Abstract:
The invention relates to an elevator installation with at least one shaft, in which at least two cars disposed one above the other can be made to travel up and down, the shaft having a plurality of shaft doors and the cars respectively comprising at least one car door, and with a safety device for blocking the travel of the cars when the shaft doors or car doors are open. In order to develop the elevator installation in such a way that its transporting capacity can be increased and the cars hinder one another as little as possible, it is proposed according to the invention that the safety device has at least two independent safety circuits, each having at least one shaft door and/or car door associated with it, it being possible for the travel of at least one car to be blocked by means of the safety circuits. A method for controlling an elevator installation is also proposed.

Description:
[0001]     This application is a continuation of international application number PCT/EP2003/004487 filed on Apr. 30, 2003.  
         [0002]     The present disclosure relates to the subject matter disclosed in international application number PCT/EP2003/004487 of Apr. 30, 2003, which is incorporated herein by reference in its entirety and for all purposes.  
       BACKGROUND OF THE INVENTION  
       [0003]     The invention relates to an elevator installation with at least one shaft, in which at least two cars disposed one above the other can be made to travel up and down, the shaft having a plurality of shaft doors and the cars respectively comprising at least one car door, and with a safety device for blocking the travel of the cars when the shaft doors or car doors are open.  
         [0004]     The invention also relates to a method for controlling an elevator installation with at least one shaft, in which at least two cars disposed one above the other can be made to travel up and down, the shaft having a number of shaft doors and the cars respectively comprising at least one car door, and it being possible for the travel of the cars to be blocked by means of a safety device when the shaft door or car door is open.  
         [0005]     In the effort to increase the handling capacity of elevator installations, it is proposed in U.S. Pat. No. 19 73 920 to make two cars travel up and down in a shaft along a common traveling path. The elevator installation has in this case a safety device, with the aid of which it can be ensured that the travel of the two cars is blocked if a car door or a shaft door is opened. For this purpose, monitoring elements which form series-connected switching contacts are disposed at all the shaft doors and car doors. If one of these switching contacts is opened on account of a shaft door or car door being opened, the supply voltage to both the cars is interrupted and consequently their travel is blocked.  
         [0006]     In the case of the cited elevator installation, the lower car is carried on the upper car. This has the consequence that both cars stop simultaneously and start up again simultaneously. If a car door and a shaft door are opened for the loading or unloading of a car, the travel of both cars is blocked by means of the safety device coming into use; since, however, both cars carry out a stop simultaneously, this blockage has no influence on the traffic flow of the elevator installation.  
         [0007]     There are also known elevator installations in which two cars can be made to travel up and down independently of one another in a shaft. In the case of such a configuration, it must be ensured by a suitable mode of operation that both cars can be braked and stopped if even only one car is to be loaded or unloaded, since otherwise, when the car door and an associated shaft door of one car open, an emergency stop would be triggered in the case of the other car on account of the safety device coming into use. This would have the consequence of a considerable risk of injury to the passengers of the other car. The stopping of one of the cars consequently leads to hindrance of the travel of the other car. This ultimately has the consequence that the transporting capacity of the elevator installation is restricted.  
         [0008]     It is an object of the present invention to develop an elevator installation and a method for controlling an elevator installation of the type stated at the beginning in such a way that the transporting capacity of the elevator installation can be increased and the cars hinder one another as little as possible.  
       SUMMARY OF THE INVENTION  
       [0009]     This object is achieved in the case of an elevator installation of the generic type according to the invention by the safety device having at least two independent safety circuits, each having at least one shaft door and/or car door associated with it, it being possible for the travel of at least one car to be blocked by means of the safety circuits.  
         [0010]     The invention incorporates the idea that mutual hindrance of the cars can be reduced by using at least two safety circuits which each independently of one another can block the travel of at least one car. This allows the control of the cars to be decoupled with regard to the state of the car doors and shaft doors, so that, if appropriate, only the travel of one car is blocked, while the other car can continue its travel undisturbed. In particular, it can be ensured that the opening of car doors and shaft doors when one car stops does not necessarily lead to the travel of another car being adversely affected.  
         [0011]     It may be provided, for example, that the shaft has at least one shaft region in which the shaft doors are associated only with one safety circuit. If one of these shaft doors is opened, only the travel of cars that are coupled to this safety circuit is blocked. As a result, the travel of cars which can be made to travel in different shaft regions can be decoupled in a simple way. In particular, it may be provided that the shaft is subdivided in the vertical direction into a plurality of shaft regions, for example into an upper shaft region and a lower shaft region. If a shaft door is opened in the upper shaft region, this merely has the consequence that the safety circuit associated with this shaft door responds, so that the cars that are coupled to this safety circuit are blocked in their travel, but not the cars that have no coupling with respect to this safety circuit.  
         [0012]     It may also be provided that shaft regions respectively associated with only one safety circuit overlap one another in the vertical direction, the individual shaft regions having respectively separate shaft doors associated with them, so that opening of these shaft doors respectively has the consequence that the associated safety circuit responds. Such a configuration provides in particular the possibility of making the cars each have different shaft doors associated with them, so that the opening or closing of the shaft doors that are associated with the one car does not have the consequence of blocking the travel of the other car.  
         [0013]     It may also be provided that the shaft has at least one shaft region in which the shaft doors are associated with at least two safety circuits. If a shaft door is opened in such a shaft region, this has the consequence that at least two safety circuits, which may be respectively coupled to different cars, respond.  
         [0014]     It is of particular advantage if the car doors of a car are associated with only one safety circuit. As a result, it can be ensured in a constructionally simple way that, when a car door is opened, only the safety circuit associated with this car responds, while the other safety circuits remain uninfluenced by this.  
         [0015]     In the case of a particularly preferred embodiment of the elevator installation according to the invention, it is provided that at least one upper car can be made to travel in an upper shaft region having shaft doors and that at least one lower car can be made to travel in a lower shaft region having shaft doors, the car doors of the at least one upper car and the shaft doors of the upper shaft region being associated with one or more first safety circuits and the car doors of the at least one lower car and the shaft doors of the lower shaft region being associated with one or more second safety circuits. Such a configuration has the advantage that, within a shaft, two cars can be made to travel independently of one another in an upper shaft region and a lower shaft region, respectively, and do not hinder one another in their travel.  
         [0016]     It has proven to be advantageous if at least one safety circuit forms a shaft-door safety circuit that only has shaft doors associated with it, it being possible by means of the shaft-door safety circuit only to block the travel of cars in whose serving region the associated shaft doors are disposed. The “serving region” of a car is to be understood here as meaning the region of the shaft that can be traveled to by a car. If the safety device has a shaft-door safety circuit, this safety circuit responds only to opening of shaft doors and can then only block the travel of those cars in whose serving region the shaft doors associated with this shaft-door safety circuit are disposed.  
         [0017]     It is of advantage if at least one safety circuit forms a car-door safety circuit that only has car doors associated with it, it being possible by means of the car-door safety circuit only to block the travel of cars whose car doors are associated with the car-door safety circuit. The at least one car-door safety circuit responds only to opening of the associated car doors, while the opening of a shaft door has no influence on the car-door safety circuit.  
         [0018]     It is advantageous if each car-door safety circuit respectively has only the car doors of one car associated with it. If a car door is opened, this has the consequence in the case of such a configuration that only the car-door safety circuit associated with this car responds, while the car-door safety circuits of the other cars remain uninfluenced by this.  
         [0019]     In the case of a particularly preferred embodiment, each car has a single car-door safety circuit and a single shaft-door safety circuit associated with it. Such a configuration makes particularly simple control of the elevator installation possible, allowing a high transporting capacity to be achieved, since the cars hinder one another only little. The car-door safety circuit respectively associated with a car monitors merely the state of its own car doors and the associated shaft-door safety circuit may be configured in such a way that it only responds to the opening of the shaft doors disposed in the serving region of this car.  
         [0020]     In the case of a constructionally particularly simple configuration of the elevator installation according to the invention, it is provided that the car-door and shaft-door safety circuits respectively associated with a car have monitoring elements, for example switching contacts, which are connected in series with one another. It can be ensured by the series connection, for example, that the travel of the car is blocked as soon as one of its car doors or one of the shaft doors situated in its serving region is opened.  
         [0021]     It is advantageous if at each shaft door there is disposed for each car traveling to this shaft door an own shaft-door monitoring element. This provides the possibility of connecting in series with one another the shaft-door monitoring elements, for example switching contacts, that are respectively associated with a shaft-door safety circuit of a car, it being possible for the series connections of switching contacts that are associated with the shaft-door safety circuits of different cars to be electrically separate from one another.  
         [0022]     It may be provided that the serving regions of the cars that can be made to travel in a shaft are separated from one another, so that no serving region has shaft doors that can be traveled to by another car. To achieve a high transporting capacity, however, it may be of advantage if the serving regions of the cars have an overlap in such a way that at least one of the shaft doors can be traveled to both by a first car and by at least one second car. In this case it is advantageous if the travel of each car can be blocked by a shaft-door safety circuit which has associated with it both at least one shaft door that is disposed in the serving region of only this one car and at least one shaft door that is disposed in the serving region of this one car and at least one other car.  
         [0023]     It is of particular advantage if, when a car is stopped in the region of a shaft door, the monitoring of the opening and closing state of this shaft door can be disabled. It can be ensured by such a configuration that the opening of a shaft door when a car stops does not lead to a safety circuit responding and, as a result, possibly the travel of another car being blocked. Rather, on the operational opening of a shaft door, its monitoring is disabled. The “operational opening” of a shaft door is understood here as meaning the opening of a shaft door when a car enters the shaft-door region with the intention of door opening. In just the same way as the opening of the car door, the opening of the shaft door may already take place here just before the flush position of the car with the shaft door is reached, for example already at a distance of approximately +/−0.3 m, if the car has a speed of less than approximately  0 . 8  m/s. The opening of the shaft door usually takes place here under the action of the opening movement of the car door, i.e., when a car stops, at least one door of the car is coupled to the shaft door, so that, with the car door, the shaft door is also opened. On account of the possibility of disabling the monitoring of this shaft door, the travel of the other cars within the shaft is consequently not adversely affected by the stopping of one car.  
         [0024]     The monitoring of the opening and closing state can preferably only be disabled by those shaft doors that are disposed in the serving region of at least two cars. The monitoring of shaft doors that are merely disposed in the serving region of a single car cannot be disabled, however, in the case of such a configuration of the elevator installation. This provides the possibility of checking the response of the safety circuit coupled to a car by traveling to and opening a shaft door for which the monitoring cannot be disabled.  
         [0025]     The monitoring of the opening and closing state of the shaft doors and car doors preferably takes place with the aid of monitoring elements of the safety device that respectively interact with a shaft door or car door. Switching contacts which can be actuated by opening of the associated shaft door or car door may be used for example as monitoring elements. In this case, with-contact or contactless actuation of the switching contacts may be provided. For example, it may be provided that the shaft doors and car doors are mechanically coupled to the switching contacts; alternatively or additionally, an inductive or capacitive coupling may be provided, or else a coupling by means of infrared or light radiation.  
         [0026]     It is advantageous if the monitoring elements of shaft doors for which the monitoring can be disabled can be rendered ineffective, for example bridged, by means of a bridging unit. The bridging unit may be disposed at the shaft door or else at one or more cars. It is of particular advantage if the bridging unit has an activating element, which can be actuated by a car stopping at the associated shaft door. If a car enters the region of the shaft door with the intention of opening the door, it can actuate the activating element of the bridging unit for bridging the monitoring elements associated with the shaft door. The actuation of the activating element may take place with contact or else contactlessly. It may be provided, for example, that the activating element is configured as a magnetic switch which can be actuated by being approached by the car.  
         [0027]     For controlling the operation of the elevator installation, the latter comprises an installation control, which is preferably coupled to input elements disposed outside the shaft for the input of a destination call by a passenger. The travel destination of the car can be inputted into the elevator control, and in this respect it is of particular advantage if the bridging units used for disabling the monitoring of the shaft doors can be activated by the elevator control. This provides the possibility of activating the bridging units of a shaft door whenever the car is made to stop at this shaft door by the elevator control.  
         [0028]     As already explained, it may be provided that the bridging units can be enabled by actuation of their activating elements by means of a car. In this respect it is particularly advantageous if the bridging units can only be activated by actuation of their activating elements whenever they are at the same time acted upon by a control signal provided by the installation control. The bridging units are consequently configured in a two-channel form, both channels having to be simultaneously effective to disable the monitoring of the respectively associated shaft door. A first channel of the bridging unit is controlled by the respective activating element, which interacts with a car stopping in the region of the shaft door, while a second channel of the bridging unit is activated by the installation control. Only in the case in which a control signal of the installation control is present and the activating element is at the same time actuated by the car is the monitoring of the shaft door disabled.  
         [0029]     It is of advantage if, when a shaft door is opened without a car being present at the stop corresponding to the shaft door, all the shaft-door safety circuits associated with this stop are responsive.  
         [0030]     For supplying energy to the driving and controlling elements of the cars, in the case of a preferred embodiment each car is connected to a separate voltage supply unit. In this respect it is advantageous if the car-door and shaft-door safety circuits of each car, that is to say the car-door and shaft-door safety circuits into which the respective car is incorporated, are connected to the respective voltage supply unit of the car, shaft-door safety circuits with shaft doors that are disposed in the serving region of at least two cars only being able to be connected to the voltage supply unit of one of the cars concerned.  
         [0031]     It has proven to be advantageous in this respect that the shaft-door safety circuits that have their associated shaft doors disposed in the serving region of a number of cars can be automatically connected in each case to the voltage supply unit of a pre-selected car, as long as this car is in operation. In the case of such a configuration, the shaft-door safety circuits that have their shaft doors disposed in the serving region of a plurality of cars are connected to a preferred voltage supply unit, the voltage supply unit being that of one of the cars traveling to the shaft doors. However, the connection only exists when this car is in operation. If this car is taken out of operation, the shaft-door safety circuits in question are automatically connected to the voltage supply unit of another of the cars traveling to the shaft doors. As a result, it can be ensured in a constructionally simple way that, for example for maintenance and repair work, the voltage supply unit of a car can be switched off, and the latter consequently taken out of operation, without the shaft-door safety circuits that have their associated shaft doors disposed in the serving region of both this car and of another car being adversely affected in their function hereby.  
         [0032]     The configuration of the elevator installation according to the invention ensures that non-operational opening of shaft doors has the consequence of hindering the travel of all the cars in the shaft in the serving region of which the opened shaft door lies. However, the opening of shaft doors does not represent the only safety-relevant event that can influence the travel of a car. The elevator installation according to the invention may have further safety-relevant switching members with the aid of which the operating states of the elevator installation can be monitored. For example, it may be provided that, if two cars are coming too close together, the elevator installation can influence the travel of at least one of the cars, for example brake or accelerate it. It is particularly advantageous in this respect if each car has a shaft monitoring circuit associated with it, by means of which the travel of the car can be blocked in dependence on the state of safety-relevant switching members, at least one switching member having a bridging element associated with it for rendering the switching member ineffective, it being possible for the travel of at least one second car to be blocked when a first car is traveling with an active associated bridging element. Such a configuration of the elevator installation according to the invention is distinguished by the fact that at least one safety-relevant switching member can be rendered ineffective, for example bridged, by means of a bridging element, it being ensured, however, that, when the first car is traveling with an active bridging element, the travel of at least one second car can be blocked. Consequently, a safety-relevant switching member, for example a proximity switch, can be selectively rendered ineffective, in order to bring two cars very close together deliberately, a responding proximity switch indeed forming a safety-relevant switching member but it being possible for it to be selectively rendered ineffective, for example bridged. When the two cars move away from one another again, the bridging is to be disabled again. To ensure that such bridging is not unintentionally retained, for example on account of a fault, it is provided according to the invention that, when a first car is traveling with an active associated bridging element, the travel of at least one second car can be blocked. This ensures that transport by means of the elevator installation is still possible even when a bridging element is damaged, although transport can only be performed by means of the one car, while the travel of the other car is blocked in order to avoid any risk of an accident being caused by the defective bridging of the safety-relevant switching member. Instead of bridging, some other form of rendering the switching member ineffective is also conceivable, for example by removal of the voltage supply.  
         [0033]     It is advantageous in this respect if the blockage of the second car can be canceled by means of the elevator control. This provides the possibility for example of canceling the blockage within the region of a specific shaft door, so that the second car can be aligned flush within the shaft door region, but cannot leave the region of the shaft door.  
         [0034]     As mentioned at the beginning, the invention also relates to a method for controlling an elevator installation with at least one shaft, in which at least two cars disposed one above the other can be made to travel up and down, the shaft having a plurality of shaft doors and the cars respectively comprising at least one car door, and it being possible for the travel of the cars to be blocked by means of a safety device when the shaft door or car door is open.  
         [0035]     To achieve a particularly high transporting capacity with greatest possible availability of the elevator installation, it is provided according to the invention in the case of such a method that the travel of each car is separately blocked in dependence on the opening and closing states of its car doors and of all the shaft doors and in dependence on the position of all the cars that can be made to travel along the shaft.  
         [0036]     The method according to the invention is distinguished in particular by the fact that, after checking the states of the individual car doors, the positions of the individual cars and the states of the shaft doors, the travel of the individual cars is released or blocked. If, for example, an upper car is located in an upper shaft region, while a lower car assumes a position in a lower shaft region, the travel of the lower car can be released even when a shaft door is open in the upper shaft region, provided that this shaft door lies outside the serving region of the lower car. If, however, it is established that a shaft door is opened within the serving region of the lower car, the travel of the lower car is blocked unless the opening of this shaft door takes place operationally by the upper car.  
         [0037]     The method according to the invention has the advantage that the travel of the cars that can be made to travel within the shaft is hindered as little as possible and, as a result, the transporting capacity of the elevator installation can be increased.  
         [0038]     The following description of preferred embodiments of the invention serves for further explanation in conjunction with the drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]      FIG. 1  shows a schematic representation of a first embodiment of an elevator installation according to the invention;  
         [0040]      FIG. 2  shows a schematic representation of a shaft-door safety unit of the elevator installation from  FIG. 1 , and  
         [0041]      FIG. 3  shows a schematic representation in extract form of a second embodiment of an elevator installation according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]     In  FIGS. 1 and 2 , a first embodiment of an elevator installation according to the invention is represented in a greatly schematized form and designated overall by the reference numeral  10 . It comprises two cars disposed one above the other in a shaft  12 , specifically an upper car  14  and a lower car  16 , which can be made to travel individually up and down along a common traveling path on guides  11 ,  13 . To achieve a better overview, in the drawing the shaft  12  is shown offset in relation to the two cars  14  and  16 .  
         [0043]     The upper car  14  is coupled to a counterweight  19  via a suspension cable  18 , and the lower car  16  is coupled to a counterweight  22  via a suspension cable  21 . Each car  14  and  16  has a separate drive associated with it in the form of an electric drive motor  24  and  25 , respectively, and in each case a separate brake  28  and  29 , respectively. The drive motors  24  and  25  in each case drive a traction sheave  31  and  32 , respectively, over which the suspension cables  18  and  21  are led.  
         [0044]     The upper car  14  has a car door  34  with two door leaves, the opening and closing state of which is monitored by monitoring means, in the exemplary embodiment represented by switching elements  35 . The switching elements  35  have electrical contacts, which are closed when the associated car door  34  is closed. The electrical contacts of the switching elements  35  are open when the associated car door  34  is not closed. All the switching elements  35  of the upper car  14  are connected in series with one another and form a car-door safety circuit  37 , explained below, of the upper car  14 .  
         [0045]     In a corresponding way, the lower car  16  has a car door  39  with two door leaves, the opening and closing state of which is monitored by monitoring means in the form of electrical switching elements  40 . The switching elements  40  have electrical switching contacts, which are closed when the associated car door  39  is closed. The electrical contacts of the switching elements  40  are open when the associated car door  39  is not closed. All the switching elements  40  of the lower car  16  are connected in series with one another and form a car-door safety circuit  42 , explained in more detail below, of the lower car  16 .  
         [0046]     At each stop at which access to one of the cars  14 ,  16  is possible, the shaft  12  has in each case at least one shaft door  14 , which in the embodiment represented comprises two door leaves. The opening and closing state of the shaft doors  44  is monitored by respectively associated monitoring means, which have electrical contacts. Each shaft door  44  additionally has blocking means, for example hook bolts, with signaling contacts. The monitoring means and blocking means are hereafter referred to together as switching elements  45 . The electrical contacts of the switching elements  45  are closed when the associated shaft door  44  is closed and locked; the contacts are open when the associated shaft door  44  is not closed or not locked. The interconnection of the electrical contacts of all the switching elements  45  of all the shaft doors  44  forms a shaft-door safety unit  47 , which is explained in more detail below and represented in  FIG. 2 .  
         [0047]     If a car  14 ,  16  enters a stop that can be served by it, in order to be loaded or unloaded there, the car door  34  or  39  is then opened in the unlocking region of the stop in a way which is known per se and therefore not represented, for example by means of an electrical door drive. In a way which is likewise known to a person skilled in the art and therefore not represented, to achieve a better overview in the present case, when a stop is entered a mechanical coupling takes place between the car door  34  or  39  and the shaft door  44  of the stop entered by the corresponding car  14 ,  16 . The movement of the opening car door  34  or  39  has the effect that the coupled shaft door  44  is unlocked and, in synchronism with the car door  34  or  39 , likewise opens. The closing operation also takes place in a corresponding way, the shaft door  44  subsequently being locked in the closed position and the mechanical coupling between the car door  34  or  39  and the shaft door  44  being canceled again.  
         [0048]     Each car  14  and  16  has associated with it a separate voltage supply unit  49  and  50 , respectively, which undertakes the voltage supply to all the controlling and driving components associated with the respective car  14  or  16 . In particular, the electrical drive motors  24  and  25  and the brakes  28  and  29  of each car  14 ,  16  are supplied with electrical energy by the respectively associated voltage supply unit  49  or  50 . For this purpose, the voltage supply unit  49  is in electrical connection with the drive motor  24  via an electrical supply line  52  and a first current path  53  and with the brake  28  of the upper car  14  via the electrical supply line  52  and a second current path  54 , a first contact block  55  and an automatic drive control  56  being connected between the electrical supply line  52  and the first current path  53 , and a second contact block  57  being connected between the electrical supply line  52  and the second current path  54 .  
         [0049]     In a corresponding way, the voltage supply unit  50  of the lower car  16  is connected to the associated drive motor  25  via an electrical supply line  60  and also a first current path  61  and to the associated brake  29  via the electrical supply line  60  and a second current path  62 , a first contact block  63  and an automatic drive control  64  being connected between the electrical supply line  60  and the first current path  61 , and a second contact block  65  being connected between the electrical supply line  60  and the second current path  62 .  
         [0050]     The first and second contact blocks  55 ,  57  and  63 ,  65  respectively associated with a car  14  or  16  can be electrically actuated by a travel contactor  67  or  69  associated with the respective car  14  or  16 . The first and second contact blocks  55 ,  57 , associated with the upper car  14 , in this case form the contactor contacts of the travel contactor  67 , and the first and second contact blocks  63 ,  65 , associated with the lower car  16 , form the contactor contacts of the travel contactor  69 .  
         [0051]     The voltage supply to the travel contactor  67  takes place via a safety chain  71 , which is associated with the upper car  14  and via which the travel contactor  67  is connected to the voltage supply unit  49  of the upper car  14 . The safety chain  71  is formed by a shaft monitoring circuit  72 , associated with the upper car  14 , the output contacts  74  of a shaft-door safety circuit  75 , associated with the upper car  14  and explained in more detail below, and also the car-door safety circuit  37 . The shaft-door monitoring circuit  72 , the output contacts  74  and the car-door safety circuit  37  are connected in series with one another. The shaft monitoring circuit  72 , associated with the upper car  14 , is configured here in a way known per se; it includes all the safety switches that are associated with the upper car  14  and are to be incorporated in a safety loop, such as for example emergency limit switches, safety gear switches, buffer switches and the like. Not incorporated in the shaft monitoring circuit  72 , however, are the switching elements  35  of the car doors  34  of the upper car  14  and the switching elements  45  of all the shaft doors  44 .  
         [0052]     A current flow from the voltage supply unit  49  to the travel contactor  67  only comes about when all the electrical contacts of the safety chain  71  that are involved in the current flow are closed. The current flow is interrupted as soon as only one contact involved in the current flow is open. If all the contacts of the safety chain  71  are closed, the brake  28  of the upper car  14  is supplied with electrical energy by the voltage supply unit  49  via the electrical supply line  52  and the second current path  54 , so that the brake  28  is open. At the same time, the drive motor  24  of the upper car  14  is supplied with electrical energy by the voltage supply unit  49  via the electrical supply line  52  and the first current path  53 , and can consequently be set in rotation in order to move the car  14 . An interruption of the current flow via the safety chain  71  to the travel contactor  67  has the consequence that the first and second contact blocks  55 ,  57  are opened and, as a result, the energy supply to the brake  28  and to the drive motor  24  is interrupted. This has the effect that no electrical drive energy is available any longer to the drive motor  24  and that the brake  28  engages, so that the drive shaft of the drive motor  24  is braked and, as a result, the car  14  comes to a stop.  
         [0053]     The travel contactor  69  associated with the lower car  16  is connected via a safety chain  78  of the lower car  16  to the voltage supply unit  50  of the latter. The safety chain  78  is configured in a way corresponding to that of the safety chain  71 ; it has a shaft monitoring circuit  79  that is known to a person skilled in the art and is connected in series with output contacts  80  of a shaft-door safety circuit  81 , associated with the lower car  16  and explained in more detail below, and also with the switching elements  40  of the car-door safety circuit  42  of the lower car  16 .  
         [0054]     The shaft monitoring circuit  79  is formed in a way corresponding to that of the shaft monitoring circuit  72 . It includes all the safety switches that are associated with the lower car  16  and are to be incorporated in a safety loop, for example emergency limit switches, safety gear switches, buffer switches and the like. Not incorporated in the shaft monitoring circuit  79 , however, are the switching elements  40  of the car doors  39  of the lower car  16  and the switching elements  45  of all the shaft doors  44 .  
         [0055]     A current flow from the voltage supply unit  50  to the travel contactor  69  only comes about when all the electrical contacts of the safety chain  78  that are involved in the current flow are closed. The current flow is interrupted as soon as only one contact involved in the current flow is open. If a current flow comes about, the associated first and second contact blocks  63 ,  65  are closed by the travel contactor  69 , so that the brake  29  is connected to the voltage supply unit  50  via the electrical supply line  60  and the second current path  62  and opens as a result, and the drive motor  25  is supplied with electrical energy via the electrical supply line  60  and the first current path  61  and is set in rotation as a result, in order to move the lower car  16 . The rotational speed of the drive motor  25  can in this case be controlled in a way known per se by the automatic drive control  64 , which may be configured for example in the form of a frequency converter.  
         [0056]     The automatic drive control  56  associated with the upper car  14  permits a corresponding automatic control of the rotational speed of the drive motor  24  and may likewise be configured as a frequency converter.  
         [0057]     If the current flow from the voltage supply unit  50  via the safety chain  78  to the travel contactor  69  is interrupted, the contacts of the first and second contact blocks  63  and  65  are opened, i.e. the energy supply to the drive motor  25  and the brake  29  is interrupted. This has the consequence that the brake  29  engages and consequently the drive shaft of the drive motor  25  is braked and, as a consequence, the lower car  16  is brought to a stop.  
         [0058]     The elevator installation  10  has an installation control  85 , which is connected via a signal line  86  to an encoder  87 , which is mounted in a rotationally fixed manner on the drive shaft of the drive motor  24  of the upper car  14  and provides the installation control  85  with displacement pulses, from which the installation control  85  can determine the position of the upper car  14  in the customary way. Via a further signal line  88 , the installation control  85  is in electrical connection with an encoder  89 , which is mounted in a rotationally fixed manner on the drive shaft of the drive motor  25  of the lower car  16  and provides the installation control  85  with displacement pulses, from which the installation control  85  can determine the position of the lower car  16  in the customary way.  
         [0059]     The installation control  85  is in electrical connection via a first control line  91  with a first bridging circuit  92 , which is associated with the upper car  14 , is connected in parallel with the car-door safety circuit  37  and can be controlled by the installation control  85 .  
         [0060]     Used for bridging the car-door safety circuit  42  associated with the lower car  16  is a second bridging circuit  93 , which is connected to the installation control  85  via a second control line  94 .  
         [0061]     The bridging of the car-door safety circuits  37  and  42  by means of the first and second bridging circuits  92  and  93 , respectively, makes it possible when a stop is entered for the car doors  34  and  39  to be opened in the unlocking region of this stop even before the car  14  or  16  has reached the level at which it is flush with the respective stop, so that the car  14  or  16  can reach the flush level with the car doors  34  or  39  already open. As already explained, opening of the car doors  34  or  39  has the consequence that the car-door safety circuit  37  or  42  is opened. The interruption of the current flow to the travel contactor  67  or  69  can be prevented, however, by means of the first or second bridging circuit  92  or  93  if a control signal is provided by the installation control  85  in dependence on the respectively ascertained position of the upper or lower car  14 ,  16 . Activation of the bridging circuit  92  or  93  means that its electrical contacts are closed and consequently the bridging is active. Consequently, an open car-door safety circuit  37  or  42  no longer leads to an interruption of the current flow to the associated travel contactor  67  or  69 . If, however, the associated bridging circuit  92  or  93  is not activated by the installation control  85 , its contacts are open and consequently the bridging is not active.  
         [0062]     The locations of the cars  14  and  16  within the shaft  12  are known to the installation control  85  on the basis of the displacement pulses provided by the respective encoders  87  and  89 , and the activation of the bridging circuit  92  and  93  takes place only whenever the respective car  14  or  16  is located in the unlocking region of the stop that is the travel destination.  
         [0063]     If, when the car doors  34  or  39  are being opened while the car  14 ,  16  is entering the stop, the switching elements  45  of the respective shaft door  44  are also actuated, the output contacts  74  and  80  of the shaft-door safety circuit  75 ,  81  associated with the respective car  14  or  16  can be bridged in addition to the respective car-door safety circuit  37  or  42  by the bridging circuits  92 ,  93 , in that the bridging circuits  92  and  93  are connected to the respective safety chain  71  or  78  not via a connecting line  96  that merely permits bridging of the associated car-door safety circuit  37  or  42 , but via a connecting line  97 , which is represented by dashed lines in  FIG. 1 , the connecting line  97  permitting not only bridging of the car-door safety circuit  37  or  42  but also bridging of the output contacts of the respective shaft-door safety circuit  75  or  81 .  
         [0064]     The shaft-door safety circuits  75  and  81  are explained in more detail below with reference to  FIG. 2 . In  FIG. 2 , the elevator installation  10  is represented in a greatly schematized form with a total of eleven stops, it being intended for the lowermost stop to be situated on the ground floor of a building and this stop having the reference numeral  100 ; the next-following stop is disposed on the first upper floor and is provided with the reference numeral  101 . The other stops are provided with  102 , etc., so that the stop on the tenth upper floor has the reference numeral  110 . To achieve a better overview, the stops of the third to sixth floors are not represented in  FIG. 2 ; however, the configuration of the corresponding stops and the electrical wiring of the corresponding components at these stops is identical to the stops  102  or  107  explained below.  
         [0065]     In the case of the exemplary embodiment represented in  FIG. 2 , it is assumed that the stop  100  situated on the ground floor can only be traveled to by the lower car  16  and the two uppermost stops  109  and  110  of the ninth and tenth floors can only be traveled to by the upper car  14 , while the stops  101  to  108  situated in between can be traveled to by both cars. The serving region of the lower car  16  consequently extends over the stops  100  through  108 , and the serving region of the upper car  14  extends over the stops  101  to  110 , and the entire shaft  12  can be subdivided into an upper shaft region with the stops  109  and  110 , a common shaft region with the stops  101  to  108  and a lower shaft region with the stop  100 . The stops of the lower and upper shaft regions can respectively be traveled to only by one of the two cars  14 ,  16 , while the stops in the common shaft region can be traveled to by both cars.  
         [0066]     In  FIG. 2 , all the stops  100  to  110  are shown with their respectively associated switching elements  45  of the respective shaft doors  44 , all the switching elements  45  having a shaft-door contact  45   a  and a bolt contact  45   b  —this being illustrated in  FIG. 2  only for the example of the switching element of the uppermost stop  110 , to achieve a better overview. The shaft-door contact  45   a  is closed when the shaft door  44  is closed, and the bolt contact  45   b  is closed when the shaft door  44  is locked.  
         [0067]     The switching elements  45  of all the shaft doors  44  form in their totality a shaft-door safety unit  47 . This has a shaft-door safety circuit  75 , which is associated with the upper car  14 , and also a shaft-door safety circuit  81 , which is associated with the lower car  16 . The shaft-door safety circuit  75  associated with the upper car  14  is connected to a switching unit  112 , which has the output contacts  74 , and the shaft-door safety circuit  81  of the lower car  16  is connected to a switching unit  114  with the output contacts  80 .  
         [0068]     The shaft-door safety circuit  81  of the lower car  16  is formed by a series connection of the switching elements  45  of the stops  101  to  108  and of the stop  100  situated on the ground floor, while the switching elements  45  of the stops  109  and  110  situated on the ninth and tenth upper floors are not incorporated in the shaft-door safety circuit  81  of the lower car  16 .  
         [0069]     The shaft-door safety circuit  75  of the upper car  14  is formed by a series connection of the switching elements  45  of the stops  101  to  110 , while the switching elements  45  of the stop  100  situated on the first floor are not incorporated in the shaft-door safety circuit  75  of the upper car  14 .  
         [0070]     The voltage supply to the shaft-door safety circuits  75  and  81  takes place via a common feed line  116  and a common return line  117 , which is connected to the switching units  112  and  114  and can selectively be connected via contact blocks  119  and  120  to the voltage supply unit  49  of the upper car  14  or the voltage supply unit  50  of the lower car  16 . The contact blocks  119  and  120  can be controlled by a switching contactor  122 , which is connected to the voltage supply unit  49  of the upper car  14 . If the upper car  14  is in operation, its voltage supply unit  49  is active. This has the consequence that the switching contactor  122  is supplied with electrical energy and the two contact blocks  119  and  120  are activated in such a way that the common feed line  116  and the common return line  117  of the two shaft-door safety circuits  75  and  81  are in electrical connection with the voltage supply unit  49 . If, on the other hand, the upper car  14  is not in operation, its voltage supply unit  49  is switched off. This has the consequence that the switching contactor  122  is not supplied with electrical energy. The contact blocks  119  and  120  then assume such a switching position that the feed line  116  and the return line  117  are in electrical connection with the voltage supply unit  50  of the lower car  16 . The shaft-door safety circuits  75  and  81  are consequently in any event only supplied with electrical energy by a single voltage supply unit  49  or  50 , the voltage supply unit  49  of the upper car  14  preferably being used whenever this car  14  is in operation.  
         [0071]     The current flow via the shaft-door safety circuit  75  associated with the upper car  14  takes place from the feed line  116 , via the series-connected switching elements  45  of the stops  101  to  108 , which can be traveled to by both cars  14  and  16 , and also via the switching elements  45  of the stops  109  and  110 , which can only be traveled to only by the upper car  14 , and subsequently via a current path  124  to the switching unit  112  and from the latter via the return line  117  to one of the two voltage supply units  49  or  50 .  
         [0072]     The current flow via the shaft-door safety circuit  81  associated with the lower car  16  takes place from the feed line  116  via the switching elements  45  of the stops  101  to  108  which can be traveled to by both cars  14 ,  16  and subsequently via a current path  125  to the switching elements  45  of the lowermost stop  100  and subsequently via the switching unit  114  to the return line  117 .  
         [0073]     If the switching units  112  and  114  are supplied with electrical energy via the respectively associated shaft-door safety circuits  75  or  81 , their output contacts  74  or  80  are closed. If the energy supply to the switching units  112  or  114  is interrupted, the respective output contacts  74  or  80  are opened.  
         [0074]     The switching elements  45  of the shaft doors  44 , which are disposed in the region of the stops  101  to  108  and can be traveled to by both cars  14  and  16 , respectively have a separate associated bridging unit  127 , which is disposed in the region of the respective stop and with the aid of which the respective switching element  45  can be rendered ineffective, to be specific electrically bridged. The identically configured bridging units  127  have in each case two control channels, in that they are in electrical connection via a first input line  128  with a fork-shaped magnetic switch  129 , disposed in the region of the respective stop  101  to  108 , and are connected via a second input line  130  to a control element  132  of the installation control  85 .  
         [0075]     The bridging units  127  respectively have a first series of contacts  134  and a second series of contacts  135 , the switching positions of which are always identical. Each first series of contacts  134  is in electrical connection with a return-signaling element  138  of the installation control  85  via an output line  137 , and the switching element  45  of the shaft doors  44  that is associated with the respective bridging unit  127  can be bridged via the second series of contacts  135 .  
         [0076]     Bridging of a switching element  45  can only take place by means of the associated bridging unit  127  whenever both the magnetic switch  129  disposed in the region of the respective stop in the shaft  12  is actuated and a control signal is provided by the control element  132  via the second input line  130 . The actuation of the magnetic switch  129  takes place by means of switching lugs  140 , which are fixed on the cars  14  and  16  and may be configured for example in the form of a sheet-metal strip. The mounting locations of the magnetic switches  129  in the shaft  12  in the region of the stop associated with the respective magnetic switches  129 , on the one hand, and the mounting locations of a switching lug  140  respectively on the cars  14  and  16 , on the other hand, are chosen such that a magnetic switch  129  can only be actuated whenever one of the two cars  14 ,  16  is in the unlocking region of the stop associated with the respective magnetic switch  129 .  
         [0077]     As already explained, the two series of contacts  134  and  135  are only closed whenever both control channels of the corresponding bridging unit  127  are activated simultaneously. Both series of contacts  134  and  135  are open if only one channel or none of the two channels is activated.  
         [0078]     A closed series of contacts  135  of a bridging unit  127  bridges the switching elements  45  of all the shaft doors  44  of the associated stop. As already explained, the series of contacts  134  always has the same switching position as the series of contacts  135 . This serves for return signaling of the switching position of the series of contacts  135 . By means of the signals of the switching positions of the series of contacts  134 , the installation control  85  is always kept informed of the switching position of all the bridging units  127  and can use this information for controlling the traffic flow of the cars  14  and  16  and, in the event of an error, intervene to correct the traffic flow.  
         [0079]     The operating mode of the bridging units  127  in connection with the traffic flow of the cars  14  and  16  is described below by way of example for the entry of the car  16  into the stop  101  situated on the first upper floor: The car  16  is initially located outside the locking region of the stop  101  and approaches this stop with the intention of stopping at the stop  101 . Once the car  16  has already reduced its speed to the extent that entry to the stop  101  can be authorized by the installation control  85 , to be specific to a speed less than 0.8 m/s, and the car  16  is in the direct proximity of the unlocking region of the stop  101 , to be specific at a distance of approximately 0.3 m from the flush level of this stop, the installation control  85  then activates the first channel of the associated bridging unit  127  via the second input line  130 . When the car  16  reaches the unlocking position of this stop, the switching lug  140  enters the fork of the magnetic switch  129 , whereby the magnetic switch  129  activates the second channel of the same bridging unit  127  via the first input line  128 . Once both channels of the bridging unit  127  associated with the stop  101  have been activated, the two series of contacts  134  and  135  of the bridging unit  127  are closed. The closed series of contacts  135  thereby bridges the switching elements  45  of all the shaft doors  44  of the stop  101 . The closed series of contacts  134  signals via the output line  137  to the return-signaling element  138  of the installation control  85  that the bridging of the switching elements  45  at the stop  101  has taken place. The installation control  85  then authorizes the opening of the shaft door  44  at the stop  101 . Although the opening of the shaft door  44  at the stop  101  has the effect that the electrical contact of the switching element  44  is opened, because of the active bridging by means of the series of contacts  135  of the bridging unit  127  this does not lead to an interruption of the activation of the switching units  112  and  114  of the two shaft-door safety circuits  75  and  81 , i.e. their output contacts  74  and  80  remain closed. In spite of the opened shaft door at the stop  101 , the respective travel of the two cars  14  and  16  is consequently not influenced.  
         [0080]     When the lower car  16  subsequently leaves the unlocking region of the stop  101 , the associated magnetic switch  129  is then no longer actuated by the switching lug  140  of the lower car  16 , i.e. the second channel of the bridging unit  127  is no longer activated, so that both series of contacts  134  and  135  of this bridging unit  127  are opened and consequently the bridging of the switching elements  45  at the stop  101  is canceled. If, on account of a fault, one of the shaft doors  44  of the stop  101  is still open, this leads to the immediate stopping of both cars  14  and  16 .  
         [0081]     If, however, in the case of the example explained above with bridged switching elements  45  of the stop  101 , in addition a shaft door  44  is for example manually opened with the aid of an emergency unlocking key at another stop, for example at the stop  107 , this leads to an interruption of the activation of the switching units  112  and  114  of the two shaft-door safety circuits  75  and  81  and consequently likewise to the immediate stopping of both cars  14  and  16 .  
         [0082]     If it is intended in the example explained above for the lower car  16 , starting from the stop  101 , to carry out a new trip to another stop, the activation of the first channel of the bridging unit  127  associated with the stop  101  is then ended by the installation control  85  before travel begins, so that both series of contacts  134  and  135  of this bridging unit  127  are opened and consequently the bridging of the switching elements  45  of this stop  101  is canceled. This has the consequence that new travel by the lower car  16 , starting from the stop  101 , can only begin when the shaft doors  44  of this stop  101  are closed.  
         [0083]     At the stops  100  and  109 ,  110 , which can each only be traveled to by one car  16  or  14 , no bridging units  127  are installed. Bridging of the switching elements  45  of the shaft doors  44  accordingly cannot take place at these stops  100 ,  109  and  110 . However, as a result of the interconnection of the shaft-door safety circuits  75  and  81  that is used, entering these stops  100 ,  109  and  110 , and the opening of the shaft doors  44  of these stops  100 ,  109  and  110  thereby taking place, only has effects on the shaft-door safety circuit  75  or  81  that is associated with the respectively entering car  14 ,  16 .  
         [0084]     Thus, if the upper car  14  enters the stop  109  or the stop  110 , only the activation of the switching unit  112  of the shaft-door safety circuit  75  of the upper car  14  is interrupted by the shaft door  44  opening in the region of this stop  109  or  110 . This merely leads to opening of the output contacts  74 , and consequently to an interruption of the safety chain  71  associated with the upper car  14 , but not to an interruption of the safety chain  78  associated with the lower car  16 . Immediate stopping of the upper car  14  when it enters the stops  109  or  110  can be prevented if, as represented by dashed lines in  FIG. 1 , the connecting line  97  is used instead of the connecting line  96 . Since, as described above, the bridging circuit  92  is only effective when the upper car  14  is in the unlocking region of the associated stop which it is to enter, this measure has no safety-relevant disadvantages.  
         [0085]     If the lower car  16  enters the stop  100 , only the activation of the switching unit  114  of the shaft-door safety circuit  81  is interrupted by the opening shaft door  44 . This leads to opening of the output contacts  80 . The shaft-door safety circuit  75  of the upper car  14  remains uninfluenced by this and its output contacts  74  remain closed. Immediate stopping of the lower car  16  when it enters the stop  100  can be prevented by using the connecting line  97  instead of the connecting line  96 , as represented by dotted lines in  FIG. 1 . Since, as already mentioned, the bridging circuit  93  is only effective whenever the lower car  16  is in the unlocking region of a stop which it is to enter, this measure has no safety-relevant disadvantages.  
         [0086]     In the case of the embodiment represented in  FIG. 2 , a single shaft-door safety unit  47  is used, with series-connected switching elements  45  of all the shaft doors  44 , and the shaft-door safety circuits  75  and  81  respectively associated with a car  14  or  16  in each case cover a sub-region of the series connection of the switching elements  45 . It may alternatively also be envisaged to use two shaft-door safety units, which are electrically separate from one another and respectively comprise a shaft-door safety circuit that is assigned to a car. For this purpose, at each shaft door  44  an own switching element  45  or  46  may be disposed for each car  14 ,  16  traveling to this shaft door  44 , the switching elements  45  and the switching elements  46  respectively forming an own series connection and consequently a separate shaft-door safety circuit. In the case of the shaft doors which can be traveled to by both cars  14  and  16 , both the switching elements  45  and the switching elements  46 , represented by dotted lines in  FIG. 1 , have in each case an associated bridging unit, which is electrically connected only to the respective switching element  45  or  46  but can be activated by all the cars  14  and  16  traveling to the respective shaft door  44 . Such a configuration has the advantage that the cars  14  and  16  could have circuits that are electrically separate from one another associated with them.  
         [0087]     In  FIG. 3 , a second embodiment of an elevator installation according to the invention is schematically represented and provided overall with the reference numeral 150. This is configured largely identically to the elevator installation  10  explained above with reference to  FIGS. 1 and 2 . Therefore, the same reference numerals as in  FIGS. 1 and 2  are used for identical components in  FIG. 3 . To avoid repetition, reference is made in this respect to the full content of the statements made above.  
         [0088]     In the case of the elevator installation  150  represented in  FIG. 3 , the upper car  14  has a proximity switch  152 , the switching contacts  153  of which are integrated into the shaft monitoring circuit  72  of the safety chain  71  associated with the upper car  14 , the switching contacts  153  being connected in series with further switching contacts, known per se, of the safety circuit  72 .  
         [0089]     The actuation of the proximity switch  152  takes place by means of a switching roller  154 , which is mounted on the outside of the upper car  14  and, when the upper car  14  comes very close to the lower car  16 , comes into contact with a switching face  155 , which is fixed on a spacer  157  projecting beyond a car roof  156  of the lower car  16 .  
         [0090]     If the upper car  14  approaches the lower car  16  to such an extent that the switching face  155  of the lower car  16  actuates the switching roller  154 , the proximity switch  152  of the upper car  14  is actuated, its switching contacts  153  being opened. This has the consequence that the power supply to the travel contactor  67  of the upper car  14  is interrupted and consequently the travel of the upper car  14  is blocked.  
         [0091]     The switching face  155  consequently forms in combination with the switching roller  154  and the associated proximity switch  152  a collision prevention device, with the aid of which collisions of the two cars  14  and  16  can be prevented.  
         [0092]     In dependence on, for example, the traffic pattern and/or in dependence on particular circumstances of the building in which the elevator installation  150  is installed, such as for example reduced floor-to-floor distances, in certain cases it may be desired for the two cars  14  and  16  to come very close together. In order to avoid adversely affecting the travel of the two cars  14  and  16  in such a situation, an invalidation element in the form of a bridging module  160 , which is mounted in a fixed location in the shaft  12  and has a normally-open contact  161  and a normally-closed contact  162 , is used in the case of the elevator installation  150 . The bridging module  160  is mounted at a predetermined location within the shaft  12  at which the two cars  14  and  16  are to be allowed to come close together, the normally-open contact  161  being connected in parallel with the switching contacts  153  of the proximity switch  152 . For actuation, the bridging module  160  has a switching cam follower  163 , which can be actuated by a switching cam  164  fixed on the outside of the upper car  16 . If the bridging module  160  is actuated by the switching cam  164  by actuation of the switching cam follower  163 , this has the consequence that the normally-open contact  161  is closed and at the same time the normally-closed contact  162  is opened. The normally-closed contact  162  is integrated into the safety chain  78  of the lower car  16  and connected in series with the latter&#39;s car-door safety circuit  42 . Together with the car-door safety circuit  42 , it can be bridged by the second bridging circuit  93 .  
         [0093]     Consequently, in spite of the use of the proximity switch  152 , the upper car  14  can be deliberately made to approach the lower car  16 , any adverse effect on the travel of the upper car  14  being prevented, since the switching contacts  153  of the proximity switch  152  are bridged by the closed normally-open contact  161 . However, it is ensured by the simultaneous opening of the normally-closed contact  162  that, when the upper car  14  is deliberately made to approach the lower car  16 , the travel of the lower car is blocked, since the normally-closed contact  162  is integrated into the safety chain  78  of the lower car  16 .  
         [0094]     The configuration explained above of the elevator installation  150  makes it possible, for example, for the floor-to-floor distance between the stop  100  situated on the ground floor and the stop  101  situated on the first upper floor to be chosen to be so small that, in the case in which the lower car  16  is at the stop  100 , although the proximity switch  152  is actuated when the upper car  14  enters the stop  101 , this actuation does not adversely affect the traffic flow. For this purpose, the mounting location of the bridging module  160  within the shaft  12  of the elevator installation  150  is chosen such that, when the car  14  enters the stop  101 , the switching cam  164  actuates the switching cam follower  163  of the bridging module  160 , so that the then closed normally-open contact  161  bridges the switching contacts  153  of the proximity switch  152  and at the same time the normally-closed contact  162  assumes its open switching position. If the upper car  14  leaves the stop  101  and at the same time the bridging module  160  remains actuated on account of a fault, the normally-closed contact  162  remains in its open position. This has the consequence that the lower car  16  can in fact still move within the stop  100 , for example can adjust, as long as both the car safety circuit  42  and the normally-closed contact  162  of the lower car  16  are bridged by the installation control  85  by means of activation of the bridging circuit  93 , but that the lower car  16  cannot leave the stop  100  with the intention of traveling to another stop, since in this case the installation control  85  no longer activates the bridging circuit  93  of the lower car  16 , whereby the bridging is canceled and consequently the open normally-closed contact  162  interrupts the current flow to the travel contactors  69 . This has the consequence of immediately stopping the lower car  16 . The travel of the upper car  14  is uninfluenced by this however.