Abstract:
An arrangement includes two series-connected switching contacts in two contactors. The tripping signal thereof is generated with a variable time offset so that the number of disconnection processes and connection processes, which are carried out first in time, are virtually the same during lengthy use.

Description:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/DE00/04059 which has an International filing date of Nov. 16, 2000, which designated the United States of America, the entire contents of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The invention generally relates to an arrangement having at least two contactors. Preferably, in relates to one whose switching contacts are connected in series and which are even more preferably each actuated by a tripping signal. 
   BACKGROUND OF THE INVENTION 
   In arrangements of this generic type, the tripping signals for the two contactors are normally predetermined such that one of the two contacts is always disconnected first, so that this contact is subjected to particularly severe contact wear. Thus, this contact must be replaced while the other contact has scarcely been consumed at all. 
   SUMMARY OF THE INVENTION 
   An embodiment of the invention is thus based on an object of providing an arrangement which ensures a longer life in terms of contact wear. 
   The object can be achieved, for example, in that the tripping signals for disconnection of the switching contacts are generated with a time offset Δt such that the equation 
           ∑     i   =   1     n     ⁢   Aai     ≈       ∑     i   =   1     n     ⁢   Abi         
 
is satisfied for the first disconnection operations Aai or Abi, respectively, in time of the two switching contacts a or b, respectively.
 
   The condition defined by the equation applies to lengthy use of the arrangement with a relatively large number of switching operations n. Subject to this precondition, the contact wear on both of the series-connected switching contacts is virtually approximately the same after a large number of disconnection operations, so that the life is twice as long as with the known arrangement. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the invention will be explained in more detail in the following text with reference to a drawing, wherein: 
     The FIGURE shows a configuration including two contactors connected in series. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The arrangement according to an embodiment of the invention and as shown in the FIGURE has two contactors Sa, Sb, whose switching contacts a and b are connected in series. The tripping signals which are supplied to the contactors Sa and Sb have the reference numbers  1  and  2 , respectively, in order to connect and disconnect the contacts a and b, respectively, as a function of them. The tripping signals  1 ,  2  are preferably generated in a control mechanism  3  such that there is a deliberate time offset Δt between them, in order that the connection operations Eai, Ebi of the two contactors Sa, Sb are in each case carried out before the other contactor. A corresponding situation applies to the disconnection operations Aai, Abi, which are carried out first in time on the contactor Sa or Sb, before the respective other contactor. The control mechanism  3  generates the tripping signals  1 ,  2  such that at least the following equation is satisfied: 
           ∑     i   =   1     n     ⁢   Aai     ≈       ∑     i   =   1     n     ⁢   Abi         
 
   Thus, over a lengthy time period, the sum i=1 . . . n of disconnection operations Aai of the contactor Sa is approximately equal to the sum i=1 . . . n of disconnection operations Abi of the contactor Sb. 
   Thus, the contact wear resulting from the disconnection operations, in particular in the event of overload currents and short-circuit currents, is approximately the same for the two contacts a, b which results in the arrangement having longer life with regard to the contact breaking off. Furthermore, as already indicated above, the control mechanism can be designed such that the connection operations which are carried out first in time satisfy the following equation: 
           ∑     i   =   1     n     ⁢   Eai     ≈       ∑     i   =   1     n     ⁢   Ebi         
 
   Thus, the sum i=1 . . . n of connection operations Eai for the contactor Sa is approximately equal to the sum i=1 . . . n of connection operations Ebi for the contactor Sb. 
   A random number generator  4 , a fixed generator  5  as indicated in the FIGURE, or a device with similar characteristics may be used by the control mechanism  3  to form the time-offset tripping signals  1 ,  2 . 
   According to the FIGURE, a further switching contact c in the third contactor Sc is connected in series with the two switching contacts a, b in the contactors Sa, Sb, and is actuated by the switching signal  6  generated in the control mechanism  3 . In this case, the switching signal  6  likewise has a time offset Δt with respect to the two other switching signals  1 ,  2 , and this is in fact changed in accordance with the conditions mentioned above, in order to distribute the contact wear resulting from the disconnection operations and connection operation virtually uniformly between all three contactors Sa, Sb, Sc. This increases the overall life of the arrangement. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.