Residual current protection circuit for detecting direct current

Abstract

Title: Residual Current Protection Circuit for DC Current Detection. Residual current protection circuit for circuit interruption, comprising a current transformer (1), an excitation circuit (31) for generating an oscillating excitation voltage, an inductor (32) for inducing a magnetic field in the current transformer (1), a first resistor (33), a second resistor (38), and a switch (39), the second resistor (38) being connected in parallel with the first resistor (38) and being able to be switched on by means of the switch (39). Figure for the abstract: Fig. 5

Description

Title of the invention: Residual current protection circuit for the detection of a direct current

[0001] The invention relates to a residual current protection circuit for the detection of a direct current and a method for detecting a direct current by means of a residual current protection circuit according to the invention.

[0002] Residual current protection switches or differential current protection switches are often also referred to as RCDs (from the English "residual current detector"). They are used to protect against residual currents that can be dangerous, particularly for operators or users of an electrical installation. An RCD that also operates on direct current is usually designated as a "Type B RCD". In particular, for Type B RCDs, it is required to guarantee correct reaction or tripping behavior, that is, on the one hand, to open safely for all dangerous residual currents, but on the other hand, to prevent false tripping (in English, the expression "nuisance tripping" is common in this case).

[0003] Technical improvements in this regard to type B RCDs are indicated for example in documents DE 102018204047 Al, DE 102019204272 Al and DE 102021202171 Al. In this case, it is mainly a question of the effect of high-frequency currents, which can cause nuisance tripping.

[0004] But we need to improve the triggering behavior of type B RCDs also with regard to other effects.

[0005] It is the purpose of the invention to contribute to that.

[0006] This is achieved through the following points of the invention:

[0007] 1. Residual current protection circuit for detecting a direct current at using a current transformer, in which, by the action of an oscillating magnetic field on the transformer, a compensating current is induced in the case of a residual direct current occurring in a controlled circuit, which is evaluated with regard to the need to interrupt the circuit,

[0008] comprising

[0009] - a current transformer,

[0010] - an excitation circuit for the production of an oscillating excitation voltage,

[0011] - an inductor for inducing a magnetic field in the transformer of current, and

[0012] - an initial resistance,

[0013] characterized in that

[0014] - the protection circuit is further formed by a second resistor and an in terrupteur, the second resistor being mounted in parallel with the first resistor and being able to be switched on by means of the switch.

[0015] 2. Residual current protection circuit according to point 1,

[0016] characterized in that

[0017] - the protection circuit further comprises a calculation unit for evaluating the current detected, which is designed to cause, by a control signal that can be given by an output, the opening and closing of the switch.

[0018] 3. Residual current protection circuit according to point 2,

[0019] characterized in that

[0020] - the excitation circuit is connected to an input of the computing unit,

[0021] and

[0022] - the protection circuit is constituted to provide, by the excitation circuit by the intermediary of the input, of information, from which a change of sign of the excitation voltage can be determined.

[0023] 4. Residual current protection circuit according to one of points 1 to 3,

[0024] characterized in that

[0025] the excitation circuit, the inductance and the first resistance are connected in series.

[0026] 5. Residual current protection circuit according to one of points 2 to 4,

[0027] characterized in that

[0028] The calculation unit is constituted to carry out a process according to one of the points 6 to 12.

[0029] 6. Method for detecting a direct current by means of a protection circuit residual current according to one of the points 1 to 5,

[0030] including the stages

[0031] - closing of the switch during the first change of sign of the voltage of excitement,

[0032] - opening of the switch during the sign change following the first of the excitation voltage,

[0033] - detection of a first induced compensation current,

[0034] - closing of the switch during a second change of sign of the voltage of excitement, the second change of sign being opposite to the first,

[0035] - opening of the switch during the sign change following the second of the excitation voltage,

[0036] - detection of a second induced compensation current, and

[0037] - formation of an average value of the first and second communication currents induced thought.

[0038] 7. Method according to point 6,

[0039] characterized by the stages

[0040] - DC residual current control of the circuit,

[0041] - upon detection of the appearance of a residual current, the steps in point 6 are carried out for determining a value of the residual current, and

[0042] - use of the average value of the first and second communication currents induced thought as a value for the residual current.

[0043] 8. Procedure according to point 7,

[0044] characterized by the additional stages

[0045] - checking whether the residual current value exceeds a threshold of permissible residual current

[0046] - emission of a circuit interruption signal if the threshold is exceeded.

[0047] 9. A method according to one of points 7 or 8,

[0048] characterized in that

[0049] - the control of a residual current in direct current in the circuit is carried out at a means of detecting an induced compensation current and monitoring its exceedance above a threshold value for triggering, and

[0050] - the determination of a residual current value is carried out according to point 7 that if the threshold value for triggering is exceeded.

[0051] 10. Method according to point 9,

[0052] characterized in that

[0053] - an additional check is carried out to ensure that a threshold value has not been exceeded for immediate triggering, and

[0054] - the determination of a residual current value is carried out according to point 7 that if the threshold value for immediate triggering is not exceeded.

[0055] 11. Procedure according to point 10,

[0056] characterized by

[0057] an immediate emission of a circuit interruption signal, if the threshold value for immediate triggering is exceeded.

[0058] 12. A method according to any one of points 6 to 10,

[0059] characterized in that

[0060] during a check of the residual current in direct current in the circuit after carrying out the stages of point 6, the effect of a magnetization thus caused of the current transformer is taken into account.

[0061] 13. Computer program, which executes a process according to one of points 6 to 12 when it is run on a computer.

[0062] 14. Computer program product having a computer program following the point 13.

[0063] According to the invention, a residual current protection circuit is proposed. (Residual current protection switch) for detecting a direct current using a current transformer. A residual current protection circuit of this type is typically part of a protective switch, for example, in a type B RCD. By the effect of an oscillating magnetic field on the transformer, a compensating current is induced in the case of a DC residual current occurring in a controlled circuit. This compensating current is evaluated based on the need to interrupt the circuit (if necessary, after preliminary processing, for example, by filtering and amplification). This residual current protection circuit can operate, in particular, according to the fluxgate principle.

[0064] The residual current protection circuit according to the invention comprises a current transformer, an excitation circuit for generating an oscillating excitation voltage, an inductor for inducing a magnetic field in the current transformer, and a first resistor (also sometimes called a "measuring resistor"). These components may also be connected in series. According to the invention, the protection circuit further comprises a second resistor and a switch (for example, a mechanical switch such as a relay or an electronic switch such as a MOSFET), the second resistor being connected in parallel with the first resistor and being able to be switched on by means of the switch.

[0065] The invention aims to create an unwanted magnetization of the transformer or the transformer core. By adding the second resistor, the intensity of the magnetic field applied to the transformer can be modified. It is thus possible to calculate, using a method according to the invention, the value of a residual current that has occurred, with the possibility of at least partially compensating for the effect of the unwanted magnetization.

[0066] Preferably, the protection circuit according to the invention is further formed by a calculation unit or a logic unit (typically an MCU) for evaluating the detected or measured current, which is further configured to open and close the switch by a control signal that can be given by an output.

[0067] For a method according to the invention, in which the opening or closing of the switch is coupled to a change in the sign of the excitation voltage, the excitation circuit can be connected to an input of the processing unit, and the protection circuit is configured so as to provide, via the input of the excitation circuit, information from which a change in the sign of the excitation voltage can be determined. This information can be, for example, a binary signal that encodes the sign or the direction of the sign change. Alternatively, one could transmit an analog signal, which is itself a measurement of the voltage, and determine the sign change by the processing unit by means of a detection of zero crossing. However, solutions are also possible that do not require connections between the excitation circuit and the input of the computing unit. This can be achieved, perhaps via the induced compensation current detected and evaluated by the computing unit, at the instant of a change in the sign of the excitation voltage.

[0068] In a residual current protection circuit configured according to the invention and having a calculation unit, the latter can be constituted to carry out the process according to the invention described below.

[0069] A method for detecting a direct current using a residual current protection circuit according to the invention is proposed, in which the following stages are carried out

[0070] a) closing of the switch upon the first change of sign of the excitation voltage,

[0071] b) opening of the switch during the change of sign following the first change of the excitation voltage,

[0072] c) detection of a first induced compensation current,

[0073] d) closing of the switch during a second change of sign of the excitation voltage, the second change of sign being opposite to the first,

[0074] e) opening of the switch during the change of sign following the second of the excitation voltage,

[0075] f) detection of a second induced compensation current, and

[0076] g) formation of an average value (preferably the arithmetic mean value) of the first and second induced compensation currents.

[0077] By switching on the second resistor (closing the switch), the magnetic field strength applied to the transformer increases. By appropriately choosing the value of the second resistor, the transformer core can be magnetized, which, after switching on the second resistor, produces a compensating current. The magnetization is carried out, and the resulting compensating current is determined for different magnetic field polarities, and then the average value is calculated. If there is no residual current, the average value is zero under ideal conditions. If there is initial magnetization of the core, this method significantly reduces the apparent residual current.

[0078] According to one embodiment, the residual DC current in the circuit is checked in the usual way. Upon detection of the appearance of a suspected residual current (i.e., an induced current that may also be caused by magnetization of the transformer core), steps a) to g) indicated above for determining a value of the residual current are then carried out. The average value thus obtained is then used as the value of the residual current. first and second induced compensation current. It can then be planned to check if the value of the residual current exceeds a threshold of the permissible residual current, and if the threshold is exceeded, to emit a circuit interruption signal.

[0079] Residual current can be controlled by direct current flowing through the circuit by detecting an induced compensating current and checking whether this current exceeds a threshold value for tripping. The residual current value is determined using steps a) - g) only if the tripping threshold value is exceeded. Preferably, a check for exceeding a threshold value for immediate tripping is also performed, and the residual current value is determined using the steps only if the immediate tripping threshold value is not exceeded. This embodiment is based on the consideration that the possible ratio of core magnetization to a detected residual current is limited.For a high residual current, it can then be decided that the residual current exceeds the threshold for tripping, even if the contribution from the magnetization of the transformer core has the maximum value. This is represented by a second threshold value ("threshold value for immediate tripping"). In this case, a circuit interruption signal is preferably emitted immediately; that is, there is no average value formation by means of stages a) to g).

[0080] According to an improvement, when controlling the residual current in direct current in the circuit by carrying out stages a) to g), the effect of a magnetization thus caused of the transformer is taken into account.

[0081] The invention will be explained more precisely below by means of the figures in the context of an exemplary embodiment. The figures represent:

[0082] [Fig. 1]: Usual type B RCD,

[0083] [Fig.2]: Hysteresis curve of a transformer core,

[0084] [Fig.3] : apparent residual current during magnetization of the transformer core by a field intensity pulse.

[0085] [Fig.4a]: Apparent residual current as a function of time during magnetization of the transformer core by a field intensity pulse,

[0086] [Fig.4b]: field intensity as a function of time for a field intensity pulse, which produces a magnetization of the nucleus,

[0087] [Fig.5]: Residual current protection circuit according to the invention,

[0088] [Fig. 6]: Apparent residual current during magnetization of the transformer core by a field intensity pulse, while the resistance Rmag of the circuit in [Fig.5] is switched on,

[0089] [Fig.7a]: residual current detected as a function of time, when proceeding, according to the invention, if a previous magnetization of the core by a pulse of intensity of field is not present,

[0090] [Fig.7b]: magnetic field as a function of time, when proceeding according to the invention, if a previous magnetization of the core by a field intensity pulse is present,

[0091] [Fig.8a]: residual current detected as a function of time, when proceeding according to the invention, if a previous magnetization of the core by a field intensity pulse is present,

[0092] [Fig.8b]: magnetic field as a function of time, when proceeding according to the invention, if a previous magnetization of the core by a field intensity pulse is present,

[0093] [Fig.9]: apparent residual current during magnetization of the transformer core by a field intensity pulse, when proceeding according to the invention,

[0094] [Fig. 10a]: residual current detected as a function of time if magnetization of the core by a field intensity pulse occurs and if the process according to the invention is then carried out,

[0095] [Fig. 10b]: magnetic field as a function of time, if magnetization of the core by a field intensity pulse occurs and if the process according to the invention is then carried out,

[0096] [Fig. 11]: a method according to the invention,

[0097] [Fig. 12]: an improvement of the process according to the invention.

[0098] For the sake of simplicity, the embodiment of the residual current protection circuit is described in the example embodiment as a protective switch specifically as a type B RCD. However, the invention is not limited to this case. Not only can the circuit according to the invention be a constituent part of switches that, in addition to the residual current protection function, perform other functions (in particular, protection functions such as overload and short-circuit protection), but it is also conceivable that the circuit according to the invention could be combined with other electronic components (for example, transformers) into a product to achieve an additional residual current protection function.

[0099] Current transformers, which are used in RCDs, usually have a core made of a softly magnetic material. This core can be weakly magnetized by high-frequency differential currents. This weak core magnetization can be interpreted as a residual DC current by the RCD's differential current detection means and lead to nuisance tripping of the RCD. This is particularly problematic in combinations of RCDs and a monitoring device—designated as "RCD-MD," where "MD" stands for "monitoring device"—which trips at low currents. Devices of this kind, which are... They trigger at a maximum amplitude of 6 mA of residual current in alternating current and already at 4.5 mA of residual current in direct current, can be used in the field of electric mobility, for example to meet the requirements of the IEC 62955 standard.

[0100] Standard RCDs use a zero current transformer (ZCT) to sum the magnetic fields of the load currents in all conductors and transform the differential current (residual current) in the ZCT's secondary winding. The current transformation principle does not work for direct currents. Therefore, the fluxgate principle is usually used for measuring residual current in direct current.

[0101] A type B RCD is schematically represented in [Fig. 1]. It is shown with phase conductors L1 to L3 and a neutral conductor N. If a differential current appears, the RCD trips and interrupts the conductors by means of switches S. An excitation circuit 31 is provided (the power supply to the excitation circuit 31 is not shown), which produces an excitation current, typically by means of an alternating voltage Vexc, which drops across the measuring resistor RsenSe or 33; that is, the excitation current is an alternating current. This periodically drives the magnetic core of a current transformer 1 ZCT by means of an inductor 32, from one saturation point to the opposite saturation point (saturation detection by means of a detector 34). This produces an average magnetic field of 0 A / m.By producing a magnetic field intensity of 0 A / m, a compensating current is also produced in the case of the presence of a residual current.

[0102] This current causes a voltage drop across the measuring resistor Rsense (reference 33). A signal processing circuit with a low-pass filter 35 compensates for the influence of the excitation current. An amplifier 36 amplifies the signal, and an adder adds an offset so that the signal can be measured using an analog-to-digital converter (input 41) mounted in a microcontroller unit 4. The residual current can then be calculated using the following formula: .... JâtU ~............-------------■?-------------

[0103] in which S is the amplification factor, VADC is the digitized voltage at the vAN converter, Vo is the added offset voltage, N2 is the secondary winding index of the transformer or the winding index of the inductance 32 and RsenSe corresponds to the ohmic resistance 33 of [Fig.1].

[0104] The micro-control unit 4 calculates the residual current value. For example, it calculates the RMS value (RMS is an abbreviation for "root mean square"). square" and designates the effective value of the current). Based on the RMS value, criteria for interrupting the current are controlled (for example, a threshold value of the RMS value possibly linked to a time criterion for exceeding the threshold value). If the conditions for an interrupt are met, the micro-control unit 4 outputs an interrupt signal to output 42, which energizes an interrupt unit 5 (this is usually also the English term "trip unit") to open switch S.

[0105] It is important to ensure that core magnetization does not produce an apparent residual current and thus trigger the RCD. The effect of transformer core magnetization is shown in the hysteresis curve in [Fig. 2]. The magnetic field strength B is plotted as a function of the magnetization field strength H. The original hysteresis curve is the lower curve. If the transformer core is permanently magnetized by a positive magnetic field with a field strength Hsurge, which is significantly greater than the field strength Hexc produced by the excitation generator, the hysteresis loop can no longer be centered around the origin B = 0, but is shifted slightly in the direction of positive field strengths B (upper curve). This results in a small difference between the positive and negative remanence values: ABr.

[0106] The magnetization of the core has the same effect on detection as a residual direct current that a corresponding magnetic field would have induced in the core. The detection circuit automatically compensates for this magnetization by means of a corresponding compensating field strength:

[0107] AHc = ABr / q

[0108] The detection circuit then detects a presumed direct current, which in reality is not even present:

[0109] Iapp=LfexAHc.

[0110] Figure 3 represents the apparent current as a function of the field impulse Hsurge and the excitation field intensity Hexc. The original working point A is shifted towards point B. The resulting partial demagnetization at the value Hexc through point C results in a residual magnetization at point D and corresponds to the apparent current I_app.

[0111] Figures 4a and 4b represent this same magnetization and its effects in the time domain. The passage from point A to point D via B and C cannot be represented in this resolution because it is much faster than the period of the intensity Hexc of the excitation field.

[0112] Possible scenarios that could lead to magnetization of the core include: large current pulses due to a sharp rise in current during a lightning strike or a short circuit in the circuit to ground. In currents of this kind (several thousand amperes) the magnetization field intensity can exceed 100,000 A / m, which is to be compared to the magnetization field intensity of Hexc of the order of 50 A / m.

[0113] When the apparent current I_app or the sum thereof Strom I_app together with a technical leakage current (which would not in itself yet cause a tripping) exceeds the RCD tripping threshold, an untimely tripping of the RCD occurs.

[0114] The value of the differential current, which can be created by the permanent magnetization of the core, is generally on the order of a few mA. For a RCD with a nominal tripping current of 30 mA, this does not generally represent a real problem, because this apparent current I_app corresponds almost to a tolerable measurement accuracy. On the other hand, for a device with a nominal tripping current of 6 mA, this current caused by the magnetization of the core represents a considerable problem, because it takes on values ​​(a few mA) that can no longer be neglected and that significantly alter the tripping behavior.

[0115] Figure 5 represents a possible embodiment of the invention. It represents a typical detection aid circuit for a type B RCD (see Figure 1) with the following additions:

[0116] - a "MAG" output (reference 44) of the MCU, which controls the state of a switch 'switch' (reference 39). This switch can provide a connection of the resistance Rmag (reference 38) in parallel with the measurement resistance Rsense (reference 33).

[0117] - a "POL" input (reference 43) of the MCU, for controlling the circuit polarity Excitation: the input is a binary signal, whose logic states correspond to a positive excitation voltage and a negative excitation voltage.

[0118] In normal operation, switch 39 is open and resistor 38 is not connected in parallel with resistor 33. The periodic field strength applied to the current transformer is then

[0119] If switch 39 is closed, the overall resistance becomes: “J — O >”

[0120] and the field strength amounts to:

[0121] The field intensity Hexc2 is substantially greater than the field intensity Hexcl, but cannot be as great as the field intensity Hsurge. The value The field intensity Hexc2 can, for example, take a value on the order of 1000 A / m. Based on what enters input 43, the MCU can open switch 39 after a positive pulse of the positive field intensity +Hexc2 or a negative pulse of the field intensity -Hexc2.

[0122] Figures 7a and 7b show that magnetization occurs after a first positive pulse and that an apparent current Iapp2+ then remains. Correspondingly, a negative apparent current Iapp2- is detected after a negative pulse of field intensity Hexc2, as shown in [Fig. 7a].

[0123] The average value Im of the apparent currents Iapp2+ and Iapp2- is almost zero and corresponds to reality. In the next working phase, the MCU must compensate for the remaining apparent current Iapp2- during the differential current calculation. If, for example, in [Fig. 7b], the procedure is finished (i.e., over time after the second negative peak), and if the RCD has not triggered, the RCD is still monitored to see if the residual current exceeds a trigger threshold. For this threshold value check, the induced magnetization is compensated. Let, for example, lapp be the current value obtained during the check. The compensation can then consist of comparing not lapp but (lapp + (Im - Iapp2-)) to the threshold value, where Im and Iapp2- are the values ​​obtained during the last time determination of Im. Figures 8a and 8b represent a scenario in which a differential current Idiff passes in the ground floor.This produces a magnetization field intensity Hdiff - Idiff / Lfe, which is compensated by the circuit and results in a current lappl, which has the same value as the current Idiff. If magnetization is performed with field intensities +Hexc2 and -Hexc2, the average value Im of Iapp2+ and Iapp2- corresponds to the current lappl and the current Idiff. The formation of the average value after magnetization in positive and negative polarities does not influence the result, and the measured current corresponds to the actual differential current.

[0124] Figure 9, along with Figures 10a and 10b, illustrates a scenario in which the core of the current transformer has been magnetized to Hsurge by a very intense differential current. Following the intense current, the MCU detects an apparent lappl current, which is caused by the spurious magnetization. At this point, the MCU cannot distinguish between the effect of magnetization and a true DC differential current. The MCU then turns to the very large excitation field strengths +Hexc2 and -Hexc2 and measures the new apparent currents Iapp2+ and Iapp2-. The average value Im of these currents is considerably smaller than the previous apparent lappl current. It can be inferred that at least the difference lappl-Im is caused by the magnetization of the core.

[0125] The average current Im may not correspond exactly to the actual value of the differential current, because the intensity Iexc2 of the excitation field is still consideredsignificantly smaller than the magnetization at Hsurge, i.e., the effect of spurious magnetization cannot be completely compensated. The remaining error has approximately the value of lappl - Iapp2+. However, the error in detecting the differential current in DC is significantly reduced by this method, if the average value Im is interpreted as the differential current in DC.

[0126] Figure 11 is a flowchart of a method according to the invention. In a first stage, a possible residual current is checked by a circuit according to Figure 5. Initially, the resistance Rmagn' is not in the circuit, and the induced current lappl is determined. Depending on whether there is an induced current, the method continues. The intensity of the determined current lappl can optionally also be used as a criterion for this purpose.This apparent lappl current can also be caused by unwanted magnetization of the transformer core. For example, during the S2 request, one could check if the induced lappl current exceeds a threshold value for RCD tripping. If so, stages S3 to S10 would be executed to prevent the possibility of unwanted tripping caused by core magnetization. In stage S3, a sign change is detected for the excitation voltage Vexe and communicated via the Pol input to the MCU. The MCU then closes switch 39 via the Mag output, thereby connecting the resistor Rmagest in parallel with the resistor RsenSe-. During the next sign change, switch 39 is opened again (stage S4). The induced lappl current Iapp2+ is then determined.Next, stages S3-S5 are repeated with a change of sign in the opposite direction (stages S6 to S8), thus obtaining the value Iapp2- of the induced current. In stage S9, the average value Im is calculated from Iapp2+ and Iapp2- and used as a criterion for tripping (S10). If a tripping limit value is exceeded, the RDC interrupts the controlled current channel (stage S1); otherwise, the control process continues. The magnetization of the transformer core is taken into account in stages S6 and S7.

[0127] Figure 12 represents a variant of the method according to the invention. The following consideration underlies this method: the method according to the invention seeks to improve tripping safety by at least partially compensating for a magnetization of the transformer core during the evaluation of the induced αppl current. This compensation is not necessary if the induced αppl current is so intense that the tripping threshold is exceeded. Let ABR > max be the maximum core magnetization, which occurs in operating situations. This gives an induced current IaPP,max according to the equation

[0128] AHc,max~ ABRmax / q

[0129] and

[0130] lapp.max—LpE X AHC>max.

[0131] Let Ippl be the tripping threshold of the residual current protection circuit. The induced current Ippl is determined according to stage S2 in [Fig. 11]. The subsequent stages S3 to S9 prevent tripping events in which exceeding the threshold value is due solely or primarily to magnetization of the transformer core. Therefore, if the determined current Ippl is so intense that there is, in any case, an excessively high residual current, tripping can occur immediately, i.e., without executing stages S3 to S9. This condition is satisfied if Ippl > 1^1 + Iappl>max. Following the improvement in [Fig. 12], another limiting value Ippl is introduced. This is chosen so as to, in principle, impose an excessively intense residual current in the event of a certain overshoot, regardless of whether the core is magnetized, for example Itripp2 > I trippl 3" lapp.max*

[0132] The maximum magnetization of the core, and thus also Iapp>max, can be determined empirically, for example, for a given residual current protection circuit. This method is implemented by stage S21 in the improvement according to claim 12. Immediate tripping occurs if the threshold value Itripp2 is exceeded. The advantage is faster tripping, particularly at high residual currents, which are potentially also dangerous.

Claims

Demands

1. Residual current protection circuit for detecting a direct current using a current transformer (1), in which, by the action of an oscillating magnetic field on the transformer (1), a compensation current is induced in the case of a residual direct current occurring in a controlled circuit, which is evaluated with regard to the need for interruption of the circuit, comprising - a current transformer (1), - an excitation circuit (31) for the production of an oscillating excitation voltage, - an inductance (32) for the induction of a magnetic field in the current transformer (1), and - a first resistor (33), characterized in that - the protection circuit is further formed by a second resistor (38) and a switch (39), the second resistor (38) being mounted in parallel with the first resistor (38) and being able to be switched on by means of the switch (39).

2. Residual current protection circuit according to claim 1, characterized in that the protection circuit is further formed by a calculation unit (4) for evaluating the detected current, which is constituted to cause, by a control signal which can be given by an output (Mag), the opening and closing of the switch (39).

3. Residual current protection circuit according to claim 2, characterized in that - the excitation circuit (31) is connected to an input (Pol) of the calculation unit (4), and - the protection circuit is constituted to have, by the excitation circuit (31) via the input (Pol), information from which a change of sign of the excitation voltage can be determined.

4. Residual current protection circuit according to any one of claims 1 to 3, characterized in that the excitation circuit (31), the inductor (32) and the first resistance (33) are mounted in series.

5. Residual current protection circuit according to any one of claims 2 to 4, characterized in that the calculation unit (4) is constituted to carry out a process according to any one of claims 6 to 12.

6. A method for detecting a direct current by means of a residual current protection circuit according to any one of claims 1 to 5, comprising the stages - closing of the switch (39) on a first change of sign of the excitation voltage, - opening of the switch (39) on the next change of sign of the excitation voltage, - detection of a first induced compensation current (Iapp2+), - closing of the switch (39) on a second change of sign of the excitation voltage, the second change of sign being opposite to the first, - opening of the switch (39) on the next change of sign of the excitation voltage, - detection of a second induced compensation current (Iapp2-), and - formation of an average value (Im) of the first and second induced compensation currents (Iapp2+, Iapp2-).

7. A method according to claim 6, characterized by the stages - control of the residual current in DC current of the circuit, - upon detection of the appearance of a residual current (lappl), the stages of claim 6 are carried out for the determination of a value of the residual current, and - use of the average value (Im) of the first and second induced compensation currents (Iapp2+, Iapp2-) as the value for the residual current.

8. Method according to claim 7, characterized by the additional stages - checking whether the residual current value exceeds an permissible residual current threshold, - emitting a circuit interruption signal if the threshold is exceeded.

9. A method according to any one of claims 7 or 8, characterized in that - the control of a residual current in direct current in the circuit is carried out by means of a detection of an induced compensation current (lappl) and the control of its exceedance of a threshold value for the tripping, and - the determination of a value of the residual current according to claim 6 is only carried out if the threshold value for the tripping is exceeded.

10. A method according to claim 9, characterized in that - an additional check is carried out to ensure that a threshold value is exceeded for immediate tripping, and - the determination of a residual current value according to claim 7 is only carried out if the threshold value for immediate tripping is not exceeded.

11. A method according to claim 10, characterized by an immediate emission of a circuit interruption signal, if the threshold value for immediate triggering is exceeded.

12. A method according to any one of claims 6 to 10, characterized in that during a check of the residual current in direct current in the circuit after having carried out the stages of claim 6, the effect of a magnetization thus caused of the current transformer (1) is taken into account.

13. Computer program, which performs a process according to any one of claims 6 to 12 when executed on a computer.

14. Computer program product having a computer program according to claim 13.

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