Abstract:
A residual current device is described that has an amplifier circuit for producing a fault voltage, which is derived from a fault current, a comparison circuit for comparing the fault voltage with a reference voltage, and a reference circuit, which is connected to a power supply unit, for producing the reference voltage as well and an operating voltage for the amplifier circuit. The residual current device also includes an arrangement that, if the output voltage from the power supply unit falls below a predetermined value, raise the reference voltage at least to the instantaneous operating voltage, at least if the fault voltage is less than the reference voltage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a residual current device. 
     BACKGROUND INFORMATION 
     A residual current device is used to ensure protection against a dangerous fault current in an electrical system. Such a fault current occurs when a live conductive part makes an electrical contact with ground. This occurs, for example, when somebody touches a live part of an electrical system. The fault current then flows via the person to ground, as a body current. The residual current devices which are used for protection against dangerous body currents have to isolate the electrical system from the mains voltage in the event of a fault current that is greater than 30 mA. 
     The design of a residual current device is described, for example, in the article “Warum Fehlerstrom-Schutzschalter mit netzspannungsunabhängiger Auslösung?,” etz, Volume 107 (1986), Issue 20, pages 938 to 945. Outline circuit diagrams and functional principles of a residual current device are described there, in particular in FIGS. 1 to  3 . In this case, a distinction is drawn between two different basic types. The residual current device (RED) is a fault-current protective device in which the electrical power required for the switching process is obtained from the fault current itself, independently of the mains voltage. A differential-current or DI circuit breaker is, in contrast, a fault-current protective device in which the electrical auxiliary energy required for the switching process is taken from the mains voltage itself. Such a DI circuit breaker thus requires a mains connection and a power supply unit to operate it. The power supply unit converts the mains voltage into the supply voltage required to operate the DI circuit breaker components. 
     In such a DI circuit breaker, the voltage induced by the fault current in the secondary winding of the core-balanced transformer is normally amplified in an amplifier. If the fault voltage that is present at the output of the amplifier exceeds a predetermined reference voltage, then the DI circuit breaker trips. This reference voltage is generally provided by a voltage divider that is connected to a power supply unit, which supplies the electronic circuits located in the DI circuit breaker with the operating voltage required to operate it. 
     Any fault current or dissipation current flowing in an electrical system that is less than the tripping fault current leads to a fault voltage at the output of the amplifier, which is less than the reference voltage associated with the tripping fault current. If the mains voltage is switched off, then the operating voltage falls, and the reference voltage thus also falls, with a time constant that is dependent on the magnitude of the smoothing capacitors. If the reference voltage is less than the fault voltage at a time at which sufficient energy is still stored in the power supply unit to trip the tripping relay, the DI circuit breaker trips incorrectly. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is a residual current device that avoids a fault operating state caused by the mains voltage being switched off. 
     The present residual current device according to the invention contains an amplifier circuit for producing a fault voltage that is derived from a fault current, a comparison circuit for comparing the fault voltage with a reference voltage, and a reference circuit, which is connected to a power supply unit, for producing the reference voltage as well as an operating voltage for the amplifier circuit, in which case means are provided which, if the output voltage from the power supply unit falls below a predetermined value, raise the reference voltage at least to the instantaneous operating voltage, at least if the fault voltage is less than the reference voltage. 
     In other words, if the fault voltage is less than the reference voltage at the time at which the output voltage from the power supply unit falls below the predetermined value, that is to say when the tripping condition for the tripping circuit of the residual current device is not satisfied at this time, the reference voltage is increased at least to the instantaneous operating voltage, which is in the process of falling. 
     Raising the reference voltage at least to the instantaneous operating voltage, preferably to the instantaneous operating voltage, ensures that the fault voltage derived from the fault current by the amplifier circuit is not less than the reference voltage as the output voltage of the power supply unit falls, since the output voltage of an amplifier circuit is, as a rule, less than the operating voltage used to operate it. This largely avoids incorrect tripping when the mains voltage is switched off. 
     In a further advantageous refinement of the present invention, the reference circuit contains a first voltage divider, which is connected to the operating voltage, for producing the reference voltage. That first voltage divider is connected to ground via a switching element which can be controlled as a function of the output voltage from the power supply unit. In this way, the reference voltage can be raised to the operating voltage by a simple switching process. 
     A transistor is preferably provided as the controllable switching element. 
     In a further advantageous refinement of the invention, the reference circuit contains a second voltage divider for producing a control voltage for the controllable switching element. 
     The second voltage divider is connected between the output voltage of the power supply unit and ground, and preferably includes a zener diode, which is connected in series, in the reverse direction, between the divider resistors of the second voltage divider, and to the base of a transistor, which is provided as the controllable switching element. 
     In a further embodiment of the present the invention, the output of the comparison circuit is connected to the reference circuit via a feedback circuit. This ensures that, in the event of a discontinuity in the supply voltage provided by the power supply unit, which may have been caused by actuating a tripping coil, the tripping condition required for the tripping coil to trip, namely that the fault voltage is greater than a reference voltage, is maintained. 
     In particular, the feedback circuit contains a diode and a resistor connected in series with it, and is connected to a control line which carries the control voltage. 
     In a further advantageous embodiment, the reference circuit contains a voltage stabilization circuit for producing a stabilized operating voltage from the output voltage. This creates reproducible and stable operating conditions for the residual current device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows a circuit diagram of a residual current device according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     According to the FIGURE, the residual current device includes a core-balanced transformer  2 , downstream of which an amplifier circuit  4  is connected. The amplifier circuit  4  converts a voltage signal UF that is induced in the secondary winding of the core-balanced transformer  2  to a rectified fault voltage UA. The amplifier circuit contains an amplifier  6  and a rectifier  8  for this purpose. The amplifier circuit  4  has a comparison circuit  10 , for example a comparator, connected downstream of it, in which the fault voltage UA is compared with a reference voltage URef provided by a reference circuit  12 . If the fault voltage UA is greater than the reference voltage URef, the base of a switching transistor T is actuated from the output of the comparison circuit  10 . A tripping relay coil  14  is connected between an output voltage UN of a power supply unit  15  and ground in the collector-emitter circuit of the switching transistor T. 
     The core-balanced transformer  2 , amplifier circuit  4 , comparison circuit  10 , as well as the circuit  16  containing the tripping relay coil  14 , and the switching transistor T form the tripping circuit of the residual current device. 
     The operating voltage UB required to operate the electronic components in this tripping circuit is provided by a reference circuit  12 , at whose output the output voltage UN provided by the power supply unit  15  is present. 
     The output voltage UN of the power supply unit  15  is regulated down to the operating voltage UB via a voltage stabilization circuit. For this purpose, the voltage stabilization circuit contains an emitter-follower circuit with a regulating transistor V 1  whose base is connected to the cathode of a zener diode D 1 , which is connected in series, in the reverse direction, between two divider resistors R 1 , R 2 . The series circuit including the divider resistor R 1 , the zener diode D 1  and the divider resistor R 2  is connected in parallel with the output of the power supply unit  15 . 
     The emitter of the regulating transistor V 1  is connected with a first voltage divider  20  (reference voltage divider) to ground. This first voltage divider  20  contains a series circuit including a divider resistor R 3 , a divider resistor R 4 , as well as a controllable switching element V 2 . The divider resistor R 3  is connected to the operating voltage UB, in the exemplary embodiment of the emitter of the regulating transistor V 1 . The controllable switch V 2  is connected between the divider resistor R 4  and ground. A tap for the reference voltage URef, which is supplied to the comparator  10 , is located between the divider resistor R 3  and the divider resistor R 4 . A smoothing capacitor C connected to ground in parallel with the first voltage divider  20  is used to smooth the operating voltage UB. 
     When the controllable switching element V 2  is closed, the reference voltage URef, which is governed by the operating voltage UB and the division ratio of the divider resistors R 3  and R 4 , is present at the reference input of the comparator in the comparison circuit  10 . 
     When the switching element V 2  is open, the entire voltage drop in the voltage divider  20  occurs across the switching element V 2 , so that the operating voltage UB is present between the divider resistor R 3  and the divider resistor R 4 . 
     In the exemplary embodiment, the controllable switch V 2  is a bipolar npn transistor, whose base is controlled by the voltage drop across the divider resistor R 2 , which is connected in series with the zener diode D 1 . If the output voltage UN of the power supply unit  15  is greater than the zener voltage UZ of the zener diode D 1 , this zener diode conducts, and a control voltage corresponding to the division ratio of the divider resistors R 1  and R 2  is applied to the base of the transistor, which is used as the controllable switching element V 2 . The series circuit including the divider resistor R 1 , the zener diode D 1  and the divider resistor R 2  forms a second voltage divider  22  (control voltage divider), whose divider voltage is used as the control voltage UT for the controllable switching element V 2 . 
     If the output voltage UN of the power supply unit  15  is greater than the zener voltage UZ of the zener diode D 1 , this zener diode conducts, and the transistor, which is provided as the controllable switching element V 2 , is switched on. If the collector-emitter saturation voltage of the transistor is ignored, the first voltage divider  20  is connected to ground, that is to say the switching element V 2  is closed. 
     If the output voltage UN of the power supply unit  15  falls below the zener voltage UZ of the zener diode D 1 , then the current flow through the zener diode D 1  breaks down, and the control voltage UT falls to ground potential. The transistor that is used as the controllable switch V 2  switches off, and the instantaneous reference voltage URef′ is raised to the level of the instantaneous operating voltage UB′. This ensures that the fault voltage UA which is present at the output of the amplifier circuit  4 , cannot exceed the instantaneous reference voltage URef′ (which is supplied to the reference input of the comparison circuit  10  when the power supply unit  15  is switched off) of the comparison circuit, so that the switching transistor T remains in the switched-off state. This reliably prevents incorrect tripping when the power supply unit  15  is switched off. 
     A further advantage of the present invention is that the influence of the temperature-dependent base-emitter voltage of the regulating transistor V 1  on the operating voltage UB can be largely eliminated, provided a physically identical transistor is used as a controllable switching element V 2 . In this case, the base-emitter voltages of the two transistors V 1  and V 2  are raised in comparison with the operating voltage UB, so that this is always approximately equal to the zener voltage UZ of the zener diode D 1 . 
     The output of the comparison circuit  10  is connected, via a feedback circuit  24 , to the control line for the controllable switching element V 2 . The feedback circuit  24  contains a diode D 2 , which is connected in series with a resistor R 5 . 
     This circuitry ensures that the residual current device responds reliably when an unacceptable fault current occurs, even in the event of undervoltage. Such an undervoltage occurs, for example, if the power supply unit  15  is only supplied, for example, with 50 V from two external conductors, instead of from a three-phase connection at, for example, 480 V. 
     If the low-resistance tripping relay coil  14  is connected to the power supply unit output voltage UN, then this can be dropped in the power supply unit  15  due to the voltage drop across the protective circuitry resistances, so that this results in the current flow via the zener diode D 1  being interrupted. 
     Without the feedback formed from the diode D 2  and the resistor R 5 , the switching transistor, which is used as the controllable switching element V 2 , would switch off immediately and the reference voltage Uref would rise to the instantaneous operating voltage UB′, so that the reason for tripping, namely that the fault voltage UA present across the comparison circuit  10  is greater than the reference voltage URef provided by the reference circuit  16 , would immediately be canceled again. Reliable tripping is no longer ensured in this situation. 
     Feedback from the output of the comparison circuit  10  to the control line for the controllable switching element V 2  allows the positive output voltage of the comparison device  10  to be used as the control voltage for the controllable switching element V 2  and prevents the latter from opening, that is to say, in the exemplary embodiment, the transistor, which is used as the controllable switching element V 2 , remains switched on even in the event of voltage interruptions caused by actuation of the tripping relay coil  14 . 
     The diode D 2 , which is connected at the feedback circuit  24 , prevents the resistor R 2  from being short-circuited in the situation when the fault voltage UA is less than the reference voltage Uref.