Arc fault detector with circuit interrupter
There is here disclosed a method and apparatus for detecting the occurrence of arcing of a conductor by monitoring the current on an AC power line. The signal detected is split and directed along four separate paths to generate four signals having separate characteristics which represent the current in the line. A first path is for a signal representative of the current flowing in the line. A second path is for a signal having a pulse for each occurrence of a positive step change in current that is significant and has a di/dt value above a predetermined value. A third path is for a signal having a pulse for each occurrence of a negative step change in current that is significant and has a di/dt value above a predetermined value. A fourth path is for a signal having a voltage level representative of the broadband noise signal on the line. Using at least one of five different methods in combination with one of three input signals, a reference signal designated as “SINE” is generated. The SINE signal generated in combination with a CURRENT input is used to produce a control waveform “DELTA”. DELTA can be represented as a relative value or as an absolute difference between the SINE and the CURRENT. Each occurring half cycle of the DELTA signal is analyzed by, for example, a micro-controller for specific identifiable characteristics found to indicate the presence of arcing. Upon the detection of arcing, an output signal can be generated to activate a circuit interrupting mechanism, sound an audio alarm and/or alert a central monitoring station.
1. An arc fault detecting device capable of being electrically connected to an electrical wiring system between line side phase and neutral conductors and load side phase and neutral conductors, said device comprising:
signal detecting means coupled to detect current in a conductor of the electrical wiring system to generate a signal representative of the detected current,
signal modifying means coupled to the signal detecting means to generate at least one modified signal which represents the filtered or unfiltered current detected in the conductor, and
a micro-controller coupled to receive the at least one modified signal wherein the micro-controller performs the following:
generates a SINE signal having half cycles and zero crossings from a raw input signal, or a signal from a finite impulse response (FIR) filter, or a signal from an infinite impulse response (IIR) filter,
compares the generated SINE signal with the at least one signals generated from the signal modifying means to generate a DELTA signal,
analyzes the DELTA signal for at least one characteristic representative of the occurrence of an arc, and
generates an output signal upon detection of the at least one characteristic representative of the occurrence of an arc.
2. The device according to claim 1 wherein the signal detecting means comprises a current transformer.
3. The device according to claim 2 wherein the signal modifying means provides a CURRENT signal representative of the current in the line.
4. The device according to claim 3 wherein the CURRENT signal is used by the micro-controller to generate the SINE signal.
5. The device according to claim 4 wherein the CURRENT signal is fed to a multipoint running finite impulse response filter to smooth the signal prior to being used to generate the SINE signal.
6. The device according to claim 4 wherein the CURRENT signal is fed to an infinite impulse response (IIR) filter to smooth the signal prior to being used to generate the SINE signal.
7. The device according to claim 3 wherein the signal modifying means for generating the CURRENT signal comprises a programmable gain amplifier controlled by the micro-controller which scales the output signal to one of a number of scales.
8. The device according to claim 7 further comprising a voltage follower located between the transformer and the programmable gain amplifier, and
a low pass filter located between the programmable gain amplifier and the micro-controller, wherein the programmable gain amplifier and the low pass filter operate as a D/A converter for generating the CURRENT signal.
9. The device according to claim 2 wherein the signal modifying means provides negative and positive di/dt signals of the detected current in the line.
10. The device according to claim 9 wherein the signal modifying means provides a signal of the high frequency noise on the line.
11. The device according to claim 2 wherein either the phase or neutral conductor is coupled to the current transformer.