as a pair of control electrodes dedicated to each of the electromagnets  101  and  111  and intended to receive a same control signal to control the switch  1 . 
       FIG.  2    shows the electrical diagram of the switch  1  when the circuit  13  connects the relays  10  and  11  and the relays  10  and  11  are inactive. 
     In this example, the first electromagnet  101  is connected to the control electrodes  131 ,  132  via the contact  112  and contact  103 . More specifically, the contact  103  and contact  112  are connected in parallel with each other. Both the contact  103  and the contact  112  are connected between the electrode  132  and a first terminal of the electromagnet  101 . A second terminal of the electromagnet  101  is connected to the other electrode  131 . 
     In this manner, the connection of the electromagnet  101  to the control electrodes  131 ,  132  is conditional on the state of the second relay  11 . 
     The second electromagnet  111  is connected here to the control electrodes  131 ,  132  via the contacts  102 ,  104  and  103  to alternately connect or disconnect the second electromagnet  111  to the control electrodes  131 ,  132 , depending on the state of the first relay  10 . 
     In addition, the energy reserve  12  is connected to the electrodes  131 ,  132  and the second electromagnet  111  via the contacts  102  and  104 . The circuit  13  is thus arranged so that the contacts  102  and  104 :
         authorize charging the energy reserve  12  from the control electrodes  131 ,  132  when the contact  105  is open, and   allow the energy reserve  12  to be discharged into the second electromagnet  111  when the first contact  105  is closed.       

     For this purpose, the contact  104  connects a terminal of the second electromagnet  111  to a first terminal of the energy reserve  12 . A second terminal of the energy reserve  12  and the other terminal of the electromagnet  111  are connected here to the electrode  131 . The contact  102  connects the first terminal of the energy reserve  12  to a first terminal of the electromagnet  101  to which the contacts  103  and  112  are connected. 
     Thus, the energy reserve  12  can only be connected to the electrode  132  through the contacts  102  or  104 . 
     The second electromagnet  111  is further connected to the control electrode  132  through the contact  113  of the second relay  11 . 
     Due to the configuration of the circuit  13 , when a control signal is received at the control electrodes  131 ,  132 , the relays  10  and  11  are switched sequentially, one after the other, to their active state. 
     Switching is prevented, however, if one of the relays  10 ,  11  is initially in an abnormal state, because one of the contacts  105  or  114  is stuck in the closed state for example. The circuit  2  then remains in the blocked configuration, which ensures that the switch circuit  1  remains in the open state. 
     The connection of the electromagnets  101  and  111  to the electrode  132  through the contacts  103  and  113 , respectively, ensures that the corresponding relay  10 ,  11  remains in the energized state once this relay has switched to the energized state and provided a control signal is present. 
     Furthermore, when the control signal ceases to be received at the electrodes  131 ,  132 , if one of the contacts  105  or  114  remains locked in the closed state, then switching the other contact  105 ,  114  is prevented. 
     Thus, a functional failure of either contact  105 ,  114 , as a result of sticking in the closed state caused by a partial melting of the contact, for example, causes the circuit  2  to switch to the “off” state. This keeps the switch  1  in a safe state. 
     In contrast, if the control signal received on the electrodes  131 ,  132  were directly applied simultaneously to the electromagnets  101  and  111  without these being conditional on the contacts of the individual relays  10  and  11 , then the switching of the relays  10  and  11  would be simultaneous regardless of the state of either relay  10 ,  11 . 
     Thanks to the invention, when the switch  1  is switched, the control of the state of the contacts  105 ,  114  is carried out intrinsically, without calling upon an external electronic control unit, and also without calling upon a mechanical device dependent on the earth&#39;s gravity for its operation. 
     In addition, the relays  10  and  11  experience different wear and tear due to the chosen switching sequence. For example, the second relay  11  tends to wear out more quickly than the first relay  10  because it undergoes current calls more frequently than the first relay  10 , particularly during the closing sequence of the safety chain. This differentiated wear prevents both relays  10  and  11  from suffering a simultaneous failure due to the same cause of wear. 
     According to a variant not shown, the second electromagnet  111  may be connected to second control electrodes. For example, the contact  113  may connect the electromagnet  111  to a second electrode separate from the electrode  132 . In a variant, the contact  102  may connect the first terminal of the power supply  12  to this second electrode. The control signal is then applied to both these second control electrodes and to the electrodes  131  and  132 . 
     An example of the operation of the switch  1  is now described, with reference to  FIGS.  2  through  5   . In this example, the circuit  2  is switched from the “off” state to the “on” state in response to a control signal. 
     As illustrated in  FIG.  2   , the relays  10  and  11  are initially inactive. The contacts  102  and  112  are in the closed state, while the contacts  103 ,  104 ,  105 ,  113 ,  114  are in the open state. No control signal is applied between the electrodes  131 ,  132 . The contacts  105 ,  114  are in the open state and the circuit  2  is therefore in a “off” state. 
     At this point, the energy reserve  12  is not able to supply power to the coil  101  to activate the first relay, in particular because the maximum voltage that the energy reserve  12  can deliver is lower than the control voltage of the coil  101 . Moreover, in practice, the energy reserve  12  is usually empty or partially discharged at this point. 
     The energy reserve  12  can then discharge into the coil  101  without being able to change the state of the relay  10 , since it cannot provide enough energy. 
     As shown in  FIG.  3   , a control signal, such as an electrical voltage Vcc, is applied between electrodes  131  and  132 . 
     On the one hand, the energy reserve  12  is connected to the electrode  132  via the contacts  102  and  112 , both of which are in the closed state. In parallel, the electromagnet  101  is connected to the electrode  132  through the contact  112 . At this point, the contact  112  is in the closed state and the contact  103  is in the open state. In the example shown in  FIG.  3   , the electrical voltage applied across the terminals of the energy reserve  12  is equal to the electrical voltage applied across the terminals of the first electromagnet  101 . This electrical voltage is greater than the control voltage of the first electromagnet  101 , for example. 
     As the coil  101  is supplied with a voltage greater than its control voltage, the relay  10  is energized. For example, the coil  101  generates an electromagnetic force that causes the contacts  102 ,  103 ,  104  and  105  to switch. 
     Thus, as shown in  FIG.  4   , the relay  10  switches to the energized state. The contact  102  opens and the contacts  103 ,  104  and  105  close. Arrow F 1  illustrates the closing of contact  105 . 
     In practice, this switching is not instantaneous, but occurs after an initial switching time, for example less than or equal to 100 ms. 
     At this stage, the control signal is maintained on the electrodes  131 ,  132 . The circuit  2  is still in an “off” state, which prevents the flow of current through the circuit  2 . 
     The electromagnet  101  continues to be powered, this time through the contact  103 , which is closed. This ensures that the relay  10  remains in the energized state as long as the control signal is supplied to the switch  1 . 
     However, due to the new configuration of the contacts  103 ,  104  and  102  after the switching of the relay  10 , the energy reserve  12  is no longer connected to the electrode  132  and therefore no longer electrically recharged from the voltage Vcc. In fact, the contact  102  is now in the open state and the contact  113  is still in the open state. 
     On the other hand, since the contact  104  is closed, the electromagnet  111  is connected with the energy reserve  12 , which allows the energy reserve  12  to discharge into electromagnet  111 , to electrically supply the latter. 
     In this way, as the voltage supplied by the energy reserve  12  is greater than the control voltage of the electromagnet  111 , the electromagnet  111  triggers the switching of the relay  11  to the energized state, as shown in  FIG.  5   . The contact  112  opens and the contacts  113  and  114  close. Arrow F 2  illustrates the closing of the contact  114 . 
     In practice, this switching is not instantaneous, but occurs after a second switching time, of less than or equal to 100 ms for example. 
     Thus, the circuit  2  switches to the “on” state, thus authorizing the flow of an electric current. 
     At the end of this switching, the electromagnet  111  continues to be powered, this time through the contact  113 , which is closed. This ensures that the relay  11  is kept in the energized state as long as the control signal is supplied to the switch  1 . 
     In addition, through the resistor  14 , the electrical voltage applied to the terminals of the energy reserve  12  is decreased to a holding voltage with a predefined value, chosen to ensure that only a small amount of energy is actually reserved in the energy reserve  12 . This ensures, among other things, that the relay  11  can be switched quickly when the control signal is interrupted, since the energy reserve  12  will not be able to hold the relay  11  in the energized state for too long. 
     When the control signal is interrupted, the electromagnets  101  and  111  cease to be energized. The relays  10  and  11  return to their inactive state. The contacts  102 ,  112  close, while applied to the terminals of the energy reserve contacts  103 ,  104 ,  105 ,  113  and  114  reopen. The circuit  2  then switches to the “off” state. 
     Although the energy reserve  12  may be transiently connected to the electromagnet  111  when applied to the terminals of the energy reserve relays  10  and  11  return to their inactive state, it does not contain sufficient energy to energize applied to the terminals of the energy reserve relay  11  again. 
     Furthermore, the energy reserve  12  is equally unable to energize the relay  10  at the end of switching, because although it is connected to the electromagnet  101  via the relay  102 , which returns to its closed state once the relay  10  returns to its inactive state, the voltage supplied by the energy reserve  12  remains lower than the control voltage necessary to energize the electromagnet  101 . 
     The operation of the switch  1  is said to be “safe” in that it ensures that the circuit  2  cannot switch to the “on” state if either contact  105  or  114  remains stuck in the closed state when the control signal is absent. 
     In particular, in this example, if the contact  105  is initially abnormally stuck in its closed state, then the contact  114  cannot be closed when a control signal is subsequently applied. Indeed, since the contacts of the relay  10  are coupled together, then the contacts  104  and  103  are closed and the contact  102  is open when the contact  105  is closed, even in the absence of power to the electromagnet  101 . In this case, the electromagnet  111  is disconnected from the electrode  132 , because the contacts  102  and  113  are open. The electromagnet  111  is only connected to the energy reserve  12 , which at this stage does not contain sufficient energy to switch the relay  11 . The electromagnet  111  cannot therefore be energized and therefore the relay  11  cannot be switched to the energized state. The circuit  2  remains in the “off” state. 
     In the case where the contact  114  is initially abnormally stuck in its closed state, then the contact  105  cannot be closed when a control signal is subsequently applied. In fact, since the contacts of the relay  11  are coupled together, then the contact  113  is closed and the contact  112  is open when the contact  114  is closed, even in the absence of power to the electromagnet  111 . In this case, the electromagnet  101  is disconnected from electrode  132 , because the contacts  112  and  103  are open. The electromagnet  101  cannot therefore be energized and therefore the relay  10  cannot be switched to the energized state. The circuit  2  remains in the “off” state. 
     Such a failure of the switch  1  therefore leads to the circuit  2  remaining in a safe configuration. 
     The probability of simultaneous failure of the contacts  105  and  114  is extremely low here, less than 10-9 occurrences per hour for example, which guarantees a good safety level for the switch  1 . 
     The embodiments and variants contemplated above may be combined with each other to generate new embodiments.