Fire protection switch, low-voltage circuit and procedures for serial arc faults

Abstract

Fire protection switch for a low-voltage circuit having a mains frequency for detecting serial arc faults, comprising: - a first sensor (SR1) for determining the level of the current (I) in the mains frequency range of the low-voltage circuit, - a second sensor (SR2) for determining the level (RSSI) in a high-frequency range of the low-voltage circuit, - a control unit (SE) connected to the first and second sensors (SR1, SR2) and designed to detect serial arc faults in the low-voltage electrical circuit and to emit an arc fault detection signal (FS) when an arc fault limit value (G4) is exceeded, characterized by that an arc fault detection signal (FS) is only emitted if a current change occurs when an arc fault event is detected, that the current change is a change in the effective value of the current (I).

Description

[0001] The invention relates to a fire protection switch for serial arc faults according to the preamble of patent claim 1, a low-voltage circuit and a method for detecting serial arc faults according to the preamble of patent claim 16.

[0002] Low voltage refers to voltages of up to 1000 volts AC or up to 1500 volts DC. Low voltage refers in particular to voltages greater than extra-low voltage, with values ​​of 25 or 50 volts AC or 60 or 120 volts DC. Low-voltage circuits, networks, or systems refer to circuits with rated currents of up to 125 amperes, more specifically up to 63 amperes.

[0003] A low-voltage circuit with a mains frequency refers, in particular, to alternating current circuits. Mains frequency refers to frequencies in the range 10 Hz to 1 kHz, particularly frequencies of 50 Hz, 60 Hz, 400 Hz, or 16 2 / 3 Hz. Frequencies of 50 Hz are used specifically in European low-voltage circuits and networks; 60 Hz is used in (US) American networks.

[0004] Arc faults are arcs that occur in the event of a fault, i.e., caused by faults in the electrical circuit. For example, due to poorly clamped or faulty connections in the electrical circuit, such as poorly clamped or poorly conductive contacts, e.g., in junction boxes, switches, or sockets in the low-voltage circuit.

[0005] This does not include (interference) arcs such as those that occur during regular operation of a network, e.g. when switching or on the brushes of a motor.

[0006] Serial arc faults refer to arcs that occur in the current path of the circuit, meaning that the current that also flows through a load, for example, flows through the arc. This means that if a current flows in a "nearly interrupted" conductor, a so-called serial arc fault occurs at the point of interruption.

[0007] In contrast, parallel arcs, i.e., arcs that occur between two conductors in a circuit, are not included. This means that if a (near) short circuit occurs with another conductor, this is referred to as a parallel arc fault.

[0008] Arc fault devices are relatively new protective devices for electrical circuits or power grids and are used to detect arc faults. Such arc faults can be caused by poorly clamped electrical connections in junction boxes or sockets, or by poor insulation and contacts. Arc fault devices can be used, particularly in domestic installations such as fuse boxes, to detect such faults. If a fault occurs or an arc fault threshold is exceeded, they can initiate an interruption of the electrical circuit, interrupt the circuit themselves, or emit an arc fault detection signal.

[0009] In low-voltage circuit installations, AFDs are often installed for each feeder used to a consumer, often at the point of the feeder, such as in a fuse box. Typically, one AFD is used per feeder.

[0010] Arc fault devices operate using a variety of detection principles, methods, and techniques. The spectrum or noise generated by an arc fault is often detected and evaluated in a high-frequency range.

[0011] Very complex analyses are often carried out here.

[0012] In practice, the problem with parallel branches or parallel lines that are protected by different fire protection devices is that if there is a fault in one branch, the parallel line or parallel branch also receives or is impressed with a fault arc spectrum (high-frequency noise of the fault arc) due to crosstalk, which is detected by the parallel fire protection device and leads to the shutdown of an otherwise fault-free branch by the fire protection device.

[0013] This should be done by Fig. 1 and Fig. 2 will be explained in more detail. Fig. Figure 1 shows a low-voltage circuit with two conductors L, N, e.g., a phase conductor L and a neutral conductor N. It has a first branch with a first load LOAD1, to which the two conductors are connected, and a second branch with a second load LOAD2, to which the two conductors are also connected. An AFDD (fire protection device) is connected upstream or between the first load LOAD1. A MCB (circuit breaker) is connected upstream or between the second load.

[0014] If a (serial) arc fault occurs in one of the two conductors to the first load LOAD1, this is detected by the fire protection device AFDD and can lead to a direct or indirect interruption of the electrical low-voltage circuit of this branch.

[0015] The example only shows a single-phase alternating current network with two conductors; the same applies to a three-phase alternating current network with 3 (phase) conductors or 3 (phase) conductors plus neutral conductor or other network types.

[0016] Fig. 2 shows an arrangement according to Fig. 1, with the difference that there is no (serial) arc fault in the conductors to the first load LOAD1; however, there is a (serial) arc fault in the conductors to the second load LOAD2. The lines of the two feeders are parallel, adjacent, or close to each other. Crosstalk couples the spectrum of the arc fault in the lines from the second load LOAD2 to the (fault-free) lines of the first load LOAD1. This is detected by the AFDD and leads to the interruption of the fault-free feeder to the first load LOAD1.

[0017] German patent application DE 10 2016 209 445 A1 discloses an arc fault detection unit. An arc fault detection unit for a low-voltage electrical circuit, comprising at least one voltage sensor for periodically determining electrical voltage values ​​(u(n), u(k)) of the electrical circuit, which is connected to an evaluation unit configured to continuously sum a first half of a first number of voltage values ​​(u(n), u(k)) to a first partial sum (TS1) and the second half of voltage values ​​(u(n), u(k)) to a second partial sum (TS2), and to determine a difference (DU) between the two partial sums.Either the difference (DU) or the amount of the difference (DU) is compared with a first threshold value (SW1) and an arc fault detection signal (SLES) is emitted if this threshold value is exceeded, or the difference (DU) is compared with a second threshold value (SW2) and an arc fault detection signal (SLES) is emitted if this threshold value is undershot.

[0018] The American patent application US 2009 / 0171603 A1 describes a method for detecting serial arcs in electrical signals. A method for detecting a serial arc fault in an AC signal is provided.A method for detecting a series arcing fault in an AC signal comprises: a) calculating discrete wavelet coefficients using a current signal as input for calculating the discrete wavelet coefficients and outputting an output signal filtered for the discrete wavelet coefficients; b) subsampling the output signal filtered for the discrete wavelet coefficients and outputting a subsampled output signal; c) calculating critical variables from the subsampled output signal; d) determining, based on the critical variables, whether a series arcing fault is present in the current signal; and e) outputting a signal indicating the presence of a series arcing fault in the current signal when substantially all critical variables satisfy a predetermined criterion.

[0019] Chinese Patent Application Laid-Open CN 1 05 629 112 A discloses an arc fault detection device and method. The arc fault detection device consists of a dual-bus circuit, a Rogowski coil, a sensing resistor, a signal processing circuit, a microprocessor, a test circuit, an acousto-optic alarm circuit, and a key. The dual-bus circuit supplies power to the device. The Rogowski coil detects the current signal from the protected line and converts it into a voltage signal via the sensing resistor. The signal processing circuit separates the voltage signal into a high-frequency signal and a power frequency signal. The envelope and power frequency current waveform of the high-frequency signal are output to a microprocessor. The arc fault detection is performed by the microprocessor processing.As soon as the microprocessor detects the arc fault as an arc fault, the sound-light alarm circuit generates a sound-light alarm and reports the alarm condition via the dual bus circuit. The key is connected to the microprocessor, and the test circuit is connected between the Rogowski coil and the microprocessor.

[0020] It is therefore an object of the present invention to improve the detection of serial arc faults, in particular in parallel and fused lines where crosstalk could occur.

[0021] This object is achieved by a fire protection switch having the features of patent claim 1, a low-voltage circuit according to patent claim 15 or a method having the features of patent claim 16.

[0022] According to the invention, a fire protection switch for a low-voltage circuit having a mains frequency for detecting serial arc faults comprises: - a first sensor for determining the level of current in the mains frequency range of the low-voltage circuit, - a second sensor for determining the level of levels in a high-frequency range of the low-voltage circuit, - a control unit connected to the first and second sensors, which is designed such that serial arc faults in the electrical low-voltage circuit are detected and an arc fault detection signal is emitted when an arc fault limit is exceeded, wherein an arc fault detection signal is only emitted if a current change occurs when an arc fault event is detected.

[0023] The current change is a change in the effective value of the current.

[0024] This provides a particularly reliable criterion for the current change.

[0025] According to the invention, it was found that when an arc fault event occurs, a current change occurs in the low-voltage circuit, namely a current reduction for passive loads and an current increase for active loads.

[0026] According to the invention, a further criterion, the current change, is advantageously used to detect arc faults. This allows arc fault signals or levels induced by adjacent lines to be distinguished from arc faults actually present in the circuit, since a current change is present when a fault signal is present in the circuit.

[0027] Advantageous embodiments of the invention are specified in the subclaims.

[0028] In an advantageous embodiment of the invention, a current change only occurs when a change threshold is exceeded.

[0029] This has the particular advantage of providing a particularly clear and simple decision.

[0030] In an advantageous embodiment of the invention, the level of the change threshold depends on the level of the current flowing before the arc fault event.

[0031] This has the particular advantage of enabling particularly precise detection of arc fault events in the circuit itself or of current changes.

[0032] In an advantageous embodiment of the invention, the change threshold is determined by an upper and a lower limit, the limit being determined on average by the level of the current flowing before the arc fault event, the upper limit of the change threshold being determined by the sum and the lower limit by the difference of the following two terms: Term 1, first summand or minuend, the magnitude of the current flowing before the arc fault event, Term 2, second summand or subtrahend, a product of the magnitude of the current flowing before the arc fault event and a quotient of 13 volts divided by the effective value voltage of the low-voltage circuit.

[0033] This has the particular advantage of enabling particularly accurate determination of current changes.

[0034] In an advantageous embodiment of the invention, the level of the change threshold depends on the level of a minimum tripping current and a selected minimum arc voltage.

[0035] This has the particular advantage of enabling a further, particularly accurate detection of arc fault events in the circuit itself or of current changes, especially if the circuit has many branches and distributed loads.

[0036] In an advantageous embodiment of the invention, the change threshold is determined by an upper and a lower limit, the limit being determined on average by the level of the current flowing before the arc fault event, the upper limit of the change threshold being determined by the sum and the lower limit by the difference of the following two terms: Term 1, first summand or minuend, the magnitude of the current flowing before the arc fault event, Term 2, second summand or subtrahend, a product of the level of the minimum tripping current and a quotient of a minimum arc voltage, in particular 13 volts, divided by the effective value voltage of the low-voltage circuit.

[0037] This has the particular advantage of enabling particularly accurate determination of current changes, especially when the circuit has many branches and distributed consumers.

[0038] In an advantageous embodiment of the invention, the mains frequency range is defined by a frequency in the range 10 Hz to 1 kHz, in particular by a mains frequency of 50 or 60 Hz.

[0039] This has the particular advantage of enabling improved detection of arc faults, particularly in alternating current networks.

[0040] In an advantageous embodiment of the invention, the high-frequency range is defined by a range from 10 kHz to 50 MHz, in particular by a range from 30 kHz to 30 MHz, and more specifically by frequencies in the range around 2 MHz. This has the particular advantage of enabling particularly good detection of arc faults.

[0041] In an advantageous embodiment of the invention, an arc fault event is defined by: 1. the exceeding of a first threshold value of the current level in the mains frequency range; 2. the exceeding of a second threshold value of the level in the high frequency range and 3. the exceeding of a third threshold value of the derivative of the level in the high frequency range with respect to time.

[0042] This has the particular advantage of enabling simple and reliable detection of arc fault events or arc faults.

[0043] In an advantageous embodiment of the invention, each (occurring) arc fault event is integrated or summed (over time) to form an arc fault sum. If a fourth threshold is exceeded, an arc fault detection signal is emitted.

[0044] This has the particular advantage that an arc fault detection signal is only emitted when several consecutive arc fault events occur, thus avoiding false tripping due to a random arc fault event.

[0045] In an advantageous embodiment of the invention, the integration speed during integration depends on the level of the current in the mains frequency range, in particular that integration is faster at higher currents.

[0046] This has the particular advantage that a faster interruption of the electrical circuit is initiated, especially at higher currents, which have a higher potential for damage.

[0047] In an advantageous embodiment of the invention, if there is no change in current when an arc fault event is detected, an integration or summation is aborted, reset or not carried out.

[0048] This has the particular advantage that crosstalk between arc fault signals prevents circuits from being interrupted.

[0049] In an advantageous embodiment of the invention, an interruption unit is provided which interrupts the current flow in the low-voltage circuit when an arc fault detection signal is present.

[0050] This has the particular advantage that if an arc fault detection signal is present, the low-voltage circuit is interrupted.

[0051] In an advantageous embodiment of the invention, the fire protection switch has a housing.

[0052] The interruption unit can be arranged within the housing, so that a compact switch is advantageously provided.

[0053] The interruption unit can be located outside the housing of the fire protection switch. It can, for example, be part of another switch, such as a miniature circuit breaker or residual current device, allowing an existing network to be retrofitted with a fire protection switch function without wasting unnecessary installation space. The arc fault detection signal can be transmitted to the interruption unit or is transmitted to the interruption unit.

[0054] In an advantageous embodiment of the invention, the second sensor determines a voltage, a current or a power in the high-frequency range as a level.

[0055] This has the particular advantage of providing a simple way to detect arc faults.

[0056] In an advantageous embodiment of the invention, the fire protection switch is arranged in a first branch of the low-voltage circuit. In an advantageous embodiment of the invention, lines of a second branch of the low-voltage circuit are arranged at least partially parallel to the first branch.

[0057] This has the particular advantage that it enables reliable detection of arc faults, particularly when there are multiple branches.

[0058] According to the invention, a low-voltage circuit comprising a fire protection switch according to the invention is also claimed.

[0059] The invention further claims a parallel method in which serial arc faults are detected in a low-voltage electrical circuit and an arc fault detection signal is emitted when an arc fault threshold is exceeded. An arc fault detection signal is only emitted if a current change occurs upon detection of an arc fault event.

[0060] All embodiments, both in dependent form referring back to the independent patent claims and referring back only to individual features or combinations of features of patent claims, result in an improvement of a fire protection switch.

[0061] 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 embodiments, which are explained in more detail in connection with the drawings.

[0062] The drawing shows: Fig. 1 a first representation of a low-voltage circuit to explain the invention, Fig. 2 a second representation of a low-voltage circuit to explain the invention, Fig. 3 a first representation of signal curves, Fig. 4 a second representation of signal curves, Fig. 5 a third representation of signal curves, Fig. 6 a fourth representation of signal curves, Fig. 7 a first block diagram of a fire protection switch, Fig. 8 a second block diagram of a fire protection switch, Fig. 9 a third block diagram of a fire protection switch, Fig. 10 a fourth block diagram of a fire protection switch.

[0063] Fig. 1 and Fig. 2 shows a schematic diagram of a low-voltage circuit with two branches, as explained above.

[0064] Fig. Figure 3 shows five signal curves over time. From top to bottom: 1. The course of the electric current I in a low-voltage circuit, for example in the first branch with the load LOAD1 according to Fig. 1.

[0065] The first section shows a fault-free case with a sinusoidal current I.

[0066] The second section shows a fault case with a series arc fault, which has an only approximately sinusoidal current I.

[0067] A first threshold value G1 is shown, each as a positive or negative threshold value of the same amount. In one embodiment of the invention, an arc fault is detected or an arc fault event only occurs when this (positive or negative) first threshold value G1 is exceeded. The first threshold value G1 can, for example, be 1 A, be in the range around 1 A, or have a value in the range from 500 mA to 5 amperes, in particular from 500 mA to 3 amperes.

[0068] 2. The RSSI level curve in the high-frequency range. The first section again shows the error-free case, with a low level. The second section again shows the error case, with a high level. A second threshold value G2 is shown.

[0069] In one embodiment of the invention, an arc fault detection or an arc fault event only occurs when this second threshold value G2 is exceeded.

[0070] The RSSI level can be a voltage, a current, a power, or an equivalent. It can be a direct level or a logarithmized level. For example, it can be the level at a specific frequency in the high-frequency range, or the level in a specific frequency range, i.e., a certain bandwidth at a frequency, in the high-frequency range.

[0071] For example, this could be the level at a frequency of 2 MHz with a bandwidth of 300 kHz. For example, the second threshold could be in the range of 300 mV to 1 volt.

[0072] The second threshold G2 can be a relative threshold, ie the height changes, for example depending on the noise or noise level on the line.

[0073] 3. The course of the first derivative of the level RSSI in the high frequency range over time dRSSI / dt.

[0074] The first section again shows the error-free case, where the time derivative has a value of zero. The second section again shows the error case, with peaks similar to a Dirac pulse at the edges of the RSSI level.

[0075] A third threshold G3 is shown, each as a positive or negative (-G3) threshold of the same amount.

[0076] In one embodiment of the invention, an arc fault detection or an arc fault event only occurs when this (positive or negative) third threshold value G3 is exceeded.

[0077] The third threshold value G3 can, for example, be 5000 volts / s.

[0078] 4. The course of an arc fault event Arc detector, which in this example is composed of the sum of the exceedance of the first threshold value G1 (the level of current I in the mains frequency range); the exceedance of the second threshold value G2 of the level RSSI in the high frequency range and the exceedance of the third threshold value G3 of the derivative of the level RSSI in the high frequency range with respect to time dRSSI / dt, for each half-wave of the electrical current I.

[0079] The first section again shows the fault-free case, where no pulses from an arc fault event are present. The second section again shows the fault case, with (continuous) pulses for an arc fault event.

[0080] 5. The arc fault integrator curve, where, in the example, each arc fault event is integrated or summed to form an arc fault sum. A fourth threshold G4 is provided, which, if exceeded, triggers an arc fault detection signal. In the example, five arc fault events must occur before the fourth threshold G4 is reached and a fault detection signal is triggered.

[0081] The integration speed can depend on the current level I. Alternatively, the fourth threshold G4 can dynamically depend on the current level. For example, an arc fault detection signal can be generated within 800 ms (slow integration speed) at a current of 2.5 A. At a current of 16 A, it can be generated within 100 ms (fast integration speed).

[0082] Fig. 4 shows a representation according to Fig. 3, with the difference that an arc fault event is present or detected, but by a level that is caused by crosstalk from another faulty line, for example according to Fig. 2, is coupled into an undisturbed line.

[0083] Fig. 4 is identical to Fig. 3, with the difference that the current waveform I is sinusoidal in both sections.

[0084] Fig. 5 shows a representation according to Fig. 3, with the difference that a current change according to the invention occurs when an arc fault event occurs or is detected. For this purpose, the effective value Irms of the current I over time is now entered in second place from the top.

[0085] The first section again presents the fault-free case, where the effective value of the current I remains constant. The second section again presents the fault case, where a current change occurs when the arc fault occurs. In the example, this represents a current reduction, as would occur with passive loads. In the case of active loads, a current increase would occur. This reduction in the effective value of the electrical current remains after the arc fault event.

[0086] A change threshold is shown in each case, designated as an upper (G5o) and a lower (G5u) threshold. This change threshold is determined by an upper (G5o) and a lower limit (G5u).

[0087] In a simple embodiment, the upper limit of the change threshold can be defined by the sum and the lower limit by the difference of the following two terms: Term 1, first summand or minuend, the magnitude of the current ip flowing before the arc fault event, Term 2, second summand or subtrahend, a product of the magnitude of the current ip flowing before the arc fault event and a quotient of 13 volts divided by the effective value voltage of the low-voltage circuit.

[0088] In this case, the change threshold is a relative value that can be continuously recalculated if necessary.

[0089] In an improved embodiment, the upper limit of the change threshold can be defined by the sum and the lower limit by the difference of the following two terms: Term 1, first summand or minuend, the magnitude of the current ip flowing before the arc fault event, Term 2, second summand or subtrahend, a product of the level of the minimum tripping current and a quotient of a minimum arc voltage, in particular 13 volts, divided by the effective value voltage of the low-voltage circuit.

[0090] The minimum tripping current is the smallest possible value for tripping. According to the standard, it is at least 2.5 amps. Depending on the device, it can be lower. For a device with a minimum tripping current of 1.5 amps, a minimum current change of 85 mA (upward or downward) results from a low-voltage circuit RMS voltage of 230 volts and a (selected) minimum arc voltage of 13 volts.

[0091] In this case, the minimum current change or change threshold is + / - 85 mA, ie is an absolute value.

[0092] Thus, the change threshold is a range around the level of the current ip flowing before the arc fault event.

[0093] If this range is exceeded or undershot, the change threshold is exceeded.

[0094] The minimum arc voltage can be assumed to be 13 volts or a value in the range 10 to 30 volts, in particular 10 to 20 volts.

[0095] The minimum tripping current can be assumed to be 2.5 A or less, or a value in the range 100 mA to 3 A, in particular 500 mA to 2.5 A.

[0096] Fig. 6 shows a representation according to Fig. 5, with the difference that the case according to Fig. 4 or Fig. 2 is shown.

[0097] This means that crosstalk is present. The effective value of the electrical current does not change when the arc fault event occurs or is detected. The change threshold is not exceeded (upper change threshold not exceeded, lower change threshold not undershot).

[0098] In one embodiment, the fault integrator can therefore be reset, as in Fig. 6 is shown.

[0099] Alternatively, the integration or summation can be aborted or not started / not carried out.

[0100] Fig. 7 shows a first block diagram of a fire protection switch to explain the invention. Fig. 7 shows two conductors L, N of a low-voltage circuit passing through a fire protection switch AFD to the load LOAD1, analogous to Fig. 1. At least one of the two conductors L, N is assigned a first sensor SR1 for determining the level of current I in the mains frequency range of the low-voltage circuit, and a second sensor SR2 for determining the level of levels RSSI in a high-frequency range of the low-voltage circuit.

[0101] The two sensors SR1, SR2 are connected to a control unit SE, which determines the presence of serial arc faults and, if arc faults are present, outputs an arc fault detection signal FS at an output.

[0102] The control unit SE can have further components, such as a central computer unit MCU, analog-digital converters ADU1, ADU2, and / or filters FI1, FI2.

[0103] The filters FI1, FI2 can perform low-pass, high-pass, and / or band-pass filtering to limit or adapt the signals from the sensors SR1, SR2.

[0104] The sensor signals, which may have been filtered in this way, can be converted from analog to digital and are further processed / evaluated by the MCU.

[0105] The sensors SR1, SR2 and control unit SE can be arranged in a first housing GEH1.

[0106] Fig. 8 shows a block diagram according to Fig. 7, with the difference that conductors L and N have an interruption unit UE for interrupting the conductors of the circuit. The arc fault detection signal FS is transmitted from the control unit SE to the interruption unit UE, so that the circuit is interrupted in the event of an arc fault.

[0107] Furthermore, the sensors SR1, SR2, the control unit SE and the interruption unit UE are shown as separate components that are connected to each other by lines.

[0108] Fig. 9 shows a block diagram according to Fig. 8, with the difference that the sensors SR1, SR2, the control unit SE, and the interruption unit UE are housed in a housing GEH2. This creates a compact device in which all components are located. This device in the housing GEH2 could be the fire protection switch AFDD according to Fig. 1 or Fig. 2 be.

[0109] Fig. 10 shows an arrangement according to Fig. 9, with the difference that the interruption unit UE is arranged externally, ie outside the housing GEH2.

[0110] Only the sensors SR1, SR2 and the control unit SE are arranged in a housing GEH3.

[0111] The interruption unit UE can be a stand-alone component to which the arc fault detection signal FS is supplied. The interruption unit UE can also be part of another switch, switching device, or circuit breaker, such as a miniature circuit breaker or residual current device, which is triggered by the arc fault detection signal FS, i.e., interrupts the circuit.

[0112] In the following, the invention is explained in more detail in further embodiments in other words.

[0113] The current in the mains frequency range (50 or 60 Hz) must preferably exceed a first threshold G1. The RSSI level must preferably exceed a second threshold G2. It is assumed that an arc fault generates strong levels. Furthermore, it is assumed that the arc fault rise occurs very quickly, usually less than 1 µs. This means that the derivative of the level must preferably be greater than a third threshold G3, at the start and end of the arc fault. All of these criteria are met in each half-wave of the mains frequency for an arc fault. To avoid nuisance tripping, these arc fault events can be integrated until a fourth threshold is reached, i.e. the fault situation has lasted long enough. In this case, an arc fault detection signal is generated that can be used directly or indirectly to interrupt the circuit.

[0114] In most electrical installations, cables from different branches are bundled together or are located in close proximity. This allows crosstalk or coupling of signals or interference from one cable to another, see Fig. 2.

[0115] If an arc fault is generated in another feeder, its level may be detected in the feeder protected by an AFD.

[0116] The fire protection switch may be triggered incorrectly.

[0117] A similar situation can occur with a consumer that generates (disturbing) high-frequency levels.

[0118] A serial arc fault can be thought of as an electrical component connected in series in a circuit. The arc fault has an impedance and arc voltage that depend on the magnitude of the current I, the distance, and other environmental conditions, such as temperature, conductor materials, the presence of smoldering insulation materials near the fault, etc.

[0119] The following equation can be used to understand the effect of the series arc: Vn=Varc+ZLoad*i Vn Effective value voltage of the circuit Varc arc fault voltage ZLoad impedance of the consumer i Power in the mains frequency range i=(Vn−Varc) / ZLoad

[0120] If the consumer or load is passive (resistor, motor without control, ...) the current must decrease, fall.

[0121] When the consumer or load is active (switching power supply, motor with control, ...) the current must rise or increase.

[0122] The current change di at the start of the arc fault is: di=iP−iA=Varc / ZLoad=iP*Varc / Vn iP current before the fault light source iA current after the start of the arc fault

[0123] The actual arc fault voltage or arc voltage is unknown, but it can be assumed that the arc fault voltage or arc burning voltage has a minimum value. For copper conductors, the minimum possible arc burning voltage is 13 V or in the range of 13 V, more generally a value between 10 and 20 volts.

[0124] With this arc fault voltage, a minimal current change can be assumed: di(minimal)=iP*13 Volt / Vn

[0125] With a mains voltage (effective value) of 230 volts: di(minimum)=iP*13 volts / 230 volts

[0126] That is, the change threshold G5 is (upper or lower change threshold): Current before the arc fault (event) + / - di times the current before the arc fault (event).

[0127] This allows for adaptive adjustment of the change threshold for better / clearer detection of arc faults.

[0128] Alternatively, in an improved variant, the upper limit of the change threshold can be defined by the sum and the lower limit by the difference of the following two terms: Term 1, first summand or minuend, the magnitude of the current ip flowing before the arc fault event, Term 2, second summand or subtrahend, a product of the level of the minimum tripping current and a quotient of a minimum arc voltage, in particular 13 volts, divided by the effective value voltage of the low-voltage circuit.

[0129] Ie: di(minimal)=Imin*Varc / Vn With: Imin Minimum tripping current, 2.5A or less, e.g. 1.5A Vn Effective value voltage of the circuit, e.g. 230 V Varc (assumed) arc fault voltage, e.g. 13 V G5=ip + / − di (minimal) G5o=ip+di(minimal) G5u=iP−di(minimal) iP current before the fault light source

[0130] In the example, the minimum current change or the change threshold is + / - 141 mA (at 2.5A) or + / - 85 mA (at 1.5A), ie is an absolute value.

[0131] Thus, the change threshold is a range around the magnitude of the current ip flowing before the arc fault event. If this range is exceeded or undershot, the change threshold is exceeded.

[0132] A new arc fault criterion has been introduced: the current value with / after the onset of the arc fault must change by at least + / - di. Otherwise, it can be assumed that the suspected arc fault levels originate from other sources. This is the case, for example, in Fig. 6: the new condition is not met, a fault integrator / fault arc sum fault integrator is reset.

[0133] The arc voltage, and thus the change threshold G5, can be selected to be greater than 13 volts. Alternatively, the value can be selected depending on the materials used.

[0134] The controller can have a microprocessor or be microprocessor-controlled. The processes can be performed in the microprocessor or its peripheral components.

[0135] Although the invention has been illustrated and described in detail by the embodiment, the invention is not limited by the disclosed examples and other variations can be derived therefrom by a person skilled in the art without departing from the scope of the invention.

Claims

[1] Fire protection switch for a low-voltage circuit having a mains frequency for detecting serial arc faults, comprising: - a first sensor (SR1) for determining the level of the current (I) in the mains frequency range of the low-voltage circuit, - a second sensor (SR2) for determining the level (RSSI) in a high-frequency range of the low-voltage circuit, - a control unit (SE) connected to the first and second sensors (SR1, SR2) and designed to detect serial arc faults in the low-voltage electrical circuit and to emit an arc fault detection signal (FS) when an arc fault limit value (G4) is exceeded, characterized by , that an arc fault detection signal (FS) is only emitted if a current change occurs when an arc fault event is detected, that the current change is a change in the effective value of the current (I). [2] Fire protection switch according to claim 1, characterized by that a current change only occurs when a change threshold (G5) is exceeded. [3] Fire protection switch according to claim 2, characterized by that the level of the change threshold (G5) depends on the level of a minimum tripping current and a selected minimum arc voltage. [4] Fire protection switch according to claim 3, characterized by that the change threshold (G5) is determined by an upper and a lower limit, whereby the upper limit of the change threshold is determined by the sum and the lower limit by the difference of the following two terms: Term 1, first summand or minuend, the magnitude of the current flowing before the arc fault event, Term 2, second summand or subtrahend, a product of the level of the minimum tripping current and a quotient of a minimum arc voltage, in particular 13 volts, divided by the effective value voltage of the low-voltage circuit. [5] Fire protection switch according to one of the preceding claims, characterized by that the mains frequency range is defined by a frequency in the range 10 Hz to 1 kHz, in particular by a mains frequency of 50 or 60 Hz. [6] Fire protection switch according to one of the preceding claims, characterized by that the high frequency range is defined by a range from 10 KHz to 50 MHz, in particular by a range from 30 KHz to 30 MHz, more specifically by frequencies in the range around 2 MHz. [7] Fire protection switch according to one of the preceding claims, characterized by that an arc fault event is defined by:

1. the exceeding of a first threshold value (G1) of the level of the current (I) in the mains frequency range; 2. the exceeding of a second threshold value (G2) of the level (RSSI) in the high frequency range and 3. the exceeding of a third threshold value (G3) of the derivative of the level (RSSI) in the high frequency range with respect to time (dRSSI / dt). [8] Fire protection switch according to claim 7, characterized by that each arc fault event is integrated or summed to form a fault integrator, and when a fourth threshold value (G4) is exceeded, an arc fault detection signal (FS) is emitted. [9] Fire protection switch according to claim 8, characterized by that during integration the integration speed depends on the level of the current in the mains frequency range, in particular that integration is faster at higher currents. [10] Fire protection switch according to claim 8 or 9, characterized by that if there is no change in current when an arc fault event is detected, an integration or summation is aborted, reset or not carried out. [11] Fire protection switch according to one of the preceding claims, characterized by that an interruption unit (UE) is provided which interrupts the current flow in the low-voltage circuit when an arc fault detection signal (FS) is present. [12] Fire protection switch according to claim 11, characterized by that the fire protection switch has a housing and the interruption unit (UE) is arranged inside or outside the housing, in particular in the case of an external interruption unit, the arc fault detection signal (UE) can be transmitted to the interruption unit. [13] Fire protection switch according to one of the preceding claims, characterized bythat the first sensor (SR1) is a current sensor. [14] Fire protection switch according to one of the preceding claims, characterized by that the second sensor (SR2) determines a voltage, a current or a power in the high frequency range as a level. [15] Low-voltage circuit comprising a fire protection switch according to one of claims 1 to 14. [16] Method in which serial arc faults are detected in a low-voltage electrical circuit and an arc fault detection signal (FS) is emitted when an arc fault limit is exceeded, characterized by , that an arc fault detection signal (FS) is only emitted if a current change occurs when an arc fault event is detected, that an arc fault event is defined by:

1. the exceeding of a first threshold value (G1) of the level of the current (I) in the mains frequency range; 2. the exceeding of a second threshold value (G2) of the level (RSSI) in the high frequency range and 3. the exceeding of a third threshold value (G3) of the derivative of the level (RSSI) in the high frequency range with respect to time (dRSSI / dt). [17] Method according to claim 16, characterized by , that a current change only occurs when a change threshold (G5) is exceeded, that the change threshold (G5) depends on the level of the minimum tripping current and a selected minimum arc voltage.

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