DE102019204272B4 - Residual current device and procedure - Google Patents

Abstract

Residual current protection unit for a low-voltage circuit, comprising: - at least one summation current transformer (ZCT), the primary side of which is formed by the conductor of the low-voltage circuit and the secondary side of which has a secondary winding (N2), - a measuring resistor (R) which is connected in series with the secondary winding (N2) - an excitation circuit (EC), which is connected in parallel to the series circuit of measuring resistor (R) and secondary winding (N2) and supplies the series circuit with an excitation voltage or excitation current, - a first filter (LP), which is connected to the measuring resistor (R ) is connected and emits a filtered differential current (VF), - a control unit (SE) to which the signal from the first filter (LP) is fed and which is designed in such a way that a fault current detection signal (FES) is emitted when a differential current limit value is reached, characterized in that a second filter (HP) is provided, which is parallel on the input side lel is connected to the measuring resistor (R) and is connected on the output side to a saturation detector circuit (SD), which is connected on the output side to the excitation circuit (EC), the saturation detector circuit (SD) when a first threshold value of the amount of the secondary winding (N2) is reached flowing excitation current (Iexc) sends a polarity reversal signal to the excitation circuit (EC) that the control unit (SE) is connected to the excitation circuit (EC) so that the control unit (SE) detects the status of the excitation circuit (EC) or the excitation voltage, that the control unit (SE) is connected to the saturation detector circuit (SD) in such a way that a signal can be transmitted from the control unit (SE) to the saturation detector circuit (SD).

Description

The invention relates to a fault current protection unit for a low-voltage circuit according to the preamble of claim 1 and a method for detecting DC and AC fault currents for a low-voltage circuit according to the preamble of claim 16.

Residual current circuit breakers for electrical circuits, in particular for low-voltage circuits, low-voltage systems or low-voltage networks, are generally known. Residual current circuit breakers are also known as FI circuit breakers or residual current devices, or RCD for short.

With low voltage are meant voltages up to 1000 volts alternating voltage and / or 1500 volts direct voltage. With low voltage are meant in particular voltages that are greater than the low voltage, with values of 25 volts or 50 volts alternating voltage and 60 volts or 120 volts direct voltage.

With circuits, especially for low voltage, circuits are meant for currents up to 6300 amps, more specifically currents up to 1600 amps, 1200 amps, 630 amps, 125 amps, 63 amps, 35 amps, 25 amps, 16 amps, 10 amps or 6 amps . The current values mentioned in particular mean nominal and / or cut-off currents, i.e. the maximum current that is normally passed through the circuit or at which the electrical circuit is usually interrupted, for example by a protective device such as a miniature circuit breaker or a circuit breaker. Residual current circuit breakers are used in particular for nominal current ranges of or up to 16, 25, 40, 63, 80 or 125 amperes.

Residual current circuit breakers determine the current sum in an electrical circuit, which is normally zero, and interrupt the electrical circuit when a differential current value is exceeded, i.e. a current sum not equal to zero that exceeds a certain (differential) current value or response current value or fault current value or fault response current value .

Almost all previous residual current circuit breakers have a summation current transformer, the primary winding of which is formed by the conductor of the circuit and the secondary winding of which emits the total current or an equivalent of the total current, e.g. in the form of a voltage, which is used directly or indirectly to interrupt the electrical circuit.

For this purpose, two or more conductors, mostly forward and return conductors or outer and neutral conductors in a single-phase alternating current network, all three outer conductors or all three outer conductors and the neutral conductor in a three-phase alternating current network, through a mostly ring-shaped core made of ferromagnetic material, current transformers guided. Only the differential current is converted from the conductors, i.e. a current deviating from the forward and return currents. Usually, i.e. in an error-free or error-free state, the sum of the current in an electrical circuit is equal to zero. In this way, fault currents can be detected.

For example, if a current flows to earth on the energy sink side or on the consumer side, this is referred to as a fault current. A fault occurs, for example, when there is an electrical connection from a phase conductor or outer conductor of the electrical circuit to earth. For example, when a person touches the phase conductor. Then part of the electrical current does not flow back via the neutral conductor or neutral conductor as usual, but via the person and the earth. This fault current can now be recorded with the help of the summation current transformer, since the amount recorded from the incoming and outgoing currents is not equal to zero. A relay or a holding magnet release, for example with a connected mechanism, causes the circuit to be interrupted, for example at least one, part or all of the lines. Residual current circuit breakers for recording AC residual currents are generally from the publication DE 44 32 643 A1 known.

The main function of residual current circuit breakers is to protect people from electrical currents (electric shock), as well as systems, machines or buildings from fire due to electrical insulation faults.

If the residual current circuit breaker or its summation current transformer is designed in such a way that the secondary-side energy is sufficient to actuate a trip unit or an interruption unit or a trip unit, then such residual current circuit breakers are called voltage-independent; otherwise depending on the mains voltage.

If a power supply unit is provided for the energy supply of a fault current detection, it is a mains voltage-dependent fault current circuit breaker. These are required, for example, to detect fault currents in DC voltage networks as well as mixed DC / AC networks or in circuits with high frequencies.

Residual current circuit breakers are available in different types, which are identified by letters or letter combinations, such as AC, A, F, G, K, S, B, B +. Each type detects a certain type of fault current. Residual current circuit breakers are currently 2-pole for phase and neutral conductors (L + N), 3-pole for three phase conductors ( L1 , L2 , L3 ) and 4-pole for three phase conductors and neutral conductor ( L1 , L2 , L3 , N) known.

For example, type AC only record purely sinusoidal fault currents. Type A detects both purely sinusoidal AC residual currents and pulsating DC residual currents. Type F are mixed frequency sensitive residual current protective devices. They detect all types of residual currents such as type A, and are also suitable for detecting residual currents that consist of a frequency mixture of frequencies up to 1 kHz. Type K includes the characteristics of type A, however, its switch-off behavior is briefly delayed. Type S are selective residual current circuit breakers, which can be graded in the rated residual current as well as in the tripping time.

Type B residual current protective devices are used, in addition to the detection of type F residual current forms, to detect smooth DC residual currents. They are also suitable for fault currents with frequencies of up to 2 kHz.

For residual current protective devices of type B + the same conditions apply as for residual current protective devices of type B. Only the frequency range for the detection of residual currents applies to an extended range up to 20 kHz. The triggering takes place within this frequency range below 420 mA.

So-called fluxgate technology is often used for residual current circuit breakers for detecting direct residual currents, such as with residual current protective devices of type B or B +.

The problem with DC residual currents is that they cannot be detected with a conventional summation current transformer due to the missing AC component.

With fluxgate technology, a voltage, ideally a square wave voltage, is applied to the secondary winding of the summation current transformer. This saturates the core of the summation current transformer when the voltage is applied. When saturation is reached, which is detected by a saturation circuit, the polarity of the voltage is changed and the core is again driven into saturation.

Here again a detection of the saturation takes place and the polarity is changed again, etc.

The saturation can take place by determining the voltage or the current via a measuring resistor connected in series with the secondary winding. If a first voltage threshold is reached, the core has reached saturation and polarity reversal can take place.

This application of an auxiliary voltage to the secondary winding, in particular a DC voltage, which becomes a square-wave voltage by reversing the polarity, allows DC residual currents to be detected on the primary side of the summation current converter.

In addition, the higher-frequency AC residual currents can also be recorded in the usual way.

These differential currents are filtered, in particular by a first filter, such as a low-pass filter, and can be compared with a differential current limit value by a control unit. If it is exceeded, a fault current detection signal is emitted, which is usually fed to an interruption unit, which interrupts the electrical circuit, in particular advantageously brings about galvanic isolation.

Due to the large frequency range to be recorded with residual current circuit breakers, tripping errors can occur if the residual current has certain frequencies. A malfunction of the detection circuit (and thus false tripping) occurs in particular when the frequency of the fault current matches and the excitation generator to be expected. The same can happen if the frequency of the fault current is a multiple of the excitation frequency. Possible causes for the disturbances are resonance effects or interactions between the excitation generator and the summation current transformer.

Incorrect tripping leads to system downtime, which regularly results in high cost losses.

• - eine Magnet-Schaltung (11), die von der Stromleitung (5) durchquert wird und eine Sekundärwicklung (12) umfasst;
• - einen Generator (13), der mit der Wicklung (12) verbunden ist, um ein elektrisches Signal an sie zu liefern,
• - eine Schaltung zur Messung des in der Wicklung (12) fließenden Stroms (14),
• - eine Verarbeitungsschaltung (15), die ein Fehlersignal liefert, wenn der Strom in der Wicklung (12) einen bestimmten Schwellenwert überschreitet.

The European patent application EP 3 232 526 A1 discloses a device for detecting a fault current occurring in a power line ( 5 ) an electrical system flows, comprising:

• - a magnetic circuit ( 11 ) coming from the power line ( 5 ) is crossed and a secondary winding ( 12th ) includes;
• - a generator ( 13th ), the one with the winding ( 12th ) is connected to provide an electrical signal to them,
• - a circuit for measuring the in the winding ( 12th ) flowing current ( 14th ),
• - a processing circuit ( 15th ), which provides an error signal when the current in the winding ( 12th ) exceeds a certain threshold.

The device comprises a control circuit ( 22nd ), which in case of saturation of the magnetic circuit ( 11 ) or a failure of the generator ( 13th ) a second error signal ( 25th ) delivers. The invention also relates to a differential protection device and the assembly of several differential protection devices.

• - mindestens einen Summenstromwandler, dessen Primärseite durch Leiter des Niederspannungsstromkreises gebildet ist und dessen Sekundärseite eine Sekundärwicklung aufweist,
• - einen Messwiderstand, der in Serie zur Sekundärwicklung geschaltet ist,
• - eine Erregungsschaltung, die parallel zur Serienschaltung aus Messwiderstand und Sekundärwicklung geschaltet ist und die Serienschaltung mit einer Spannung versorgt,
• - eine Sättigungsdetektorschaltung, die eingangsseitig parallel zum Messwiderstand angeschlossen ist und ausgangsseitig mit der Erregungsschaltung verbunden ist, die bei Erreichen eines ersten Spannungsschwellwertes ein Umpolungssignal an die Erregungsschaltung sendet,
• - ein erstes Filter, der eingangsseitig mit dem Messwiderstand verbunden ist und einen gefilterten Differenzstrom abgibt,
• - einer Steuerungseinheit, der das Signal des ersten Filters zugeführt wird, und die derart ausgestaltet ist, dass bei Erreichen eines Differenzstromgrenzwertes ein Fehlerstromerkennungssignal abgegeben wird. Die Steuerungseinheit ist mit der Erregungsschaltung verbunden, so dass die Höhe der Spannung der Erregungsschaltung änderbar ist.

The German Offenlegungsschrift DE 10 2018 204 047 A1 discloses a residual current device for a low voltage circuit, comprising:

• - At least one summation current transformer, the primary side of which is formed by the conductors of the low-voltage circuit and the secondary side of which has a secondary winding,
• - a measuring resistor connected in series with the secondary winding,
• - an excitation circuit that is connected in parallel to the series circuit comprising the measuring resistor and secondary winding and supplies the series circuit with a voltage,
• - a saturation detector circuit, which is connected on the input side in parallel to the measuring resistor and on the output side is connected to the excitation circuit, which sends a polarity reversal signal to the excitation circuit when a first voltage threshold value is reached,
• - a first filter, which is connected on the input side to the measuring resistor and emits a filtered differential current,
• a control unit to which the signal from the first filter is fed and which is designed in such a way that a fault current detection signal is emitted when a differential current limit value is reached. The control unit is connected to the excitation circuit so that the level of the voltage of the excitation circuit can be changed.

The object of the present invention is to improve a residual current protection unit, in particular a residual current protection unit for detecting DC residual currents, in particular type B or B + residual current protection units, in order to avoid false tripping in particular.

This object is achieved by a residual current protection unit with the features of claim 1 or by a method according to claim 16.

• - mindestens einen Summenstromwandler, dessen Primärseite durch Leiter des Niederspannungsstromkreises gebildet ist und dessen Sekundärseite eine Sekundärwicklung aufweist,
• - einem Messwiderstand, der in Serie zur Sekundärwicklung geschaltet ist,
• - eine Erregungsschaltung, die parallel zur Serienschaltung aus Messwiderstand und Sekundärwicklung geschaltet ist und die Serienschaltung mit einer Erregungsspannung versorgt, die einen Erregungsstrom durch die Sekundärwicklung erzeugt,
• - einem ersten Filter, das eingangsseitig mit dem Messwiderstand verbunden ist und einen gefilterten Differenzstrom abgibt,
• - ein zweites Filter, das eingangsseitig parallel zum Messwiderstand angeschlossen ist und ausgangsseitig mit einer Sättigungsdetektorschaltung verbunden ist, die ausgangsseitig mit der Erregungsschaltung verbunden ist, wobei die Sättigungsdetektorschaltung bei Erreichen eines ersten Schwellwertes des Betrages des durch die Sekundärwicklung fließenden Erregungsstromes ein Umpolungssignal an die Erregungsschaltung sendet, der erste Schwellwert kann hierbei insbesondere entweder ein Stromschwellwert sein oder ein Spannungsschwellwert, der der Höhe des Stromes entspricht, beispielsweise indem der Strom durch einen (proportionalen) Spannungsabfall über einen Widerstand ermittelt wird,
• - einer Steuerungseinheit, der das Signal des ersten Filters bzw. der gefilterte Differenzstrom zugeführt wird, und die derart ausgestaltet ist, dass bei Erreichen eines Differenzstromgrenzwertes ein Fehlerstromerkennungssignal abgegeben wird.

According to the invention, a residual current protection unit for a low-voltage circuit is provided, having:

• - At least one summation current transformer, the primary side of which is formed by the conductors of the low-voltage circuit and the secondary side of which has a secondary winding,
• - a measuring resistor connected in series with the secondary winding,
• - an excitation circuit which is connected in parallel to the series circuit comprising the measuring resistor and secondary winding and supplies the series circuit with an excitation voltage that generates an excitation current through the secondary winding,
• - a first filter, which is connected on the input side to the measuring resistor and emits a filtered differential current,
• - a second filter, which is connected on the input side in parallel to the measuring resistor and on the output side is connected to a saturation detector circuit which is connected on the output side to the excitation circuit, the saturation detector circuit sending a polarity reversal signal to the excitation circuit when a first threshold value of the amount of the excitation current flowing through the secondary winding is reached, the first threshold value can in particular either be a current threshold value or a voltage threshold value that corresponds to the magnitude of the current, for example by determining the current by means of a (proportional) voltage drop across a resistor,
• a control unit to which the signal of the first filter or the filtered differential current is fed and which is designed in such a way that a fault current detection signal is emitted when a differential current limit value is reached.

According to the invention, the control unit is connected to the excitation circuit, so that the control unit detects the status of the excitation circuit or the excitation voltage, in particular its polarity. According to the invention, the control unit is also connected to the saturation detector circuit, in particular in such a way that a signal can be transmitted from the control unit to the saturation detector circuit.

This has the particular advantage that this unit can be used to change the excitation frequency in a targeted manner, in particular periodically. In this way, detection errors that can occur at certain frequencies can be averaged out or avoided. Tripping errors are thus avoided.

Advantageous refinements of the invention are specified in the subclaims.

In an advantageous embodiment of the invention, a third filter, which is in particular a high-pass filter, is provided between the control unit and the saturation detector circuit.

This has the particular advantage that there is in particular a DC voltage separation between the control unit and the saturation detector circuit.

In an advantageous embodiment of the invention, the residual current protection unit is designed in such a way that the control unit emits a control signal to the saturation detector circuit in order to generate a polarity reversal signal to the excitation circuit.

This has the particular advantage that there is a simple solution for generating a polarity reversal. For example, the control signal can be fed in parallel to the signal from the measuring resistor or the second filter of the saturation detector circuit, which can be designed as a comparator.

In an advantageous embodiment of the invention, the residual current protection unit is designed in such a way that the control unit uses the control signal to generate the polarity reversal signal of the saturation detector circuit in such a way that a phase shift of the excitation voltage, in particular based on the beginning or polarity change of the previous excitation voltage, is generated.

This has the particular advantage that detection errors can be avoided in a targeted manner.

In an advantageous embodiment of the invention, a phase shift of plus or minus 90 ° is generated.

This has the particular advantage that, in total, a phase behavior is generated so that detection errors are maximally averaged out.

In an advantageous embodiment of the invention, the residual current protection unit is designed in such a way that a half-period is defined by the achievement of two successive first threshold values of the amount of the excitation current through the secondary winding.

The control signal is generated after one eighth of the period of the excitation current, in particular after one eighth of the period after the first threshold value has been detected, so that the polarity of the excitation voltage is reversed, so that the polarity of the excitation current flowing through the secondary winding is reversed.

This has the particular advantage that detection errors are identified particularly well.

In an advantageous embodiment of the invention, when the first threshold value of the excitation current with a positive value or sign is exceeded, an excitation voltage, subsequently an excitation current, with a negative value or sign and vice versa is generated.

This has the particular advantage that there is a clear change in polarity.

In an advantageous embodiment of the invention, the first filter is followed by an amplifier, so that the amplified signal of the first filter, i.e. the filtered differential current, is fed to the control unit.

This has the particular advantage that the level of the differential current signal is matched or amplified.

In an advantageous embodiment of the invention, the signal of the first filter, i.e. the filtered differential current, is fed to an offset adder before it reaches the control unit.

This has the particular advantage that an offset can be added in order, for example, to convert the negative component of the signal with an analog-digital converter.

For example, you can generally add 1.65 volts, because a standard analog-digital converter or control unit with a microcontroller with an analog-digital converter input can convert voltage values between 0 and 3.3 volts. The added offset 1.65 V is then of course subtracted from the calculation of the (DC) fault current in the control unit, for example in its firmware.

In an advantageous embodiment of the invention, the control unit has an analog-digital converter to which the signal of the first filter, i.e. the filtered differential current, is fed.

This has the particular advantage that the differential current signal can be digitally processed, which enables greater flexibility.

In an advantageous embodiment of the invention, the control unit is followed by an interrupt unit to which the fault current detection signal is fed. This enables the current flow of the low-voltage circuit or the conductors of the low-voltage circuit to be interrupted.

This has the particular advantage that if a fault current detection signal is present, the low-voltage circuit is interrupted.

In an advantageous embodiment of the invention, by reversing the polarity of the excitation voltage, in particular direct voltage, an alternately positive or negative voltage is generated, in particular an approximately square-wave voltage, in particular with changing period duration or alternating period duration.

This has the particular advantage that it enables the core to pass through saturation cleanly.

In an advantageous embodiment of the invention, the control unit is designed in such a way that the effective value formation of the filtered differential current is carried out. The effective value is compared with the residual current limit value. If the limit is exceeded, the residual current detection signal is issued.

This has the particular advantage that a particularly simple and reliable fault current detection is made possible, in which short-term differential current peaks are averaged out.

In an advantageous embodiment of the invention, a fault current detection signal is only emitted when it is exceeded twice, three, four, five or more times.

This has the particular advantage that a further reduction in false tripping is achieved, since an arc fault detection signal is only emitted when the differential current value is consecutively exceeded several times.

In an advantageous embodiment of the invention, the residual current protection unit is arranged in a housing in which the interruption unit is arranged internally or externally.

This has the particular advantage that there is a compact residual current circuit breaker in the case of an internal interruption unit.

In the case of an external interruption unit, for example in another switching device, such as a line circuit breaker or circuit breaker, to which the fault current detection signal is fed, the functionality of this switching device can advantageously be increased, for example in that the fault current protection unit is designed as a module, in particular for the switching device.

According to the invention, a corresponding method for detecting direct or alternating fault currents in a low-voltage circuit is also claimed, with a summation current transformer whose primary side is formed by conductors of the low-voltage circuit and whose secondary side has a secondary winding which emits a differential current,
a measuring resistor connected in series with the secondary winding,
that the series circuit of measuring resistor and secondary winding is subjected to an excitation voltage, in particular direct voltage, to generate an excitation current flowing through the secondary winding,
that when a first voltage threshold value is reached at the measuring resistor or the first current threshold value, the polarity of the excitation voltage is inverted (so that a square-wave voltage is generated)
that the voltage of the measuring resistor is filtered, in particular low-pass filtered, and is fed to a control unit which emits a fault current detection signal when a differential current limit value is reached.

According to the invention, a half-period is determined by reaching two successive first threshold values by the amount of the secondary winding ( N2 ) flowing excitation current.

A polarity reversal is generated after one eighth of the period of the excitation current, in particular after one eighth of the period after the first threshold value of the amount of the excitation current has been reached, so that the excitation current flowing through the secondary winding is reversed.

All configurations, both in dependent form based on patent claim 1 or 16, as well as based solely on individual features or combinations of features of patent claims, bring about an improvement in a residual current protection unit, in particular to avoid false tripping.

The described properties, features and advantages of this invention and the manner in which they are achieved will become clearer and more clearly understandable in connection with the following description of the exemplary embodiments, which are explained in more detail in connection with the drawing.

• 1 zeigt ein erstes Blockschaltbild einer Fehlerstromschutzeinheit gemäß dem Stand der Technik,
• 2 zeigt ein zweites Blockschaltbild einer erfindungsgemäßen Fehlerstromschutzeinheit,
• 3 zeigt erste Zeitverläufe elektrischer Größen im Rahmen der Erfindung,
• 4 zeigt zweite Zeitverläufe elektrischer Größen im Rahmen der Erfindung,
• 5 zeigt dritte Zeitverläufe elektrischer Größen im Rahmen der Erfindung.

The drawing shows:

• 1 shows a first block diagram of a residual current protection unit according to the prior art,
• 2 shows a second block diagram of a residual current protection unit according to the invention,
• 3 shows first time curves of electrical quantities within the scope of the invention,
• 4th shows second time courses of electrical quantities within the scope of the invention,
• 5 shows third time profiles of electrical quantities within the scope of the invention.

• - mindestens einen Summenstromwandler ZCT, dessen Primärseite N1 durch einen Phasenleiter L und einen Neutralleiter N gebildet ist. Im Fall ein Drei-Phasen Drehstromnetz mit 3 Phasen- bzw. Außenleitern können die Phasen- (und ggfs. der Neutralleiter N) durch den Kern des Summenstromwandlers ZCT geführt sein. Der Kern weist sekundärseitig eine Sekundärwicklung bzw. Sekundärwindung N2 auf;
• - einem Messwiderstand R, der in Serie zur Sekundärwicklung N2 geschaltet ist, im Beispiel ist er einerseits mit einem ersten Ende der Sekundärwicklung verbunden, andererseits mit einer gemeinsamen Masseverbindung;
• - eine Erregungsschaltung EC, die parallel zur Serienschaltung aus Messwiderstand R und Sekundärwicklung N2 geschaltet ist, im Beispiel ist sie einerseits mit dem zweiten Ende der Sekundärwicklung N2 verbunden und andererseits mit der gemeinsamen Masseverbindung.

1 shows a block diagram of a residual current protection unit, comprising:

• - at least one summation current transformer ZCT , its primary side N1 is formed by a phase conductor L and a neutral conductor N. In the case of a three-phase network with 3 phase or outer conductors, the phase (and, if applicable, the neutral conductor N) can go through the core of the summation current transformer ZCT be led. The core has a secondary winding or secondary winding on the secondary side N2 on;
• - a measuring resistor R in series with the secondary winding N2 is connected, in the example it is connected on the one hand to a first end of the secondary winding, on the other hand to a common ground connection;
• - An excitation circuit EC, which is parallel to the series circuit of measuring resistor R and secondary winding N2 is connected, in the example it is on the one hand with the second end of the secondary winding N2 connected and on the other hand to the common ground connection.

• - eine Sättigungsdetektorschaltung SD, die eingangsseitig parallel zum Messwiderstand R angeschlossen ist, einerseits mit dem Verbindungspunkt zwischen Messwiderstand und ersten Ende der Sekundärwicklung N2 und anderseits mit der gemeinsamen Masseverbindung. Ausgangsseitig ist die Sättigungsdetektorschaltung SD mit der Erregungsschaltung EC verbunden. Bei Erreichen eines ersten Spannungsschwellwertes, über dem Messwiderstand R, wird vom Sättigungsdetektor SD ein Umpolungssignal an die Erregungsschaltung EC gesendet, die die Polarität der insbesondere Gleichspannung als Erregungsspannung ändert;
• - der Sättigungsdetektorschaltung SD kann ein zweites Filter HP, insbesondere Hochpassfilter, vorgeschaltet sein, wie dargestellt, d.h. es befindet sich zwischen dem Verbindungspunkt, von Messwiderstand R und Sekundärwicklung N2, und der Sättigungsdetektorschaltung SD, ferner ist es mit dem gemeinsamen Masseanschluss verbunden;
• - ein erstes Filter LP, insbesondere ein Tiefpass, das im Beispiel durch zwei Filtereinheiten LPF1, LPF2 realisiert ist. Das erste Filter ist eingangsseitig mit dem Messwiderstand R verbunden, einerseits mit dem Verbindungspunkt zwischen Messwiderstand R und ersten Ende der Sekundärwicklung N2 und andererseits mit der gemeinsamen Masseverbindung. Das erste Filter LP gibt einen gefilterten Differenzstrom bzw. Differenzstromsignal VF ab. Die Grenzfrequenz des im Beispiel Tiefpassfilters sollte vorteilhafterweise viel kleiner sein, als die Frequenz der Erregungsspannung, damit insbesondere das Erregungssignal ausreichend gefiltert wird. Die Frequenz der Erregungsspannung ist im Beispiel im Bereich von 2 - 6 kHz; könnte aber auch höher sein. Die Grenzfrequenz des Tiefpasses kann im Bereich 100 - 1000 Hz liegen, insbesondere bei 300 Hz; muss jedoch immer mindestens zweimal kleiner sein wie die Erregungsfrequenz.

The excitation circuit supplies this series circuit with an excitation voltage that generates an excitation current through the secondary winding N2 causes, which also flows through the measuring resistor R; the level of the excitation current can be determined from the associated voltage drop across the measuring resistor R;

• - A saturation detector circuit SD, which is connected on the input side in parallel to the measuring resistor R, on the one hand with the connection point between the measuring resistor and the first end of the secondary winding N2 and on the other hand with the common ground connection. On the output side, the saturation detector circuit SD is connected to the excitation circuit EC. When a first voltage threshold value is reached above the measuring resistor R, the saturation detector SD sends a polarity reversal signal to the excitation circuit EC, which changes the polarity of the, in particular, direct voltage as the excitation voltage;
• A second filter HP, in particular a high-pass filter, can be connected upstream of the saturation detector circuit SD, as shown, ie it is located between the connection point of the measuring resistor R and the secondary winding N2 , and the saturation detection circuit SD, further connected to the common ground terminal;
• - A first filter LP, in particular a low pass, which is implemented in the example by two filter units LPF1, LPF2. The first filter is connected on the input side to the measuring resistor R, on the one hand to the connection point between the measuring resistor R and the first end of the secondary winding N2 and on the other hand with the common ground connection. The first filter LP outputs a filtered differential current or differential current signal VF. The cut-off frequency of the low-pass filter in the example should advantageously be much smaller than the frequency of the excitation voltage, so that in particular the excitation signal is sufficiently filtered. In the example, the frequency of the excitation voltage is in the range of 2 - 6 kHz; but could also be higher. The cutoff frequency of the low pass can be in the range 100-1000 Hz, in particular at 300 Hz; however, it must always be at least twice as small as the excitation frequency.

In the example, an amplifier Amp is connected downstream of the first filter LP to amplify the filtered signal VF. The amplifier Amp is i.B. an offset adder OA connected downstream for adding an offset value to the filtered or amplified differential current signal. A control unit SE is provided, for example with a microprocessor MCU, to which the signal of the first filter, possibly amplified and / or offset-added, is supplied. On the input side, the differential current signal can be converted into the digital range by an internal or external analog-digital converter ADC. The control unit SE is designed in such a way that the possibly digitally converted differential current signal is compared with a differential current limit value. If the limit is exceeded, a fault current detection signal FES is issued. For example, to an internally or externally arranged interruption unit TU, which can have contacts for interrupting the low-voltage circuit, in particular its conductors L, N. Here, at least some of the conductors can be interrupted. Advantageously, all conductors L, N can be interrupted in order to achieve galvanic separation.

The control unit SE can also be designed in such a way that an effective value calculation of the supplied differential current signal is carried out, for example by an effective value calculation unit or function. This effective value is compared with a differential current threshold value, for example with a comparator unit or function. In addition, an exceedance can be carried out with regard to the presence of two, in particular successive, effective values being exceeded. If the differential current threshold value is not reached, the next differential current value is used for the comparison, for example by a return unit or function.

If the differential current value is exceeded, a counter can be incremented, for example with a counting unit or function. A fault current detection signal is emitted if the values are exceeded twice, three times or, depending on the definition, n times.

This fault current detection signal FES is fed in particular to the internal or external interruption unit TU.

A connection is shown between the control unit SE and the excitation circuit EC. This can have a frequency unit FC. This could be used to change the frequency of the excitation voltage.

The duration of the curve of the core of the summation current transformer to saturation is influenced by the level of the excitation voltage. With a low voltage the curve is traversed more slowly, with a higher voltage the curve is traversed faster. When saturation is reached, which is detected by the saturation detector circuit or saturation detector SD, for example when the voltage across the measuring resistor reaches a first voltage threshold value or the excitation current reaches a first current threshold value, the polarity of the excitation voltage, in particular DC voltage, is reversed and the value is repeated Curve begins. The frequency of this voltage, ideally a square-wave voltage, is determined by the level of the voltage. This also determines the frequency of the secondary differential current, which consists of the primary DC residual current or AC residual current translated by the summation current transformer and the superimposed excitation voltage.

In the example according to 1 a fault caused by a lightning strike is shown between the phase conductor L and earth Earth. As a result, a differential current Id or id flows through the summation current transformer ZCT is detected and a current Icomp in the secondary winding N2 generated. In addition, there is the excitation voltage of the excitation circuit EC, which generates an excitation current Iexc through the secondary winding N2 generated.

After the first filter LP, both are present as a filtered error voltage or filtered differential current signal VF and are optionally amplified and / or fed to the control unit SE or microprocessor MCU with offset. The microprocessor MCU carries out an evaluation as described.

2 shows an arrangement according to 1 , with the difference that the frequency unit FC is not provided. A connection is provided between the excitation circuit EC and the control unit SE, so that the control unit SE detects the status of the excitation circuit EC or the excitation voltage. Furthermore, the control unit SE is connected to the saturation detector circuit SD, in particular via a third filter HP2, in order to generate an (artificial) saturation signal for the saturation detector SD by means of a control signal Vc in order to subsequently generate a polarity reversal by the excitation circuit EC.

The third filter HP2 is, for example, a high-pass filter.

According to the invention, the saturation detector SD can be designed, for example, as a comparator. The (voltage) signal fed to the comparator is compared with the first threshold value, and when it is exceeded, a polarity reversal signal, for example a direct voltage with a first value or sign, is fed to the excitation circuit EC. The excitation circuit can do this, for example, directly or amplified as an excitation voltage for the secondary winding N2 use.

According to the invention, for example, the voltage across the measuring resistor R, especially high-pass filtered through the second filter HP, and the control signal Vc, especially high-pass filtered, especially through the third filter HP2, can be connected in parallel to the control unit SE before or in the saturation detector circuit SD.

Functionally, the control unit SE outputs a control signal Vc, in particular via the third filter HP2, to the saturation detector circuit SD. The saturation detector circuit SD then emits a polarity reversal signal to the excitation circuit EC, so that, for example, the polarity of the DC voltage is reversed as the excitation voltage.

According to the invention, the polarity reversal signal of the saturation detector circuit SD is generated by means of the control unit SE and the control signal Vc in such a way that a phase shift of the excitation voltage, in particular based on the beginning of the previous excitation voltage or the excitation current, is generated.

In particular, a phase shift of plus or minus 90 ° is generated.

This can be done, for example, by the fact that a period half-duration is achieved by reaching two successive first threshold values (by the amount of the secondary winding ( N2 ) flowing excitation current Iexc) is defined. This half-period can be recorded by measurement, for example by measuring the time interval between the occurrence of the exceeding of two successive threshold values (the measurement of the voltage across the measuring resistor R).

The control signal Vc is generated in particular after one eighth of the period or period duration of the excitation current, in particular after one eighth of the period after detection of the reaching of the first threshold value (by the amount of the excitation current), so that a polarity reversal of the excitation voltage or as a result of the through the secondary winding N2 flowing excitation current Iexc takes place.

When the first threshold value is exceeded by the excitation current Iexc with a positive value or sign, an excitation voltage or an excitation current with a negative value or sign and vice versa is generated by polarity reversal.

According to the invention, the switchover takes place after an eighth of the period in order to generate a 90 ° phase shift in such a way that after a (regular) polarity reversal of the excitation voltage (exceeding the first threshold value due to saturation of the core of the summation current transformer), the excitation current flows in the opposite direction, after an eighth, ie 45 °, there is a polarity reversal according to the invention, after again an eighth, ie 45 °, the core is again in the (previous) saturation (first threshold value of the excitation current exceeded), whereby a (regular) polarity reversal of the excitation voltage takes place. As a result, a 90 ° phase shift of the excitation voltage is generated by the two one-eighth long periods (45 °).

According to the invention, this process is repeated periodically.

3 shows five characteristic curves over time. The first, uppermost characteristic curve shows the level of the excitation voltage VL for the secondary winding N2 over time. The excitation voltage that is generated (or amplified) by the excitation circuit EC is a square-wave voltage formed from a direct voltage, in particular with a constant period duration or partial period duration.

The second characteristic curve shows the course of the excitation current Iexc through the secondary winding over time. Due to the core of the summation current transformer, the excitation current exhibits such a current impulse or “jagged” behavior over time.

This characteristic is synchronized with the first characteristic. At the time of the cycle change of the first characteristic, a current pulse or current excursion occurs.

At the level of the second characteristic curve, there is a parallel diagram of a hysteresis curve, a coil with a core, like the summation current transformer ZCT , displayed synchronized.

The magnetic flux density B (T) is shown on the horizontal X-axis of the hysteresis curve and the magnetic field strength H (A / m) is shown on the vertical Y-axis. The diagram shows a common hysteresis curve. The change in magnetization or the end of magnetization (maximum of the magnetic flux density reached) correspond to the magnitude of the current pulses or current excursions of the excitation current Iexc at the respective point in time.

verknüpft, wie eingezeichnet (bzw. dessen negatives Pendant: -H = N2 × Iexc/LFE )
wobei LFE die Induktivität der Sekundärwicklung N2 mit (Eisen-)Kern ist.The magnetic field strength H (A / m) is related to the excitation current Iexc via the relationship: $$\text{H}=\text{N2}\times \text{Iexc / LFE}$$

linked as shown (or its negative counterpart: -H = N2 × Iexc / LFE)
where LFE is the inductance of the secondary winding N2 with (iron) core.

The third characteristic shows the course of a fault current Id over time. A DC residual current is shown in the example. This has a constant current value over time.

The fourth characteristic curve shows the course of the sum of the excitation current Iexc and the secondary differential current Icomp (this "compensated" differential current Icomp in the secondary winding is determined by the primary Fault current Id generated), as it is at the secondary winding N2 is present over time. A DC residual current is shown in the example. Analogous to this, as in the second characteristic curve, the diagram of the hysteresis curve.

wie eingezeichnet, wobei N1 die Windungszahl der Primärseite ist (oft N1 = 1) und N2 hierbei die Windungszahl der Sekundärwicklung N2 angibt.The fault current Id or id causes the fourth characteristic to be shifted upwards compared to the second characteristic by the amount: $$\text{N1}\times \text{Id / LFE}=\text{N2}\times \text{Icom / LFE}$$

as shown, where N1 is the number of turns on the primary side (often N1 = 1) and N2 is the number of turns of the secondary winding N2 indicates.

The fifth, lowest characteristic curve shows the course of the resulting filtered difference signal VF over time.

3 shows a normal course without any further effects.

In the following, the invention is explained in more detail once again in other words with reference to the other figures.

A residual current device, in particular of the RCD type B, uses a special circuit to detect and convert differential DC currents. An excitation circuit EC applies an excitation voltage to the secondary winding N2 of the current transformer ZCT at. The inductance of the coil with core limits the rate of rise of the excitation current Iexc until the permeability drops and the magnetic core is saturated. The high rate of rise of the excitation current Iexc at saturation is detected with the second filter HP, for example a high-pass filter, and the saturation detector circuit SD, for example a comparator. When the core is in saturation, the excitation polarity of the excitation voltage is inverted. This leads to a periodic excitation voltage with the following frequency: $$\text{Fexc}=\text{Vexc /}\left(\text{4 * N2 * Bs * A}\right)$$

The excitation frequency Fexc depends on the excitation voltage Vexc, the cross section A of the core, the saturation flux density Bs and the number of secondary turns N2 away.

In the event of a differential current id in the primary circuit, the excitation circuit adjusts the excitation voltage in order to compensate for the value of the differential current. The reference is the magnetic hysteresis of the core, which is symmetrical. The compensated current has the following value: $$\text{Icomp}=\text{N1}*\text{id}/\text{N2}$$

The current through the secondary winding N2 flows through a measuring resistor R and there generates a voltage drop Vs. This is proportional to the compensated current (differential current) and the excitation current in the secondary winding N2 . The excitation signal, ie the excitation current, is strongly attenuated with the first filter LP, for example a low-pass filter.

After the filter, a (filtered) differential current signal VF is available, which if necessary is amplified, digitized and evaluated or analyzed by the control unit SE in order to determine the value of the fault current. $$\text{Id}=\text{N2}*\text{VF}/\left(\text{N1}*\text{R.}\right)$$

A remaining signal with the frequency Fexc is still present in the differential current signal, since the first filter LP cannot remove the excitation voltage 100%. The design of the first filter LP and the excitation circuit EC must ensure that its contribution is negligible.

This residual current protection unit correctly converts the differential current, the frequency of which is lower than the frequency Fexc of the excitation voltage. A residual current device for the RCD type B has a Defined tripping characteristic for a residual current from 0 Hz to 1000 Hz (standard EN 62423), therefore Fexc must be greater than 1000 Hz. It is generally in the range from 2.5 kHz to 3.5 kHz.

In the case of residual currents with a frequency above 1000 Hz, the tripping behavior is not defined. However, it is recommended to implement the RCD with low sensitivity in order to avoid tripping operation in applications in which fault currents (or leakage currents) in the range of many kHz (e.g. due to frequency converters) are expected for technical reasons. Such a residual current protection unit should not trip with a leakage current of 500 mA in such frequency ranges. In addition, there is an EMC test (EN 61543, Part T.2.6) in which the FI circuit breaker does not trigger with a differential current at 50 Hz with a value of 30% of the rated residual current and an interference current of up to 150 kHz and 66mA see table below. Table: HF test current according to EN 61543, part T.2.6 Frequency range RMS (root mean square) current value for level 2 l _Δn <30 mA RMS (root mean square) current value for level 3 l _Δn ≥ 30 mA 1 kHz to 1.5 kHz 2 mA ^a) 6.6 mA ^a ) 1.5 kHz to 15 kHz 2 mA to 20 mA ^b) 6.6 mA to 66 mA ^b) 15 kHz to 150 kHz 20 mA ^a) 66 mA ^{a) a)} Constant current value over the entire range. ^b) The current value increases from 1.5 kHz to 15 kHz at 20 dB / decade.

This system has the disadvantage that if there is a fault current (or leakage current) with twice or n times twice the excitation frequency (n is a natural number), there is a disadvantageous synchronization between the peaks of the excitation current and the peaks of the differential current, which leads to an aliasing-like behavior.

4th shows such behavior. 4th shows characteristics according to 3 , with the difference that four of the five characteristics are shown, the second to fifth characteristics. Furthermore, in the third characteristic (numbering according to 3 ) the fault current Id is an AC fault current with a sinusoidal time curve.

This results in a course of the sum of excitation current Iexc and differential current Icomp in the fourth characteristic curve, as it is at the secondary winding N2 is present, over time with certain peaks or peaks as well as notches or dips, as shown.

As in 4th shown, the current peak at saturation does not always have the same level. They differ in the case of a positive or negative half-wave of the differential current. The reason is the dependence between the rate of rise of the current and the rate of change of the permeability in the saturation region. This leads to a DC fault current signal at the output of the first filter LP, which is interpreted by the control unit SE as a DC fault current. This can trigger the residual current device. This aliasing effect leads to a low-frequency differential signal when an HF interference current is present.

It is possible according to the excitation voltage 1 to change, in the case of the detection of a differential signal, in order to avoid synchronization between the excitation signal and the differential current. This frequency change works well for normal FI protective devices with a rated residual current Idn of greater than or equal to 30 mA. For very sensitive residual current protection units, such as for inline charging cables for vehicles or residual current protection units with a sensitivity level of 6 mA, this is often no longer sufficient. Often because there is a high probability that a frequency, for example in the range of up to 150 KHz, will be found which leads to a disadvantageous synchronization for both excitation frequencies.

5 shows characteristics according to 3 respectively. 4th , with the difference that a sixth characteristic curve of the control signal Vc over time is shown in the first, uppermost position.

Furthermore, a sinusoidal alternating fault current Id is shown in the fourth characteristic curve. Furthermore, that the invention was applied, which can be seen in the curves.

By generating a phase jump in the excitation voltage VL according to the second characteristic in 5 are the current peaks of the excitation current Iexc of the third characteristic curve at other times. This results in the fifth characteristic curve of the sum of excitation current Iexc and differential current Icomp that the current peaks are added to the current in such a way that they compensate each other on average, in particular over several periods, in particular over six half-periods or three full periods.

By controlling the synchronization of the excitation signal and the fault current / differential current, the effect of the aliasing phenomenon is to be reduced. An additional excitation voltage change can be generated by the saturation detector through normal saturation of the magnetic core and additionally through a control signal Vc of the control unit SE (in particular with the third filter, in particular the high-pass filter) (according to FIG 5 ). The actual status of the excitation voltage is detected by the control unit SE, and the control unit SE can trigger the saturation detector SD in order to generate or introduce a phase shift of -90 ° or + 90 ° in the excitation voltage or in the excitation current Iexc.

This phase shift leads to a new synchronization between the excitation current Iexc and the differential current Icomp, but with the difference that the effect of the aliasing phenomenon is inverted every half cycle of the control signal, according to FIG 5 . As a result of the control signal Vc, this leads to a remaining signal at the output of the first filter (especially low-pass filter) with 2/3 of the original excitation frequency, which can be easily filtered and does not generate a triggering process.

The control signal Vc is output by the control unit SE (in particular MCU) in order to trigger the saturation detector SD via an (HP) filter.

• 1. Die halbe Periode des Erregungssignals wird zwischen zwei Stromspitzen - Überschreitung des ersten Schwellwertes - gemessen (Ausgabe der Zustandsänderungen des Sättigungsdetektors). Die Periode bzw. Periodendauer $$\text{Texc}=\text{1 / Fexc}$$
  
  kann (daraus) ermittelt werden.
• 2) Das Steuersignal Vc (bzw. die Steuerspannung Vc) wechselt zur Zeit Texc / 8, also nach einem Achtel der Periodendauer, nach Erfassung der Stromspitze (Überschreitung des ersten Schwellwertes). Die Umschaltung ist von niedrig nach hoch, wenn die Stromspitze (Überschreitung des ersten Schwellwertes) bzw. der Peak negativ war (negatives Vorzeichen bzw. Wert), oder von hoch nach niedrig, wenn die Stromspitze (Überschreitung des ersten Schwellwertes) bzw. der Peak positiv war (positives Vorzeichen bzw. Wert).
• 3) Ein neue Stromspitze (Überschreitung des ersten Schwellwertes) bzw. Peak mit demselben Vorzeichen wie der letzte Peak tritt auf. Die Zeit zwischen beiden Peaks (Überschreitung des ersten Schwellwertes) beträgt dann Texc / 4 und entspricht der Zielphasenverschiebung von -90°.
• 4) Die nächste Stromspitze (Überschreitung des ersten Schwellwertes) bzw. der nächste Peak hat eine entgegengesetzte Polarität und eine neue Messung der Erregungshalbperiode wird durchgeführt, wodurch ein aktualisierter Wert von Texc erhalten wird.
• 5) Das Steuersignal Vc (Steuerspannung Vc) wird zum Zeitpunkt Texc / 8 wie in Schritt 2 erneut umgeschaltet.

The sequence can be:

• 1. Half the period of the excitation signal is measured between two current peaks - the first threshold value is exceeded - (output of the changes in state of the saturation detector). The period or period duration $$\text{Texc}=\text{1 / Fexc}$$
  
  can be determined (from this).
• 2) The control signal Vc (or the control voltage Vc) changes at the time Texc / 8, i.e. after one eighth of the period, after the current peak has been detected (the first threshold value has been exceeded). The switchover is from low to high when the current peak (exceeding the first threshold value) or the peak was negative (negative sign or value), or from high to low when the current peak (exceeding the first threshold value) or the peak was positive (positive sign or value).
• 3) A new current peak (exceeding the first threshold value) or peak with the same sign as the last peak occurs. The time between the two peaks (exceeding the first threshold value) is then Texc / 4 and corresponds to the target phase shift of -90 °.
• 4) The next current peak (exceeding the first threshold value) or the next peak has an opposite polarity and a new measurement of the excitation half-cycle is carried out, whereby an updated value of Texc is obtained.
• 5) The control signal Vc (control voltage Vc) is switched again at the time Texc / 8 as in step 2.

A control signal Vc with a frequency of 2/3 of the original excitation frequency can thus be achieved. This can easily be filtered.

This is one way of avoiding fault current detection. If an aliasing-like effect (disadvantageous synchronization between excitation signal and leakage current) cannot be avoided by changing or switching the excitation frequency, its effect can be avoided by controlling a synchronization by periodically introducing a phase shift, in particular of -90 ° or + 90 ° will.

With a residual current protection unit according to the invention, the determination of residual currents is less disturbed by high-frequency signal components and the resistance to false tripping is increased. The invention is particularly advantageous for residual current circuit breakers of type B or B +. The solution is very easy to implement. Usually only small hardware changes are required.

Although the invention has been illustrated and described in more detail by the exemplary embodiment, the invention is not restricted by the disclosed examples and other variations can be derived therefrom by the person skilled in the art without departing from the scope of protection of the invention.

Claims (16)

Residual current protection unit for a low-voltage circuit, comprising: - at least one summation current transformer (ZCT), the primary side of which is formed by the conductor of the low-voltage circuit and the secondary side of which has a secondary winding (N2), - a measuring resistor (R) which is connected in series with the secondary winding (N2) - an excitation circuit (EC), which is connected in parallel to the series circuit of measuring resistor (R) and secondary winding (N2) and supplies the series circuit with an excitation voltage or excitation current, - a first filter (LP), which is connected to the measuring resistor (R ) is connected and emits a filtered differential current (VF), - a control unit (SE) to which the signal from the first filter (LP) is fed and which is designed in such a way that a fault current detection signal (FES) is emitted when a differential current limit value is reached, characterized in that a second filter (HP) is provided, the input side g is connected in parallel to the measuring resistor (R) and is connected on the output side to a saturation detector circuit (SD), which is connected on the output side to the excitation circuit (EC), the saturation detector circuit (SD) when a first threshold value of the amount of the through the secondary winding (N2 ) flowing excitation current (Iexc) sends a polarity reversal signal to the excitation circuit (EC) that the control unit (SE) is connected to the excitation circuit (EC) so that the control unit (SE) detects the status of the excitation circuit (EC) or the excitation voltage that the control unit (SE) is connected to the saturation detector circuit (SD) in such a way that a signal can be transmitted from the control unit (SE) to the saturation detector circuit (SD). Residual current protection unit according to Claim 1 , characterized in that a third filter (HP2) is provided between the control unit (SE) and the saturation detector circuit (SD). Residual current protection unit according to Claim 1 or 2 , characterized in that the residual current protection unit is designed such that the control unit (SE) emits a control signal (Vc) to the saturation detector circuit (SD) to generate a polarity reversal signal to the excitation circuit (EC). Residual current protection unit according to Claim 3 , characterized in that the residual current protection unit is designed such that the control unit (SE) generates the polarity reversal signal of the saturation detector circuit (SD) with the control signal (Vc) in such a way that a phase shift of the excitation voltage is generated. Residual current protection unit according to Claim 4 , characterized in that a phase shift of plus or minus 90 ° is generated. Residual current protection unit according to one of the preceding Claims 1 until 4th , characterized in that the residual current protection unit is designed in such a way that a half-period is defined by the reaching of two successive first threshold values by the amount of the excitation current (Iexc) flowing through the secondary winding (N2), so that the control signal (Vc) after one eighth of the period of the excitation current (Iexc) is generated so that the polarity of the excitation voltage (V _L ) applied to the secondary winding (N2) is reversed. Residual current protection unit according to Claim 6 , characterized in that when the excitation current (Iexc) exceeds the first threshold value with a positive sign, an excitation voltage with a negative sign and vice versa is generated. Residual current protection unit according to one of the preceding claims, characterized in that an amplifier (AMP) is connected downstream of the first filter (LP), so that an amplified filtered differential current is fed to the control unit (SE). Residual current protection unit according to one of the preceding claims, characterized in that the filtered differential current of the first filter (LP) is fed to an offset adder (OA) before it reaches the control unit (SE). Residual current protection unit according to one of the preceding claims, characterized in that the control unit (SE) has an analog-digital converter (ADC) to which the filtered differential current of the first filter (LP) is fed. Residual current protection unit according to one of the preceding claims, characterized in that the control unit (SE) is followed by an interruption unit (TU) to which the residual current detection signal (FES) is fed to interrupt the conductors or the current flow of the low-voltage circuit. Residual current protection unit according to one of the preceding claims, characterized in that an alternating positive or negative voltage is generated by reversing the polarity of the excitation voltage. Residual current protection unit according to one of the preceding claims, characterized in that the control unit (SE) is designed in such a way that an effective value formation (RMS) of the filtered differential current is carried out, that the effective value is compared with the differential current limit value, that the residual current detection signal (FES) is generated if it is exceeded. is delivered. Residual current protection unit according to Claim 13 , characterized in that a fault current detection signal (FES) is only emitted when it is exceeded twice, three, four, five or more times. Residual current protection unit according to one of the preceding claims, characterized in that the residual current protection unit is arranged in a housing in which the interruption unit (TU) is arranged internally or externally. Method for the detection of direct or alternating fault currents in a low-voltage circuit, with a summation current transformer (ZCT) whose primary side is formed by the conductor of the low-voltage circuit and whose secondary side has a secondary winding (N2) that emits a differential current, a measuring resistor (R), which in Is connected in series to the secondary winding (N2) that the series connection of measuring resistor (R) and secondary winding (N2) is acted upon with an excitation voltage to generate an excitation current (Iexc) flowing through the secondary winding, so that when a first threshold value of the excitation current (Iexc ) the polarity of the excitation voltage (VL) is inverted, so that the voltage across the measuring resistor (R) is filtered and fed to a control unit (SE) which, when a differential current limit value is reached, emits a fault current detection signal (FES) that a period half-duration by reaching two successive The first threshold value of the amount of the excitation current (Iexc) flowing through the secondary winding (N2) is characterized , characterized in that a polarity reversal of the excitation voltage is generated after one eighth of the period,

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