Methods and devices for residual current protection monitoring using a single transformer

Abstract

Device (10) for use with an alternating current system comprising a live conductor (L), a neutral conductor (N) and a transformer (12), wherein the live conductor (L) and the neutral conductor (N) are each connected between a source and a load and the neutral conductor (N) is connected to earth at the source, wherein the transformer (12) comprises a first primary winding connected in series with the live conductor (L), a second primary winding connected in series with the neutral conductor and a secondary winding (20), wherein the device comprises: a first circuit connected to the secondary winding, wherein the first circuit (14) provides a first detection signal if a current from the outer conductor (L) to earth exceeds a first predetermined threshold; and a second circuit connected to the secondary winding (20), wherein the second circuit (18) includes an AC signal source (90) which supplies an AC signal with a defined amplitude and a defined frequency to the secondary winding, and wherein the second circuit (18) provides a second detection signal if an impedance between the neutral conductor (N) and earth is less than a second predetermined threshold, wherein the AC signal source (90) comprises an AC voltage source, and wherein the second circuit provides the second detection signal if a current in the secondary winding (20) exceeds a predetermined threshold.

Description

REFERENCE TO RELATED REGISTRATIONS

[0001] This application claims the benefits of the preliminary US patent application filed on May 14, 2009, with serial number 61 / 216,234, which is incorporated herein in its entirety by reference. BACKGROUND

[0002] This application relates generally to methods and devices for monitoring ground fault circuit interrupts (GFCI). In particular, this application relates to methods and devices for GFCI monitoring using a single transformer to monitor earth fault currents and grounded neutral fault currents in electrical current branches of AC systems.

[0003] In single-phase AC power systems, power cables are typically used, containing a live conductor and a neutral conductor connected between a power source and a load, with the neutral conductor grounded at the source. Residual current devices (RCDs) are installed in such circuits to interrupt the power supply upon detection of earth fault currents from the live conductor to earth at the load, as well as fault currents between the neutral conductor and earth at the load (e.g., fault currents due to low-impedance connections). RCDs ensure safety by protecting against electric shock and are primarily used in electrical outlets in kitchens, bathrooms, and outdoor areas where water or moisture could pose a risk of electric shock. RCDs are also used in circuit breakers, which protect these areas.

[0004] Residual current devices (RCDs) typically use a differential current transformer to detect current imbalances in the live and neutral conductors resulting from an earth leakage current from the live conductor returning to the source via an unintended earth fault path other than the neutral conductor. To prevent injury from electric shock, the RCD must trip a circuit breaker when the current difference between the live and neutral conductors is as small as 5 milliamperes.

[0005] If a fault current occurs in the grounded neutral conductor, the residual current transformer may not detect the actual magnitude of the earth leakage current. Specifically, because the neutral conductor is typically grounded at the source, some of the earth leakage current can flow back to the source via the neutral conductor. Consequently, the current differential in the residual current transformer would not accurately reflect the actual magnitude of the earth leakage current. Thus, a fault current in the grounded neutral conductor can reduce the sensitivity of the residual current sensor, causing the residual current device to trip only in response to significantly higher levels of earth leakage current.

[0006] To address this problem, many existing residual current devices (RCDs) use a second transformer on the neutral conductor to detect fault currents from the grounded neutral. In such devices, if a fault current is present through a low-impedance connection between the neutral and ground, the RCD generates an oscillator whose output signal is coupled to the residual current transformer using the second transformer. The oscillator signal is then used to detect fault currents from the grounded neutral. Upon detection of a grounded neutral fault current, the RCD interrupts the current in the AC system. Such dual-transformer RCDs require considerable space to accommodate both transformers and are also correspondingly more expensive due to the additional cost of the second transformer.

[0007] To overcome these disadvantages, some known residual current devices (RCDs) use a single transformer to monitor earth fault currents and grounded neutral fault currents. For example, US Publication US 4,001,646 A, titled "Ground Fault Circuit Interrupter Utilizing a Single Transformer," describes an RCD that uses a single transformer to monitor earth fault currents and grounded neutral fault currents. Specifically, Howell uses a negative resistance network to generate an oscillating signal that increases unless a fault current is present through a low-impedance connection between the neutral and ground. In this respect, Howell's RCD is quite complex and cumbersome.

[0008] Furthermore, existing residual current devices (RCDs) with two transformers and with one transformer typically offer limited control over the magnitude or frequency of the oscillator signals used to monitor grounded neutral fault currents. Consequently, these devices may not provide consistent and reliable earth fault monitoring. Therefore, improved residual current devices are desirable.

[0009] From US patent 4,378,579 A, ​​a residual current protection circuit is known which uses a single differential transformer consisting of a ferrite toroid with a secondary winding of eleven turns.

[0010] From EP 0 069 655 A1 a differential protection device for detecting grounded fault currents in an electrical device is known. SUMMARY

[0011] In a first aspect of the invention, a device for use with an alternating current system is provided, which has a live conductor, a neutral conductor, and a transformer, wherein the live conductor and the neutral conductor are each connected between a source and a load, the neutral conductor at the source is connected to earth, and the transformer has a first primary winding connected in series with the live conductor, a second primary winding connected in series with the neutral conductor, and a secondary winding. The device has a first circuit connected to the secondary winding, wherein the first circuit provides a first detection signal if a current from the live conductor to earth exceeds a first predetermined threshold.The device also features a second circuit connected to the secondary winding, containing an AC signal source that supplies an AC signal with a defined amplitude and frequency to the secondary winding. This second circuit also provides a second detection signal if the impedance between neutral and ground is less than a second predetermined threshold.

[0012] In a second aspect of the invention, a residual current device (RCD) is provided for use with an alternating current system comprising a live conductor, a neutral conductor, and a transformer. The live conductor and the neutral conductor are each connected between a source and a load, the neutral conductor at the source is connected to earth, and the transformer has a first primary winding connected in series with the live conductor, a second primary winding connected in series with the neutral conductor, and a secondary winding. The device includes an earth fault current monitoring circuit connected to the secondary winding. The earth fault current monitoring circuit provides a first detection signal if a current in the secondary winding exceeds a first predetermined threshold.The device also features a grounded neutral fault detector circuit connected to the secondary winding. The grounded neutral fault detector circuit: (a) contains an AC signal source that applies an AC voltage of a defined amplitude and frequency to the secondary winding, (b) monitors a load current in the secondary winding, and (c) provides a second detection signal if the load current exceeds a predetermined threshold.

[0013] In a third aspect of the invention, a method for residual current protection monitoring is provided for use with an AC system comprising a live conductor, a neutral conductor, and a transformer, wherein the live conductor and the neutral conductor are each connected between a source and a load, and the neutral conductor is connected to earth at the source. The transformer has a first primary winding connected in series with the live conductor, a second primary winding connected in series with the neutral conductor, and a secondary winding.The method comprises: (a) for a first predetermined time interval, monitoring a current in the secondary winding and providing a first detection signal if the monitored secondary current exceeds a first predetermined threshold; and (b) for a second predetermined time interval, applying an AC voltage or AC current of a specified amplitude and frequency to the secondary winding, monitoring a load current in or voltage across the secondary winding, and providing a second detection signal if the load current or load exceeds a predetermined threshold or if the load voltage is less than a predetermined threshold.

[0014] Further features and aspects of the present invention will become even clearer from the following detailed description, the attached claims and the associated drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The features of the present invention will become even clearer from the following detailed description, which is given in conjunction with the following drawings, in which the same reference numerals consistently denote the same elements and in which: Fig. 1 a block diagram of an exemplary residual current protection device according to this invention; Fig. 2 a more detailed block diagram of an exemplary residual current protection device according to this invention; Fig. 3 a block diagram of an alternative exemplary residual current protection device according to this invention; Fig. 4 a block diagram of another alternative exemplary residual current protective device according to this invention; and Fig. 5 a block diagram of another alternative exemplary residual current protection device according to this invention. DETAILED DESCRIPTION

[0016] The present invention provides improved residual current devices (RCDs) that use a single transformer to detect earth fault currents and grounded neutral fault currents in an AC system. Specifically, RCDs according to this invention comprise a single transformer, an earth fault current monitoring circuit, and a grounded neutral fault current monitoring circuit. During a first predetermined time interval, the earth fault current monitoring circuit monitors a first current in a secondary winding of the transformer to determine whether an earth leakage current exceeds a first predetermined threshold. If the first threshold is exceeded, the RCD disconnects the AC system.

[0017] After the first predetermined time interval, the residual current monitoring circuit of the grounded neutral conductor applies an AC signal (e.g., an AC voltage) of a defined magnitude and frequency to the secondary winding of the transformer and monitors a signal (e.g., a second current or voltage) in the secondary winding to determine whether the low impedance between earth and neutral is less than a second predetermined threshold. The monitored AC signal can be used to determine the impedance between earth and neutral. If the monitored AC signal indicates that the impedance between earth and neutral is less than the second predetermined threshold, the residual current device interrupts the AC system.

[0018] After the second predetermined time interval, the grounded neutral fault current monitoring circuit stops energizing the transformer's secondary winding, and the earth fault fault current monitoring circuit resumes monitoring the primary current in the secondary winding for another predetermined time interval to detect earth fault currents. This process continues iteratively to continuously detect both earth fault currents and grounded neutral fault currents using a single transformer.

[0019] With reference to Fig. A first exemplary residual current device according to this invention is described in Figure 1. A residual current device 10 comprises a single transformer 12, an earth fault residual current monitoring circuit 14, a residual current monitoring circuit of the earthed neutral conductor 16, and an interruption circuit 18. The transformer 12 is a differential current transformer with a toroidal core and a secondary winding 20. It is clear to the person skilled in the art that other differential current transformers can also be used.

[0020] A live conductor L and a neutral conductor N of an alternating current system run through the toroidal core of transformer 12 as primary windings with one turn each. It is clear to the average person that the live conductor L and the neutral conductor N can also be configured as primary windings of transformer 12 with multiple turns. The live conductor L and the neutral conductor N are each connected between a source ("SOURCE") and a load ("LOAD"), and the neutral conductor N is connected to earth at the SOURCE.

[0021] The secondary winding 20 is connected to an input terminal of the earth fault residual current monitoring circuit 14 and an output terminal of the residual current monitoring circuit of the grounded neutral conductor 16. The secondary winding 20 can be a single winding with N s coils, as in Fig. 1 shown. Alternatively, the secondary winding 20 can be a first secondary winding with N S1windings connected to an input terminal of the earth fault current monitoring circuit 14, and a second secondary winding with N S2 The secondary winding 20 includes turns connected to an input terminal of the grounded neutral conductor's residual current monitoring circuit 16. In other embodiments, the secondary winding 20 may contain a single winding with a first tapped output connected to an input terminal of the earth fault residual current monitoring circuit 14, and a second tapped output connected to an input terminal of the grounded neutral conductor's residual current monitoring circuit 16. It is clear to the person skilled in the art that other, similar winding configurations may also be used.

[0022] According to this invention, the residual current device detects 10 phase earth faults at the load, as R L2Gspecified, and low impedances between earth and neutral at the load, as R G2N specified by monitoring signals at the secondary winding 20 of the transformer 12.

[0023] In particular, an earth leak current I leads to L2G , caused by an external conductor earth fault R L2G flows, resulting in a differential current through transformer 12. The differential current causes the secondary winding 20 to draw a current I. s conducts, which is equal to the earth leakage current I L2G , divided by the number of turns N s the secondary winding 20 is: Is=IL2GNs

[0024] Therefore, by monitoring the secondary current I s and because the winding ratio N s The earth leakage current I is known L2G can be determined from equation (1).

[0025] Furthermore, low impedances between earth and neutral conductor R G2NThe value can be determined by applying an AC signal (e.g., a voltage or a current) to the secondary winding 20 and monitoring a corresponding load signal (e.g., a current or a voltage) at the secondary winding 20 of the transformer 12. G2N This appears as a relatively low impedance at the secondary winding 20. In particular, it appears under the assumption that R L2G significantly larger than R G2N is the impedance Z s of transformer 12 at the secondary winding as: Zs=Hs2×RG2N

[0026] Thus, if the secondary winding 20 is connected to an alternating voltage V ac is subjected to a fixed quantity and frequency, the load current I ac , which is required to apply voltage to the secondary winding 20: Iac=VacZs

[0027] Therefore, R can G2N by monitoring the load current I accan be calculated from equations (2) and (3).

[0028] According to this invention, the earth fault current monitoring circuit 14 detects an earth leakage current I L2G by monitoring the differential current in transformer 12. In particular, during a first predetermined time interval T1, the earth fault residual current monitoring circuit 14 monitors the current I s in the secondary winding 20. The first predetermined time interval T1 can be approximately 100 milliseconds, more generally between approximately 1 and 560 milliseconds, although other time intervals can also be used. By monitoring the secondary current I s and because the winding ratio N s The earth leakage current I is known L2G can be determined from equation (1). Thus, when the monitored secondary current I is generated s indicates that the earth leakage current I L2GWhen a first predetermined value has been exceeded, the earth fault current monitoring circuit 14 sends a first detection signal GCF to an output terminal, which causes the interruption circuit 18 to open the outer conductor L and thereby interrupt the AC system.

[0029] For example, the GCF signal may normally be LOW, but can switch from LOW to HIGH when the earth leakage current I L2G exceeds the first predetermined value. It is clear to the average expert that the GCF signal can also normally be HIGH, but can switch from HIGH to LOW when the earth leakage current I exceeds the threshold value. L2G exceeds the first predetermined value.

[0030] After the first predetermined time interval T1, the fault current monitoring circuit of the grounded neutral conductor 16 applies an AC signal to the secondary winding 20 during a second predetermined time interval T2 to detect low impedances between earth and neutral conductor R. G2N to monitor. The second predetermined time interval T2 can be approximately 5 milliseconds, more generally between approximately 0.1 and 17 milliseconds, although other durations can also be used. For example, during the second predetermined time interval T2, the fault current monitoring circuit of the grounded neutral conductor 16 applies an alternating voltage of a specified value V to the secondary winding 20 of the transformer. ac and a fixed frequency f d has.

[0031] The load current I ac The voltage required to energize the secondary winding 20 is, as shown above in equation (3), equal to V. ac, divided by the impedance Z s of the transformer 12. Based on equations (2) and (3), the load current I can be ac to be written as follows: Iac=Vac(Ns2×RG2N)

[0032] Ideally, R G2N infinite, and the load current I ac is zero. However, if there is a low impedance between ground and neutral, R G2N not infinite and can be quite low. If R G2N As the load current decreases, the load current I increases. ac Thus, when the load current I ac exceeds a predetermined value (which indicates that R G2N (has decreased below a second predetermined threshold), the fault current monitoring circuit of the grounded neutral conductor 16 at an output terminal sends a second detection signal GNF, which causes the interrupt circuit 18 to open the outer conductor L and thereby interrupt the AC system.

[0033] For example, the GNF signal can normally be LOW, but can switch from LOW to HIGH when the load current I ac exceeds the second predetermined threshold. It is clear to the average expert that the GNF signal can also normally be HIGH, but can switch from HIGH to LOW when the load current I exceeds the threshold. ac exceeds the second predetermined threshold.

[0034] It is clear to the average expert that the fault current monitoring circuit of the grounded neutral conductor 16 can instead also supply the secondary winding 20 with an alternating current of a defined quantity I. ac and a fixed frequency f d exhibits, and the control voltage V ac The residual current monitoring circuit of the grounded neutral conductor 16 can then generate the second detection signal GNF when the monitored drive voltage V is exceeded.ac smaller than a predetermined value.

[0035] After the second predetermined time interval T2, the fault current monitoring circuit of the grounded neutral conductor 16 stops energizing the secondary winding 20, and the earth fault fault current monitoring circuit 14 resumes monitoring the differential current flowing in the transformer 12. This process is repeated iteratively to continuously detect phase-to-earth faults and low impedances between earth and neutral.

[0036] As described above, the interrupt circuit is used to open the phase conductor L based on the first and second detection signals GCF and GNF, respectively, thereby interrupting the AC system. Specifically, the interrupt circuit 18 has a first input terminal connected to the phase conductor L at the SOURCE, a second input terminal connected to an output terminal of the earth fault current monitoring circuit 14 and to an output terminal of the grounded neutral conductor fault current monitoring circuit 16, and an output terminal connected to the primary winding of the transformer 12 associated with the phase conductor L. The first input terminal is normally connected to the output terminal unless the signal at the second input terminal has a predetermined value (e.g., HIGH).Thus, when GCF and GNF are both LOW, the phase conductor L at the SOURCE is connected to the primary winding of transformer 12 assigned to phase conductor L. When GCF or GNF changes to HIGH (indicating a ground fault current or a grounded neutral fault current, respectively), the phase conductor L at the SOURCE is disconnected from the primary winding of transformer 12 assigned to phase conductor L.

[0037] It is clear to the average person skilled in the art that the interrupting circuit 18 can also be arranged between the transformer 12 and the load. In such an alternative embodiment, when GCF and GNF are both LOW, the primary winding of the transformer 12 associated with the live conductor L is connected to the live conductor L at the load. If GCF or GNF changes to HIGH (indicating a ground fault current or a fault current of the grounded neutral conductor, respectively), the primary winding of the transformer 12 associated with the live conductor L is disconnected from the live conductor L at the load.

[0038] Residual current devices according to this invention, such as the residual current device 10, can be implemented in digital circuits, in analog circuits, or in a combination of digital and analog circuits. It is now referred to Fig. 2 An exemplary residual current device 10a is described, which is implemented using digital circuits. The residual current device 10a comprises a single transformer 12, an earth fault residual current monitoring circuit 14a, an earthed neutral conductor residual current monitoring circuit 16a, and a breaker circuit 18a.

[0039] The exemplary earth fault current monitoring circuit 14a includes a buffer (or amplifier) ​​30 (referred to here as "BUFFER / AMB. 30"), a multiplexer ("MUX") 32, an analog-to-digital converter ("ADC") 34, a processor 36, and a controller 38. The buffer / amplifier 30 has an input terminal connected to the secondary winding 20 and an output terminal connected to a first input terminal IN1 of the MUX 32. The MUX 32 has a second input terminal IN2, a control terminal connected to the controller 38, and an output terminal connected to an input terminal of the ADC 34. The ADC 34 also has a control terminal connected to the controller 38 and an output terminal connected to an input terminal of the processor 36.

[0040] During the first predetermined time interval T1, the control device 38 deactivates the fault current monitoring circuit of the grounded neutral conductor 16a, the control signal V ac is zero, and the load current I ac is zero. Therefore, the output of buffer / amplifier 30 is the monitored secondary current I. s , which is connected to input terminal IN1 on MUX 32. The control unit 38 supplies MUX 32 with a control signal to select input IN1 as the output of MUX 32. The ADW 34 samples the buffered (or amplified) secondary current signal I s and delivers a digital count value that corresponds to the secondary current I sThis corresponds to the processor 36. The processor 36 can be a conventional integrated microprocessor circuit or another similar processor. Alternatively, the ADW 34 and the processor 36 can be combined in a single device containing a microprocessor and an analog-to-digital converter, such as an ATTINY461A microcontroller from Atmel Corporation, San Jose, California, or an MSP430s microcontroller from Texas Instruments Incorporated, Dallas, Texas. The processor 36 processes the converted data from I s , to determine whether the earth leakage current I L2G exceeds the first predetermined threshold.

[0041] If the earth leakage current I L2GWhen the first predetermined threshold is exceeded, the digital fault current detector 36 delivers an output signal to the control unit 38, indicating that a ground fault current is present. The control unit 38 then delivers a first detection signal GCF to the interrupt circuit 18a. In the illustrated embodiment, the interrupt circuit 18a includes a thyristor 60, which is connected to a trip coil 62 and has a control input connected to the control unit 38. The interrupt circuit 18a also has contacts 64, which are configured to trigger the solenoid coil 62. The first detection signal GCF causes the thyristor 60 to become conductive, which in turn energizes the trip coil 62, causing the contacts 64 to open and thus interrupting the AC system.

[0042] As mentioned previously, during the first predetermined time interval T1, the control device 38 deactivates the residual current monitoring circuit of the grounded neutral conductor 16a. After the first predetermined time interval T1, the control device 38 activates the residual current monitoring circuit of the grounded neutral conductor 16a for the second predetermined time interval T2.

[0043] In particular, the fault current monitoring circuit of the grounded neutral conductor 16a includes a digital signal generator 40 with an output terminal connected to an input terminal of a digital-to-analog converter (“DAW”) 42, a first low-pass filter (“TPF”) 44 with an input terminal connected to an output terminal of the DAW 42, a driver 46 with an input terminal connected to an output terminal of the TPF 44, an optional rectifier 48 with an input terminal connected to an output of the driver 46, and a second TPF 50 with an input terminal connected to an output terminal of the rectifier 48, wherein the MUX 32 has a second input terminal IN2 connected to an output of the TPF 50. The residual current monitoring circuit of the grounded neutral conductor 16a also includes the ADW 34, the digital residual current detector 36 and the control unit 38, which are described above.

[0044] After the first predetermined time interval T1, the control device 38 switches on the digital signal generator 40, which supplies digital data to the DAW 42 to generate an analog output signal with a defined amplitude and frequency. According to this invention, the digital signal generator 40 and the DAW 42 can be used to generate an AC signal with a defined amplitude and frequency, each of which can be controlled separately.

[0045] The output of the DAW 42 is smoothed by the TPF 44, and the output of the TPF 44 is routed to the transformer 12 via the driver 46. In the exemplary embodiment of Fig. 2 The fault current monitoring circuit of the grounded neutral conductor 16a applies an alternating voltage V to the secondary winding 20. ac and monitors the load current I ac , which is conducted through the secondary winding 20.

[0046] In particular, the driver 46 can contain a current mirror which provides a mirrored load current I' ac delivers, which is essentially equal to the load current I ac ' is. The mirrored load current I' ac The optional rectifier 48, which can be a full-wave rectifier or another similar rectifier circuit, can process the signal to provide a rectified output signal. This signal is smoothed by the TPF 50 and fed to the second input IN2 of the MUX 32. Thus, IN2 receives a signal corresponding to the mirrored load current I'. ac corresponds.

[0047] During the second predetermined time interval T2, the control unit 38 supplies the MUX 32 with a control signal to select input IN2 as the output of the MUX 32. The ADW 34 samples the signal of the mirrored load current I'. ac and delivers a digital count value, which corresponds to the mirrored load current I' acThis corresponds to the digital fault current detector 36. The digital fault current detector 36 processes the converted I' ac -Data to determine whether the load current I ac exceeds a predetermined value (indicating that R G2N (has decreased below a second predetermined threshold).

[0048] If R G2N If the value is less than the second predetermined threshold, the digital fault current detector 36 sends an output signal to the control unit 38, indicating that a fault current is present in the grounded neutral conductor. The control unit 38 then sends a second detection signal GNF to the interrupt circuit 18a. The second detection signal GNF causes the thyristor 60 to become conductive, which in turn energizes the trip coil 62, causing the contacts 64 to open and thus interrupting the AC system.

[0049] After the second predetermined time interval T2, the control unit 38 switches off the digital signal generator 40, and the earth fault current monitoring circuit 14 resumes monitoring the differential current flowing in the transformer 12. This process is repeated iteratively to continuously detect phase-to-earth faults and low impedances between earth and neutral.

[0050] As mentioned previously, residual current devices according to this invention can alternatively also be implemented using analog circuits. It is now described with reference to Fig. 3 An exemplary residual current device 10b is described, which is implemented using analog circuits. The residual current device 10b comprises a transformer 12, an earth fault residual current monitoring circuit 14b, an earthed neutral residual current monitoring circuit 16b, a breaker circuit 18a, and a controlled switch 94. The transformer 12 has a secondary winding 20 with a first tapped output 20a connected to N sa windings and a second tapped output 20b with N sb Twists and turns.

[0051] The exemplary earth fault current monitoring circuit 14b includes a buffer (or amplifier) ​​72 (referred to here as "BUFFER / AMB. 72"), which has an input terminal connected to the secondary winding 20 via the controlled switch 94, and an output terminal connected to an input terminal of a TPF 74. The TPF 74 has an output terminal connected to an input terminal of a peak detector 76, which has an output terminal connected to an input terminal of a comparator 78. The comparator 78 has an output terminal connected to an input terminal of the thyristor 60.

[0052] During the first predetermined time interval T1, a timer circuit (timer 92, described below) closes the controlled switch 94 and deactivates the fault current monitoring circuit of the grounded neutral conductor 16b. As a result, the control signal V acequal to zero, and the load current I ac is zero. Therefore, the output of buffer / amplifier 72 is the monitored secondary current I. s , which is smoothed by the TPF 74, and the signal of the smoothed secondary current I s is supplied to the input of the peak detector 76. The peak detector 76 provides an output DC voltage equal to the peak value of the secondary current I. s The output of the peak detector is fed to comparator 78 as an input.

[0053] The comparator 78 provides an output signal whose state changes (e.g., from LOW to HIGH) when the peak secondary current I s has a value which is the first predetermined threshold divided by N sa , exceeds (i.e., when the earth leakage current I L2G(exceeds the first predetermined threshold). Thus, the comparator 78 supplies the interrupt circuit 18a with a first detection signal GCF. The first detection signal GCF causes the thyristor 60 to become conductive, which in turn energizes the trigger coil 62 and causes the contacts 64 to open, thereby interrupting the AC system.

[0054] As mentioned previously, the grounded neutral conductor residual current monitoring circuit 16b is inactive during the first predetermined time interval T1. After the first predetermined time interval T1, the earth fault residual current monitoring circuit 14b is inactive, and the grounded neutral conductor residual current monitoring circuit 16b is active during the second predetermined time interval T2.

[0055] In particular, the fault current monitoring circuit of the grounded neutral conductor 16b includes a differential amplifier 80, a bandpass filter (“BPF”) 82, a peak detector 84, a comparator 86, a timer circuit 92, an AC signal generator 90, a driver 88, and a sampling resistor R. SENSE and a capacitor C RES . After the first predetermined time interval T1, the timer circuit opens the controlled switch 94, thereby effectively deactivating the earth fault current monitoring circuit 14b and activating the AC source 90.

[0056] The AC source 90 can be an oscillator that supplies an AC signal (e.g., an alternating voltage or an alternating current) to the driver 88. The AC source 90 can generate a sine wave, or it can instead generate a non-sinusoidal waveform, such as a square wave, ramp wave, or other similar waveform. According to this invention, the AC source 90 generates an AC signal that has a defined amplitude and a defined frequency, each of which can be controlled separately.

[0057] At the in Fig. In the embodiment shown in section 3, the AC source 90 supplies an alternating voltage signal to the driver 88, which outputs an AC output signal V. ac to the second tapped output 20b of the secondary winding 20. Thus, during the second predetermined time interval T2, the fault current monitoring circuit of the grounded neutral conductor 16b supplies the secondary winding 20 with an AC voltage signal V. ac .

[0058] The differential amplifier 80 measures the voltage across the sampling resistor R. SENSE , to determine the load current I ac to monitor the one supplied by driver 88. R SENSE The resistor can have a value between 1 and 10 ohms, although other resistance values ​​can also be used, depending on the number of turns of the secondary winding, the driver, and the sensitivity of the detection circuit. The output of the differential amplifier 80 is filtered by the BPF 82, and the filtered output is fed to the peak detector 84 and the comparator 86. The peak detector 84 provides a DC output voltage equal to the peak value of the load current I. ac The output of the peak detector is fed to comparator 86 as an input.

[0059] The comparator 86 provides an output signal whose state changes (e.g., from LOW to HIGH) when the peak value of the load current Iac has a value which is Vac, divided by the second predetermined threshold multiplied by N sb 2 , exceeds (i.e., when the low impedance between earth and neutral conductor R G2N (is less than the second predetermined threshold). Thus, the comparator 86 supplies the interrupt circuit 18a with a second detection signal GNF. The second detection signal GNF causes the thyristor 60 to become conductive, which in turn energizes the trigger coil 62 and causes the contacts 64 to open, thereby interrupting the AC system.

[0060] With reference to Fig. Section 4 now describes another alternative, exemplary residual current device 10c, implemented using analog circuits. Residual current device 10c is similar to residual current device 10b, except that the peak detectors 76 / 84 and comparators 78 / 86 are replaced by window comparators 98 and 100, respectively, which trip at both positive and negative peak values. Therefore, residual current device 10c can react more quickly to earth fault currents and fault currents of the grounded neutral conductor.

[0061] With reference to Fig.Section 5 now describes another alternative, exemplary residual current device 10d, which is implemented using analog circuits. The residual current device 10d is similar to the residual current device 10b, except that the peak detectors 76 and 84 are implemented with rectifiers 102 and 108, respectively, and low-pass filters 104 and 110. The rectifiers 102 and 108 can be full-wave or half-wave rectifiers.

[0062] The above description merely illustrates the principles of this invention, and a person skilled in the art can make various modifications without departing from the scope and idea of ​​this invention.

Claims

[1] Device (10) for use with an alternating current system comprising a live conductor (L), a neutral conductor (N) and a transformer (12), wherein the live conductor (L) and the neutral conductor (N) are each connected between a source and a load and the neutral conductor (N) is connected to earth at the source, wherein the transformer (12) comprises a first primary winding connected in series with the live conductor (L), a second primary winding connected in series with the neutral conductor and a secondary winding (20), wherein the device comprises: a first circuit connected to the secondary winding, wherein the first circuit (14) provides a first detection signal if a current from the outer conductor (L) to earth exceeds a first predetermined threshold; and a second circuit connected to the secondary winding (20), wherein the second circuit (18) includes an AC signal source (90) which supplies an AC signal with a defined amplitude and a defined frequency to the secondary winding, and wherein the second circuit (18) provides a second detection signal if an impedance between the neutral conductor (N) and earth is less than a second predetermined threshold, wherein the AC signal source (90) comprises an AC voltage source, and wherein the second circuit provides the second detection signal if a current in the secondary winding (20) exceeds a predetermined threshold. [2] Device (10) according to claim 1, which further comprises an interruption circuit (18) connected to the outer conductor (L), wherein the interruption circuit (18) interrupts the current in the alternating current system if the first circuit provides the first detection signal and / or if the second circuit provides the second detection signal. [3] Device (10) according to claim 2, wherein the interrupt circuit (18) is arranged between the source and the transformer (12). [4] Device (10) according to claim 2, wherein the interrupt circuit (18) is arranged between the load and the transformer (12). [5] Device (10) according to claim 1, wherein the secondary winding (20) comprises a single winding which is used jointly by the first circuit and by the second circuit. [6] Device (10) according to claim 1, wherein the secondary winding (20) comprises a tapped output which is connected to the second circuit. [7] Device (10) according to claim 1, wherein the secondary winding (20) comprises a first secondary winding connected to the first circuit and a second secondary winding connected to the second circuit, wherein the first secondary winding and the second secondary winding are separate windings. [8] Device (10) according to claim 1, wherein the AC signal source comprises an alternating current source and wherein the second circuit provides the second detection signal if a voltage across the secondary winding (20) exceeds a predetermined threshold. [9] Device (10) according to claim 1, wherein the first circuit comprises a digital circuit arrangement. [10] Device (10) according to claim 1, wherein the first circuit comprises a digital earth fault current monitoring circuit (14) which provides the first detection signal if a current in the secondary winding (20) exceeds the first predetermined threshold. [11] Device (10) according to claim 1, wherein the second circuit comprises a digital circuit arrangement. [12] Residual current device (10a) (Ground Fault Circuit Interrupt device, “GFCI” device) for use with an alternating current system comprising a live conductor (L), a neutral conductor (N) and a transformer (12), wherein the live conductor (L) and the neutral conductor (N) are each connected between a source and a load and the neutral conductor (N) is connected to earth at the source, wherein the transformer (12) has a first primary winding connected in series with the live conductor (L), a second primary winding connected in series with the neutral conductor (N), and a secondary winding, wherein the device comprises: an earth fault current monitoring circuit (14a) connected to the secondary winding, wherein the earth fault current monitoring circuit (14a) provides a first detection signal if a current in the secondary winding exceeds a first predetermined threshold; and a residual current monitoring circuit (16a) of the earthed neutral conductor (N) connected to the secondary winding, wherein the residual current monitoring circuit (16a) of the earthed neutral conductor: (a) includes an AC signal source which applies an AC voltage of a specified amplitude and frequency to the secondary winding, (b) monitors a load current in the secondary winding, and (c) provides a second detection signal if the load current exceeds a predetermined threshold. [13] Residual current protection device (10a) according to claim 12, which further comprises an interruption circuit (18) connected to the phase conductor (L), wherein the interruption circuit (18) interrupts the current in the AC system if the earth fault residual current monitoring circuit (14a) provides the first detection signal and / or if the residual current monitoring circuit (16a) of the earthed neutral conductor provides the second detection signal. [14] Device (10a) according to claim 13, wherein the interrupt circuit (18) is arranged between the source and the transformer (12). [15] Device (10a) according to claim 13, wherein the interrupt circuit (18) is arranged between the load and the transformer (12). [16] Device (10a) according to claim 12, wherein the secondary winding comprises a single winding which is used jointly by the earth fault current monitoring circuit (14a) and by the second circuit. [17] Device (10a) according to claim 12, wherein the secondary winding comprises a tapped output connected to the fault current monitoring circuit (16a) of the grounded neutral conductor (N). [18] Device (10a) according to claim 12, wherein the secondary winding comprises a first secondary winding connected to the earth fault current monitoring circuit (14a) and a second secondary winding connected to the earthed neutral conductor fault current monitoring circuit (16a), wherein the first secondary winding and the second secondary winding are separate windings. [19] Device (10a) according to claim 12, wherein the AC source comprises an alternating voltage source and wherein the fault current monitoring circuit (16a) of the grounded neutral conductor provides the second detection signal if a current in the secondary winding exceeds a predetermined threshold. [20] Device (10a) according to claim 12, wherein the AC source comprises an alternating current source and wherein the fault current monitoring circuit (16a) of the grounded neutral conductor provides the second detection signal if a voltage across the secondary winding exceeds a predetermined threshold. [21] Method for residual current protection (ground fault circuit interrupt, “GFCI”) monitoring for use with an alternating current system comprising a live conductor (L), a neutral conductor (N) and a transformer (12), wherein the live conductor (L) and the neutral conductor (N) are each connected between a source and a load and the neutral conductor (N) is connected to earth at the source, wherein the transformer (12) comprises a first primary winding connected in series with the live conductor (L), a second primary winding connected in series with the neutral conductor (N), and a secondary winding, the method comprising: (a) for a first predetermined time interval: (i) Monitoring a current in the secondary winding, and (ii) Providing a first detection signal if the monitored secondary current exceeds a first predetermined threshold; and (b) for a second predetermined time interval: (i) Applying an AC voltage or AC current with a specified amplitude and frequency to the secondary winding (20); (ii) Monitoring a load current in or a voltage across the secondary winding (20); and (iii) Providing a second detection signal if the load current or load exceeds a predetermined threshold. [22] The method of claim 21, which further comprises iteratively repeating step (a) and then step (b). [23] Method according to claim 21, which further comprises interrupting the current flow in the alternating current system in response to the first detection signal and / or the second detection signal. [24] Method according to claim 21, wherein the first predetermined time interval is between about 1 and 560 milliseconds. [25] Method according to claim 21, wherein the first predetermined time interval is about 100 milliseconds. [26] Method according to claim 21, wherein the second predetermined time interval is between about 0.1 and 17 milliseconds.

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