Abstract:
A compact electronic circuit breaker is simple to assemble. The circuit breaker has an insulating housing, a switch contact for reversibly contacting a load power circuit to be monitored, a triggering magnet acting by way of a triggering mechanism on the switch contact, and triggering electronics for actuating the triggering magnet. The switch contact, the triggering magnet, and the triggering electronics are fixedly mounted on a printed circuit board and they form a preassembled component together with the printed circuit board. The preassembled component can thereby be inserted in the housing as a unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation, under 35 U.S.C. §120, of copending international application No. PCT/EP2010/003362, filed Jun. 2, 2010, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2009 025 513.3, filed Jun. 19, 2009; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF INVENTION 
     Field of the Invention 
       [0002]    The invention relates to an electronic circuit breaker. A circuit breaker such as this is used to automatically open an electrical load circuit when a tripping condition occurs, that is to say to electrically interrupt it. The tripping condition is normally an overcurrent (short circuit or overload). Additionally or alternatively, however, a circuit breaker may also be designed to trip in response to a different tripping condition, in particular an undervoltage or overvoltage. 
         [0003]    In the case of traditional, electrical circuit breakers, the presence of the tripping condition is identified by a thermal and/or magnetic principle of operation. In general, thermal circuit breakers comprise a tripping element in the form of a bimetallic strip or expanding wire through which the load current flows and whose temperature-dependent shape change trips the circuit breaker. In the case of magnetic circuit breakers, tripping is generally carried out by the direct energizing of a solenoid coil by the load current itself. By way of example, one electrical overcurrent circuit breaker using the thermal tripping principle is known from commonly assigned U.S. Pat. No. 5,451,729 and its counterpart European patent EP 0 616 347 B1. A further electrical circuit breaker with additional undervoltage tripping is known from commonly assigned U.S. Pat. No. 5,834,996 and its counterpart European patent EP 0 802 552 B1. 
         [0004]    In contrast to this, the tripping condition in an electronic circuit breaker is identified by an electronic circuit. The tripping electronics produce a tripping signal when a tripping condition is identified. The tripping signal then once again leads to the operation of, say, a magnetic release. 
         [0005]    An electronic circuit breaker generally consists of a multiplicity of individual parts. On the one hand, it is therefore often comparatively difficult to produce in large quantities. On the other hand, an electronic circuit breaker can frequently be assembled only with a comparatively great amount of effort. 
       SUMMARY OF THE INVENTION 
       [0006]    It is accordingly an object of the invention to provide an electronic circuit breaker which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a compact electronic circuit breaker which can be assembled easily. 
         [0007]    With the foregoing and other objects in view there is provided, in accordance with the invention, an electronic circuit breaker, comprising: 
         [0008]    an insulating housing; 
         [0009]    a switching contact for reversible contact-making in a load circuit to be monitored; 
         [0010]    a tripping magnet configured to act on said switching contact via a tripping mechanism; 
         [0011]    tripping electronics for operation of said tripping magnet; and 
         [0012]    a printed circuit board having firmly mounted thereon said switching contact, said tripping magnet, and said tripping electronics and forming therewith a preassembled assembly, with said preassembled assembly configured for insertion into, or inserted in, said housing as a unit. 
         [0013]    In other words, the object of the invention is achieved by a circuit breaker assembly with an insulating housing, a switching contact for reversible contact-making, that is to say opening and closing of a load circuit to be monitored, a tripping magnet which acts on the switching contact via a tripping mechanism, as well as tripping electronics for operation of the tripping magnet. The switching contact, the tripping magnet and the tripping electronics are in this case firmly mounted on a common printed circuit board. The printed circuit board therefore forms a preassembled unit, or component unit, which could be preassembled in accordance with the requirements outside the circuit breaker housing and can be inserted as an entity into the housing during the course of final assembly of the circuit breaker. The preassembly of the switching contact, the tripping magnet and the tripping electronics on the common printed circuit board considerably simplifies the assembly effort for the circuit breaker overall. In this case, it should be noted in particular that the printed circuit board with the components to be mounted on this is considerably more accessible outside the circuit breaker housing than in the installed state, thus considerably simplifying manufacture of the preassembled assembly by machine, or half by machine. In particular, the components of the preassembled assembly can be completely electrically wired up outside the housing, by being mounted on the common printed circuit board. The electrical and electronic operation of the circuit breaker can in this way be tested even before the printed circuit board has been inserted into the housing, thus identifying production faults at an early stage, and avoiding consequential costs resulting from increased production scrap or subsequent repair of defective circuit breakers. 
         [0014]    Furthermore, the preassembly of the switching contact, the tripping magnet and the tripping electronics on the common printed circuit board also allows a spatially particularly advantageous arrangement of these components, a system spatially particularly compact implementation of the circuit breaker. 
         [0015]    In order to further simply assembly, contact rails are also preferably mounted firmly in advance on the printed circuit board within the preassembled assembly, and are used for the connection of the switching contact, of the tripping magnet and of the tripping electronics to external power lines, and which to this extent project out of the circuit breaker housing, in the final assembled state of the circuit breaker. The preassembled assembly in this embodiment advantageously contains all of the parts of the circuit breaker which carry current and/or voltage, thus reducing the final assembly of the circuit breaker to purely mechanical manufacturing steps. 
         [0016]    Both with respect to simple assembly capability and with respect to a spatially particularly advantageous arrangement, because it is compact, of the circuit breaker components, in one preferred embodiment of the circuit breaker, the housing is formed essentially by a housing trough and a housing cover which can be fitted to the latter, with the printed circuit board, together with the parts already fitted to it, being held approximately parallel to the housing cover in the circuit breaker housing in the final assembled state. In the final assembled state, the printed circuit board is in this case expediently immediately adjacent to the housing cover. All further functional parts of the circuit breaker, in particular the moving parts of the tripping mechanism, are thus arranged in the interior of the housing trough, on the side of the printed circuit board facing away from the housing cover, in the final assembled state. 
         [0017]    In accordance with a preferred embodiment of the invention, the tripping magnet is a holding magnet. The tripping magnet is therefore coupled to the tripping mechanism such that it keeps the circuit breaker in an energized state in a non-tripped position. The circuit breaker is therefore tripped by deactivation or disconnection of the tripping magnet, and not by energizing it. The embodiment of the tripping magnet as a holding magnet allows this magnet to have comparatively small dimensions, not least because no active magnetic energy pulse need be applied to trip the circuit breaker. In fact, when the tripping magnet is in the form of a holding magnet, the circuit breaker trips as a consequence of an elastic resetting force of the tripping mechanism. The compact physical form of the tripping magnet, which is in the form of a holding magnet, advantageously furthermore contributes to reducing the physical size of the circuit breaker. 
         [0018]    A further preferred embodiment of the circuit breaker, in which the longitudinal axis of the tripping magnet is aligned essentially at right angles to the movement direction of the switching contact during opening and closing, is also advantageous for achieving a particularly compact design. In addition or as an alternative to this, the longitudinal axis of the tripping magnet is aligned essentially at right angles to the longitudinal direction of the housing—once again in order to achieve a particularly compact design. In this case, the longitudinal direction of the housing is that direction in which the housing has its greatest extent. This is normally that direction which connects a housing front face to a housing rear face. In this case, the housing front face is that housing face on which a control element, in particular a control knob or a switching rocker, projects outward from the housing. The housing rear face is that housing face on which electrical contact can be made with the circuit breaker, that is to say on which, in particular, the contact rails described above project outward. 
         [0019]    The circuit breaker is preferably an overcurrent circuit breaker, which trips when an overcurrent which exceeds a predetermined current threshold occurs. In one preferred embodiment, the circuit breaker in this case trips after different holding times, depending on the load. In one expedient refinement, the tripping electronics are in this case designed to disconnect after short holding times in the event of very high short-circuit currents, and to disconnect after longer holding times when lower overcurrents occur (overload). The tripping electronics preferably take account of the magnitude of the load current level for short-circuit tripping. In contrast, for overload tripping, the tripping electronics expediently take account of the square of the load current level as a measure of the electrical power of the load current. The tripping electronics are preferably subdivided once again into a number of disconnection steps, which each have different holding times depending on the load, for short-circuit tripping and/or overload tripping. 
         [0020]    In addition or as an alternative to overcurrent tripping, in one preferred embodiment, the circuit breaker has an undervoltage tripping function and/or overvoltage tripping function. It is also possible to provide for the circuit breaker additionally or alternatively also to trip when some other, in particular thermal, tripping condition occurs. 
         [0021]    In addition to single-pole embodiments, multipole embodiments of the circuit breaker according to the invention are also envisaged. These have a plurality of switching contacts, the number of which corresponds to the number of poles and which can be opened and closed simultaneously, reversibly, via coupled tripping mechanisms, in particular in a common housing. For economic production capability reasons, a separate printed circuit board is expediently provided for each pole within such multipole embodiments, on which printed circuit board the switching contact and respective tripping electronics, associated with this pole, are fitted in advance. Furthermore, the contact rails which are required for connection of the switching contact and of the tripping electronics to external power lines are optionally already fitted permanently in advance on each printed circuit board. In contrast, for reasons relating to weight, physical space and material saving—only one (a single one) of these printed circuit boards expediently has an associated tripping magnet, which acts on all the switching contacts via the coupled tripping mechanisms. The plurality of tripping electronics are in this case connected in parallel with the tripping magnet, for operating purposes. 
         [0022]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0023]    Although the invention is illustrated and described herein as embodied in an electronic circuit breaker, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0024]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0025]      FIG. 1  is an exploded, perspective illustration of an electronic circuit breaker according to the invention; 
           [0026]      FIG. 2  shows a section illustration of the circuit breaker in an OFF position; 
           [0027]      FIG. 3  shows an illustration corresponding to  FIG. 2  of the circuit breaker in an ON position, in the non-tripped state; 
           [0028]      FIG. 4  shows an illustration corresponding to  FIG. 2  of the circuit breaker in the ON position, but in the tripped state; 
           [0029]      FIG. 5  is a sectional view of the circuit breaker taken along the line V-V in  FIG. 2 ; 
           [0030]      FIG. 6  shows a block diagram of operating electronics for operating the circuit breaker; 
           [0031]      FIGS. 7 and 8  each show current/time graphs of the time profile of a control method, which is implemented in the operating electronics as shown in  FIG. 6 , for tripping the circuit breaker in the event of a short circuit or overload; and 
           [0032]      FIG. 9  shows a time/current graph of two characteristics, which characterize the tripping behavior of the circuit breaker in the event of a short circuit or overload. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    Like elements and parts, as well as functionally and structurally corresponding parts and variables are identified with the same reference symbols throughout the figures. 
         [0034]    Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown an exploded illustration of an electronic circuit breaker  1 . In this case, the circuit breaker  1  is in the form of an overcurrent circuit breaker. In addition, the circuit breaker  1  trips when a predetermined undervoltage threshold is undershot. 
         [0035]    The circuit breaker  1  has a housing  2  composed of insulating plastic, which in turn has a housing trough  3  and a housing cover  4 . The closed housing  2  is substantially in the form of a flat cuboid, which is closed on three narrow faces. In the assembled state, a switching rocker  6  which can be tilted for activation or deactivation of the circuit breaker  1  is used as a control element on the fourth narrow face, which is referred to in the following text as the front face  5 . A narrow face of the housing  2  opposite the front face  5  is referred to in the following text as its rear wall  7 . The two adjacent (mutually opposite) narrow faces of the housing  2  form its side walls  8  and  9 . 
         [0036]    The housing trough  3  is formed substantially by a housing base  10 , the rear wall  7  and the side walls  8 ,  9 , while the housing cover  4  is formed essentially by a rectangular plate  11 , which is provided on the edges with latching eyes  12 , which are integrally formed approximately at right angles, for latching to corresponding latching tabs  13  on the side walls  8  and  9 . Furthermore, pins  14  which project at right angles and can be inserted into complementary slots  15  in the rear wall  7 , such that they fit very accurately, are integrally formed on the plate  11 , in the area of its edge which faces the rear wall  7 . 
         [0037]    The circuit breaker  1  furthermore has a printed circuit board  20  which is inserted into the housing  2  substantially parallel to the housing cover  4  in the assembled state. 
         [0038]    Three electrical contact rails  21 ,  22  and  23 , as well an electromagnet  24  which acts essentially as a tripping element for the circuit breaker  1 , are soldered onto the printed circuit board  20 . Furthermore, tripping electronics  25 , which will not be described any further here, for operating the electromagnet  24  are arranged on the printed circuit board  20 . 
         [0039]    The contact rails  21  and  23  are used to make contact with a load circuit  26  to be monitored ( FIGS. 3 ,  6 ). The contact rail  22  acts as a printed circuit board connection for the voltage supply for the tripping electronics  25  and the electromagnet  24 . 
         [0040]    The circuit breaker  1  furthermore has a tripping mechanism  30  for operation and tripping. The tripping mechanism  30  in turn has a switching lever  31 , a tripping lever  32  and a plunger  33 , in addition to the switching rocker  6 . 
         [0041]      FIG. 2  shows a sectioned side view of the circuit breaker  1  in an assembled state. For orientation, a longitudinal direction Y, which is parallel to the side walls  8 ,  9 , and a lateral direction X, which is directed from the side wall  8  to the side wall  9 , are indicated here. 
         [0042]    As can be seen from  FIG. 2 , in their main area extent, the contact rails  21 ,  22  and  23  are each aligned approximately parallel to the side walls  8  and  9 , and therefore approximately at right angles to the area extent of the printed circuit board  20 . 
         [0043]    In this case, the contact rails  21  and  23  are each arranged in the immediate vicinity of one of the side walls  8  or  9 , while the contact rail  22  is arranged approximately centrally between the two other contact rails  21 ,  23 . For connection purposes, each of the contact rails  21 ,  22 ,  23  has a free end  34 ,  35 ,  36  which is in each case passed to the outside through a corresponding slot  37  in the rear wall  7 . In the assembled state, each slot  37  is also closed by one of the pins  14 , on the side facing the housing cover  4 . 
         [0044]    A contact spring  41 , which is in the form of a leaf spring, projects approximately at right angles and once again has a contact surface  42  at the free end, is fitted to the contact rail  21  in the area of its fixed end  40 , which is remote from the free end  34 . 
         [0045]    A contact surface  45 , which likewise projects approximately at right angles and corresponds to the contact surface  42 , is integrally formed on the contact rail  23 , at the corresponding fixed end  44 . The assembly which is formed from the contact spring  41 , the contact surface  42  and the contact surface  45  is referred to in the following text as the switching contact  46 . 
         [0046]    The contact spring  41  extends approximately in the lateral direction X over the housing width, such that the contact surfaces  42  and  45  can be brought into contact, in order to reversibly close the load circuit  26 . 
         [0047]    The electromagnet  24  is arranged between the two contact rails  21  and  23 , and the longitudinal axis  50  of its coil former  51  is directed approximately along the lateral direction X, that is to say in the longitudinal extent of its magnet core  52 . The electromagnet  24  is soldered onto the printed circuit board  20  by means of solder contacts  53 . The magnet core  52  projects out of the coil former  51  on its side facing the side surface  9 . 
         [0048]    Seen in the longitudinal direction Y, the tripping lever  32  is arranged between the electromagnet  24  and the contact spring  41 . The tripping lever  32  has an approximately rectangular shape with a long limb  55  (approximately in the lateral direction X) and a short limb  56  (approximately in the longitudinal direction Y). The point where the two limbs  55 ,  56  meet is referred to in the following text as the knee  57 . In the area of the knee  57 , the tripping lever  32  is borne such that it can pivot on a pin  59  (shown by dashed lines) on the housing  2 . 
         [0049]    The plunger  33  is fitted to the long limb  55  via a film hinge  60 , such that it can pivot, at its end remote from the knee  57 . The plunger  33  extends in the longitudinal direction Y, as far as the switching rocker  6 , starting from the long limb  55 . 
         [0050]    Seen in the longitudinal direction Y, the switching lever  31  is arranged above the contact spring  41 . It is formed by an essentially approximately triangular, rigid part, which is guided by a pin  61  in an elongated hole guide  62  in the housing  2 . 
         [0051]    The switching rocker  6  has a body  63  in the form of a shell, as well as a shaft  64  which projects into the housing  2 . The switching rocker  6  is borne on a pin  66  on the housing  2 , such that it can pivot, by means of a bushing  65  in the shaft  64 . 
         [0052]    The switching rocker  6  is coupled to the switching lever  31  via a pin  67 , which is arranged at the free end of the shaft  64  and engages in a guide  69  ( FIG. 3 ), which is roughly in the form of a hockey stick, on the switching lever  31 . The guide  69  is optionally in the form of a groove or elongated hole. Furthermore, the switching rocker  6  corresponds with the tripping lever  32  via the plunger  33 . 
         [0053]    The switching lever  31  once again acts on the one hand by means of a holding tab  70  with a holding shoulder  71  on the short limb  56  of the tripping lever  32 . On the other hand, the switching lever  31  acts on the contact spring  41  via an effective surface  72 . 
         [0054]    The tripping lever  32  corresponds with the magnet core  52  of the electromagnet  24  via a magnet yoke  73 , which is snapped thereon by means of two latching brackets  74 , and is sprung by means of a compression spring  75 , which is clamped in between the magnet yoke  73  and the tripping lever  32 . 
         [0055]      FIG. 2  shows the circuit breaker  1  with its switching rocker  6  in an OFF position. In the OFF position, the switching rocker  6  is prestressed by the spring force of a spring clip  81  in the tilted position illustrated in  FIG. 2 . 
         [0056]    In the OFF position, the switching lever  31  is released, that it to say it does not act either on the contact spring  41  or on the tripping lever  32 . The contact spring  41  is in a rest position, in which the contact between the contact surfaces  42  and  45  is interrupted. 
         [0057]    In the OFF position, the switching rocker  6  furthermore presses the plunger  33  downward by acting on the free plunger end  87  in the longitudinal direction Y, thus bringing the magnet yoke  73  into contact with the magnet core  52 . 
         [0058]    When current is passed through the electromagnet  24  via the tripping electronics  25 , then the magnet yoke  73  and the tripping lever  32  are held by magnetic force from the electromagnet  24  in the position illustrated in  FIG. 2 . If the switching rocker  6  is now tilted to an ON position as illustrated in  FIG. 3 , then the holding tab  70  on the switching lever  31  first of all strikes the holding shoulder  71  on the tripping lever  32 . As a result of the two-point bearing on the holding shoulder  71  and the pin  67 , which is inserted in the guide  69 , the switching lever  31  is pivoted, the switching rocker  6  being tilted further (in the clockwise direction as shown in  FIG. 3 ). Its effective surface  72  thus strikes the contact spring  41  and pushes it downward in the longitudinal direction Y until contact is made between the contact surfaces  42  and  45 . In this state, the load circuit  26  is closed via the contact rails  21  and  23  and via the contact spring  41 . 
         [0059]    When tripping occurs, the electromagnet  24  is deactivated by the tripping electronics  25 , that is to say the current fluid is disconnected, and the magnet yoke  73  is therefore released. As a consequence of this, the tripping lever  32  is pivoted in the counterclockwise direction about the knee  57  to the position illustrated in  FIG. 4 , under the influence of a spring clip  92 . 
         [0060]    In consequence, the holding tab  70  on the switching lever  31  is decoupled from the holding shoulder  71  on the tripping lever  32 . Because of the lack of mutual coupling, the switching lever  31  is pivoted in the counterclockwise direction to the position illustrated in  FIG. 4 , in which it once again releases the contact spring  41 , as a result of which the contact surfaces  42  and  45  are separated. This tripping mechanism also takes place in particular when the switching rocker  6  is blocked in the ON position as shown in  FIG. 4  (freetripping). 
         [0061]    If the switching rocker  6  is not blocked in the ON position, it tilts back to the OFF position, as shown in  FIG. 2 , under the influence of the spring clip  81 . 
         [0062]      FIG. 5  shows an angled cross section taken along the line V-V of the circuit breaker  1  as shown in  FIG. 2 . As can be seen from this illustration, a first edge  96  of the printed circuit board  20  rests approximately on the rear wall  7 , and an edge  97  of the printed circuit board  20  opposite this projects into the switching rocker  6 . As can likewise be seen from  FIG. 5 , the moving parts of the tripping mechanism  30 , specifically the switching rocker  6 , the switching lever  31  and the tripping lever  32  together with the plunger  33  including the associated springs  81  and  82 , are all arranged on the side of the printed circuit board  20  facing away from the housing cover  4 . 
         [0063]    The printed circuit board  20  is assembled outside the housing  2  with the contact rails  21 ,  22 ,  23  of the contact springs  41  and the electromagnet  24  to form a fixed cohesive preassembled assembly. This preassembled assembly, which comprises all the parts of the circuit breaker  1  which carry current or voltage, is inserted as an entity into the housing trough  3  with the tripping mechanism  30  inserted therein. All that is then necessary is to clip the housing cover  4  onto the housing trough  3 , in order to complete the assembly process—which is therefore not complex overall. 
         [0064]    In the illustrated exemplary embodiment, the tripping electronics  25  are formed at least essentially by a microcontroller. A control program  100 , which is illustrated in more detail in  FIG. 6 , is implemented in the software form in the microcontroller and automatically carries out a method, as will be described in more detail in the following text, for tripping the circuit breaker  1  in the event of a short circuit or overload. 
         [0065]    The control program  100  comprises two parallel functional sections, specifically a (short-circuit tripping) section  101  and an (overload tripping) section  102 , which branch off from a common section  103 . 
         [0066]    First of all the (load) current level i in the load circuit  26  is determined as an input signal by means of a current sensor  104  in the common section  103 . The current sensor  104  (for example formed by a shunt or a current transformer) emits as an output signal an analogue current measurement signal i A  in the form of a voltage which is proportional to the current level, to a downstream analogue/digital (A/D) converter  106 . The analogue current measurement signal i A  is converted to a digital current measurement signal i D  in the A/D converter  106 , which is preferably an integral component of the microcontroller, in time with a (measurement) clock frequency f m  with a resolution of nm bits (in this case nm=8). 
         [0067]    The current measurement signal i D  is produced such that: 
         [0068]    i D =0 corresponds to a measured current level i=−C·I N , 
         [0069]    I D =2 nm-1  corresponds to a measured current level i=0, and 
         [0070]    I D =2 nm  corresponds to a measured current level i=+C·I N . 
         [0071]    I N  in this case denotes the rated current level of the circuit breaker  1 . The constant C is fixed at values between about 3 and 20, for example at C=15, depending on the tripping sensitivity of the circuit breaker  1 . 
         [0072]    The circuit breaker  1  is intended primarily for monitoring an alternating-current load circuit. The measurement clock frequency f m  is therefore set to a multiple of, in particular to 20 times, the normal mains frequency f N  (that is to say to fm=1 kHz when the mains frequency is f N =50 Hz). In addition to this, the circuit breaker  1  may, however, be used to monitor a direct-current load circuit without having to modify the control program  100  for this purpose. 
         [0073]    A digital (current) magnitude signal i B  which corresponds in essence to the absolute magnitude of the load current level i is produced by a magnitude module  107 , which in software terms is connected downstream from the A/D converter  106 , using the equation 
         [0000]        i   B   =|i   D −2 nm-1 |
 
         [0000]    The magnitude signal i B  flows as an input variable into the section elements  101  and  102  of the control program  100 . 
         [0074]    In a zero test stage of the short-circuit tripping section  101 , the sample value of the magnitude signal i B , determined in each measurement clock cycle, is compared in a comparison module  110   0  at the clock frequency f m  with a discrete characteristic point k 0  on a stored (short-circuit tripping) characteristic K ( FIG. 9 ). The comparison module  110   0  remains inactive provided that the sample value of the magnitude signal i B  does not exceed the characteristic point k 0  (i B ≦k 0 ). Otherwise (i B &gt;k 0 ), the comparison module  110   0  outputs a tripping signal A, on the basis of which the current flow through the electromagnet  24  is interrupted, and the circuit breaker  1  is therefore tripped. 
         [0075]    The current measurement signal i D ), to be precise the magnitude signal i B , therefore contains digital sample values of the current level i at discrete sampling times, which follow one another at a time interval of f m   −1 . 
         [0076]    The characteristic point k c , reflects the so-called immediate tripping threshold. The value of the characteristic point k 0  is a measure of the maximum permissible overcurrent level averaged over a holding time t H  ( FIG. 9 ). In this case, the holding time t H  corresponds to the reciprocal of the clock frequency f m  or the simple (measurement) clock time t m  ( FIG. 7 ) (t H =t m =f m   −1 ; in this case t H =0.001 s). A single measured value of the magnitude signal i B  which exceeds the characteristic point k 0  is therefore sufficient to trip the circuit breaker  1 . 
         [0077]    In a—subsequently—first test step in the short-circuit tripping section  101 , the respectively determined sample value of the current magnitude i B  is written to a first (FIFO, first-in-first-out) memory  113   1  with a total of (in this way by way of example: two) memory locations, at the clock frequency f m , that is to say in each measurement clock cycle. 
         [0078]    Whenever a number of measurement clock cycles corresponding to the number of memory locations has passed—indicated by the clock symbols  115 —a sum module  120   1  forms a rounded mean value i M1  from the sample values of the magnitude signal i B  stored in the memory  113   1 . If there are two memory locations, the mean value i M1  is therefore formed at half the clock frequency f m /2=500 Hz. A sample value of the magnitude signal i B  which is stored in the memory  113   1  is therefore only ever taken into account once in the averaging process. In simple terms, the memory  113   1  is only ever evaluated when it has been completely filled with new sample values of the magnitude signal i B . 
         [0079]    The mean value i M1  is supplied as a test variable to a downstream comparison module  110   1 . The comparison module  110   1  in turn compares this mean value i M1  with an associated characteristic point k 1  on the characteristic K and—analogously to the comparison module  110   0 —outputs the tripping signal A if the value of the mean value i M1  exceeds the characteristic point k 1  (i M1 &gt;k 1 ). The characteristic point k 1  is a measure of the average maximum permissible overcurrent level over a holding time t H , which corresponds to twice the clock time t m (t H =2·t m =2·f m   −1 ; in this case t H =0.002 s). 
         [0080]    The mean value i M1  in the first test step is supplied as an input variable to a second test step which, analogously to the first test step, has a further (first-in-first-out) memory  113   2 , a further sum module  120   2  and a further comparison module  110   2 . The operation of the second test step is also the same as that of the first test step, with the difference that the mean value i M1  from the first test step is supplied to the memory  113   2 , rather than the magnitude signal i B , and that a mean value i M2 , produced by the sum module  120   2 , is produced at the clock frequency f M /4, that is to say f m /4=250 Hz. A characteristic point k 2  which is associated as a tripping criterion with the comparison module  110   2  is therefore a measure of the maximum overcurrent level on average over a holding time t H  which corresponds to four times the clock time t m (t H =4·t m =4·f m   −1 ; in this case t H =0.004 s). 
         [0081]    The second test step is followed in cascade form by one or more n-th order (n=3, 4, . . . ) further test steps, whose configuration and function once again correspond to those of the second test step, and which are each formed by a (first-in-first-out) memory  113   n , a further sum module  120   n  and a further comparison module  110   n . As an input signal, the memory  113   n  in this case always receives the mean value i M(n-1)  from the directly preceding (n−1)th test step. The sum module  120   n  in the n-th test step always produces a mean value i Mn  at the clock frequency divided by 2 n , that is to say f m /2 n , and this mean value is compared with a characteristic point k N  in the comparison module  110   n . The characteristic point k n  is a measure of the maximum overcurrent level on average over a holding time t H  which corresponds to 2 n  times the clock time t m (t H =2 n ·t m =2 n ·f m   −1 ). 
         [0082]    The principle of this cascade-like averaging process is illustrated once again in  FIG. 7 , in which the profile of the magnitude signal i B  and of the mean values i M1  and i M2  is compared over the time t in synchronous graphs, which are arranged one above the other. As can be seen directly from this illustration, the cascade-like averaging process results in the hierarchically successive test steps checking for changes in the load current on respective timescales which increase exponentially with the order of the step. A measure for the timescale associated with the respective test set is in this case the holding time t H  of the respective test step: 
         [0000]        n -th test step( n= 0, 1, 2, . . . ):  t   H =2 n   ·t   m =2 n   ·f   m   −1    
         [0083]    As shown in  FIG. 6 , a square signal p, where p=i B ·i B , is first of all calculated from the magnitude signal i B  in a squaring module  130  in the overload tripping section element  102 , as a measure of the power of the load current. 
         [0084]    This square signal p is read at the clock frequency f m  to a (first-in-first-out) memory  131  in a zero test step of the section element  102 . The memory  131  has a total number q of memory locations—once again for use of the circuit breaker  1  for protection of an alternating-current load circuit—, which corresponds to the ratio of the clock frequency f, to the normal mains frequency f N  or to a multiple thereof: 
         [0000]        q=j·f   m   /f   N  where  j= 1, 2, 3, . . . 
         [0085]    In particular, the memory  131  has q=20 memory locations for a mains frequency of f N =50 Hz and a clock frequency of f m =1 kHz. 
         [0086]    After a number of measurement clock cycles corresponding to the number q—indicated by the clock symbols  133 —a sum module  132  which follows the memory  131  always calculates a rounded mean value p M0  from the values of the square signal p stored in the memory  131 . The mean value p M0  in this case represents a measure of the root mean square power of the load current. If the memory  131  has 20 memory locations, the mean value p M0  is formed at a clock frequency f e =f N = 1/20·f m , which corresponds to the mains frequency f N . A value of the square signal p stored in the memory  131  is in consequence once again only ever taken into account once in the averaging process. 
         [0087]    The mean value p M0  is compared in a downstream comparison module  136   0  with a characteristic point u 0  on a stored (overload-tripping) characteristic U ( FIG. 9 ), with the comparison module  136   0  producing the tripping signal A if the value of the mean value p M0  exceeds the square of the characteristic point u 0  (p M0 &gt;u 0   2 ). The square u 0   2  of the characteristic point u 0  therefore represents a measure of the maximum permissible root mean square power of the load current. 
         [0088]    Analogously to the section element  101 , hierarchically successive test steps are also provided in the section element  102 , whose design and function correspond to those of the corresponding test steps in the section element  101 . Each of these test steps comprises:
       a (first-in-first-out) memory  138   n  with two memory locations, which are supplied as an input variable with the mean value p M(n-1)  from the respectively previous test step,   a sum module  140   n , which calculates a mean value p Mn  of the values contained in the memory  138   n  at ½ n -times the clock frequency ½ n ·f e , and   a comparison module  136   n , which compares this mean value p Mn  with the square u n   2  of an associated characteristic point u n , and produces the tripping signal A if p Mn &gt;u n   2 .       
 
         [0092]    The numerical variable n=1, 2, 3, . . . in this case once again denotes the hierarchical order of the respective test step. 
         [0093]    In one exemplary embodiment of the control program  1 , the section element  101  has five test steps (n=0, 1, . . . ,4) while the section element  102  has thirteen test steps (n=0, 1, . . . ,12). 
         [0094]    Analogously to  FIG. 7 ,  FIG. 8  shows the time profile of the square signal p and of the mean values p M0  and p M1  in the form of a comparison. As can be seen from this illustration, the test steps in the second section element  102  test for changes in the power of the load current—with the exception of the zero test step—once again using time scales which grow exponentially with the step order: 
         [0000]        n -th test step( n= 1, 2, . . . ):  t   H =2 n   ·f   e   −1    
         [0095]    The modules  107 ,  110   n  (n=0, 1, 2, . . . ),  120   n  (n=1, 2, . . . ),  130 ,  132 ,  136   n  (n=0, 1, 2, . . . ), and  140   n  (n=1, 2, . . . ) are software modules in the control program  100 . The (first-in-first-out) memories  113   n  (n=1, 2, . . . ),  131  and  138   n  (n=1, 2, . . . ) are preferably software-allocated (that is to say reserved) areas in a common main memory in the microcontroller which runs the control program  100 . 
         [0096]      FIG. 9  shows the characteristics K and U plotted on a log-log graph against the holding time t H  (in this case plotted on the ordinate). The current level i is plotted as a percentage of the rated current level I N  on the circuit breaker  1  on the abscissa of the graph. 
         [0097]    Corresponding to the respective number of test steps, the characteristic K comprises four characteristic points k 0 , k 1 , . . . , k 4 , while the characteristic U is formed from thirteen characteristic points u 0 , u i , . . . , u 12 . As can be seen from  FIG. 9 , the characteristics K and U cover a holding time interval of 10 −3  s≦t H ≦10 2  s, without any overlap. The characteristic K in this case defines the tripping behavior of the circuit breaker  1  on timescales below the reciprocal of the mains frequency (t H &lt;f N   −1 =20 ms), while the characteristic U defines the tripping behavior of the circuit breaker  1  on timescales above the reciprocal of the mains frequency (t H ≧f N   −1 =20 ms). 
         [0098]    The current values (tripping values) of the characteristic points k n  and u n  may be chosen freely—contrary to the example shown in  FIG. 9 . However, the characteristic points k n  and u n  are expediently chosen such that the characteristics K and U each fall strictly monotonally, as a result of which the holding time t H  is always shorter the higher the current value of the respective characteristic point k n  or u n . 
         [0099]    In principle, the number of characteristic points k n  and u n  can also be chosen freely for each of the characteristics K and U. The number of test steps in the branch elements  101  and  102  must in this case always be matched to the number of characteristic points k n  and u n  on the respectively associated characteristic K or U, with the respectively associated holding time t H  for each characteristic point k n  or u n  corresponding to a test step in the section element  101  or  102 , respectively. However, alternatively, it is also feasible
       to provide more test steps within a section element  101  or  102  when the associated characteristic has characteristic points k n  or u n , and/or   to choose at least some of the characteristic points k n  and/or u n  such that the holding time t H  associated with these characteristic points k n  or u n  does not match the holding time t H  associated with a test step.       
 
         [0102]    In these situations, rather than supplying the test steps with the characteristic points k n  or u n , they are supplied with threshold values which are derived by interpolation or extrapolation from the characteristic points k n  or u n  on the basis of the holding times t H  associated with the test steps. 
         [0103]    In an alternative embodiment of the invention, the exponential increase in the holding time t H  as the step order n rises can also be varied by defining the successive memories  113   n  (n=1, 2, . . . ) or  138   n  (n=1, 2, . . . ) to have a varying number of memory locations within the same section element  101  or  102 . 
         [0104]    By virtue of its design, the circuit breaker  1  has a passive undervoltage tripping function, with the tripping mechanism  30  necessarily being tripped when the voltage which is present between the contact rails  21  and  22  is no longer sufficient to supply enough electrical energy to the electromagnet  24  and/or the tripping electronics  25 . In particular, this function can be used to trip the circuit breaker  1  by remote control, by means of a switch connected downstream from the contact rail  22 . 
         [0105]    Furthermore, optionally, the circuit breaker  1  has an active overvoltage tripping function which, in particular, is implemented in the form of software in an undervoltage tripping block (which is not illustrated) in the control program  100 . For the purposes of this active undervoltage tripping, the control program  100  continuously and in parallel with the running of the program part illustrated in  FIG. 6 , records the magnitude (the root mean square magnitude in the case of an AC voltage) of the electrical voltage which is present between the contact rails  21  and  22 , and compares the recorded voltage magnitude with a stored threshold value. In this case, the control program  100  produces the tripping signal A if the recorded voltage magnitude undershoots the threshold value.