EP0334027B1 - Dynamic auto-controlling circuit for flame detection - Google Patents

Description

The invention relates to a dynamic self-monitoring circuit for flame monitors.

In addition to a high sensitivity, a high degree of intrinsic safety and availability must be required from flame monitors. A flame monitor is considered to be intrinsically safe or "failsafe" if all possible component faults that would lead to a pretense of flame if the flame was extinguished always result in the signaling of the "flame off" state. The availability is high if the probability of lockouts despite the flame is low. Flame monitors with two flame sensors which absorb the flame radiation are known, which either increase the safety or responsiveness by appropriate linking or improve the selectivity in multi-burner systems by appropriate alignment to certain parts of the flame or at a certain angle to one another. Circuits have been developed for self-monitoring of such flame monitors working with two flame sensors, in which not only the flame sensor itself but also the entire flame monitor the evaluation circuit and the flame relay are designed redundantly and errors can be detected by comparing the output signals. However, such circuits are not intrinsically safe because active defects simulating the flame signal cannot be recognized immediately during operation.
DE-OS 350 825 3 specifies a flame monitor with two flame sensors, which are directed at successive positions of the flame and which detect the temperature difference of the flame. There is only one transmission channel for the electrical flame signal, which can be separated for self-monitoring by a clock-controlled interrupter, which means that the transmission channel is used in constant alternation to transmit the flame signal and to control the switch-off function.
The disadvantage here is that the two flame sensors are not included in the self-monitoring.
A self-monitoring circuit that works according to this principle, to check the functionality of the circuit during the periodic interruptions of the transmission channel, is specified in DE-OS 33 21 166. Such self-monitoring circuits have the disadvantage that within a cycle time that must always be less than the required safety shutdown time (for gas, for example <1 sec.), A compromise must always be made between the one for transmitting and evaluating the flame signal and that for checking the shutdown function available time interval, which is a compromise between availability and security. If the time for the transmission and evaluation of the flame signal is chosen too short, there is a risk that, for example, in the start-up phase, where the burner to be monitored is operated at a low load level, no stable flame signal can develop and the flame monitoring for "no flame" recognizes what limits the availability of the system. If the time for checking the switch-off function is too short, that is limited error detection. Further disadvantages of the circuits mentioned can be seen in the additionally required separate monitoring circuit for the clock signal and the relatively high circuit complexity which ultimately limits availability.

The invention seeks to remedy this. The object of the invention is therefore to provide a dynamic self-monitoring circuit for flame monitors, in particular for selective flame monitoring in multi-burner systems, which enables the uninterrupted transmission and evaluation of flame signals and does not require separate monitoring of the system clock and of flame sensors.

This object is achieved according to the invention by a dynamic self-monitoring circuit for flame monitors with one or two flame sensors which absorb the flame radiation, in particular for selective flame monitoring in multi-burner systems, two transmission channels being provided and conjunctively linked at one output and alternately with that of the flame sensor using a clock-controlled device emitted flame signal and each have an evaluation circuit for the flame signal and a flame relay, and that in series with the flame relay switches are arranged, which are connected via diodes to the outputs of both evaluation circuits, the switching state evaluating control circuits are controllable, and that a start-up circuit is provided.

Preferred embodiments of the subject matter of the invention are set out in the dependent claims.

The creation of two independent, alternately activatable transmission channels allows the uninterrupted transmission and evaluation of flame signals, with the control of the switch-off function taking place in the respective non-activated transmission channel, including the relevant flame sensor. When the flame signal is recorded and the circuit is intact, the circuit arrangement is located in a dynamically stable state in which the flame relays are activated and the switches are closed. A flame break is then registered in a known manner via the flame relay of the currently active transmission channel.
The invention makes it possible that not only all component errors, interruptions in the transmission channels and vibrations of the evaluation circuits, but also the running away of the clock frequency from the tolerance range lead to disturbance of the dynamic stability and thus to lockout by opening at least one switch. When starting up or switching on again, a switch-on aid is required to achieve the dynamically stable state, which is referred to in the registration as a start-up circuit.
In a further specific embodiment of the invention, liquid-crystal cells are used as clock-controllable means for covering the radiation-absorbing elements, and the clock signals that trigger the electro-optical effect are in exactly opposite phase. Furthermore, the two control circuits each consist of a relay with a drop-out delay and a diode connected in series, the two diodes being arranged antiparallel and the switches being make contacts of these relays. It is advisable to use the flame signal itself as the triggering criterion for the start-up circuit. One appropriate form of implementation of the start-up circuit which corresponds to the requirement for intrinsic safety is to arrange threshold switches at the inputs of the control circuits, to bridge the switches in series with the flame relays with start-up resistors and to connect the excitation circuits of the flame relays to one another by means of a holding resistor. In order to ensure the presence of the flame presence signal at the output of an evaluation circuit each time the channel is switched, the evaluation circuits must have a switching hysteresis.

In a departure from the use of two flame sensors which receive the flame radiation, the dynamic self-monitoring circuit according to the invention can of course also be used if only one flame sensor is provided. Then, according to a further feature of the invention, the transmission channels linking conjunctively at the output can advantageously be alternately connected to the flame sensor via a clock-controlled switchover device, wherein the clock-controlled switchover device can be an electronic switch.

Fig. 1

eine dynamische Eigenüberwachungsschaltung für Flammenwächter unter Verwendung von zwei die Flammenstrahlung aufnehmenden Flammenfühlern und

Fig. 2

einen Ausschnitt der dynamischen Eigenüberwachungsschaltung nach Fig. 1 unter Verwendung nur eines Flammenfühlers

Die Fig. 1 zeigt eine dynamische Eigenüberwachungsschaltung in Verbindung mit einem Flammenwächter mit zwei die Flammenstrahlung aufnehmenden Flammenfühlern 41, 42,die zur Erhöhung der Selektivität und Sicherheit gegen Flammenvortäuschung aus verschiedenen Richtungen auf einen bestimmten Punkt der Flamme 1 gerichtet sind. Die Schaltungsanordnung besteht aus zwei jeweils den Flammenfühler 41, 42, die Bewertungsschaltung 10, 20 für das Flammensignal und das Flammenrelais 11, 21 enthaltende am Ausgang A über die Arbeitskontakte 12, 22 der Flammenrelais 11, 21 konjunktiv verknüpften Übertragungskanälen 2, 3, wobei die beiden Flammenfühler 41, 42 durch die gegenphasigen Taktsignale T, T steuerbare Flüssigkristallzellen 31, 32 zum Abdecken der Fühlerelemente aufweisen. Weiterhin sind in Reihe mit den Flammenrelais 11, 21 die Schalter 13, 23 angeordnet, die mittels der jeweils über die Dioden 17, 27 mit den Ausgängen der beiden Bewertungsschaltungen 10, 20 verbundenen, deren Schaltzustand auswertenden Ansteuerschaltungen 110, 210 steuerbar sind. Außerdem enthält die Schaltungsanordung die durch das Flammensignal auslösbare Anlaufschaltung 4. Als Bewertungsschaltungen 10, 20 für das Flammensignal sind an sich bekannte frequenzselektive Anordnungen eingesetzt, über deren Ausgang ein Flammenrelais 11, 21 ansteuerbar ist und die eine Schalthysterese aufweisen, wodurch die Einschaltverzögerung kleiner ist als die Ausschaltverzögerung. Die beiden Ansteuerschaltungen 110, 210 bestehen aus den Relais 15, 25 mit den in Reihe geschalteten zueinander antiparallel angeordneten Dioden 16, 26 wobei den Relais 15, 25 Verzögerungskondensatoren 19, 29 und Freilaufdioden 111, 211 parallel geschaltet sind. Die Schalter 13, 23 sind Arbeitskontakte dieser Relais 15, 25. Die durch das Flammensignal auslösbare Anlaufschaltung 4 besteht aus den an den Eingängen der Ansteuerschaltungen 110, 210 angeordneten Schwellwertschaltern 18, 28, den Anlaufwiderständen 14, 24 und dem die Erregerstromkreise der Flammenrelais 11, 21 miteinander verbindenden Haltewiderstand 5.
Im folgenden soll die Funktionsweise der Schaltungsanordnung beschrieben werden.
Bei aufgenommenem Flammensignal und intakter Schaltung befindet sich die Schaltungsanordnung in einem dynamisch stabilen Zustand, bei dem alle vier Relais 11, 21, 15, 25 angezogen sind. Die Taktsignale T, T schalten die Flüssigkristallzellen 31, 32 im Wechsel mit der Taktfrequenz f_T von dem gesperrten in den für die aufzunehmende Strahlung durchlässigen Zustand. Anstelle der Flüssigkristallzellen 31, 32 sind auch in bekannter Weise elektrisch steuerbare mechanische Blenden oder Klappen zum Abdecken der Flammenfühler 41, 42 einsetzbar.
Die Flammenstrahlung aktiviert somit mit der Taktfrequenz f_T die Flammenfühler 41, 42, deren elektrische Flammensignale an die Eingänge der frequenzselektiven Bewertungsschaltungen 10, 20 gelangen, an deren Ausgängen beispielsweise L-Pegel bei anliegendem Flammensignal und H-Pegel bei Nichterfüllung des Bewertungskriteriums liegt. Infolge der Schalthysterese entsteht eine zeitliche Überlappung der Signalpegel an den Ausgängen der Bewertungsschaltungen 10, 20 zum Zeitpunkt der Kanalumschaltung, so daß immer an einem der beiden Ausgänge L-Pegel vorhanden ist. Die beiden Dioden 17, 27 sind jeweils in Durchlaßrichtung bei Flammenanwesenheit gepolt. Zunächst sei mittels des Taktsignales T während der Zeit

die Flüssigkristallzelle 31 für die Flammenstrahlung durchlässig und somit der Übertragungskanal 2 aktiviert. Am Ausgang der Bewertungsschaltung 10 liegt L-Pegel und am Ausgang der Bewertungsschaltung 20 H-Pegel, so daß die Diode 27 sperrt. Das Flammenrelais 11 erhält über den geschlossenen Schalter 13 die voll Betriebsspannung U_B.The invention is explained in more detail below with reference to drawings which illustrate two exemplary embodiments. Show it:

Fig. 1

a dynamic self-monitoring circuit for flame monitors using two flame sensors and

Fig. 2

a section of the dynamic self-monitoring circuit of FIG. 1 using only a flame sensor

1 shows a dynamic self-monitoring circuit in connection with a flame monitor with two flame sensors 41, 42 which receive the flame radiation and which are directed to a certain point of the flame 1 from different directions in order to increase the selectivity and security against flame simulation. The circuit arrangement consists of two each containing the flame sensor 41, 42, the evaluation circuit 10, 20 for the flame signal and the flame relay 11, 21 at the output A via the make contacts 12, 22 of the flame relay 11, 21 conjunctively linked transmission channels 2, 3, the two flame sensors 41, 42 by the counter-phase clock signals T, T controllable liquid crystal cells 31, 32 for covering of the sensor elements. Furthermore, the switches 13, 23 are arranged in series with the flame relays 11, 21, which can be controlled by means of the respective control circuits 110, 210 which are connected to the outputs of the two evaluation circuits 10, 20 via the diodes 17, 27. In addition, the circuit arrangement contains the starting circuit 4 which can be triggered by the flame signal. Known frequency-selective arrangements are used as evaluation circuits 10, 20 for the flame signal, via whose output a flame relay 11, 21 can be controlled and which have a switching hysteresis, as a result of which the switch-on delay is less than the switch-off delay. The two control circuits 110, 210 consist of the relays 15, 25 with the diodes 16, 26 arranged in series with one another in antiparallel, the relays 15, 25 delay capacitors 19, 29 and free-wheeling diodes 111, 211 being connected in parallel. The switches 13, 23 are normally open contacts of these relays 15, 25. The starting circuit 4, which can be triggered by the flame signal, consists of the threshold switches 18, 28 arranged at the inputs of the control circuits 110, 210, the starting resistors 14, 24 and the excitation circuits of the flame relays 11, 21 interconnecting holding resistor 5.
The mode of operation of the circuit arrangement will be described below.
When the flame signal is recorded and the circuit is intact, the circuit arrangement is in a dynamically stable state in which all four relays 11, 21, 15, 25 are energized. The clock signals T, T switch the liquid crystal cells 31, 32 alternately with the clock frequency f _T from the blocked state to the state permeable to the radiation to be recorded. Instead of the liquid crystal cells 31, 32, electrically controllable mechanical shutters or flaps can also be used in a known manner to cover the flame sensors 41, 42.
The flame radiation thus activates at the clock frequency f _T the flame sensors 41, 42, the electrical flame signals of which reach the inputs of the frequency-selective evaluation circuits 10, 20, the outputs of which are, for example, L level when the flame signal is present and H level when the evaluation criterion is not met. As a result of the switching hysteresis, there is a temporal overlap of the signal levels at the outputs of the evaluation circuits 10, 20 at the time of the channel switchover, so that there is always an L level at one of the two outputs. The two diodes 17, 27 are polarized in the forward direction when the flame is present. First, let the clock signal T be used during the time

the liquid crystal cell 31 is transparent to the flame radiation and thus the transmission channel 2 is activated. L level is at the output of the evaluation circuit 10 and H level at the output of the evaluation circuit 20, so that the diode 27 blocks. The flame relay 11 receives the full operating voltage U _B via the closed switch 13.

während der der Übertragungskanal 2 aktiviert ist. Das gilt analog für den Verzögerungskondensator 19. Mit der Umschaltung der Flüssigkristallzellen 31, 32 auf den Übertragungskanal 3 nach Ablauf der Zeit

wechseln die Pegel an den Ausgängen der Bewertungsschaltungen 10, 20, so daß in analoger Weise nunmehr das Flammenrelais 21 die volle Betriebsspannung U_B erhält und die Relais 25 und 11 in Reihe an der Betriebsspannung U_B liegen. Das Relais 15 wird jetzt durch den Verzögerungskondensator 19 gehalten und der Verzögerungskondensator 29 wird nachgeladen.
Bei der Inbetriebnahme oder beim Widereinschalten der Flammenüberwachung wird der oben beschriebene dynamisch stabile Zustand nicht ohne zusätzliche Hilfe erreicht, da zu diesem Zeitpunkt alle Relais 11, 21, 15, 25 abgefallen sind. An die als Einschalthilfe wirkende Anlaufschaltung ist die Forderung zu stellen, selbst oder im Zusammenwirken mit den anderen Teilen der Gesamtschaltung eigensicher zu sein. Weiterhin ist es zweckmäßig, das Flammensignal selbst als Auslösekriterium für die Anlaufschaltung zu verwenden.
Dem wird die in Fig. 1 enthaltene Anlaufschaltung 4 gerecht, deren Funktionsweise im folgenden beschrieben wird.The start-up circuit 4 should initially be disregarded for the description of the dynamically stable state. Thus, the operating voltage is also above the series circuit consisting of the relay 15 of the diode 16 and the flame relay 21. The part of the operating voltage dropping via the relays 15, 21 is sufficient to leave it in the holding state. At the same time, the delay capacitor 19 is recharged. The relay 25 remains in the hold state through the delay capacitor 29. In order to prevent this relay 25 from falling before the next channel changeover, the delay capacitor 29 must be dimensioned such that the fall delay time T _{A is} greater than the time

during which the transmission channel 2 is activated. This applies analogously to the delay capacitor 19. With the changeover of the liquid crystal cells 31, 32 to the Transmission channel 3 after the time has expired

change the levels at the outputs of the evaluation circuits 10, 20, so that the flame relay 21 now receives the full operating voltage U _B in an analog manner and the relays 25 and 11 are connected in series to the operating voltage U _B. The relay 15 is now held by the delay capacitor 19 and the delay capacitor 29 is recharged.
When the flame monitoring is started up or switched on again, the dynamically stable state described above is not achieved without additional help, since at this point in time all relays 11, 21, 15, 25 have dropped out. The start-up circuit, which acts as a switch-on aid, must be intrinsically safe, either itself or in cooperation with the other parts of the overall circuit. Furthermore, it is expedient to use the flame signal itself as a trigger criterion for the start-up circuit.
The start-up circuit 4 contained in FIG. 1, the functioning of which is described below, does this justice.

wobei T_F die Fehlerabschaltzeit, d.h. die maximal mögliche Zeit vom Auftreten eines Defektes bis zur Störabschaltung darstellt.
Bei Auftreten eines Flammenabrisses erfolgt innerhalb der mit der Bewertungsschaltung 10 bzw. 20 realisierten Sicherheitsabschaltzeit T_S das Umschalten des Ausganges der im Moment des Flammenabrisses gerade über die durchlässige Flüssigkristallzelle 31, 32 und dem betreffenden Flammenfühler 41, 42 aktivierten Bewertungsschaltung 10, 20 auf den H-Pegel und das Abfallen des betreffenden Flammenrelais 11, 21. Die Schaltungsanordnung ist eigensicher bezüglich der in betracht zu ziehenden Defekte wie Unterbrechungen von Leitungen und Widerständen, Schluß und Unterbrechung von Kondensatoren und Dioden, Parameterveränderungen der Schwellwertschalter, möglichem Schwingen der Bewertungsschaltungen, Defekten der Flammenfühler und Flüssigkristallzellen. Diese Fehler bewirken eine Störung des weiter oben beschriebenen dynamisch stabilen Zustandes, indem je nach Art des Defektes entweder paritätische Pegel in den beiden Kanälen auftreten oder infolge verminderter Ladung der Verzögerungskondensatoren 19, 29 die Zeitrelation

nicht mehr zutrifft und die Relais 15, 25 abfallen.
Darüber hinaus wird mit dieser Schaltungsanordnung zugleich eine Überwachung der Taktfrequenz f_T erreicht. Ein unzulässiges Ansteigen der Taktfrequenz f_T führt infolge der Hysterese der Bewertungsschaltungen 10, 20 zu paritätischen Signalpegeln am Ausgang der Bewertungsschaltungen 10, 20 das Absinken der Taktfrequenz f_T bewirkt das Abfallen der Relais 15, 25 da die in den Kondensatoren 19, 29 gespeicherte Ladung nicht mehr ausreicht, während der größeren nichtaktiven Zeit jedes Übertragungskanals 2, 3 die Relais 15, 25 zu halten.At switch-on, all relays 11, 21, 15, 25 initially dropped out and the two threshold switches 18, 28 are connected to the operating voltage U _B via the starting resistors 14, 24 and the flame relays 11, 21. Depending on the instantaneous value of the clock, for example via the transparent liquid crystal cell 32, the flame radiation reaches the flame sensor 42 and activates the transmission channel 3, the output of the evaluation circuit assuming 20 L levels.
At the input of the threshold switch 28, the threshold value is therefore fallen below and the voltage difference that arises at the outputs of the threshold switches 18, 28 leads to the charging of the delay capacitor 29 and to the relay 25, which also activates the flame relay 21 with the full operating voltage. The over the holding resistor 5, the starting resistor 14 and the flame relay 11 current flowing causes a voltage drop across the holding resistor 5, the amount of which falls at the level at the input of the threshold switch 18, whereby the threshold value is not fallen below. The starting resistor 14, which only has the task of supplying the H level for the threshold switch 18 when the switch 13 is turned on or when the switch 13 is open, must be chosen to be large enough to limit the current flowing through the flame relay 11 in such a way that with certainty no triggering can take place. When the flame signal is switched to the input of the evaluation circuit 10, the L level which arises at the output thereof falls below the threshold value of the threshold switch 18. At the same time as the level changes at the evaluation circuit 20, the flame relay 21 is held in the holding current range via the holding resistor 5. The associated raising of the input level of the threshold switch 28 above the threshold value leads to its switching. The potential reversal achieved in this way on the control circuits 110, 210 causes the delay capacitor 19 to be charged and the relay 15 and the flame relay 11 to be energized as a result of the closing of the normally open contact 13. This achieves the dynamically stable state described above without taking the start-up circuit 4 into account. The threshold switches 18, 28 additionally cause a potential separation of the control circuits 110, 210, which are thereby operated with the full voltage swing. The holding current for the flame relay of the respective non-active transmission channel is obtained via the holding resistor 5 after the start-up circuit 4 has been added. The two free-wheeling diodes 111, 211 protect the threshold switches 18, 28 against inductive peaks. The relationship applies to the clock frequency f _T to be used and for the drop delay time T _{A of} the relays 15, 25

where T _{F represents} the fault shutdown time, ie the maximum possible time from the occurrence of a defect to fault lockout.
If a flame break occurs, within the safety shutdown time T _S realized with the evaluation circuit 10 or 20, the output of the evaluation circuit 10, 20 activated at the moment of the flame break via the transparent liquid crystal cell 31, 32 and the relevant flame sensor 41, 42 is switched to the H -Level and the falling of the relevant flame relay 11, 21. The circuit arrangement is intrinsically safe with regard to the defects to be considered, such as interruptions in lines and resistors, closure and interruption of capacitors and diodes, parameter changes in the threshold switches, possible oscillation of the evaluation circuits, defects in the flame sensors and liquid crystal cells. These errors cause a disturbance of the dynamically stable state described above, in that, depending on the type of defect, either parity levels occur in the two channels or, due to the reduced charge of the delay capacitors 19, 29, the time relation

no longer applies and relays 15, 25 drop out.
In addition, monitoring of the clock frequency f _{T is also} achieved with this circuit arrangement. An inadmissible increase in the clock frequency f _T leads to equal signal levels at the output of the evaluation circuits 10, 20 due to the hysteresis of the evaluation circuits 10, 20, the drop in the clock frequency f _T causes the relays 15, 25 to drop because the charge stored in the capacitors 19, 29 is no longer sufficient to hold the relays 15, 25 during the longer non-active time of each transmission channel 2, 3.

The dynamic self-monitoring circuit according to FIG. 2 corresponds to that according to FIG. 1, but here the start-up circuit 4 and the control circuits 110, 210 are not shown any further. As can be seen from this figure, only one flame sensor 6 is provided. In this case, the transmission channels 2, 3, which are conjunctively linked at the output A via the make contacts 12, 22 of the flame relays 11, 21 from the two evaluation circuits 10, 20 for the flame signal and the flame relay 11, 21 are alternately connected to the clock-controlled switching device 7 Flame sensor 6 can be switched on. This means that the flame signal received by the flame sensor 6 with the clock frequency f _T alternately to the inputs of the evaluation circuits 10, 20, at the outputs of which there is, for example, L level when the flame signal is present and H level when the evaluation criterion is not met, by the clock-controlled ones Switching device 7 is switched. The further functioning of this dynamic self-monitoring circuit corresponds to that according to FIG. 1.

Claims (9)

1. A dynamic self-monitoring circuit for automatic flame guards, comprising one or two flame sensors (6, 41, 42) receiving the flame radiation, more particularly for selective flame monitoring in multi-burner plants, two transmission channels (2, 3) being provided and linked by an AND-circuit at an output (A) and alternately supplied via a clock-controlled device (7, 31, 32) with the flame signal delivered by the flame sensor (6, 41, 42) and each comprising a flame relay (11, 21) and a circuit (10, 20) for evaluating the flame signal, and switches (13, 23) are disposed in series with the flame relays (11, 21) and are actuated by actuating circuits (110, 210) connected respectively via diodes (17, 27) to the outputs of the two evaluating circuits (10, 20) and evaluating their switching state, and a starting circuit (4) is provided.
2. A dynamic self-monitoring circuit according to claim 1, characterised in that a single flame sensor (6) is provided and is alternately connected to the transmission channels (2, 3) via the clock-controlled device, which is in the form of a change-over device (7).
3. A dynamic self-monitoring circuit accordign to claim 2, characterised in that the clock-controlled change-over device (7) is an electronic change-over switch.
4. A dynamic self-monitoring circuit according to claim 1, characterised in that two flame sensors (41, 42) are provided, each flame sensor (41, 42) being connected to a transmission channel (2, 3), and the clock-controlled device has means (31, 32) for alternately covering the flame sensors (41, 42).
5. A dynamic self-monitoring circuit according to claim 4, characterised in that the clock-controllable means for covering the flame sensors (41, 42) are liquid crystal cells (31, 42) giving control clock signals (T, T) in exact phase opposition and triggering an electro-optical effect of the liquid crystal cells (31, 32).
6. A dynamic self-monitoring circuit according to at least one of the preceding claims 1 to 5, characterised in that the two actuating circuits (110, 210) each comprise a relay (15, 25) with drop-out delay and a diode (16, 26) connected in series, the two diodes (16, 26) being disposed anti-parallel and the switches (13, 23) being working contacts of the relays (15, 25).
7. A dynamic self-monitoring circuit according to at least one of the preceding claims 1 to 6, characterised in that the starting circuit (4) comprises threshold-value switches (18, 28) disposed at the inputs of the actuating circuit (110, 210), starting resistors (14, 24) bridging the switches (13, 23), and a holding resistor (5) interconnecting the energisation-current circuits of the flame relays (11, 21).
8. A dynamic self-monitoring circuit according to at least one of the preceding claims 1 to 7, characterised in that the evaluating circuit (10, 20) has an overlap.
9. A dynamic self-monitoring circuit according to at least one of the preceding claims 1 to 8, characterised in that the relation

   

   holds for the clock frequency (f_T) and the drop-out delay time (T_A) of the relays (15, 25) in relation to the fault break time (T_F) and the safety break time (T_S).

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