Abstract:
A safety switching apparatus for the failsafe disconnection of an electrical load has a first and a second terminal for supplying a first and a second signal from a signaling device, such as an emergency-off button. The safety switching apparatus also has a first and a second switching element which together produce a redundant output switching signal for disconnecting the load. The first switching element has a first control circuit to which the first signal is supplied. The second switching element has a second control circuit to which the second signal is supplied. The safety switching apparatus further has circuitry providing a first current path, an alternative second current path and at least one further switching element. The at least one further switching element is designed to select either the first or the second current path, depending on a polarity of the second signal, in order to route the second signal to the second control circuit.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of international patent application PCT/EP2007/000765 filed on Jan. 30, 2007 designating the U.S., which international patent application has been published in German language and claims priority from German patent application DE 10 2006 010 106.5 filed on Mar. 1, 2006. The entire contents of these prior applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a safety switching apparatus for failsafe disconnection of an electrical load, such as an electrical motor or electromagnetic valve in an automatically operated installation. 
         [0003]    Safety switching apparatuses in terms of the present invention are typically used to shut-down a dangerous machine or installation in a failsafe manner when this is necessary in order to protect people. The safety switching apparatuses typically monitor signals generated from or via emergency-off buttons, guard door switches, light barriers, light grids or other safety signaling devices. The safety switching apparatuses are designed to interrupt a power supply path to the monitored machine or installation as a function of or depending on these signals. 
         [0004]    Depending on a machine&#39;s or an installation&#39;s potential for danger, there are different safety categories stipulating what requirements a safety switching apparatus needs to meet for a particular use. For example, European standard EN 954-1 defines four safety categories, with category 4 imposing the highest safety requirements in terms of intrinsic failsafety of the safety switching apparatus. Apparatuses which comply with category 4 are usually designed with multichannel redundancy. In such a case, the signals must also be in multichannel-redundant form, i.e. they must include at least a first and a second signal having a correlated information content. 
         [0005]    In addition, category 4 requires the safety switching apparatus to identify a short on the signal lines which transmit these signals, because a short means that single-fault safety is lost. For applications below category 4, however, it is possible to dispense with short-circuit identification, which simplifies the wiring of the safety switching apparatus and makes installation less expensive. 
         [0006]    A prior art safety switching apparatus is sold by the assignee under the brand name PNOZ X8P. This prior art safety switching apparatus can be used either with or without short-circuit identification, with the wiring connection of the signal lines for supplying these signals being different in the two cases. For an application with short-circuit identification, the second channel of the signaling device (which produces the second signal) has to be connected to different terminals than for an application without short-circuit identification. To allow flexible use, the known safety switching apparatus therefore requires a relatively large number of connection terminals. 
         [0007]    German Patent Publication DE 44 23 704 C1 discloses a safety switching apparatus wherein a first signal is supplied to the control circuit of a first switching element via a first terminal, while a second signal is routed to the control circuit of a second switching element via a second terminal. Due to the fact that the control circuits of the two switching elements are further connected to a common ground potential and the signals have a positive and a negative signal level relative to the common ground potential, it is possible to identify a short between the two signals. However, this prior art safety switching apparatus does not allow operation without short-circuit identification, and it is therefore limited in terms of the flexibility of use. Furthermore, this known safety switching apparatus requires an AC supply, which also restricts the opportunities for use. 
       SUMMARY OF THE INVENTION 
       [0008]    Against this background, it is an object of the present invention to provide a safety switching apparatus of the type mentioned at the outset, which offers flexible options for use and can be accommodated in a compact and small installation space. 
         [0009]    According to one aspect of the invention, there is provided a safety switching apparatus for failsafe shut-down of an automatically operated installation in response to a first and a second signal from a two-channel signaling device, with the first and the second signal each having a defined polarity, the safety switching apparatus comprising a first terminal for supplying the first signal and a second terminal for supplying the second signal, a first and a second switching element, which together are designed to produce a redundant output switching signal for shutting-down the installation, and circuitry having a first current path, an alternative second current path and at least one further switching element, wherein the first switching element has a first control circuit to which the first signal is supplied, wherein the second switching element has a second control circuit to which the second signal is supplied, and wherein the at least one further switching element is designed to automatically rout the second signal to the second control circuit either via the first current path or via the second current path, depending on the polarity of the second signal relative to the polarity of the first signal. 
         [0010]    According to another aspect of the invention, there is provided a safety switching apparatus for failsafe disconnection of an electrical load, comprising a first and a second terminal for supplying a first and a second signal, comprising a first and a second switching element, which together are designed to produce a redundant output switching signal for disconnecting the load, and circuitry having a first current path, an alternative second current path and at least one further switching element, wherein the first switching element has a first control circuit to which the first signal is supplied, wherein the second switching element has a second control circuit to which the second signal is supplied, and wherein the at least one further switching element is designed to connect either the first or the second current path, depending a polarity of the second signal, in order to route the second signal to the second control circuit. 
         [0011]    Embodiments of the novel safety switching apparatus thus have at least two alternative current paths which are preferably arranged at least partially between the second terminal and the second control circuit. This allows the signal at the second terminal to be routed to the second control circuit differently. Since the selection between the at least two alternative current paths is dependent on the polarity of the signal at the second terminal, the safety switching apparatus is able to process signals of different polarities at the second terminal. In other words, signals of different polarities may be supplied on the second terminal. Depending on the polarity used, wiring with or without short-circuit identification is possible, although the signals can be supplied via the same connection terminals in both cases. 
         [0012]    Consequently, the novel safety switching apparatus can be implemented with a smaller number of connection terminals than the assignee&#39;s PNOZ X8P safety switching apparatus mentioned above. Accordingly, it can also be implemented in a smaller device housing, because the number of required connection terminals affects the physical size. The terminal-saving implementation of the novel safety switching apparatus therefore allows the installation space to be reduced without limiting the flexibility of use. Therefore, the above object is completely achieved. 
         [0013]    In addition, the novel safety switching apparatus allows a very simple and inexpensive implementation because the signals can be directly used for activating or deactivating the switching elements. In other words, the signals from the signaling devices can supply the power for turning the switching elements on and/or off, which is actually preferred in exemplary embodiments of the invention. Therefore, loss of a signal from a signaling device automatically results in the associated switching element being turned off and hence in the electrical load being disconnected. 
         [0014]    The novel safety switching apparatus can therefore be implemented very inexpensively, and it is particularly suitable for “small” applications in which only a few safety functions need to be monitored. 
         [0015]    In a preferred refinement, the at least one further switching element comprises a plurality of diodes which form a bridge rectifier. 
         [0016]    This refinement is a very inexpensive way of supplying power to the second control circuit regardless of the polarity of the signal. Furthermore, this refinement allows a reduction in the number of the required components, since polarized components, such as electrolytic capacitors or optocouplers which are often required for supplying power to and monitoring the second control circuit, each need to be provided only once. 
         [0017]    In a further refinement, the bridge rectifier has a first and a second bridge terminal, the second control circuit being arranged between them. 
         [0018]    In this refinement, the second control circuit is arranged in the bridge path of the bridge rectifier. This refinement allows a very simple and inexpensive implementation. 
         [0019]    In a further refinement, the bridge rectifier has a third and a fourth bridge terminal, wherein the third bridge terminal is coupled to the second terminal and wherein the fourth terminal is connected to a defined reference potential. In this case, the reference potential may be a fixed or variable reference potential, such as a switchable potential. 
         [0020]    This refinement provides for a defined flow of current through the bridge rectifier depending on the signal at the second terminal of the safety switching apparatus. Depending on the polarity of the second signal, different current paths are conductive within the bridge rectifier, with these current paths being switched by the switching elements (diodes) in the bridge rectifier. In addition, this refinement also provides at least a coarse monitoring of the signal level of the second signal, because a sufficient flow of current through the paths of the bridge rectifier is possible only if a defined potential difference exists between the reference potential and the second signal. This refinement is therefore a simple and inexpensive implementation which affords additional safety. 
         [0021]    In a further refinement, the second signal alternates between a high first and a low second signal level, wherein the defined reference potential is substantially in the mid between the first and second signal levels. In this refinement, one of the signal levels may also be a high-impedance, unused state at the second terminal. 
         [0022]    This refinement provides a simple way of automatically operating the novel safety switching apparatus with two signals of different polarity. It is sufficient for the user (for example an installer wiring up the novel safety switching apparatus in a machine or installation) to connect the signal line which transmits the second signal to the second terminal. The novel safety switching apparatus of this refinement can be started up without further adjustments to the mode of operation (with or without short-circuit identification). This refinement is therefore very convenient for the user. 
         [0023]    In a further refinement, the circuitry comprises at least one switch is designed to connect the second control circuit either to a first or to a second reference potential. Preferably, the first and second reference potentials are fixed potentials. In a particularly preferred embodiment, the first reference potential is a DC operating voltage and the second reference potential is ground. 
         [0024]    In addition, it is preferred if the circuitry has at least two switches, preferably in the form of transistors, in a half-bridge arrangement, which switches are arranged between the internal operating voltage of the safety switching apparatus and ground. 
         [0025]    These refinements are particularly preferred if the signals also alternate between operating voltage and ground, with the operating voltage signaling the safe state. By connecting the second control circuit either (depending on the polarity of the second signal) to the first or to the second operating potential, the control circuit can be operated in the same way in each case, regardless of the polarity of the second signal. The control circuit can operate independently of the selected mode of operation, and it is provided with maximum switching power in each case, which ensures very reliable operation of the novel safety switching apparatus. In addition, this refinement requires few internal potentials, which facilitates a failsafe design. 
         [0026]    In a further refinement, the novel safety switching apparatus comprises a control unit which is designed to connect the second control circuit alternately to the first or to the second reference potential until a flow of current is detected in the second control circuit. 
         [0027]    In this refinement, the novel safety switching apparatus can also automatically adjust itself to the signal at the second terminal, since a flow of current in the second control circuit indicates that the relevant switching position of the second further switching element is the “correct” switching position for evaluating the applied second signal. Therefore, this refinement is also a very convenient solution for the user which simplifies installation of the novel safety switching apparatus. 
         [0028]    Preferably, this refinement also comprises a current detector which is connected to the second control circuit in order to detect the flow of current in the second control circuit, with the current detector then being coupled to the control unit. 
         [0029]    In a further refinement, the novel safety switching apparatus comprises an enabling unit which is designed to disable the control unit as soon as a flow of current is detected in the second control circuit. 
         [0030]    This refinement allows the first and second switching elements to be used as “fault memory”, since the switching position of the second switching element cannot be changed again by the control unit once a current flow is detected in the second control circuit. Hence, this refinement facilitates the design of particularly simple and inexpensive safety switching apparatuses. 
         [0031]    In a further refinement, the safety switching apparatus comprises a control element having at least two control positions, wherein the control element is operationally coupled to the at least one switch in order to connect the second control circuit either to the first or to the second reference potential. Preferably, the control element is designed to be manually operated. 
         [0032]    In this refinement, the user has the option of selectively setting the “type” of the second signal. This refinement therefore dispenses with the advantage of the variants discussed above wherein the safety switching apparatus identifies the type of the second signal automatically. On the other hand, the present refinement has the advantage that both single-pole and two-pole signals can be correctly processed. A single-pole signal is a signal which alternates between a defined potential and a high-impedance state. Typically, single-pole signals are produced by contact-based signaling devices, such as emergency-off buttons. Two-pole signals alternate between a high and a low potential, for example between 24 V and 0 V (ground). Such signals are typically supplied by light barriers and other signaling devices with semiconductor outputs. The present refinement increases the flexibility of the novel safety switching apparatus. This reduces production and storage costs. 
         [0033]    In a further refinement, at least the second switching element is an electromechanical switching element, preferably an unpolarized electromechanical switching element, and even more preferred a relay having a plurality of make contacts which are positively guided. 
         [0034]    This refinement is advantageous because an electromechanical switching element provides a floating output switching signal for disconnecting the load. An output switching signal of this kind can be used in a plurality of environments, which means that the scope of application of the novel safety switching apparatus is increased. An unpolarized electromechanical switching element is advantageous because a control current can effectively flow in both directions. This refinement facilitates practical implementation. 
         [0035]    It goes without saying that the features which have been mentioned above and those which are yet to be explained below can be used not only in the respectively specified combination but also in other combinations or on their own without departing from the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0036]    Exemplary embodiments of the invention will be explained in more detail in the description which follows and are illustrated in the drawing, in which: 
           [0037]      FIG. 1  shows a schematic illustration of an automatically operated installation wherein exemplary embodiments of the novel safety switching apparatus are used, 
           [0038]      FIG. 2  shows a simplified illustration of an exemplary embodiment of the novel safety switching apparatus, 
           [0039]      FIG. 3  shows a simplified equivalent circuit diagram including the second switching element in accordance with a further exemplary embodiment, and 
           [0040]      FIG. 4  shows a simplified equivalent circuit diagram of a further exemplary embodiment. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0041]    In  FIG. 1 , an installation where exemplary embodiments of the invention are used is denoted by the reference numeral  10  in its entirety. 
         [0042]    The installation  10  comprises a robot  12  whose operating space is protected by means of a guard door  14 . An actuator  16  is arranged on the guard door  14 , which actuator interacts with a guard door switch  18 . Guard door switch  18  is seated on a frame on which the moving part of guard door  14  rests in the closed state. By way of example, actuator  16  may be a transponder which can communicate with the guard door switch  18  only when guard door  14  is closed. 
         [0043]    The guard door switch  18  is connected to a safety switching device  20  which processes the signals from the guard door switch  18 . There is a second safety switching device  22  arranged in series with safety switching device  20 , with an emergency-off button  24  being connected as a signaling device to the second safety switching device  22 . The safety switching devices  20 ,  22  are compact safety switching apparatuses in terms of the present invention, and they have a predefined scope of functions implemented by the manufacturer. In principle, however, the invention may also be used for more complex, programmable safety controllers. 
         [0044]    Reference numerals  26 ,  28  denote two contactors whose make contacts are arranged in the power supply path to the robot  12 . The control circuits of contactors  26 ,  28  are supplied with power via the safety switching devices  20 ,  22 , so that each of the safety switching devices  20 ,  22  is able to disconnect the robot  12  via contactors  26 ,  28 . An operational controller controlling the normal operations of robot  12  is not shown for the sake of simplicity. 
         [0045]      FIG. 2  shows a preferred exemplary embodiment of the invention with respect to safety switching device  22 . Same reference symbols denote the same elements as before. 
         [0046]    Safety switching device  22  has a device housing  34  including a plurality of device or connection terminals for connecting the emergency-off button  24 , the contactors  26 ,  28  and possible further signaling devices and actuators (not shown here). Reference symbol  36  denotes a first connection terminal, to which a first break contact  24   a  of the emergency-off button  24  is connected. Reference numeral  38  denotes a second terminal, to which a second break contact  24   b  of emergency-off button  24  is connected. The safety switching apparatus  22  thus receives a first signal, which is dependent on the switching position of the signaling contact  24   a , via terminal  36  and receives a second signal, which is dependent on the switching position of the signaling contact  24   b , via terminal  38 . 
         [0047]    In the embodiment shown here, break contact  24   a  is also connected to a terminal  40 , which carries an operating voltage U B  of 24 V, for example, from inside of the device. When break contact  24   a  is closed, the terminal  36  therefore receives the operating voltage U B  as a signal. When signaling contact  24   a  is open (not shown here), terminal  36  “sees” a high impedance state. 
         [0048]    In this exemplary embodiment, break contact  24   b  is likewise connected to a terminal  42  which is grounded from inside of the device. Accordingly, the terminal  38  is connected to the device ground when the break contact  24   b  is closed. When the break contact  24   b  is open, the terminal  38  is likewise at a high impedance state. 
         [0049]    Provision of the signals, which are routed via break contacts  24   a ,  24   b , on output terminals  40 ,  42  of the device is a typical implementation for safety switching devices of the this type. However, the invention is not limited to this implementation. Rather, the terminals  36 ,  38  may also receive signals which are produced externally using another devices, for example using a light barrier (not shown here). In this case, the terminals  40 ,  42  may remain unused. 
         [0050]    In the variant shown, the safety switching device  22  can identify a short between the signaling lines to the emergency-off button, because the signal at terminal  36  corresponds to the operating voltage U B  in the safe state, while the signal at the second terminal  38  corresponds to the ground potential in the safe state. A short between the signal lines carrying the signals would result in the potential at the first terminal  36  being pulled to ground, which is detected in a manner described below. 
         [0051]    Alternatively, the safety switching apparatus  22  can also be operated without short-circuit identification on the signaling lines, if this is acceptable in accordance with a lower safety category. In this case, the second signaling contact  24   b  of the emergency-off button  24  would be connected to a terminal  44 , as shown in a dotted line by the reference numeral  24   b ′. Terminal  44  is connected to the operating voltage U B  from inside of the device, so that both signaling contacts  24   a ,  24   b ′ would then carry the operating voltage U B  in the safe state. Short-circuit identification is not possible in this case. 
         [0052]    As an alternative to this illustration, the second signaling contact  24   b ′ could also be arranged between the terminals  38  and  40 , i.e. the input-side connections of the signaling contacts  24   a ,  24   b ′ would be connected in parallel at the terminal  40 . This implementation allows terminal  44  to be dispensed with in order to save further installation space. 
         [0053]    Two further terminals  46 ,  48  serve for supplying the safety switching device  22  with the operating voltage U B  of 24 V by way of example. Further terminals  50 ,  52  are provided for connecting the contactors  26 ,  28  and possible further loads. 
         [0054]    In the exemplary embodiment shown, the safety switching device  22  has floating outputs in the form of relay contacts, which are connected in series with one another between the terminals  50 ,  52 . In such a case, terminals  50  may be connected to a positive potential of 24 V, for example, and the contactors  26 ,  28  may be connected to terminals  52 . The relay contacts of relays K 1 , K 2  can then be used to supply the contactors  26 ,  28  with current, or to interrupt the flow of current. Alternatively or in addition, the present invention can also be used for safety switching apparatuses which have semiconductor outputs related to a specific potential, however. 
         [0055]    Relays K 1 , K 2  each have a control circuit, which is denoted by the reference numerals  56 ,  58  in  FIG. 2 . In this case, the control circuits  56 ,  58  are the energizing coils of respective relays K 1 , K 2 . As shown in  FIG. 2 , the control circuit (energizing coil)  56  of relay K 1  has one terminal connected to ground, with this path also containing a collector-emitter path of a transistor  60 , which a microcontroller  62  can use to interrupt the flow of current through energizing coil  56 . The second terminal of control circuit  56  is routed via a resistor  64  and a make contact  66  of relay K 1  to the terminal  36 . Resistor  64  is a symbolic equivalent resistance which is representative of further components and line resistances which are not shown here for sake of simplicity. The make contact  66  implements self-latching for relay K 1 , which has been used for many years in safety switching devices of the present type and is therefore known to the skilled person. 
         [0056]    In summary, the signal from terminal  36  is thus routed via control circuit  56  of relay K 1  and it delivers the power for latching relay K 1 . (In order to energize relay K 1 , the power may be supplied via a starting circuit, which is not shown here for sake of simplicity.) If the signal at terminal  36  corresponds to the operating voltage U B  and relay K 1  is in a self-latched state (make contact  66  closed), a control current flows through the energizing coil of relay K 1  and the operating contacts of relay K 1  are closed (not shown). 
         [0057]    Likewise, control circuit  58  of relay K 2  is connected to the second terminal  38  via a symbolic equivalent resistance  68  and a make contact  70  (for self-latching). However, control circuit  58  of relay K 2 —unlike control circuit  56  of the relay K 1 —is located in a diode bridge in this exemplary embodiment, which diode bridge comprises four diodes  72 ,  74 ,  76 ,  78  in a bridge circuit. The diode bridge has a first and a second bridge terminal  80 ,  82 , between which the control circuit  58  of relay K 2  and an optocoupler  84  are arranged. In this case, optocoupler  84  is used as a current detector, which microcontroller  62  can use to determine whether or not a current is flowing in the bridge path between bridge connections  80 ,  82 . Instead of an optocoupler, it would also be possible to use another element for current detection, such as a shunt resistor or Hall element. 
         [0058]    In addition, the diode bridge has a third and a fourth bridge terminal  86 ,  88 . The third bridge terminal  86  is connected to terminal  38  via resistor  68  and the make contact  70 . (Alternatively, the resistor  68  and the make contact  70  could also be arranged in the bridge path of the diode bridge.) The fourth bridge terminal  88  is connected to the collector of a pnp transistor  90  and to the collector of an npn transistor  92 . Transistors  90 ,  92  form a half-bridge which is arranged between the operating voltage U B  and ground. Accordingly, the emitter of the pnp transistor  90  is connected to the operating voltage U B  and the emitter of the npn transistor  92  is connected to ground. The control connections (bases) of the transistors  90 ,  92  are coupled to the microcontroller  62  via an enabling unit  94 . 
         [0059]    The diode bridge provides a first and an alternative second current path to the control circuit  58  of relay K 2 . If the second signal at terminal  38 , as shown in  FIG. 2 , corresponds to the ground potential and if also pnp transistor  90  is on and npn transistor  92  is off, the diodes  74 ,  78  are forward-biased and a flow of current is enabled via pnp transistor  90 , the fourth bridge terminal  88 , the diode  74 , the control circuit  58  of relay K 2 , the optocoupler  84 , the diode  78 , the bridge terminal  86 , the resistor  68 , the make contact  70  and the signaling contact  24   b  to ground. 
         [0060]    By contrast, if the signaling contact  24   b ′ is connected up to the operating voltage U B , and if also the transistor  90  is off and the transistor  92  is on, a flow of current is enabled via the signaling contact  24   b ′, the make contact  70 , the resistor  68 , the diode  72 , the control circuit  58  of relay K 2 , the optocoupler  84 , the diode  76  and the transistor  92 . In both cases, a control current flows through control circuit  58 , which control current results in the make contacts of relay K 2  being able to close. 
         [0061]    The diodes  72 ,  74 ,  76 ,  78  are therefore switching elements which form different current paths to the control circuit  58  of relay K 2  depending on the polarity of the signal at terminal  38 . In addition, the transistors  90 ,  92  are switching elements which the microcontroller  62  can use to activate and deactivate the different current paths in a selective manner. 
         [0062]    Reference numeral  96  denotes a potentiometer as an example of a manually actuatable control element. Potentiometer  96  has one terminal connected to the operating voltage U B  and a second terminal connected to ground. The potentiometer&#39;s tap is supplied to the microcontroller  62 . Instead of a potentiometer, it would also be possible to use a rotary switch, DIP switch or another manually actuatable control element here. Microcontroller  62  can identify the mode of operation in which the safety switching device  22  is intended to operate from the control position of control element  96 . 
         [0063]    In the present embodiment, microcontroller  62  particularly identifies whether the safety switching device  22  is intended to operate with or without short-circuit identification between the signal lines at the terminals  36 ,  38 . If the potentiometer  96  has been used to select a mode of operation with short-circuit identification, the microcontroller  62  turns on pnp transistor  90  and turns off npn transistor  92 . This means that the bridge terminal  88  is connected to operating voltage U B , and a flow of current is produced through the bridge path and via the control circuit  58  of relay K 2  when a ground potential is applied to the second terminal  38  (additionally, an operating voltage potential must also be applied to the first terminal  36  so that relay K 2  can energize, but this is not shown in more detail here for sake of simplicity). By contrast, if potentiometer  96  is used to select an alternative mode of operation which operates without short-circuit identification on the signal lines, microcontroller  62  turns off pnp transistor  90  and turns on npn transistor  92 . In this case, terminal  38  must have an operating voltage potential applied to it so that a control current can flow through the bridge path and via control circuit  58  of relay K 2 . 
         [0064]    In the exemplary embodiment shown here, short-circuit identification between the signal lines at terminals  36 ,  38  is implemented by means of microcontroller  62  recording the potentials at the two terminals  36 ,  38  by measurement, which is shown in a simplified fashion by means of two measuring lines  98 ,  100 . In a preferred embodiment, microcontroller  62  has integrated A/D converters for recording the potentials at terminals  36 ,  38  via measuring lines  98 ,  100 . A plausibility comparison for the potentials allows a short to be identified. This type of short-circuit identification is the subject matter of earlier German patent application DE 10 2005 055 325.7-32 of the present applicant, which is incorporated by reference here for the purposes of further explanation. 
         [0065]    Alternatively, the short-circuit identification may also be implemented in a conventional manner, such as in the case of the safety switching device PNOZ X8P mentioned at the outset. 
         [0066]    In the exemplary embodiment shown in  FIG. 2 , the potential tapped at terminal  38  is also supplied to the enabling unit  94 . The operation of the enabling unit  94  is explained in more detail further below with reference to  FIG. 4 . In other exemplary embodiments, enabling unit  94  may be dispensed with. 
         [0067]    Finally, a preferred embodiment of the novel safety switching device  22  comprises a single-channel microcontroller  62 . Alternatively, however, it is also possible to use two or more redundant microcontrollers  62 , as shown in  FIG. 2  in simplified form. Furthermore, instead of microcontroller  62 , it is also possible to use other logic chips, such as an FPGA or an ASIC, or the safety switching device can be constructed entirely from discrete components using the present invention. 
         [0068]      FIG. 3  shows a modification of the embodiment of  FIG. 2  in a simplified equivalent circuit diagram which essentially shows the bridge rectifier including the second control circuit and the alternative terminal options for the second signaling contact  24   b / 24   b ′. Aside from that, same reference symbols denote the same elements as before. 
         [0069]    In the modification shown in  FIG. 3 , the signaling contact  24   b  or  24   b ′ is again connected between bridge terminal  86  of the diode bridge, and, selectively, either operating voltage U B  or ground. In this case, however, the opposite bridge terminal  88  is connected to a potential which corresponds to half the operating voltage U B . This implementation allows transistors  90 ,  92  to be dispensed with. 
         [0070]    Since half the operating voltage U B /2 is always present across the bridge path between the bridge connections  80 ,  82 , a control current can flow via control circuit  58  of the relay K 2  in both alternatives without the need for manual changeover. Only the diodes  72  to  78  connect the different current paths depending on the wiring or the potential at bridge terminal  86 , with any changeover taking place automatically. This variant of the invention therefore provides automatic identification of the respective type of wiring chosen. 
         [0071]      FIG. 4  shows another variant which allows automatic identification of the type of wiring. Again, same reference symbols denote the same elements as before. The internal wiring of this variant corresponds to the solution shown in  FIG. 2  including the two transistors  90 ,  92 . If microcontroller  62  now toggles the two transistors  90 ,  92  on and off, bridge terminal  88  has operating voltage U B  or ground applied to it alternately. If the signaling contact  24   b / 24   b ′ is now connected to bridge terminal  86 , a control current flows through the bridge path and via the optocoupler  84  as soon as the bridge connections  86 ,  88  are at different potentials. Microcontroller  62  can identify the flow of current using optocoupler  84 , and it then maintains the latest switching position of transistors  90 ,  92 . 
         [0072]    To prevent microcontroller  62  from restarting the “search” for kind of wiring of the signaling contact  24   b / 24 ′ once the connected signaling contact is opened, enabling unit  94  ( FIG. 2 ) is provided. This is in the form of a self-latching mechanism such that it prevents microcontroller  62  from accessing transistors  90 ,  92  after a defined potential is identified at bridge terminal  86  and/or a flow of current is identified in the bridge path. The enabling unit  94  does not allow microcontroller  62  to access transistors  90 ,  92  again until the safety switching device  22  is restarted following discontinuation of the operating voltage U B  at terminal  46 . 
         [0073]    In another exemplary embodiment (not shown here), the potential at bridge terminal  88  can be changed over manually in order to selectively change between the alternative current paths of the bridge rectifier.